on the early history of spinning and spin research in the ... · the direction of research on...

Journal of Aeronautical History Paper 2019/05

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On the early history of spinning and spin research in the UK

Part 3: the period 1940 to 1949

Brian Brinkworth

Waterlooville UK

Abstract

This third part of a study of the history of spinning and spin research in the UK covers the decade of

the 1940s, which was dominated by almost five years of the Second World War. New types of

aircraft were required to replace obsolete ones and to fill changing operational needs, though they

were subject to essentially the same spin testing procedures as in the pre-war period. Testing with

dynamic models continued in the vertical Free Spinning Tunnel at the Royal Aircraft Establishment,

and at full-scale at the Aeroplane and Armament Experimental Establishment. In the later years of

the war, the first squadrons of jet-propelled types were formed, followed by the appearance of

aircraft with new configurations for flight in the compressible range.

Although little fundamental research on spinning could be undertaken in wartime conditions,

progress continued, mainly through empirical developments in the model testing methods. These

included refinement of the modelling by, for example, representing the angular momentum of

engines and propellers, and of the test procedures to improve the agreement between the outcome of

a model test and that of the corresponding aircraft test at full-scale. These were significant

advances, which were made at the expense of greater complexity in the methods employed.

1. Introduction

1.1 Spinning and recovery

The development in Britain of an understanding of the spinning of aircraft and of means of

recovering from spins has been reviewed previously in this journal, covering the earlier periods

from 1909 to 1929 (1) and from 1930 to 1939 (2). This is continued here for the decade of the 1940s,

which include most of the years of World War Two (WW2). By way of introduction, a brief outline

is given here of key elements of that understanding, and of the situation as it stood at the end of the

1930s.

The spin had been a known hazard to manned flight from its earliest days, generally following a

stall, with one wing dropping. The aircraft then descends rapidly along a vertical helical path in a

combination of falling and rotating, while remaining deeply stalled. Two distinct types of spin had

been identified - the steep spin, in which the incidence of the aircraft to its path lies roughly in the

range 30 o

to 50 o

, and the flat spin, where it can be 70 o

or more. The rate of rotation is higher in the

flat spin, sometimes taking less than 2 seconds per turn, and it is rarely possible to recover from it.

The spin is a steady state, with the inertia of the dynamic motion in equilibrium with the

aerodynamic forces and moments caused by the airflow over the aircraft. A complete theoretical

representation of this state had been established before the end of the 1920s. But in the deeply-

stalled condition of the spin the airflow over the aircraft is separated, and the aerodynamics of that


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situation had not been investigated by the end of the 1930s. Thus, estimation of the applied forces

and moments in the spin could be made only from empirical data.

Actions by the pilot to bring about recovery from the spin had been established by trial and error

during the Great War (WW1), and were duly incorporated as standard procedures in pilot training.

This was usually to centralise the controls, then apply opposite rudder to slow the rotation, followed

by moving the stick forward to unstall the wing and begin a pull-out. The forward speed on emerging

from a spin can be high, and care is needed in this phase to avoid inducing a high normal

acceleration. When aircraft were small and light, recovery actions could be a sequence of

independent measures, but as the mass and moments of inertia grew with aircraft development, they

tended to merge into one progressive movement.

1.2 The position in the late 1930s

By the end of the 1930s the body of measurements that had been gathered with models on rotating

balances in wind tunnels and in flight at full-scale allowed some advice to be given to designers on

features of an aircraft that could reduce its tendency to spin and increase the chance of recovery if a

spin occurred.

For new aircraft to be considered for acceptance into service with the RAF and the Fleet Air Arm

(FAA), prototypes were required to be evaluated by the Aeroplane and Armament Experimental

Establishment (A&AEE), then at Martlesham Heath, or the Marine Aircraft Experimental

Establishment (MAEE) at Felixstowe. The trial programmes conducted there included testing in the

spin. The RAF requirement for acceptance of a fighter aircraft was that it should be coming out of

the spin within two further turns after moving the controls to the positions specified for recovery. If

the type was ordered into production, examples of the first aircraft to be completed were checked

again by A&AEE for the Release to Service. It was not unusual for problems in handling, including

irregularity in the spin, to appear at this stage. Advice on possible means of rectification was often

offered to the manufacturer, or where the reasons for failure were not clear, aircraft could be sent to

RAE at Farnborough for more detailed examination.

The direction of research on spinning had taken a new direction in the latter part of the decade, with

the opening of the vertical Free Spinning Tunnel at the RAE (3). The tunnel is described in part 2 of

this paper (2). It allowed models, correctly scaled geometrically and dynamically (in terms of

inertia), to be set spinning in an up-going airstream matched to the rate of fall. Their motions could

then be observed and measured, as the spin developed and in a prolonged spin. Then the controls

were moved to represent the standard method of recovery. Models were observed to behave in

ways that were sufficiently similar to those found in flight at full-scale for this procedure to become

a major advance.

It was considered that there could be factors that could affect the scaling of model results to

represent the behaviour of a given aircraft accurately. Accordingly, for model testing two measures

were taken routinely to bias the situation and build a factor of safety into the procedure. One was to

modify the model so that its moment of inertia in pitch was 10% larger than the value given by

scaling the data for the full-scale aircraft, and to position the centre of gravity 6% of the mean chord

aft of the normal rearward limit. By this the stability in pitch was reduced, a factor known to make

a transition to the flat spin more likely. The second measure was to attach a vane to the tip of the


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wing that was to be innermost in the spin, to apply an additional turning moment in the pro-spin

direction. An arbitrary unit had been adopted for moments, such that ten units would be roughly

equivalent to that applied by a fully-deflected rudder at full-scale. Initially, the behaviour

considered to be satisfactory was for recovery to take place within the full-scale equivalent of 10

seconds of the controls being activated, with an applied moment equivalent to 10 units at full-scale.

Tests were also repeated with increasing pro-spin moments, to establish the value beyond which

recovery became impossible.

The spinning characteristics of a new design could now be estimated as soon as its shape and mass

distribution had been established sufficiently for a representative model to be made. If the spin and

recovery in a model test was unsatisfactory, corrective measures could be tried on the model to

advise the designers. From a combination of theory and experience, these were usually changes

intended to increase the aerodynamic moment caused by side forces on the rear fuselage and the fin

and rudder, which arose from the displacement in yaw experienced by the aircraft in a spin. In

recovery, the moment produced by movement of the rudder was vital, as this was the first action to

be undertaken in the standard procedure for recovery taught to pilots.

An important further advance in this direction was put forward by RAE at the turn of the 1930s (4, 5).

In this the factors considered to have the greatest influence on the spinning behaviour and recovery

of aircraft were represented approximately by three non-dimensional coefficients:

X, the Inertia Coefficient, based on the difference (C - A) between its moments of inertia

about the normal (yaw) axis and the longitudinal (roll) axis respectively*,

Y, the Body Damping Ratio, representing the restoring moment of forces on the projected

side area of the rear fuselage and empennage in a displacement in yaw, and

Z, the Unshielded Rudder Volume Coefficient, expressing the effectiveness of the rudder

in applying a restoring moment to begin the recovery.

(Symbols representing these coefficients were not assigned originally. X, Y and Z were used in Part 2

of this study (2) and are continued in use here).

Coefficients similar to Y and Z were familiar to aerodynamicists from their use in estimation of

stability and control, though in Z the term 'unshielded' referred to the part of the rudder that lay

outside the estimated path of the wake shed from the tailplane at the incidence expected in a spin.

The coefficients are non-dimensional so that a model that is correctly scaled dynamically has the

same numerical values for them as does the full-size aircraft. When values of Y and Z from full-

scale and model tests were plotted against X, it was found that the points for aircraft with

satisfactory and unsatisfactory spin behaviour were sufficiently separated to allow some provisional

'pass/fail' boundaries for Y and Z to be laid down for given values of X. With further experience, it

was hoped that this approach would at last meet the objective of enabling designers to have an idea

of the risk of a new type developing a dangerous spin and being unable to recover from a spin if one

occurred. That could be checked routinely during the design and development once values for X, Y

and Z could be estimated.

* The generation of inertia couples in pitch and yaw during a spin is explained and illustrated in

sections 5.1.2 to 5.1.5 of part 2 of this paper (2).


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2. The early 1940s

2.1 A new situation

When World War 2 opened on 3rd September 1939, plans that had been prepared for this eventuality

were set in motion promptly. It had been recognised since WW1 that air power would be the major

factor in any future conflict, and the means of projecting British capability in that area had been

embodied in the War Potential plan for aircraft production that was revised periodically. By the

1930s this envisaged that a total monthly output of 2,000 aircraft of all types would be required to

sustain a war if it was prolonged. After events in Europe showed that Germany was likely to

become the enemy again, the initial stages of an expansion of industry to implement War Potential

were begun in 1936 (6).

The strategic aim for the RAF would include the effective disruption of the enemy's arms production

and infrastructure. Orders were placed for the development of the long-range heavy bombers needed

for this role, but it was anticipated that, even with maximum effort, it would take several years to

reach the rate of production required to sustain that. Meanwhile, an enemy was being faced which

already had substantial air resources, strengthened by secret aircrew training schemes and operational

experience in the Spanish Civil War. The immediate needs were for the protection of the homeland,

requiring further development of the country's integrated air defence system by the inclusion of

radar and emphasis on the output of fighters for the operational arm. Facilities would also have to

be built up for the training of the great expansion in the number of aircrew required by the plan.

Among many other factors, it would be necessary to ensure that spinning would not be a significant

hazard for those involved.

Procedures for the procurement of a new aircraft that had taken years in peacetime were now

telescoped, to the point of ordering 'off the drawing board'. It had been usual for rigorous testing to

take place before any order was placed for production in quantity. The loss of a sole prototype in

test flying had previously set back programmes considerably, so now two or more were required,

and preparations for production would often be started before they had flown. The first spins were

part of the contractor's trials, detailed in specifications and production contracts, and if difficulties

were encountered at that stage, companies were encouraged to consult RAE for suggestions about

modifications to correct them. The A&AEE and MAEE, where the final acceptance tests for release

of a new type into service were carried out, had both been located on the East Coast. There they

would be particularly vulnerable to enemy action, so they were moved in 1939 to Boscombe Down

near Salisbury and Helensburgh on the Firth of Clyde respectively. RAE remained at Farnborough

in northeast Hampshire.

Formerly, the practice in spin testing had been to let a spin continue for eight turns to ensure that it

was fully developed before recovery action commenced. By 1940 this had been reduced for fighter

aircraft to just two turns, the part known as the 'incipient spin' region. This was reckoned to be

more representative of the current situation, when trained pilots were expected to recognise quickly

that they had entered a spin and to begin recovery action promptly. For an aircraft to be cleared to

go into service with the RAF, recovery had then to take place within two further turns after the

controls had been moved to begin recovery (7). Standard spinning trials were just a small part of

very wide-ranging evaluations of the suitability of a new type to be accepted into service. As well

as all aspects of its performance and handling, these tests reviewed its suitability as a workplace,

covering matters such as how the layout would help aircrew to carry out their duties efficiently,

safety in operation, and in leaving the aircraft in emergencies. A new type or mark of aircraft could


137

not be delivered to RAF operational units unless a certificate of Release to Service had been issued

by A&AEE. The accompanying report of test results often included a list of modifications required

to be made to the design of the aircraft concerned. The significance of this mechanism for the

feedback of good practice to designers, based on the ever-growing experience of independent testers,

has not been generally appreciated.

This vital safeguard of quality was exercised with rigour and professionalism despite the great

pressures of the wartime period. During the six years of the war the A&AEE tested 1,500 different

types and marks of British and American aircraft (7). Also tested were individual 'rogue' aircraft that

were reported by training and operational stations to have shown irregular characteristics. Aircraft

could be further remitted onward to RAE when more intensive investigations needed to be carried

out.

The first part of the review that follows concerns aircraft that were in service during the early 1940s,

with particular reference to types for which problems with spinning had been reported and which

provide illustrations of the methods employed at that time to inquire into and counter those problems.

A few types that were tested but not ordered into production are mentioned also if they had shown

some deficiencies in their spinning characteristics.

2.2 Aircraft types in the early wartime period

2.2.1 Single-engined training aircraft

The spinning characteristics of training aircraft were of particular concern, as they would be

handled by pilots with little experience, who would be most likely to enter a spin inadvertently and

to become confused about the recommended procedures for recovering from it, or slow in applying

the recovery procedures. Accordingly, recovery action for these types was to be delayed during

testing into the region where the spin had stabilised, known as 'prolonged' spinning. Where the

behaviour was satisfactory relative to the required standard, but close to the acceptable limit, it was

more likely in the case of trainers that there would be a comment in the subsequent report,

indicating that a wider margin should be provided.

The basic (or ab initio) and intermediate stages of pilot training in the RAF proceeded in steps,

generally taking place at training stations employing aircraft specifically ordered for those duties.

Advanced training was taken on types that would be used in service, often in units located on

stations where these were operational. Despite the ever-pressing demands for pilots, the training

programme was substantially maintained throughout the war, with only minor reductions of hours at

a few points.

Spinning and recovery were included in the training of pilots from the ab initio level onwards. The

type generally employed for that stage was the de Havilland DH2 Tiger Moth. Originally a popular

civil type, of largely wooden construction, it had been used widely in flying clubs for training and

for sports flying generally. Adopted by the RAF as its principal basic trainer, it was employed in

great numbers, both at training squadrons in the UK and throughout the Dominions participating in

the Commonwealth Air Training Plan. It had first been tested at Martlesham Heath before the war

and reckoned to be generally docile. On use of the standard procedure, it recovered normally from

a spin, though it was noted that the response to movement of the controls had been slow. Subsequent

events provide an example of how types that were well-established in service could display


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problematic features arising from minor changes of use or the introduction of modifications that

would not have been expected to have troublesome consequences.

At the beginning of WW2 many of the Tiger Moths in service were of the Mark II variety, for which

the official specification T.7/35 had been issued, as shown in Figure 1. Following reports of crashes

at Training Schools, three aircraft of this type were sent to Boscombe Down in 1940 for investigation

of its behaviour in the spin, that had been described as 'difficult' (7). Extensive trials there confirmed

that all three were taking up to four turns to begin recovery, but no specific reason had been found

for that. The aircraft were then remitted to RAE for more intensive study, with only an observation

that the more dangerous spins had occurred when the entry to the spin had been somewhat

mishandled.

The investigation of the spinning issue by RAE illustrates the thoroughness with which problems

were followed up, even in the most critical stages of the war (8). Many full-scale spinning tests were

made in this case, with the three aircraft from A&AEE and two more obtained from training

squadrons. It was found that all five developed a conventional steep spin with the normal method

of entry, but a flatter one could be induced if a small amount of opposite aileron had been applied at

that time. This was recognised as something that might easily be done inadvertently by a trainee.

Standard recoveries could be obtained by the normal routine for all except one aircraft, where the

incidence rose to 50 o, effectively into the region of the flat spin. Recovery was obtained for this

case also, though only after 13 turns, and by use of (unspecified) 'emergency action'.

In pursuit of reasons for the behaviour observed, it was noted by RAE that the evolution from the

Mark I (essentially the civil type) into Mark II had increased the moment of inertia coefficient X.

The main contributors to that were the strengthening of parts of the structure and the addition of

mass balance weights to the ailerons and rudder. It was also said that typical service navigation

Figure 1. de Havilland Tiger Moth Mk II ab initio trainer


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lights had been added at the wing tips, and that the mass and location of these had been enough to

affect the inertia significantly (9). Some of the Mk II aircraft had been fitted with rails to carry eight

small bombs, located below the fuselage at its junction with the lower wing. Training in targeting

with these was a measure introduced in the months prior to the opening of the war, when any option

was considered if it might contribute to thwarting an expected invasion. It had been retained

afterwards, perhaps to provide an early assessment of the capacity of a trainee to maintain the

precise tracking required in the run-up to the release of bombs.

RAE's next move was to procure a civil Tiger Moth and modify it to bring its moments of inertia

and centre of gravity position up to the values of the Mark II version, by adding weights in the

wings, fitting balance weights to the controls and adding bomb rack rails. In experiments with this

aircraft, it was found that with application of full opposite aileron a flat spin could be induced,

though recovery from it could be obtained by the standard procedure after eight turns. Systematic

removal of the added items, with repeated testing in between, showed that the bomb rack rails had

the greatest effect, followed by the balance weights for the control surfaces. Flight trials were then

made with the rails and weights removed, and various palliative measures applied that previous

experience had shown to be helpful in deterring the development of the flat spin. Fitting strakes to

the top of the rear fuselage ahead of the tailplane generally had the most effect (2).

In conclusion, it was recommended that the rails and balance weights should be removed from all

aircraft of that type in service and that strakes should be added to those in subsequent production.

Figure 2 shows the form of the strakes applied for testing at RAE.

Another ab initio trainer that was in service at the beginning of the war, the Miles Magister, was a

single-engined low-wing monoplane of wooden construction, supplied by Phillips & Powis Aircraft

Ltd (later Miles Aircraft Ltd) (10). Like the Tiger Moth, it had been derived from a successful civil

type, though it had some more modern features such as flaps and wheel brakes. It too had

Figure 2. RAE drawing showing anti-spin strakes for tests of Tiger Moth Mk II


140

experienced sporadic episodes of unsatisfactory spinning behaviour. Modification of the shape of

the rear fuselage and the fitment of strakes ahead of the tailplane had resolved the problem. Though

the effectiveness of this measure had been found empirically some years previously, the actual

mechanism of its action had not been fully researched. Details of such modifications were rarely

reported, but the form of the strakes shown in Figure 2 for the trials of the Tiger Moth Mk II at RAE

suggests that its action was probably effected by stabilising the flow separating at high incidence

from the upper surface of the rear fuselage. This would take the form of two narrow sheets trailing

from the outward-facing edges of the strakes. While they remained apart, these sheets would then

have interfered less with the action of the rudder than had the broad wake formed behind the

unmodified fuselage.

From 1939 Miles also supplied the Master trainer, that bridged the intermediate and advanced

stages of preparation of pilots for operation in single-seat fighter aircraft, as shown in Figure 3 (10).

Designed to resemble a fighter, and with a performance to match, this provided the trainee with a

front cockpit laid out with the controls, instrumentation and other equipment that would be met on

operational types. With a light weight, a Rolls-Royce Kestrel engine and three-bladed constant-

speed propeller, it was claimed to be the fastest training aeroplane in the world. Produced in quantity,

partly on the first moving track assembly line in Britain at Woodley near Reading, and later at other

plants, it was followed by further marks using the Bristol Mercury and Pratt & Witney Twin Wasp

engines (10, 11). The Mark 1A was fitted with flaps, retractable undercarriage and a reflector gunsight

for its single Browning gun with a gun camera for training purposes. Having handling characteristics

similar to those of the RAF's Hurricane and Spitfire fighters, the Master helped to ease the transition

for thousands of pilots into operating these and other types.

Two Masters had been at Martlesham Heath in 1939 and they moved with A&AEE to Boscombe

Down. In the acceptance trials the spin behaviour was found to be normal, perhaps helped by

modifications made before the start of production, which included some of the measures to counter

undesirable spins that were now being recognised. These included lengthening and deepening the

rear fuselage and enlarging the fin and rudder (to increase the side forces in yaw and their moment

arms) and raising the position of the tailplane from the top of the fuselage to a location on the fin

(where in this case about half the rudder area now lay below it and would not be shielded by its

wake at high incidence).

Figure 3. Miles Master intermediate / advanced trainer (Miles Aircraft Collection)


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Just prior to the war, Miles had also designed another ab initio trainer, the M.18, shown in Figure 4,

which was intended to build on the experience obtained with the Magister. It was not ordered into

service, and was not given a name, but it could be noted here because trials with one of the prototypes

had raised concerns about its spinning characteristics, which again led to modifications of kinds now

effectively becoming standard procedures. Though its spinning had been found to be 'satisfactory'

at Martlesham Heath, it was also fitted subsequently with strakes on the rear fuselage, as for the

Magister. However, in company tests, its spin was found to be still very flat, and although it could

be recovered, the process was prolonged. When spinning-tunnel model tests were made for it at

RAE, it could be recovered against a pro-spin moment of 14 units, but that was considered to be

borderline for an ab initio trainer (12). Accordingly, another measure was recommended, that the fin

and rudder be moved forward by 24 in relative to the tailplane, which would take most of the rudder

clear of its wake in a spin. This was done on one of the prototypes (though actually by 22 in),

moving the rudder post to a position at the leading edge of the tailplane. No further spinning trouble

was reported. At A&AEE in May 1941 an M.18 was said to be 'impossible to spin' (7). The forward

position of the fin and rudder is a very prominent feature, and as used for some other aircraft

mentioned below, it was sometimes introduced during the design phase, specifically as a precaution

against spinning.

In pre-war years Percival Aircraft Ltd had been a rival to Miles in producing aircraft of wooden

construction for the fields of sports flying and air racing. The Proctor, a version of its Vega Gull

machine, was produced to specification T.20/38 for radio training and general communication

purposes (see Figure 5). With the growing importance of VHF transmission for ground control,

landing aids and other purposes, this type became the main RAF trainer for radio operations

throughout the war period. For the initial model testing in October 1940, the inertia coefficient X

was found to be adequately low, but in relation to that the damping coefficient Y was very small,

indicating that the spin recovery would be poor (13). On test, it failed to meet the required pass

criterion 'by a large margin', recovery being possible against a pro-spin moment of four units but

impossible against five. The prototype had flown a year previously, but the contractor's trials had

not included spinning, so the recommendation was that they should 'not be carried out'. Unusually,

Figure 4. Miles M.18 proposed successor to the Magister ab initio trainer

(Miles Aircraft Collection)


142

modifications to obtain better spinning and recovery were not suggested, perhaps because its duties

largely involved only point-to-point flying without manoeuvres, and so it remained a type for which

spinning was simply prohibited. In service, it was developed into several Marks, with more than a

thousand built in total.

The North American Harvard was a 2-seat single-engined monoplane for the advanced training of

fighter pilots, used in Britain by the RAF and the FAA and throughout the Commonwealth Air

Training Plan. An order for Harvards had been placed by the British Purchasing Commission

shortly before the war, and an early example was under test at Martlesham Heath in September

1939 (7) (see Figure 6). It was reported to have 'excessive propeller noise' (which would be endorsed

by anyone who heard a Harvard in flight subsequently) and 'an undesirable wing drop at the stall'.

Figure 5. Percival Procter

Figure 6. North American Harvard trainer


143

The latter was a known precursor to a spin, so this aspect of performance was revisited by A&AEE

as successive Marks of the type came for approval. The Mark I could be recovered, though with a

loss of height that was considered to be marginal for a trainer aircraft, but the conclusion was that

further spin testing would not be needed. Complaints were received from the RAF that the Mark III

had a high rate of rotation in the spin. Although this was believed to be an indication of the

propensity to develop a flat spin, the behaviour was considered at Boscombe Down to be 'only a

little worse than normal'. No representations seem to have been made to the manufacturer on this

issue, but in any case it was known that American firms were reluctant to make any modifications

after the start of delivery of a type, unless they had arisen in items specifically required in the

contract (2). At this time there was no equivalent of A&AEE in the US to carry out independent

routine testing of American-designed aircraft.

2.2.2 Twin-engined trainers

At the start of the war, the RAF had two versatile aircraft for training aircrew for multi-engined

operation, the Avro Anson and the Airspeed Oxford, shown in Figure 7. Of roughly the same size

and shape, both were low-wing monoplanes based on pre-existing commercial types, with twin air-

cooled radial engines, and flaps and retractable undercarriage (initially hand-operated). The Anson

had been ordered for land-based maritime reconnaissance duties, but it took on many other roles

before being transferred to operations as a trainer just prior to the war. The Oxford, designed to

specification T.23/36, entered service at about the same time. As well as serving the RAF these

became preferred types for the Commonwealth Air Training Plan, more than 8,000 of each type

being produced in Britain, and more built under licence in Canada and Australia.

Like the Master, both aircraft were provided with instrumentation and other fittings and equipment

representing those of the (bomber) types currently in service, and, notably with the Oxford, with

internal arrangements that could be quickly changed to suit particular training needs. Additions to

the facilities for pilots, navigators and wireless operators were bomb-sights and racks for practice

bombs for training bomb-aimers and dorsal turrets for air gunners. This versatility could enable an

entire aircrew to be given initial experience in operating as a team, though the capacities of the

interiors of the aircraft were rather cramped for that. Vertical camera installations provided training

for photographic reconnaissance, and the Oxford could be rapidly converted into an air ambulance.

For all-purpose aircraft with multiple occupation, it might be expected that the possibility of spinning

problems would be a concern. The Anson had cleared the acceptance tests for the maritime

reconnaissance role at Martlesham Heath before the war and was in service at 26 squadrons by the

outbreak of WW2. The Oxford, tested as a model at RAE, had failed to reach the standard required

for spin recovery, despite some modifications being made to the design. A second model, made

with a twin-finned empennage, met the requirements, probably due to the outer surfaces of the fins

and rudders being clear of the wake of the tailplane in a spin. However, to avoid delay in the

rearmament build-up, the Oxford was put into production with the single fin, with the proviso that

deliberate spinning would be forbidden.


144

In 1940, Airspeed fitted a full-scale Oxford with twin fins as originally proposed, and this was

submitted for spin testing at RAE, with an anti-spin parachute as an extra safeguard (14). This test

aircraft is described throughout the RAE report as having 'twin rudders', not the usual term for one

with twin fins, though strictly correct since each had its own rudder. The aircraft was flown by

several pilots and spin recovery was found to be 'rapid and straightforward'. However, one pilot,

flying solo, experienced a flatter spin than usual but obtained recovery by use of the engine on the

inside of the spin, with the outer one idling. This differential use of the thrust of engines on opposite

sides of the central axis provided an anti-spin yawing moment that was not affected in the same way

as were the control surfaces by the deep stall experienced in the spin. It was recommended that this

procedure be made part of the training for the Oxford, and perhaps was to be extended to all multi-

engined aircraft. Though the twin-fin version was not put into production, this method of spin

recovery became recognised as a regular procedure for larger aircraft.

Figure 7. Avro Anson (upper) and Airspeed Oxford (lower) advanced trainers


145

2.2.3 Fighter aircraft

At the beginning of the War the RAF was only beginning to be equipped with fighter types that

could be called modern in design. There had been differing opinions within the Air Ministry during

the 1930s as to the future operational requirements for fighters and the technical features that they

would need to meet those, including their armament. One outcome had been the concept of the 'turret

fighter', exemplified by the Bolton-Paul Defiant. The presence of a 4-gun dorsal turret on a fighter

hugely degraded its performance, but the RAF had to commit the squadrons of those that it had in

1940 to engage in ground attack, in which losses to ground fire and fast fighters were very great.

There would be a similar story for the 'light bomber' the Fairy Battle.

Confusion and delays in initiating the processes that would normally lead to orders produced the

perilous situation in which leading firms in the industry had clearer perceptions about what would

be required than the Ministry did. Having been presented with specifications which they considered

to be unrealistic, these firms proceeded with prototype designs of their own as private ventures.

Fortunately, there were those in the Air Ministry who kept in contact with them and encouraged this.

Orders for types based on these were eventually placed, though not until a time of such urgency that

it was almost too late.

Thus it was that the eight-gun Hawker Hurricane and Supermarine Spitfire, shown in Figure 8, came

to define the latest conception of the high-speed single-seat interceptor fighter of the late 1930s.

When these types began entry into service in December 1937 and August 1938 respectively, there

was great pressure to get effective numbers of them out to the squadrons, with enough trained pilots

ready to take them into combat (6).

Model spinning tests of both aircraft were made before the prototypes had flown, and it was

considered from those that their behaviour in this respect would be at best borderline in both cases (15).

As recommended by RAE, production Hurricanes were fitted with a supplementary fin under the

rear fuselage, beginning forward of the tailwheel and merging with a downward extension of the

rudder. When tests of both types were made at A&AEE in 1938, their spin characteristics were

judged to be acceptable and the anti-spin parachutes that had been fitted as a precaution against

failure to recover had not needed to be deployed.

After further Spitfire model tests were made at RAE, it was suggested that its rear fuselage should

be lengthened and the tailplane raised, though the manufacturers did not make these modifications.

However, in contractor’s tests after the prototype had been fitted with armament and developed

towards production standard, the test pilot Jeffrey Quill reported a disagreeable aspect to the spin in

the form of 'a series of convulsive kicks' (16). Irregularities continued to be noted in various Marks

of the Spitfire in later years of testing at A&AEE (7). In 1942, it was recorded of a Mk IX that spins

had been accompanied by 'unpleasant pitching and buffeting', and of a Mk XII, the first with the

Griffon engine, that spinning produced 'the usual pitching and buffeting'. By 1945 the spinning of

the Seafire Mk XV was just described as 'acceptable'. Fortunately, for both Hurricane and Spitfire,

all Marks had proved recoverable from spins in service.


146

Also existing early in the war were prototypes under specification F.18/37 of a potential fighter to

succeed the Hurricane. This would employ a new engine in the 2,000 HP range, with a maximum

speed of 400 mph and ceiling of 35,000 ft. A contract had been given to Hawker, where design

along similar lines was already in progress, for a fighter to be powered by the Rolls-Royce Vulture

24-cylinder engine. At Hawker's suggestion another prototype was also ordered, to be provided

with a different engine of similar rating that was currently under development, the Napier Sabre.

The two machines bore some resemblance to the Hurricane, but differed from each other in the

wing position, span and area.

In model tests at RAE in October 1939, both versions had shown similar spin recovery, which

exceeded the requirements in the clean condition, but failed to reach it with undercarriage and flaps

down (17).

Figure 8. Hawker Hurricane (upper) and Supermarine Spitfire (lower) single-seat fighters


147

Given the pressures of the time, orders were placed for the type with the Vulture engine, to be built

at other factories in the Hawker Siddeley group, and the first production aircraft, to be known as the

Tornado, flew in August 1941. But there were problems with the reliability of the Vulture engine

and it was abandoned by Rolls-Royce, also bringing the production of the Tornado to an end. Work

on the Sabre engine continued at Napier (D Napier & Son Ltd, from 1942 part of the English Electric

group). In due course, the two Hawker types made their appearance in production, a revised

Tornado with the Sabre engine, to be called Typhoon, and the Tempest in several marks using three

different engines. Spin characteristics of these are reported in Section 3.3.

2.2.4 Twin-engined fighters

As seen above, twin-engined trainers had been evaluated for spinning behaviour, but quite early in

the wartime period twin-engined fighters made their appearance also. The first British type was the

third in a sequence of Bristol aircraft, the Beaufighter, shown in Figure 9. Initially based on the

Beaufort medium bomber, with many common parts and the use of jigs and fixtures of that type to

speed production, it entered service in July 1940, at the height of the Battle of Britain. Being much

larger than the single-engined fighters, and with a two-man crew, it was fitted by its greater range,

endurance and armament to be developed subsequently for multi-role usage in RAF Fighter and

Coastal Commands. It was seen that, having to be fast and manoeuvrable in these roles, it was more

likely to experience conditions that might lead inadvertently to entry to a spin than other twin-

engined types.

A model of the Mk I version was tested at RAE in 1941 as being representative of the heavy fighter

type (18). Although its inertia coefficient was satisfactory, there were concerns about its spin

recovery, due to the damping and rudder volume coefficients being very low. This expectation was

confirmed when at the simulated altitude of 15,000 ft it could not be recovered against a pro-spin

moment of more than 12 units within the 10 seconds (full-scale) specified at the time. Following

this failure, the opportunity was taken to extend the tests to cover variations in the model's overall

Figure 9. Bristol Beaufighter


148

weight, mass distribution, equivalent height, direction of spin and position of controls during spin

and recovery. At the equivalent of 30,000 ft, recovery in 10 seconds full-scale could not be obtained

against a moment of more than 7 units. Perhaps by way of encouragement, the report concluded

that there should be no difficulty in recovery 'below 10,000 ft', and the potential for differential use

of engine thrust to assist in recovery at altitude was mentioned again in connection with this type.

Although no remedial measures were suggested after the model tests, when full-scale Beaufighters

were tested later at A&AEE the spin recovery was found to be satisfactory for all Marks (7). In

service it proved to be a rugged and formidable weapon in various theatres and in many different

roles. About 5,500 were built in Britain and under licence in Australia.

Another type tested in 1940 was a model of the Gloster F.9/40 (19). Described only as 'a twin-engined

low wing monoplane intended for high speed fighter duties', this was to become the Meteor, the first

British jet-propelled type to enter service. The data given showed that the inertia coefficient was

favourable and although the fuselage damping coefficient was also low, the model was expected to

make a good recovery from the spin, and this proved to be the case. With the worst conditions of

loading, recovery was obtained within 10 seconds full-scale against an applied yawing moment of

17 units flaps up, and 15 units flaps down. Thus the design passed the model spinning standard for

the time.

From November 1941 the RAF began to receive the de Havilland Mosquito, the two-man twin-

engined type that became famous for its exceptional performance and versatility. When tested at

A&AEE later, no adverse spin characteristics were reported. However, as can be seen in Figure 10

the design placed the fin in a forward position, with the rudder post about level with the leading edge

of the tailplane. The tall rudder meant that the major part of its area was clear of the position assumed

for the wake of the tailplane in the spin, giving a generous value of the unshielded rudder coefficient

Z. With the further option to use differential engine thrust, the aircraft would be expected to be well

placed for spin recovery, though no test results have been found. About 7,800 Mosquitos of many

marks and varieties entered service, some built at DH subsidiary companies in Canada and Australia.

Figure 10. de Havilland Mosquito


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Government papers for this period include inter-departmental correspondence on testing the

Beaufighter and Mosquito, which indicate that there was reluctance by officials to cause concern to

manufacturers about spin characteristics of heavy fighters (20). This centred on the potential wing

loading due to rotation in the spin. It had not been shown then that these types experienced about

the same normal acceleration as had been found in the types on which the spin criteria had long

been based. But it was seen that specifying a possibly unnecessary loading case in contract

documents might result in strengthening of the structure, requiring additional weight that would

adversely affect performance. Evidently it was noted that these types had not been prone to spinning

troubles in service, and no action was taken in this regard. In the last letter of the file, dated in 1945,

the writer states that 'It is a requirement of twin-engine fighters that it should be possible to recover

from incipient spins, but I do not think that any have ever been tested at A&AEE for this

characteristic'.

2.2.5 Naval aircraft

Among aircraft operated by the FAA at the beginning of WW2 was the remarkable Fairey Swordfish,

shown in Figure 11. This biplane had been ordered originally to specification S.9/30 and entered

service in 1936 for varied duties, with a three-man crew as a fleet spotter (hence category S.) for

registering the fall of shot from naval gunnery, and for general maritime reconnaissance. With a

crew reduced to two it could carry a torpedo for attack on shipping and surfaced submarines.

Though slow and of an obsolete configuration, it remained in service throughout the war, operating

with good effectiveness in a great diversity of theatres and operations, with RAF squadrons as well

as the FAA. About 2,400 were built, between the parent company and Blackburn Aircraft Ltd. It

was not referred to A&AEE or RAE for problems in the spin.

Other Blackburn aircraft with an input from Fairey were the Skua and Roc, originally to

specifications O.27/34 and O.30/35 (the duty under 'O.' had initially been observation, but later

covered a variety of functions, mainly for carrier-borne operations).

Figure 11. The Fairey Swordfish


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Model testing for the Skua at RAE in 1938 had shown satisfactory recovery from a spin, but there

was concern that the body damping and rudder coefficients might be inadequate with the high value

of the moment of inertia coefficient for this type. Because of space limitations in handling and

stowage on carriers, the fuselage could not be lengthened, a measure often advised to enhance the

body damping and rudder coefficients by extending their moment arms. It was recommended instead

that the fin and rudder coefficients could be usefully enhanced if the fin was moved forward, which

would take it out of the wake from the tailplane and would also clear most of the rudder. A

supplementary fin was fitted below the rear fuselage, where it would not lie in a separated flow in

the spin. This arrangement is apparent in the upper image in Figure 12.

The Roc was in effect a variant of the Skua, armed with a 4-gun dorsal turret as for the RAF Defiant,

conceived at about the same time. After model tests on the Roc just prior to the war, RAE suggested

that both types should be fitted with wing tip slats (to limit the liability to wing-dropping at the onset

of the stall) and that deliberate spinning should be prohibited. This suggests that the conclusions

from tests were pessimistic, for there seems to have been no indication in subsequent service that

these aircraft were particularly prone to entering the spin or resistant to recovery from it. Further,

from about this time, slats were becoming unpopular with pilots, due to their tendency to open

unexpectedly when gusts were encountered and differentially during manoeuvres.

Figure 12. Blackburn Skua (upper) and Fairey Fulmer (lower)


151

The Roc was handicapped by the dorsal turret and is generally considered to have been an

unsatisfactory concept. A more successful Fairey naval aircraft becoming available near the

beginning of the war was the Fulmar, also shown in Figure 12. A sturdy two-man carrier-borne

aircraft with a long endurance for reconnaissance and shadowing, it also carried eight guns for

interception as required. It had been commissioned to specification O.8/38, though the design was

based on a fighter submitted for an earlier RAF requirement, for which the spin of the model had

been described as 'vicious' (21). The fin was not placed forward as in the Skua, so in the spin the

rudder was significantly exposed to the wake of the tailplane, giving a low value for the rudder

volume coefficient. In the haste for production there had been no prototypes, so no spin trials were

made until the first production aircraft arrived at A&AEE for the Release to Service review in May

1940. It was however generally well received, and, perhaps unexpectedly, the report refers to its

'instantaneous spin recovery' (7).

The Lockheed Hudson, intended as a replacement for the Anson for maritime patrol and attack with

RAF Coastal Command, was the first American type ordered before the war by the British

Purchasing Commission. This was a development of a pre-existing twin-engined civil type, to be

fitted with a bomb-bay, a dorsal gun turret and forward-facing machine guns. In the inter-war years,

involvement in foreign conflicts had been forbidden under the American Neutrality Act, so these

aircraft had to be exported via Canada and fitted with their armament after arriving in Britain. All

Marks from I to VI were tested by A&AEE between 1939 and 1943, mainly for clearance with

various weapons, including bombs and rocket projectiles. The Mark III could also carry a large

dinghy under the fuselage for Air/Sea Rescue work (7). There is a reference for this type to a violent

stall with the left wing dropping, but no spin problems seem to have been investigated.

2.2.6 Bombers

In the inter-war years there had been a class of 'light bombers', with the specification code P. The

3-man Fairey Battle, with a bomb load of 1,000 lb, exemplified this. It was a streamlined all-metal

single-engined monoplane, with flaps and retractable undercarriage, considered an advanced design

in the early 1930s. But by the time of its delayed entry into service in 1937 it proved to be seriously

underpowered for its size and weight. When it was deployed in ground attack in support of the

British Expeditionary Force, with minimal armament it suffered very heavy losses. The era of the

light bomber effectively passed with the Battle, though it continued in production, serving in large

numbers in the Commonwealth Air Training Plan, as a single-engined type able to carry an

instructor as well as a pilot under training. For tactical purposes the light bomber was replaced by

the new varieties of fast and manoeuvrable twin-engined fighter-type aircraft described in sub-

section 2.2.4 above, equipped with heavy gun (and later rocket) armament and sometimes carrying

bombs.

Aircraft generally described as 'medium bombers' were twin-engined, though still classified under

specification code P. Some of this type designed in the early 1930s were in service at the beginning

of the war and were in action from the first day. Among these, the Bristol Blenheim was also

pressed into service in ground support for the BEF, but suffered heavily. They were followed in the

bombing role by types such as the Bristol successor to the Blenheim, the Beaufort, and the Vickers

Wellington. These carried the war against the enemy effectively at the time, with much lower losses

by operating at night-time.


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The medium bombers had range enough to reach the concentrations of industry in the valleys of the

Ruhr and Rhine and on to Berlin. But to serve fully the strategic aims of the RAF, the class of 4-

engined 'heavy bombers', to specification code B were required. A large 4-engined type had been in

service with Coastal Command since 1938, the Short Sunderland, a long-range reconnaissance and

attack flying-boat to specification R.2/33. Experience in producing this type had been of value at

Shorts when designing the Stirling, the first of the heavy bombers to be delivered, in 1938. Two

others of this class, the Handley Page Halifax and Avro Lancaster, came into service with Bomber

Command in 1940 and 1942 respectively.

Use of more than one engine had to be considered of potential significance for spinning behaviour,

due to their contributions to the moments of inertia of the aircraft. The importance of these would

show up in their effects on the inertia coefficient X. This involves the difference (C - A) between

the moments of inertia about the normal and longitudinal axes of the aircraft respectively (See Part

2 sections 5.1.2 to 5.1.4 for details (2)). Though it had not seemed necessary to revise the working

boundaries used in assessing the spin coefficients when applied to twin-engined types, the arrival of

the heavy bombers brought a configuration that moved further from those of the types on which

they were originally based.

A general treatment from this time of the effects on spin characteristics of having wing-mounted

engines has not been found, but it can be readily illustrated by reference to Figure 13. This shows

the location of an engine in relation to the three principal axes of inertia of the aircraft, centred on

its overall centre of gravity G. For simplicity, it is supposed that the centre of gravity of an engine

Ge lies within the plane xy. The perpendicular distances of Ge from the longitudinal and lateral axes

are then y and x respectively, and its distance from the normal axis is shown as r.

Figure 13. Moment of inertia of a wing-mounted engine


153

If the mass of the engine is Me, then in the standard nomenclature, its moments of inertia about the

three axes are roll Ae = Mey2, pitch Be = Me x2 and yaw Ce = Me r2. The contribution Xe made by

the engine to the inertia coefficient for spinning, proportional to the difference (Ce - Ae), is

Me (r2 - y2). Further, r, y and x form a right-angled triangle, so this can be written as Mex2. This is

equal to Be, the moment of inertia of the engine about the lateral axis, and the result is generally

used in that form.

The engines are often the most massive individual components of a multi-engined aircraft, so for

reasons of balance, their distance x ahead of the overall centre of gravity G cannot be large. As for

the three RAF 4-engined bombers, their outer engines lay a short distance aft relative to the inner

ones, so the distance x, and hence the contribution Be to the spin inertia coefficient, was somewhat

smaller for those than for the inboard engines.

Though the contribution of the engines to the spin characteristics of twin and four-engined types is

not negligible, it was soon realised from considerations such as this that it would not be as

troublesome as had been feared. The need for long bomb-bays to cater for the loads required would

mean that the moment arms for the fuselage damping and unshielded rudder volume coefficients

would be ample for their purposes.

3. Developments during the early wartime period

3.1 The RAE Free Spinning Tunnel

Immediate operational needs during the first part of the war left little room for developments in

spinning theory, but there was further exploration of the relationship between the behaviour of a

scaled model in the spinning tunnel and that of the corresponding aircraft in its full-scale operational

situation. There would be associated refinements in the procedures for model testing to provide

more confidence in the representation it gave, while preserving a margin of safety in what was

essentially a pass/fail process. At this time the main responsibility for work on spinning in

Aerodynamics Department fell to Dr G E Pringle.

Models built for spinning trials had to be scaled both geometrically and dynamically. The conditions

for achieving this were reviewed in Part 2 of this study (2). Briefly, geometrical scaling is the usual

procedure as for other wind tunnel models, requiring the dimensions of the external shape to be

everywhere in a fixed proportion to that of the aircraft, say 1 to n. The RAE Free Spinning Tunnel

had a diameter of 12 ft and models up to about 3 ft span could spin in it without their motion being

influenced by the presence of the walls. In its earliest use, the linear scale n would be typically

around 12 to 16, at which size models could be made accurately. The airspeed in the tunnel had to

oppose the scaled value of the vertical rate of descent of the aircraft in the spin. For dynamic scaling

it was shown that n should be proportional to the wing loading w of the aircraft. This quantity was

increasing steadily with developments in aircraft technology, as could be illustrated by the front-line

machines produced by the Hawker company. When the tunnel came into service in 1931, that was

the Hart, with a wing loading of about 13 lb/ft2, but for the Hurricane in 1937 the wing loading was

about 30 lb/ft2 and for the first Typhoon of 1941 it was 41 lb/ft2. This meant that models had to

become progressively smaller. Skilled modellers could work to such scales, but it became more


154

difficult to produce the accurate representation of the distribution of mass in the model that would

correctly scale the moments of inertia.

The first development in this area was the fitting of a more powerful fan motor to the tunnel in 1938,

by which the maximum air speed was increased from 35 to 56 ft/s (22). However, it was to be

expected that the trend to higher wing loading was bound to continue, and it would not be long

before representations were being made for the construction of a new vertical tunnel (23). The

existing tunnel took in air at the bottom and discharged to the atmosphere at the top. To limit the

required fan power, the tunnel now proposed would be of the closed return-flow type that

recirculated the air. It would have a concentric form, as shown in Figure 14. At the working section

the diameter would be 15 ft, and to cover a wide range of operating conditions, the air could be

pressurised up to a maximum of 4 atmospheres. A fan power of 1,000 HP would be required to give

a maximum tunnel speed of 140 ft/s at ground level pressure and 87 ft/s at 4 atm. These conditions

would allow the testing of a model at 1/20 scale of an aircraft with a maximum wing loading of

55 lb/ft2 operating at altitudes up to 40,000 ft.

The exigencies of wartime did not allow further work to proceed at the time on the building of a

new tunnel, so for the period covered by this Part, the operating conditions remained as in 1938.

There was a return to the subject after the war for the construction of a vertical tunnel at the

proposed National Aeronautical Establishment near Bedford, later to become RAE Bedford.

3.2 Developments in spin testing

As experience was gained with model spinning the focus of attention turned increasingly to the

effectiveness of relating the results obtained there to the behaviour of the aircraft at full-scale. The

first aspect of this had been the extent to which the model itself could be representative, which

became more questionable when models had to be made to quite small scales. The practice had

been introduced of increasing two of the basic features of the model to reduce the possibility that

minor failures to reproduce exactly the characteristics of the full-size aircraft might result in a

dangerously optimistic test result. Accordingly, the changes were made in directions that would

tend to worsen the spin behaviour.

One change was an increase in the inertial coefficient in pitch X by 10%, shown to be equivalent to

making B, the moment of inertia about the lateral axis, 10% bigger than the scaled value quoted for

the aircraft. The second adjustment was to the permitted range of the position of the centre of

gravity G. This was extended rearwards by 6% of mean chord, and most of the spinning tests were

done with G located at this extended aft limit. The effect of that is somewhat to reduce the stability

of the model in pitch, which was known to be a factor in the tendency of an aircraft to move to the

high incidence of the flat spin.

These changes had been made to allow for possible imperfections in producing the model. A more

difficult uncertainty concerned the extent to which the airflow over the model might differ from that

over the full-sized aircraft. This was the 'scale effect', a term already familiar in conventional wind-

tunnel testing, but at that time no theoretical representation of the flow had been obtained for an

aircraft in a spin as a basis for assessing it. For normal flight, the aerodynamic characteristics are

largely governed by the development of boundary layers on the wings and empennage, and by that

time, the scaling of this in terms of the Reynolds number was quite well understood. Conventional


155

wind tunnel work had developed by establishing empirical correction factors to measured results

when scaling them up to representative values, and adjustments along a similar line were expected

to be needed here. But in the spin, the flow is fully separated from most or all parts of the aircraft,

leaving it in a broad turbulent wake. There was a feeling that this pattern would not vary so much

with scale, though practical difficulties had prevented the acquisition of data that would enable an

understanding of it to be built up.

Pringle and Alston conducted a major review of the current arrangements for spin tunnel testing in

1941, with reference to cases where the results of model spinning tests had not agreed well with

those at full-scale (24). One was that of the Vickers Wellesley single-engined light bomber. It came

into service in 1937, eventually equipping six squadrons of Bomber Command. One squadron was

allocated to the RAF Long-range Development Flight, and in November 1938 three aircraft of this

type flew non-stop from Egypt to Australia, a record distance of over 7,000 miles.

The Wellesley had unusual proportions, as shown in Figure 15a. The wing span of over 74 ft was

nearly twice the length of the fuselage. Since the span appeared in the denominator of the inertial

coefficient X, it was rendered low (in a favourable direction), though the short length of the rear

fuselage was detrimental to the other coefficients Y and Z. During a standard test of the aircraft for

lateral stability before delivery, the company test pilot Jeffrey Quill experienced a flat spin which

had not been encountered previously. Nothing that Quill tried disturbed the spin, and after many

turns, he was obliged to abandon the machine at a height between three and four thousand feet (on

the second attempt, having got back into the cockpit in the first, to switch off the engine when he

realised that otherwise he might be hit by the propeller (24). Model tests at RAE had failed to

reproduce this kind of spin, and in service the type had not been particularly prone to problems in

that area.

In the new study, it was suspected on the basis of the known coupling between yaw and roll in

stability theory that something similar might occur in the spin for an aircraft with so much of its

mass in the wings. This had been neglected when the wing tip vane to apply an additional yawing

moment had come into use in spin testing.

Figure 15a. Vickers Wellesley light bomber


156

To investigate this, many more spins were carried out with the Wellesley model, now fitted with an

additional vane to apply a series of moments in roll. These tests were filmed, to allow the angles of

the orientation of the model to be recorded at every stage. A frame from one test is included in

Figure 15b. The data obtained showed that there was a clear relation between rolling and yawing

moments, such that in the original spinning test the roll had been accompanied by an additional yaw

of significant magnitude. This acted in a direction that negated part of the yawing moment applied

by the tip vane, with the result that the margin of safety was significantly lower than had been

estimated in this case.

It was concluded however that the Wellesley was an extreme example, and a general adoption of a

procedure using an additional rolling vane need not be proposed at this point. Rather, 'a watch

would be kept for further anomalies resulting from the present routine methods and ultimately it

may be possible to revise the standards of the test.'

Comparisons between model tests and full-scale experience continued to indicate that model testing

under the prevailing procedure tended to give a more optimistic assessment of the behaviour than

would be found when the type was tested at full-scale. An example was that of another private

venture trainer developed by Percival Aircraft Ltd (25). In 1942 this passed the model test

comfortably, with recovery in 6 seconds against 15 units of yawing moment, under the 'worst

conditions of loading' at an equivalent altitude of 10,000 ft, appropriate for the duty. It was

subsequently ordered to specification T.23/43 as the Prentice, but early aircraft showed poor

directional response. The consequent modification to the empennage took an unusual form, with

cut-outs at the inboard ends of the elevators that provided better flow to the rudder, which was also

modified. More urgent requirements and production difficulties delayed the entry into service of

this type until 1947, but several hundred were subsequently delivered, latterly after production was

transferred to Blackburn Aircraft Ltd.

Pringle continued work in the area of spinning and drafted a significant proposal in 1943 (26), which

Figure 15b. A model of the Wellesley in the spinning tunnel


157

was also combined with reference 24 and issued as an ARC R&M (27). This expressed the

relationship between the performance of the model and that of the full-scale aircraft in terms of the

'threshold', the limiting value of the applied yawing moment beyond which recovery became

impossible (Gates's earlier term for that had been the 'precipice' (2)). A new test procedure was

proposed, in which the threshold would be determined for the model by a series of tests with vanes

applying a sequence of increasing pro-spin yawing moments. This would be repeated for left and

right hand spins and the average taken. The average would then be corrected for the effect of the

associated rolling moment by a series of tests with a second vane added to the wing tips, as shown

in Figure 16. A further correction for the effects of probable errors in the reproduction of the

moments of inertia in the model was also suggested. If the corrected threshold was found to be 17

units or greater, the aircraft would be expected to pass the standard full-scale spin test, as applied by

A&AEE.

It was recognised that the procedure for spin model testing would be considerably lengthened by the

addition of extra tests using rolling vanes. Accordingly, an approximate formula was worked out

for calculating the effect of the rolling moment on the moment applied by the yawing vane. This

could be expressed in terms of a correction to the model threshold involving the ratio B/A of the

moments of inertia about the lateral and longitudinal axes respectively. It had been formulated in

the hope that it might be used in place of tests with rolling vanes, but consideration of the further

uncertainty in the process caused by it led to the recommended lower limit of the threshold being

raised to 21 units, and this alternative seems not to have been used in practice.

Figure 16. Twin vanes for spinning model tests


158

3.3 The new test procedure

Several aircraft bridged the change from the previous test standard to this new one. For example,

two fighter types had been put forward by Hawker as reported in Section 2.2.3. Models of both

versions were tested in the RAE vertical tunnel in October 1939 (17). They showed similar spinning

characteristics, recovering in a clean condition against pro-spin yawing moments of around 20 units,

but with flaps and undercarriage down, this fell to 13 for one and 10 for the other. Though this was

technically unsatisfactory, under the urgency of the time the Air Ministry placed initial orders for

the first of the types, to be named Typhoon, powered by the Napier Sabre engine.

In full-scale spin tests, the Typhoon was found to be borderline at 20,000 ft, and on one occasion the

pilot deployed the spin parachute to obtain recovery. Further tests were carried out by the company

test pilot P G Lucas, and an account of this was included in the R&M version of the report reviewed

above (27). It was found that for this aircraft the spin was accompanied by violent pitching and

yawing, though it could often be recovered by use of the standard procedure. But on applying full

opposite rudder after 2½ turns of a spin to the left, the nose had risen suddenly and the stick came

hard back. With use of both hands, the stick could be moved a short distance and by rocking it

forwards and backwards an oscillation in pitch was built up, in which the nose was forced down and

the aircraft recovered, though only after falling between six and seven thousand feet.

Models of the Typhoon were tested again in the spinning tunnel in the spring of 1943, when Pringle's

new procedure was under consideration (28). Recovery was found to be smoother if begun early in

the spin, though this depended on using the full range of control movements from the beginning,

confirming the observations made at full-scale. RAE recommended enlarging the rudder or fitting a

fin extension below the aft end of the fuselage.

More urgently, attention had been needed to intractable structural weakness of the rear fuselage of

this type and unreliability of the engine, but the empennage was also modified, eventually by fitting

a larger assembly that had been developed for the successor Hawker aircraft, the Tempest. Ultimately

over 3,000 Typhoons were produced, and served very successfully, in particular becoming a

formidable ground-attack aircraft, for which it was heavily armed with various guns, cannons,

rockets and bombs.

Another requirement for this period was for a heavy fighter, designed to specification F.7/41, having

a pressurised cabin to enable the engagement of bombers flying at 40,000 ft and above. The

Luftwaffe had made sporadic sorties over Britain at this height with the modified Junkers Ju 86R,

and the Air Ministry feared that there was to be a renewed bomber offensive, operating well above

the height at which interception was possible at that time.

Two twin-engined aircraft submitted for this duty were the Westland Welkin and the Vickers 432. A

model of the Welkin, a derivative of the Whirlwind fighter, was tested for spin in March 1942,

showing excellent recovery within the specified 10 seconds full-scale against a pro-spin yawing

moment of 31 units at a simulated 15,000 ft. and 19 units at 20,000 ft (29). A model of the Vickers

Type 432, tested in June 1943 (30), recovered within 10 seconds full-scale against a moment of


159

14½ units, and with an enlarged fin and

rudder, this threshold was raised to 17½

units. At this later time, the new test

procedure was being adopted; applying the

corrections for the rolling moment and

random errors in the moments of inertia,

requiring a threshold of 17 units for a pass,

but in the tests only 11 was reached.

Several modifications of the model were

investigated, though none was sufficient to

obtain a satisfactory recovery.

An order for the Welkin was placed and a

small number delivered (Figure 17).

Meanwhile, however, high-altitude

interceptions had been made by specially

modified and lightened Spitfires. The

German incursions then ended, and the

threat of a new bomber offensive faded, so

new aircraft for this duty were no longer

required.

3.4 Loading cases

When setting the strength of an airframe, the formal requirements for the loading cases that

designers must apply (arising in take-off, landing, manoeuvres, etc.) were laid down in the Air

Ministry publication AP970, which was subject to revision as new information became available. It

had been concluded in the earliest stages of spin investigation that the normal acceleration

experienced in spinning would not represent a significant loading case for aircraft (1), and this had

remained the accepted view. Some reconsideration was caused by the arrival of the heavy twin-

engined fighters, and in 1943 Pringle wrote a Technical Note to provide an interim assessment of

the rate of rotation to be expected in the spin of these types (31). The only data available were from

observations on models in the vertical tunnel, covering just five types. It is of interest to note that

this included the Gloster Meteor, that was to be the first British jet-propelled type to enter service.

The analysis was confined to the rate of rotation about the (vertical) axis of the spin. This varied

over the range from about 1.7 to 2.9 rad/s, roughly from 4 to 2 seconds per turn. The ratio of the

rate of rotation to the rate of descent V was made non-dimensional as a coefficient = s/V, where

s is the semi-span. It was found that upper limits of did not vary much with altitude, being about

0.35 at the equivalent altitude of 15,000 ft and 0.30 at 30,000 ft. When combined with an existing

expression for V, this gave a simple working relationship for determining .

Work was concluded at this stage, though two further steps would have been required to give the

normal acceleration and hence a possible loading case. First, to include the radius of the path R of

the centre of gravity of the aircraft, to give the acceleration 2 R towards its centre, then the

incidence to give the component of that in the direction of the normal axis, 2 R sin . Methods

for obtaining typical values of these quantities from film had already been developed for the work

Figure 17. Westland Welkin


160

on the Wellesley. Perhaps Pringle had written this Technical Note just for the record, as he was to

return to the question of loading in the spin at a later date.

Another aspect of loading reviewed at this time was that applied when a tail parachute had to be

deployed to obtain recovery in a spin (32). The design load to be used for this case was expressed in

AP970 in terms of the rate of descent of the aircraft at the point of recovery, but when the parachute

first opened there was generally a 'snap' or abrupt load, followed by a rise to a steadier load when it

was fully inflated. This was the beginning of a dynamic process, in which the aircraft's speed was

finally brought to a new equilibrium value. Backed by some measurements at full-scale and others

in the spinning tunnel, Pringle developed an approximate calculation procedure for this process.

Incidental observations arising from this were that if the parachute had not stopped the rotation

within two or three turns, it was 'unlikely to succeed at all', and that during a successful recovery,

the path was made steeper, so that the diving speed at exit was increased.

A suitable approximation for the maximum load for design purposes for this phase was given by

3.5 w d 2, where w is the wing loading of the aircraft and d the diameter of the parachute canopy

when fully inflated. It was clear that parachutes designed for other purposes were being fitted for

this duty, often of unnecessarily large diameter, and that loads could become dangerously high.

Pringle argued that it was imperative that parachutes of approved design should be available for this

means of spin recovery.

4 The later wartime period

4.1 Angular momentum in model testing

Pringle's next contribution concerned differences noted between left and right-handed spins of the

same aircraft (33). In acceptance tests at full-scale the times to recovery were obtained for spins in

both directions, and commonly found to differ. This was also the case for the threshold values in

model spin testing. Contributing factors would be those that produced pitching and yawing moments

that were asymmetric, that is, acted in the same direction in spins of opposite rotation.

It would be expected that one cause of asymmetric moments would be the gyroscopic couple due to

the angular momentum of an engine and propeller. This had the same sense relative to the aircraft

whatever the direction of the spin. The largest effect was likely to be a pitching moment, which was

favourable to recovery when the rotation was in the same sense as that of the spin, but adverse in

the opposite case. The situation with respect to the yawing moment was more complex, as it

depends on the direction of tilt of a spinning aircraft, which could be outward or inward.

Calculations were performed for two single-engined fighter-type aircraft, one modern and the other

the Bristol fighter, for which relevant data were available, both having 4-bladed propellers. One

with a turbojet engine also made an early appearance here. [The latter was called a 'gyrone',

Whittle's original name for a turbojet that persisted for a time.] The effects on recovery of using

one or both engines of a twin-engined type with propellers were also examined. Estimated results

were made for the effects of the propellers, based on the earlier values for a single-engined one.

This involved further assumptions, and little of general application could be concluded beyond

confirmation that the most important effect is the favourable yawing moment produced when thrust

from the inner engine is brought into use. The lift component of the propeller thrust at the high

incidence is also important. An item requiring to be investigated was the response of the constant-

speed unit of the propeller of the inner engine to the reduced forward speed there, with the


161

possibility that this might not be sufficient to move the blade angles beyond the minimum setting.

Comparative figures showed that despite the high rotational speed of the centrifugal jet engine, the

angular momentum of the modern piston engine with propeller was nearly three times as great.

Another factor recognised was the momentum given to the air, which is the means of propulsion in

both cases. That passing through a propeller is external to the airframe, and in a spin is deflected

away from it by the high incidence of the airflow. The flow through a jet engine is internal to the

airframe and its momentum vector is carried with it round the spin axis, bringing a Coriolis

acceleration into effect. With typical rates of spin, the magnitude of the force involved would not

usually be important, but its moment could be significant, depending on the location of the engine

relative to the centre of gravity of the aircraft.

Pringle presents some simplified expressions for obtaining the contribution of gyroscopic couples to

the behaviour in pitch, roll and yaw, but this required having to assume values for many quantities,

such as angles of orientation and aerodynamic derivatives, taken from measurements in spins of

single-engined aircraft in the past. This suggested that representation of the gyroscopic effects of

engines and propellers should be included in model testing.

The scaling laws show that if the rotor of a jet engine is to be correctly represented on a small model,

its rate of rotation would have to be impracticably high. It was not necessary to represent the moment

of inertia and angular velocity of a propeller or rotor separately, just their product, to give the

requisite angular momentum. An apparatus was devised that would do this, as shown in Figure 18.

A flywheel that would represent the effect of both engines and could be accommodated in the

fuselage was designed to provide a wide range of conditions when the driving wheel was run in the

range up to 2,000 rpm. The representation of moment of momentum was checked by suitably

suspending the flywheel unit and

measuring its rate of precession under a

known applied moment. This was first

used with the 1:32 scale model of the

Meteor as shown. To represent both

engines running at full thrust, the

flywheel was first run up outside the

tunnel to 17,700 rpm, to give time to

launch the model and allow the spin to

develop fully before it ran down to

15,000 rpm, the value required for

correct simulation for the aircraft at

15,000 ft.

The rotation of the flywheel was left-

handed, and as expected it was found

that when the engines were represented

as running the left-handed spin was

steeper than if they had been idling.

With engines idling it was flatter for

both directions of spin. Though the

difference between the thresholds for

no recovery from right and left-handed

Figure 18. Spinning rig for engine angular momentum


162

spins was small with engines idling, at full thrust it was substantial - about ten units of pro-spin

moment in favour of the left-hand spin.

It was clear that the contribution of the angular momentum of the engine was a significant factor in

the difference between recovery from spins of opposite hands, but the apparatus shown in Figure 18

could not simulate a situation with just one engine running. It was estimated that the effect in that

case would be equivalent to a change of 3.6 units of applied pro-spin moment, helpful or otherwise,

according to the direction of the spin.

Brief considerations were given also to two aspects of the situation at the start of a spin. One is of

the gyroscopic effects in a steep turn, from which a stall and spin might develop, and the other a

more general view of how aircraft characteristics and conditions in the incipient stage might favour

one or the other direction of spin. At this stage, these were mainly reminders that at some point

there would have to be a consideration of the continuity of effects throughout the whole process of

the spin, from conditions prior to entry to the fully developed state and recovery.

4.2 Model tests

Tests in the later years of the war included models of other early jet-propelled types. An RAE report

dated in January 1944 by Pringle and associates gives results of spin tests for the 'Gloster Tourist' (34).

The juxtaposition of items in this report is very odd. 'Tourist' was one of the code names used for

the E.28/39, the first aircraft to fly with Whittle's turbojet engine, when it was making test flights

from Edge Hill in the spring of 1942. The drawings of the models used in the tests show four types,

including two with twin fins. All of these come from the earliest stages of the design, that would

have dated from the winter of 1939, but on one the fin is shown with the shape and location well

forward on the tailplane, as in the final design of the aircraft, rolled out in April 1941. A model of

the aircraft as built is not included. Further confusion is added by the reported fitting of a flywheel

to represent the angular momentum of the engine, an arrangement first described much later, as

shown above.

Having regard to the sequence of events in the preparation of this historic aircraft, any model spin

tests would most likely have been made early in 1940 (35). The results given in Reference 34 are in

fact in accord with the requirements as they were at that date (recovery within 10 seconds of moving

the controls, against a pro-spin yawing moment of 10 units, applied by a wing tip vane). It is

conjectured that these tests had been made then, but not formally reported at the time, perhaps due

to the programme having been given the highest security classification of Most Secret. Other works

done during wartime but considered to have been important enough to be recorded were published

afterwards in special volumes of the R&M series of the ARC. These results were not included in

those. It seems probable that the report of 1944 had been based on material in a file of miscellaneous

aspects of the E.28/39 programme held at RAE Aerodynamics Department, and the anachronisms

had been overlooked when they were written up.

Overall, the results had shown that the aircraft fell somewhat below the specified requirements, but

it was not considered necessary to proceed with spin tests at full-scale. It is noted that the designer of

the E.28, (Wilfred) George Carter, seems to have had the spin very much in mind, having positioned

the fin well ahead of the tailplane, with a generously proportioned rudder, and had provided an anti-

spin parachute, to be deployed from the rear fuselage. There were no reports of any spin problems


163

with the two prototypes of this aircraft.

The Gloster Meteor that followed it was now about to enter service with the RAF, and model tests

were reported of the second turbine-powered aircraft for service, the de Havilland Type 100 to

specification E.6/41, later to become the Vampire (36). Shown in Figure 19, this had twin booms

carrying the empennage, and models with three variations of the fins and rudders were tested. It

was uncertain whether the spin characteristics could be assessed by the coefficients currently used,

which had been conceived for monoplanes with a conventional rear fuselage and empennage, but

the value of the standard inertia coefficient so obtained was low enough to be encouraging. In the

tests, all models passed the 17 unit threshold by small margins.

One model was fitted with a flywheel to represent the engine. With left rotation, the left-handed

spin was steeper and recovery better, as expected. The induced yawing motion from the flywheel

was assessed to be equivalent to about 3½ units full-scale and 1½ when idling. It was noted that as

the cg of the engine was close to the overall cg position, it would experience gyroscopic couples but

a negligible Coriolis force. This model was also tested in an inverted spin, but as the rudder was

fully effective in that case the recovery was prompt.

Other aircraft with models tested in this period included the Hawker Tempest, now fitted with two

types of engine, the Mk II with a Bristol Centaurus air-cooled radial engine and the Mk V with the

Napier Sabre liquid-cooled engine. As shown in Figure 20, these differed considerably in appearance,

due to the Centaurus having an annular intake around the engine for cooling air, and the Sabre

requiring an intake and radiator in a prominent chin-mounted housing. The inertia coefficients for

both marks were higher than for the Typhoon, so good spin recovery was not to be expected. The

tests showed that it was slightly better for the Mk II than the Mk V, but that neither could be passed

as satisfactory (37). Tests were made with miniature anti-spin parachutes, indicating that with those

recovery should be acceptable for both in an emergency.

Figure 19. de Havilland Vampire


164

At the end of 1944, tests were made to investigate recovery from a spin by parachutes attached to

wing tips (38). This had become necessary because several British firms had been working on the

design of tailless aircraft, which had no rear fuselage to which recovery parachutes could be

attached as in previous practice. A Tailless Aircraft Advisory Committee had been set up in July

1943, and the model used as an example was of one of four unpowered machines of this type

commissioned from the firm of General Aircraft Ltd, which had experimented with gliders from

pre-war times.

Various individuals had obtained stable flight at low speeds from tailless aircraft since the early

days of manned flight. The aircraft consisted principally of the wings, with no rear fuselage or

empennage and at best a small cabin for the pilot (e.g. Geoffrey Hill's Pterodactyl). These generally

had wings that were swept back, so that the lifting surfaces had some extension in the longitudinal

direction. The wings were then twisted so that the incidence was progressively reduced towards the

tips. With sweepback, this provided a forward position of the centre of lift, while the rear portion of

the wings, with lower incidence and longer moment arm, acted to provide stability in pitch in a

similar way to that of a tailplane in the orthodox arrangement.

The testers soon appreciated that it was best to fit parachutes to both wing tips for this variety of

aircraft. These provided a high rate of retardation without a large asymmetry that could draw the

aircraft into an undesirable attitude. During the spin, the cables were found to take up large angles

fore-and-aft and sideways, as sketched in Figure 21. Methods for calculating the loads applied at

the wing tips were given, and work recommended on ensuring that in use the two parachutes would

be deployed and jettisoned simultaneously.

Attention to load determination during the spin was addressed in another report, in which a compilation

of recommended expressions was set out for design calculations for aircraft of conventional

configuration (39). These covered the determination of operating conditions in the spin for a

representative flat spin and steep spin, required to obtain the drag forces and pitching moment.

Tables are given for factors such as the incidence, indicated forward speed, rate of rotation, radius of

spin path, wing drag coefficient and tailplane normal force coefficient. Angles that could be assumed

for the towing cable after parachute deployment were tabulated, concluding with a value for the

forward speed in the dive following recovery.

Figure 20. Hawker Tempest Mk.II on left, Mk.V on right


165

This practical support for strength calculation was very helpful, given the now-extensive array of

research reports on spinning from which designers would otherwise have much labour in extracting

what was needed. Nevertheless, this material was still to be regarded as provisional, until further

evidence could be obtained from flight trials.

5. The early post-war period

5.1 A new era

As WW2 drew to an end in 1945, a new era in aeronautics was opening as the results of research

that had been classified during the hostilities became more widely known and applied. The falling

efficiency of propellers as aircraft speeds approached the transonic region had been limiting, but

that would be removed by the availability of turbojet propulsion. Captured German aircraft and

records of research work done in Germany began to be revealed, with considerable impact in the

countries of the Allies. This encouraged consideration of what were at first termed 'unorthodox

configurations' to obtain better aerodynamic efficiency in the new operating conditions, including

forms with sharply swept back and delta wings. But the wind tunnel, the standard research tool for

aerodynamics of past decades, could not be made to function in the transonic region. A flow at high

subsonic speeds could be produced, but the further acceleration of the air around a model would

produce regions where it would locally reach the speed of sound. Disturbances in the flow would be

carried downstream, accompanied by shock waves reflected form the tunnel walls. The effects caused

the flow to 'choke' at the working section, preventing any further increase in speed. It would take

almost another decade before ways had been found to circumvent that. New wind-tunnel designs

were needed to enable models to be routinely tested throughout the transonic range. Meanwhile it

was necessary to prepare for higher speeds largely by experimentation in flight.

Figure 21. Wing tip spin recovery parachutes


166

Though firms were planning the next generation of turbine-propelled aircraft to enter service after

the Meteor and Vampire, the gestation period of types conceived earlier meant that most of those

coming forward for spinning tests in this period were still driven by the combination of piston-

engine and propeller.

5.2 More on wing tip parachutes

One 'unorthodox' type that well represented the ambitions of the time was the de Havilland DH 108,

shown in Figure 22 with the contemporary DH 103 Hornet. Commissioned to specifications in the

Experimental category from 1945, successive developments of the 108 put up notable performances,

including obtaining a record average speed of 605 mph over a 100 km closed course, and in a

shallow dive becoming the first British aircraft to exceed the speed of sound. But crashes of all

three of the type that were flown, fatal to the pilot in each case, were reminders of the harsh realities

of advances at this time.

A report on spinning model testing of the DH108 was not published until 1948, as reviewed later.

But the aircraft had featured before that in a further report on wing parachutes

(40) presented as Part

II of Reference 39 and reviewed in Section 4.2 above. Both parts were later published together as

an ARC R&M (41).

Wing parachutes had been installed on the aircraft at both tips and had been streamed on one

occasion when it was accidentally taken into a spin. Although both parachutes were trailing at the

limits of their cables, the pilot could see that their canopies had not opened, and the aircraft was

subsequently recovered from the spin by use of the controls. As it recovered from the spin, one of

the parachutes opened fully when the forward speed reached 310 ft/s. Both were then jettisoned

and the machine dived away safely. Before being released, the canopies had been rotating in a

coning motion, showing that there had been vortices in the regions of the wake of the stalled wings

into which the parachutes were streamed.

This behaviour was investigated in the RAE 24ft tunnel with half-scale parachutes on cables of

varying length trailed from the tip of a half wing of 12 ft semi-span, swept back at 45 o and mounted

at 45 o incidence. These confirmed that the mean air speed in the wake of the stalled wing was

substantially reduced below the free air speed in the tunnel, and that this had interfered with the

opening of the parachutes. The effects diminished at the longer cable lengths employed.

Figure 22. de Havilland Hornet (left) and 108 Swallow (right)


167

Accompanying tests were made in the vertical tunnel with a model DH108, using miniature

parachute canopies made in paper.

The aerodynamicists were joined in this study by John Picken of the Parachute Section of Mechanical

Engineering Department. That Section had a retired bomber aircraft for research into the behaviour

of parachutes, which could be launched by an observer in the rear turret. The parachute was carried

on a long cable, with the end fixed to a glider-towing point under the rear fuselage. The cable was

progressively pulled out of a bag and the parachute deployed as the cable became taut. The opening

behaviour was filmed over a range of speeds with a high-speed cine camera, and a record obtained

of the force in the cable from a strain-gauged link at the towing point. These trials had shown that

there were two critical forward speeds for a given design of parachute. One was the opening speed,

below which the canopy merely fluttered, while its mouth remained closed. Over a range of speeds

above the opening speed the canopy was fully inflated, but a second critical speed could be reached,

at which it closed again. This was due to the growing inward radial components of the forces

applied by the inclined rigging lines overcoming the outward forces on the canopy in the inflated

state.

It was concluded that the events with the DH108 had been due to the air speed in the wake at the

location of the parachutes during the spin being below their critical opening speed, so that the

canopies trailed behind unopened, providing little drag. After recovery by use of the controls, the

increasing forward speed and reducing incidence in the dive brought the air speed in the wake

above the critical opening speed, allowing the canopies to inflate.

The recommendation was that the towing points should be placed close to the wing tips and the

cable length should be as long as possible, so that the opening of the parachutes could occur where

the air speed in the wake was higher. As shown in Figure 21, the helical path taken by the spinning

aircraft and the inertia of the parachutes meant that they would lag behind the wings, displaced above

and sideways. Their combined effect was to apply a drag to stabilise the descent and a yawing

moment in the anti-spin direction, of particular value for recovery from a flat spin.

With due regard to the uncertainties of calculating the positioning of the parachutes, it was estimated

that the cable lengths could be up to 1½ times the semi-span without risk of them becoming

entangled. The ongoing work in the Parachute Section on design to reduce the opening speed and

increase the closing speed of parachutes was endorsed in the report.

5.3 Other spin tests

With the easing of the immediate pressures of wartime, it became possible to extend the testing of a

model by varying the conditions being represented and making modifications to the models in ways

that had been found to affect the behaviour in a spin. It was hoped that data could be gathered in

this way that would help with the understanding of the scale effects that were believed to be the

cause of the behaviour at full-scale sometimes being significantly different from that predicted on

the basis of model tests. A representative model of a new type could not be made until the design

had reached at least the stage at which the prototype could be defined. Towards the end of the

decade there were more cases where model results were available, but there had not yet been any

corresponding experience at full-scale with which the effects could be compared.


168

The DH103 Hornet, a long-range fighter development of the Mosquito, which is also shown in

Figure 22, was still of conventional design, but with unusual features. The forward fuselage was

short, and the twin engine nacelles projected well ahead of the cabin, to accommodate Merlin

engines with some components repositioned behind them so that the outer diameter could be

reduced. It was wondered if the presence of the large nacelles forward would reduce the effect of

the fuselage damping coefficient, which had been calculated on the basis of side area aft of the cg.

In the model spinning trials at a scaled altitude of 15,000 ft, the spin was found to be flat, with an

incidence of 60 o and rate of rotation of 3.1 rad/s (42). But recovery was straightforward, up to a

threshold of 23 units of applied pro-spin moment. The effects of several variations were then

examined. Increase in the overall weight made the rotation faster, with little change in recovery, but

increase in the inertia difference coefficient made it slower, with adverse effect on the threshold. A

dorsal fin, fitted with the intention of increasing the fuselage damping, had no effect, it was thought

because it had been effectively shielded by the fuselage at the high incidence. Various propeller

arrangements were represented. Relative to the case without any, two propellers of the same

rotation changed the threshold by 3 or 4 units, but with opposite rotations (as they were to be on this

aircraft in service), the threshold was essentially unchanged, as expected since the moments of

momentum of the two engines and propellers cancelled.

The next prototype from Hawker to be tested was the Seahawk, to specification N.7/45, shown in

Figure 23, which was to become the FAA's standard fighter and strike aircraft (43). This had straight

wings and was powered by the Roll-Royce Nene turbine engine, with wing-root intakes and

bifurcated tailpipes that discharged just aft of the wing roots. In standard conditions the rate of spin

was 2.1 rad/s with an incidence of 55 o, and recovery was obtained against pro-spin moments of 14 -

15 units. Though increases in weight, moment of inertia difference and altitude were adverse, its

spin and recovery were considered to be satisfactory for prototype flying.

Figure 23. Hawker Sea Hawk


169

Air Ministry specification T.7/45 was for a 3-seat advanced trainer to replace the Harvard, and to be

the first to be powered by a turboprop engine. Avro submitted the Athena and Boulton Paul the

Balliol, which became competitors for a production order. Both were flown as prototypes, the

Balliol with an Armstrong Siddeley Mamba engine, the first single-engined turboprop aircraft to fly.

The specification was revised considerably under T.14/47, to be for a two-seater aircraft, with the

Merlin engine, but the models for spin testing were of the first prototypes, designed to the earlier

requirement.

Under the standard conditions, neither prototype showed satisfactory spin recovery, though the

Athena was considered to be just acceptable with a small fin extension below the fuselage and an

increase in rudder area below the tailplane (44). The Balliol could be brought up to borderline level

by moving the tailplane back by 14.6 in full-scale. There was concern about the fast flat spin of this

aircraft, at 4.2 rad/s with 70 o incidence (45). The revised designs under T.14/47 were not tested as

models during the 1940s, though the Balliol was chosen for production later, both for the RAF and

for the FAA for carrier operation as the Sea Balliol.

The Westland Wyvern was a single-seat strike aircraft for carrier operations with the FAA, originally

conceived to specification N.11/44, though in its ultimate form it did not enter service until the next

decade. It was originally to be powered by the Rolls-Royce Eagle engine, but when this was

withdrawn, it was fitted with an Armstrong Siddeley Python turbine engine, in both cases driving

contra-rotating propellers. Tests were first carried out in April 1948 on a model of the TF.1 pre-

production version (46). It was noted that the body damping and unshielded rudder coefficients were

low, so it was expected that recovery would not be satisfactory, and this was confirmed when the

threshold moment was found to be only 3½ units. Of several modifications tried, the most effective

was raising the tailplane, but a borderline threshold level could be reached only by making an

impracticable rise of 32 in at full-scale.

Two months later, a model was tested of the TF.2 version, shown in Figure 24. Although the type

would not enter service in the decade covered here, the model was notable for the fitment of

miniature contra-rotating propellers with internal electric motor drives (47), shown in the lower part

of Figure 24. The 2-inch scale provides an indication of the continuing ingenuity and craftsmanship

brought to bear at RAE on providing the most realistic testing conditions that could be obtained

with very small models.

The spin coefficients for the TF.2 version were little changed from those of the model tested earlier.

Though the spin was mild, with incidence between 45 and 55 o and rotation of 2 rad/s, recovery was

possible only up to an applied moment of 5½ units. Tests with raised positions of the tailplane, as

with the TF.1 model, could obtain a satisfactory recovery only with it near the top of the fin. A

contra-rotating pair of propellers should not apply a nett moment to the aircraft, nor produce a

rotation in the slipstream, and here it was indicated that the spin was not noticeably different if the

drive was on or off. As reported below, the effect of propeller rotation was explored more

thoroughly later, when the contribution of the engine could also be represented.

The main production model of the Wyvern was the TF.4 (later S.4), which entered service in 1953,

so any further measures lie beyond the scope of this Part of the study.


170

Two models tested in July 1948 were of developments of the Hawker Tempest II, the Fury and Sea

Fury, to specifications F.2/43 and N.7/43 for the RAF and FAA respectively. Figure 25 shows the

Sea Fury. This differed from the Fury mainly by having folding wings and catapult and arrester

Figure 24. Westland Wyvern II and propeller drives for spinning models

FIG3. POWER UNIT WITH TWO MOTORS

Figure 25. Hawker Sea Fury

FIG4. POWER UNIT WITH SINGLE MOTOR


171

fittings with some local strengthening of the fuselage. Fitted with the Centaurus 2-row radial engine,

these were the most powerful propeller-driven single-engined fighters produced in Britain. The

spins for both models were quite flat, with incidence around 60 o and full-scale rotation of 2.7 rad/s.

There was a difference of 10½ units of threshold between left hand and right hand spins due to the

gyroscopic moment of the propeller. A feature of the spin tunnel tests was the inclusion of an

inverted spin with the Sea Fury model. This could then be recovered up to a threshold of 74 units,

showing the major improvement to the result in this position, when the rudder and fin were not

being affected by the wake from the rear fuselage and tailplane. Clearance was given for spinning

with both prototypes. The Fury order for the RAF was later cancelled, but the Sea Fury served with

the RN, latterly with a five-bladed propeller.

Another naval aircraft modelled at this time was the Fairey Firefly, a substantial two-seat multi-

purpose aircraft powered by a Rolls-Royce Griffon engine, as shown in Figure 26. Originally

conceived early in the war, it was delayed in production, and the Mark I version entered service

only in 1944. When model tests were made in 1948, aircraft up to Mark 4 had served well in many

theatres, notably in the Far East. The model tested had been made for the Mark I, and in an

unmodified state recovery could be obtained only up to 2 units of pro-spin moment. Production

aircraft of all marks had been modified, by moving the tailplane 18in forward relative to the fin, and

when the appropriate change had been made to the model the threshold had moved up to 9 units (49).

For the fully-representative Mk4 version this was raised to 12 units. With the forward position of

the tailplane, the fin was still largely in its wake, so the damping coefficient was lower than

considered desirable, though the rudder was largely aft of the wake, giving a good value of the

unshielded volume coefficient. On the basis of the accumulated experience, this combination of

coefficients was expected to lead to a flat spin, with a fast rotation, and the measured values were an

incidence of 67 o and rotation of about 2½ rad/s at full-scale. The conclusion was that it might be

difficult for the pilot to meet the requirement for fighters of recovery in the incipient stage of the

spin, but that it would be straightforward if the rotation was allowed to develop for five seconds

before moving the controls.

Figure 26. Fairey Firefly


172

It has been seen that direct comparisons between model spinning and full-scale experience with the

type came only rarely. An opportunity now arose with the Percival Prentice trainer, shown in

Figure 27. Designed to specification T.23/43, this resembled its predecessor the Proctor, shown in

Figure 5, though now of all-metal construction, having side-by-side dual control with a third seat

behind to serve a variety of duties. An early model spin test had indicated that spin recovery at full-

scale should be satisfactory, but some difficulties in the spin were experienced when the prototype

was taken through the prescribed trials by the contractor. For a training aircraft, these required at

least eight turns to be made before the controls were moved to begin recovery. It was noted that

over this period the spin had become flatter, and control movements became ineffective. After more

turns the anti-spin parachute had been used, finally obtaining a successful outcome.

An investigation with models was then undertaken at RAE (50). The model of the aircraft as

originally designed was first tested, with and without strakes along the top of the rear fuselage, a

popular and usually effective measure at that time, as reported earlier. The values of the inertial

difference and unshielded rudder volume spin coefficients were not greatly changed, but as

experience had shown, with strakes fitted the damping had been significantly raised. From recent

work it had been concluded that an aircraft with acceptable damping but very low unshielded rudder

volume would be likely to develop a flat spin. In this case, with strakes the incidence had risen from

49 o to 65 o, though with a similar rate of rotation of around 2½ rad/s. The model was then modified

to more closely resemble the prototype, and when fitted with strakes it was found to have a very

similar spin to that of the original model with strakes.

Tests were then made on the model with a series of modifications to provide data for analysis at

RAE. These covered the original design and addition of the fuselage strakes, a dorsal fin extension,

raising the tailplane and moving the fin rearwards so that the rudder would be clear of the tailplane

wake. All had a useful effect on recovery, with the exception of the fin extension, thought to be due

to its having been shielded by the wake from the quite broad rear fuselage. In the later tests the

model developed a flatter spin than the original, though with a somewhat lower rate of rotation.

Figure 27. Percival Prentice ab initio trainer


173

Values for the incidence and rate of rotation estimated during the contractor's trials were considered

by RAE to be insufficiently accurate to provide a direct comparison with the model results, so a

production Prentice was obtained for full-scale trials at Farnborough. These clearly showed that it

had two distinct spin states. Sometimes, after two or three turns there was a sudden change to a

flatter spin, from which recovery was more difficult.

In the report, it was pointed out that a steady spin required that the aerodynamic and inertial

moments must be in balance. Theoretically this could occur for more than one state, though that

had rarely been observed. The inertial moment in pitch, which was generally the most important

one, was proportional to the square of the rate of rotation and to sin 2α, where α is the incidence in

the spin. The development leading to this result can be found in Part 1 of this study (1). When the

damping was high the rate of rotation would be low, so α would then be expected to tend towards

45 o, where the sine term had its maximum value. This would be the case when strakes were fitted.

It was thought that a scale effect had been shown, such that when the Reynolds number was much

higher at full-scale, the damping was less effective and the tendency was for the spin to settle at a

higher rate of rotation where the moment would still be sufficient if the spin was flatter and the

incidence greater than 45 o. The effect of scale was less when the strakes were in position, and the

wake more turbulent, so there was little difference in that case between the original and the

prototype.

It had not been possible to determine the cause of a change to a steeper spin that occurred suddenly,

as noted at full-scale, but from a consideration of angles, it was thought that this could have

happened when the tailplane first became fully stalled.

The Prentice entered service as a replacement for the RAF's Tiger Moths, with modifications

including the fitting of fuselage strakes, enlarged fin and rudder, tailplane moved forward, and

upturned wing tip sections, in which form it remained in production up to 1949. As to further work

arising from the tests, it was recommended that there should be a reconsideration of the effect on

the damping of the cross-sectional shape of the rear fuselage. It had been shown by Irving and

others by 1935 (see Part 2 of this study) that a shape that was basically square, with a semi-circular

fairing above it - one that was commonly chosen - had an undesirable effect on spin and recovery,

but the effects of scale on this had not been investigated at that time.

Work done in 1946 on the experimental DH108, shown earlier in Figure 22, was for some reason

not reported until the end of 1948. Reference has been made in Section 5.2 to the parts of this

relating to the use of anti-spin parachutes, but there were also tests of the standard kind in the Free

Spinning Tunnel (51).

The value of the inertia difference coefficient for the aircraft was within the range normally

acceptable for single-engine monoplane fighters, but with the short fuselage the damping coefficient

was quite inadequate and the unshielded rudder coefficient 'practically zero'. Even if the best

control positions were used and response to the incipient spin made immediately, recovery of the

model could be obtained only against applied moments up to 6½ units. As seen earlier, however,

anti-spin parachutes streamed from the wing tips could be very effective, even for prolonged spins.

Inverted spins were checked, to see the effect of the fin and rudder when being fully unshielded in

that case. With the control directions appropriately reversed, the spin developed normally, with

initial incidence of 40 o rising to 68 o. The rate of rotation ranged from 1.4 to 3.75 rad/s (1.7 seconds


174

per turn). In the dive following recovery from the spin, the speed was higher than that found for the

conventional configurations due to the low drag coefficient of the type. It had been designed to an

experimental specification which did not include requirements with regard to spinning, so no

recommendations were made in that respect.

The last RAE report on spinning issued in this decade gave a comprehensive cover of the range of

Meteor variants that served with the RAF by that time - Marks 2, 3, 4, 6 & 7 - and the experimental

turbopropeller test-bed aircraft, shown in Figure 28 (52). It had been possible to build a basic model

that could be modified successively to provide results for all versions. Tests were run at the

equivalent of 15,000 ft and 30,000 ft, indicating that all the full-size aircraft should be recovered

from incipient spins at both altitudes and all except the Mark 6 from sustained spins at 15,000 ft.

The rates of rotation and incidence in the spin were similar for all, lying between 2 and 2.5 rad/s

and 45 to 55 o respectively at full-scale. Representation of the angular momentum of the engines

produced a measurable difference between the thresholds in left and right-handed spins as expected,

though this was not reckoned sufficient to be troublesome at full-scale. Finally, comparisons with

full-scale spin tests made with Meteors of Marks 3, 4, and 7, confirmed that the predictions from the

model tests were realistic.

6 Closure – the decade

A continuous thread in the developing understanding of the spin and of recovery from it had been

traced in earlier Parts of this study from 1909 onwards. In the decade of the 1940s reviewed here,

wartime conditions left few opportunities for contributing to spin theory, so the leading topic of new

work on spinning was of further improvements to the testing of models in the vertical Free Spinning

Tunnel at RAE. This included the representation of the angular momentum of engines and propellers.

Studies of the use of spin recovery parachutes, initially a single one attached to the rear fuselage and

later pairs attached to the wing tips, also included work at model scale.

It had been expected in the past that there would be 'scale effects' that could cause the spin and

recovery behaviour of models to differ from that of the corresponding aircraft at full-scale.

Changes intended to render the tests more representative in this respect were introduced but also

Figure 28. Gloster Meteor variant with turboprop engines


175

made them more elaborate and time-consuming to carry out. Although these measures remained

largely empirical, they continued to be formulated with due regard to theoretical principles. It

seemed likely that the effects of scale would not be understood fully unless more knowledge could

be obtained of the generation of the aerodynamic forces and moments experienced by an aircraft in

the deeply-stalled condition of the spin.

The later years of the 1940s were the start of the era of jet propulsion. The first aircraft entering

squadron service with the RAF and the FAA were straight-winged and had spin characteristics

similar to those of the late piston-engined era. But the arrival of the turbine engine had opened the

way into the transonic region of flight, and the first aircraft with new configurations being considered

for this regime began to appear. It was indicated that the effects of their 'unorthodox' shapes on spin

characteristics were likely to be a leading area of concern in the next decade.

Acknowledgements

Leslie Ruskell (Farnborough Air Sciences Trust) rendered much assistance in accessing RAE

reports from the period on microfilm

Photographs are from the Mary Evans Picture Library/Royal Aeronautical Society Collection unless

otherwise stated. Thanks to Tony Pilmer of the National Aerospace Library making them available.

References

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period 1909 - 1929 J Aero Hist, 4, 2014, 106 - 160

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period 1930 - 1940 J Aero Hist, 5, 2015, 168 - 240

3. Finn, E. Analysis of routine tests of monoplanes in the Royal Aircraft Establishment Free

Spinning Tunnel, ARC R&M 1810, July 1937, HMSO 1939

4. Gates, S B. Note on model spinning standards, RAE Report BA 1436, Oct 1937

5. Tye, W and Fagg, S V. Spinning criteria for monoplanes, RAE Report AD 3131, May 1940

6. Brinkworth, B J. On the planning for British aircraft production for the Second World War

and reference to James Connolly. J Aero Hist, 8, 2018, 233 - 298

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Publishing Ltd, Manchester, 2010

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Nov 1941

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1981, Private communication via Peter Amos, Miles Aircraft Collection

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11. Amos, P. Miles Aircraft - The Post-War Years, 1945 - 1948, Air Britain Publishing Ltd,

Tonbridge, 2016

12. Finn, E and Bigg, F J. Model Spinning Tests on the Phillips and Powis M.18, RAE Report

BA 1611, July, 1940

1.3 Model spinning tests of Percival Proctor, RAE Report BA 1635, Oct 1940

14 Full-scale spinning tests on twin rudder Oxford N.6327, RAE Report BA 1638, Nov 1940

15. Brinkworth, B J. Spitfire 'tailplane protection' and spinning trials, J Aero Hist, 7, 16-24

16. Quill, J. Spitfire, John Murray (Publishers) Ltd, London, 1983

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1315, Nov 1943

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spinning tunnel, RAE Report B A 1693, July 1941

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1943

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1967, May 1943, HMSO 1952

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Aero 1819, May 1943

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30. Pringle, G E and Warren, V G. Model spinning test of the Vickers F.7/41, RAE Report Aero

1832, June 1943

31. Pringle, G E. Note on rates of turning in the spin of two twin-engined types, RAE Tech Note

Aero 1317, Nov 1943

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1943

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Aero 1915, Jun 1944


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34. Pringle, G E and Warren, V G. Model Spinning test of the Gloster Tourist (E.28/39), RAE

Report Aero 1909, Jan 1944

35. Brinkworth, B J. The Gloster E.29/39 - Fin arrangement and spinning characteristics, J Aero

Hist, 5, 2015, 65 - 82

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RAE Report Aero 1939, Apr 1944

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Note 1559, Dec 1944

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1576, Jan 1945

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spin. Part II: Wake phenomena, RAE Aero Tech Note 1881, Mar 1947

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RAE Report Aero 2231, Nov 1947

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T.4/45], RAE Report Aero 2267, May 1948

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(Boulton Paul Balliol T.7/45), RAE Report Aero 2253, Mar 1948

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(Westland Wyvern) - plus addendum - some measurements of the effects of applied rolling

moments on the model recovery, RAE Report Aero 2262Apr 1948

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single engined naval strike aircraft (Westland Wyvern 2), RAE Report Aero 2271, Jun 1948

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F.2/43 and Sea Fury N.22/43), RAE Report Aero 2273, Jul 1948

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Firefly Mks 1 and 4) - plus amendment Oct 1948, RAE Report Aero 2286, Aug 1948

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Proctor), RAE Repot Aero 2298, Nov 1948

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E.18/45)(with Addendum and Corrigendum), RAE Report Aero 2305, Dec 1948

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The author

Brian Brinkworth read Mechanical Engineering at Bristol University. He worked first on defence

research at the Royal Aircraft Establishment Farnborough during the 1950s. There, he was assigned

part-time to be Secretary of the Engineering Physics Sub-Committee of the Aeronautical Research

Council (ARC), and after moving into Academia in 1960, he was appointed an Independent

Member and later Chairman of several ARC Committees and served on the Council itself.

Thereafter he was appointed to committees of the Aerospace Technology Board.

At Cardiff University he was Professor of Energy Studies, Head of Department and Dean of the

Faculty of Engineering. For work on the evaluation of new energy sources he was awarded the

James Watt Gold Medal of the Institution of Civil Engineers. In 1990 he was President of the

Institute of Energy and elected Fellow of the Royal Academy of Engineering in 1993.

Since retiring, he has pursued an interest in the history of aviation, contributing papers to the

journals of the RAeS, which he joined in 1959. He holds a Private Pilot’s Licence.

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