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Progress in Aerospace Sciences 41 (2005) 175191

A review of out-of-ground-effect propulsion-induced

interference on STOVL aircraft

A.J. Saddington, K. Knowles

Aeromechanical Systems Group, Department of Aerospace, Power and Sensors, Cranfield University, RMCS, Shrivenham, Swindon,

Wiltshire, SN6 8LA, UK

Available online 14 June 2005

Abstract

This paper provides a review of the out-of-ground-effect propulsion-induced interference on the aerodynamics of jet-

lift short take-off and vertical landing (STOVL) aircraft. Two main flight regimes are discussed; hover and transition

wherein the main fluid dynamics phenomena that cause the interference are presented. Where possible, an engineering

assessment is made of the effect of jet and/or airframe configuration parameters on the observed interference effects.

r 2005 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1762. The free jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

3. Hover out of ground effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

3.1. The effect of jet decay rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

3.2. The effect of nozzle pressure ratio (NPR). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

3.3. The effect of nozzle length and projection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

3.4. The effect of planform shape and position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

3.5. The effect of off-axis nozzle positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4. Transition out of ground effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.1. The free jet in a cross-flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.1.1. Correlation parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.1.2. Jet trajectory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

4.2. Jet-induced interference effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1834.2.1. The effect of velocity ratio,Ve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

4.2.2. The effect of area ratio, AnS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

4.2.3. The effect of nozzle geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

4.2.4. The relative location of wing and jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

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Corresponding author.

E-mail addresses: [email protected] (A.J. Saddington), [email protected] (K. Knowles).
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4.2.5. The effect of nozzle vector angle, dn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

4.2.6. Lift improvement devices (LIDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

4.3. Jet and intake-induced interference effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

1. Introduction

For over 40 years engineers have struggled with the

concept of giving a jet-powered fixed-wing aircraft the

capability to hover and transition to and from wing-

borne flight. From the tens of designs studied, only

about 20 jet-powered STOVL aircraft made it to the

flight test stage and of these, only two (the Harrier and

Yak-38) became operational, a testimony to the

difficulty in designing a successful STOVL aircraft.

With development of the Joint Strike Fighter (JSF)

nearing completion it is perhaps poignant to reflect on

the lessons learnt throughout the development of theseaircraft.

This paper presents a review of the main out-of-

ground-effect propulsion-induced aerodynamic interfer-

ence encountered by jet-lift STOVL aircraft. It will draw

from several earlier surveys [16] as well as presenting

more recent research results.

2. The free jet

Many generalised studies of axisymmetric free jets

have been made and their characteristics are well

documented [79]. Studies focussing on particular

aspects of free jets such as the self-similar region

downstream of the nozzle [1012], the effect of initial

conditions[1316], the effect of a co-flowing stream[17]

and large-scale structures within the jet[18,19]have also

been made.

With regard to STOVL aircraft applications high-

speed (in particular under-expanded) free jets are of

relevance and these too have been well documented

[2024]. For a circular convergent nozzle, three varia-

tions of the flow pattern are possible, depending on the

nozzle pressure ratio (NPR, the ratio of settling chamber

total pressure, Pc to ambient static pressure, pa). Thethree variations are the subsonic jet, the under-expanded

jet and the highly under-expanded jet [23]. Note that

with a convergent-divergent nozzle an additional flow

pattern exists, that of the overexpanded jet. This occurs

when the nozzle is operating off-design at too high a

NPR[25].

Jet spreading rate and entrainment characteristics

have been the subject of a number of studies. Anderson

and Johns [20] considered a range of jet exit Mach

numbers from 1.4 to 3.53. The jets were found to have a

mean cone half angle of 4:6 with this value being

independent of NPR. Later studies by Moustafa [24]

Nomenclature

An nozzle exit area

dn nozzle diameter

dne equivalent nozzle diameter

D angular mean planform diameteri index (see Eq. (3))

k1, k2 constants in Eqs. (2), (4) and (7)

L lift

m constant in Eq. (10)

_mn nozzle exit mass flow rate

_mx mass flow rate at a streamwise distance x

max maximum

M pitching moment

Mn nozzle exit Mach number

M1 freestream (cross-flow) Mach number

n constant in Eq. (10)

NPR nozzle pressure ratiopa ambient static pressure

Pc settling chamber total pressure

Pertotal total jet perimeter

qn nozzle exit dynamic pressure

qx dynamic pressure at a streamwise distancex

r radius

rn nozzle radius

S planform area

T thrustuc mean jet centreline velocity

Ve effective velocity ratio (see Eq. (9))

Vn nozzle exit velocity

V1 freestream (cross-flow) velocity

x streamwise distance from nozzle exit

z distance perpendicular to nozzle exit

x x=rna;b constants in Eq. (10)dn nozzle vector angle

D incremental

y angular measurement (see Eq. (8))

k constant in Eq. 1re ra=rnrn nozzle exit static density

ra ambient static density

A.J. Saddington, K. Knowles / Progress in Aerospace Sciences 41 (2005) 175191176


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and Lau et al. [26]indicated that jet spreading rate is in

fact strongly influenced by NPR and jet exit Mach

number. Nozzle geometry has also been found to

influence spreading characteristics with elliptical, rec-

tangular and lobed nozzles all showing greater spreading

rates than jets from a circular nozzle [27].

Ricou and Spalding [28] summarise results for the

mass entrained by a jet, based on measured velocity

profile data and augmented with analytical solutions,

showing that for given jet exit conditions air is entrained

into a jet at a constant rate. Witze [29] making use of

Kleinsteins [30] theoretical formulation and the results

of 13 experimental studies, derived an empirical

formulation (Eq. (1)) for the decay in the centreline

velocity for varying jet exit Mach numbers.

uc x 1 exp 1

k x re0:5 0:7

!, (1)

where k is a proportionality constant given by

k 0:081 0:16Mn re0:22 Mno1,

k 0:063M2n 10:15 Mn41.

The influence of temperature on the characteristics of

free jets has also been widely investigated [29,3133],

particularly with respect to spreading rates [31] and

entrainment rates [33].

3. Hover out of ground effect

The basic flow-field associated with a jet-lift STOVL

aircraft hovering out of ground effect is illustrated in

Fig. 1 [34]. In all cases, the lifting jets issuing from the

aircraft mix with ambient air to generate a complicated

three-dimensional flow-field. In general, when one or

more jet-lift engines are installed in an aircraft fuselage

or wing, and exhaust vertically downwards, a small lift

loss is induced by the entrainment action of the jet(s).

For a simple circular planform with a single central

round jet having a uniform exit velocity distribution

(Fig. 2), the reduction in lift is about 2% of the installed

thrust T, for a nozzle area to planform area ratio

An=S 0:01 [35,36]. This lift loss reduces steadily, butnon-linearly, as the ratioAn=S increases, becoming onlyabout 0.5% for An=S 0:11 and negligible if An=S isgreater than 0.25[35,36]. The lift losses on the plate arise

from the entrainment action of the jet, which mixes with

the surrounding air and sets up a cross-flow over the

bottom of the plate, thereby inducing a suction pressure.

The four-jet configuration of Otis[37]indicated that lift

losses of between 3 and 4 per cent could be encountered

for typical multiple-jet configurations.

3.1. The effect of jet decay rate

Gentry and Margason [38] made lift loss measure-

ments using a circular and a rectangular plenum

chamber. The latter was designed to fit inside the

fuselage of a wind tunnel model and was smaller than

ideally desired. A comparison of the loads induced on a

circular plate by a single nozzle fitted to the circular and

rectangular plenum chambers (Fig. 3(a)), indicated that

the loads induced on the circular plate mounted on the

Entrained air

Intake flow

Fig. 1. The flow field surrounding a V/STOL aircraft in hover

[34].

L

Fig. 2. The lift loss generated on a flat plate by a single nozzle

[35].

A.J. Saddington, K. Knowles / Progress in Aerospace Sciences 41 (2005) 175191 177


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rectangular plenum were three to four times as large as

those induced on the same plate-nozzle arrangement

with the circular plenum chamber.

Measurements of the velocity profile obtained from

the rectangular plenum indicated a distorted velocity

distribution with a deficit at the centre, whereas the flowfrom the circular plenum had a uniform top hat

velocity profile. The flow through the rectangular

plenum was found to be very turbulent and it was

suspected that the high turbulence levels were causing

the higher induced loads. To confirm the hypothesis,

fairings and baffles were added to the rectangular

plenum chamber to improve the flow quality and a

strut was inserted in the circular plenum to degrade the

flow quality. This also produced a much greater

distortion of the exit velocity profile than was present

on the original rectangular plenum chamber. Fig. 3(a)

shows that these changes increased the lift loss for the

circular plenum chamber and greatly reduced the losses

for the rectangular plenum chamber.

The lift losses did not correlate with the exit velocity

distribution, however, they were found to correlate withthe rate of decay of the jet with distance downstream

from the nozzle exit (Fig. 3(b)). These curves show the

peak non-dimensional dynamic pressure in the jet at a

non-dimensional distance downstream of the nozzle. The

circular plenum chamber gave the smallest lift loss and

had the lowest decay rate. The original rectangular

plenum produced the greatest lift loss and had the highest

decay rate. The modifications made to the circular and

rectangular plenum chambers produced jets with similar

decay rates, giving similar lift losses. The correlation was

also found to hold true for multiple-jet configurations.

Fig. 4 compares the lift loss and decay rates for threedifferent jet arrangements (all using the modified

rectangular plenum chamber). An empirical equation

was developed to predict the lift loss per unit thrust based

on the jet decay rate and model geometry[38].

DL

T

ffiffiffiffiffiffiS

An

r k1k2, (2)

where k1 0:009 and

k2

ffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffi

qqx=qn

@x=dne

max

x=dnei

vuuut , (3)

(S/An)

L/T

0 1 2 3 4 5 6 7 8 9 10-0.05

-0.04

-0.03

-0.02

-0.01

0

Single jet

Four jet

Eight jet

Four slot jet

Single jet

Four jet

Eight jetFour slot jet

x/dne

qx

/(Pc-pa

)

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

(a)

(b)

Fig. 4. (a) Induced load and (b) jet decay for single-jet and

several multiple-jet configurations NPR 1:89 [38].

NPR

L/T

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4-0.04

-0.03

-0.02

-0.01

0

Cylindrical plenum, clean nozzle

Cylindrical plenum, restriction in nozzle

Original rectangular plenum

Modified rectangular plenum

Cylindrical plenum, clean nozzle

Cylindrical plenum, restriction in nozzleOriginal rectangular plenum

Modified rectangular plenum

x/dn

qx

/(Pc-pa

)

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

(a)

(b)

Fig. 3. The effect of plenum chamber and nozzle configuration

on (a) induced load on several plate shapes and (b) on jet decay

NPR 1:64 [38].



5/17

wherexis the distance along the nozzle axis, the origin of

which is at the exit plane. The lift loss is therefore

proportional to the square root of the maximum rate of

decay of the jet divided by the distance to the point at

which this occurs. The correlation showed good agree-

ment with the test data.

It is clear that the use of rapid mixing nozzles, if

installed on a STOVL aircraft, will have a detrimental

effect on the aircrafts hover performance. These

nozzles, which are currently being developed for reduced

radar and infra-red signatures [39] and for minimising

ground erosion problems, are designed to promote high

decay rates of the jet potential core and may employ

techniques including: geometric modifications such as

lobes, notches or castellations; excrescences such as tabs

or vortex generators and fluidic techniques such as swirl

or shear layer excitation. An extensive literature review

of jet mixing enhancement is given by Wong [40], to

which the reader is referred.When multiple jets are dispersed over the planform,

the percentage lift loss is greater than for the corre-

sponding single jet of the same equivalent diameter

ratio,dne [35]. This is partly due to the increased mixing

rate, but also to the additional pressure drop produced

on the lower surface between the jets because of the

constricting effect of the jets themselves on the entrained

flow. This does not appear, however, to be a strong

effect and the lift loss seems unlikely to exceed 5%

unless the jets are arranged in rows or elongated narrow

slots, thus tending to enclose a significant amount of the

planform area.

Using a Pratt and Witney J85 engine equipped with

two alternative jet pipes; one for a single jet investiga-

tion, and one with a special exhaust ducting to produce a

four-jet configuration, McLemore [41] ran tests with

three different sizes of rectangular plates to produce

three different ratios of planform area to jet area An=S.The comparison of these real engine data with the model

results of Gentry and Margason [38] shows good

agreement (Fig. 5).

Using data from Fleming [42] and Higgins et al.

[43,44],Kuhn and McKinney[45](and later Hammond

[46]), present a comparison of experiments conducted

using jets of different temperature. These suggest thattemperature does not appear to have an appreciable

effect on the jet decay rate except for a circular nozzle

(Fig. 6).

The studies on jet decay rate demonstrate that if

accurate lift loss data are to be obtained from wind tunnel

models then the decay rate of the simulated jets should

accurately match those of the full size installation.

3.2. The effect of nozzle pressure ratio (NPR)

Gentry and Margason observed that the measured lift

losses showed a reduction with increasing NPR for a

constant nozzle to planform area ratio[38]. At the time a

theoretical model was not available to explain this. NPR

effects were looked at, however, in an extensive study of

jet-induced interference effects undertaken by Shumpert

and Tibbetts[47]. Tests were conducted on a 14 per cent

scale model of a jet VTOL aircraft with a fuselage-

mounted lift system at pressure ratios of 1.4, 1.7, 2.0 and

2.3. It was discovered that the lift loss did not follow the

correlation proposed by Gentry and Margason (Eq. (2)).

The results showed that there was a measurable

(S/An)

L/T

0 1 2 3 4 5 6 7 8 9 10-0.05

-0.04

-0.03

-0.02

-0.01

0

Single jet, small scale

Four jet, small scale

Single jet, J85

Four jet, J85

Single jet, small scale

Four jet, small scale

Single jet, J85

Four jet, J85

x/dne

qx

/(Pc-pa

)

0 1 2 3 4 5 6 7 8 9 10 11 120.0

0.2

0.4

0.6

0.8

1.0

1.2

(a)

(b)

Fig. 5. Comparison of (a) induced load and (b) jet decay for the

small-scale results of Ref. [38]and a J85 turbojet [41].

x/dne

qx

/(Pc-pa

)

0 2 4 6 8 10 12 14 160.0

0.2

0.4

0.6

0.8

1.0

1.2Circular nozzle (70F)Circular nozzle (1200F)Single slot nozzle (70F)Single slot nozzle (1200F)Four slot nozzle (70F)Four slot nozzle (1200F)

Fig. 6. The effect of temperature and nozzle configuration on

jet decay[45].



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discrepancy between the lift losses at different NPRs for

a constant nozzle area and a modification to Eq. (2) was

proposed.

DL

T

ffiffiffiffiffiffiS

An

r NPR0:64k1k2. (4)

The constant k1 was found to be aircraft configuration

dependent and for the tests conducted, varied between

0:01 and 0:016. The parameter k2 was unchanged(see Eq. (3)). As this correlation suggests, an increase in

NPR is seen to result in a decrease in percentage lift loss.

This effect was found to be similar for all configurations.

The NPR effect described above is explained by the jet

entrainment rate of different NPR jets. At an axial

distance, x downstream of the nozzle exit plane, the

ratio of mass flow rate in the jet to the mass flow rate at

the nozzle exit is given by[28]

_mx

_mn 0:32

x

dn

ffiffiffiffiffira

rn

r . (5)

Eq. (5) shows that as NPR is increased (and hence the jet

density), the mass flow rate of the jet at a given

streamwise location is reduced. The decay rate of a high

NPR jet will, therefore, be less than that of a lower NPR

jet. Comparison with data collected by Schwantes [48]

and reported by Ransom and Smy [49] suggests that

although Eq. (5) gives the correct trend for jet

entrainment, the magnitude calculated may not be relied

upon.

Shumpert and Tibbetts also conducted experiments at

constant thrust, i.e. the nozzle area was reduced as theNPR was increased. Varying NPR at constant thrust

had no measurable effect on lift loss [47].

A more direct, easier to use method for estimating

hover lift losses is presented by Kotansky[50]. Correla-

tion of data from various single and multiple jet

configurations[51]resulted in the following equation:

DL

T 0:0002528

ffiffiffiffiffiffiS

An

r

NPR0:64 Pertotal

dne

1:581. 6

Eq. (6) implicitly accounts for the higher decay rate ofmultiple jet configurations in terms of equivalent nozzle

diameter but does not account for higher decay rates

caused by jet exit conditions involving high entrainment

rates i.e. rapid mixing nozzles. If higher than normal

turbulence and decay rates are involved, Kotansky

suggests that Eq. (4) should be used [50].

3.3. The effect of nozzle length and projection

Gentry and Margason [38] investigated the effect of

nozzle length and projection on induced loads for a

single round jet with the modified rectangular plenum

chamber described in Section 3.1 and with rectangular-

planform plates. In these experiments, nozzle length

was defined as the distance from the bottom of the

settling chamber (the start of the nozzle profile) to

the nozzle exit plane. Nozzle projection, by contrast,

was the distance the nozzle exit plane extended below

the planform surface. The findings are summarised in

Table 1for NPRs of 1.5 and 2.0.

Extending the nozzle length caused a slight reduction

in the lift loss for the case of zero projection.

Measurements of the jet decay rate for the different

nozzle lengths showed that the shortest nozzle had the

highest decay rate. A further reduction in lift loss was

obtained by projecting the nozzle through the circular

plate distances of 0:5dn and 1:0dn. This reduction is dueto the increased distance between the plate and the free

surface of the jet which is entraining air.

3.4. The effect of planform shape and position

In order to determine the effect of planform shape on

the induced lift loss, Gentry and Margason tested four

plate shapes on their circular plenum chamber [38]. The

four shapes were circular, square, rectangular (aspect

ratio 1.52) and triangular (equilateral). In all cases the

ratio of nozzle exit area to plate area, An=Swas 0.0144and the centre of the nozzle was located at the centroid

of the plate. There was no measurable effect of planform

shape on the lift loss. The effect of wing height was also

investigated. As would be expected, a low wing

configuration had more lift loss than a mid-mountedwing which in turn performed worse than a high wing.

This is due to the relative proximity of the wing to the

mixing region of the jet.

The effect of planform lip shape was investigated by

Ing and Zhang [52]. An experiment was carried out to

study the parameters affecting single-jet ground-effect

hover lift loss and to identify the cause behind the large

discrepancies in lift loss between the experiments of

Wyatt[53]and Corsiglia et al.[54],commonly known as

Table 1Lift loss for various nozzle lengths and projections, An=S 0:0105[38]

Nozzle length

dn

Nozzle

projection

dn

DL=TNPR 1:5

DL=TNPR 2:0

0.248 0.0 0.0187 0.0175

0.910 0.0 0.0163 0.0155

1.410 0.0 0.0163 0.0148

0.910 0.5 0.0130 0.0129

1.410 0.5 0.0133 0.0126

1.410 1.0 0.0116 0.0107



7/17

the Wyatt anomaly. The cause of the discrepancies was

traced to a single geometrical parameter, namely the

planform plate edge geometry, which significantly

affected the flow separation underneath the plate. The

out of ground effect lift loss was, however, found to be

insensitive to the edge geometry.

3.5. The effect of off-axis nozzle positions

The effect of positioning a nozzle away from the

fuselage centre-line was part of a study carried out by

Welte[55]. Tests on a model of the VTOL transporter,

Dornier Do-31, indicated that a centrally located jet will

produce more lift loss than a peripherally located jet.

The podded lift engines installed at the wing tips induced

a lift loss of 2.2 per cent compared with 3.6 per cent for

the lift/cruise engine installed below the inner wing

section. Welte postulated that the constant k1 in Eq. (3)

is universal and an additional term could be introducedinto the equation to take into account configuration

effects. The following equation was developed [55].

DL

T

D

dneNPR0:64k1k2, (7)

where D is the angular mean planform diameter and is

given by

D 1

p

Z 2p0

ry dne

2

dy, (8)

where k1 0:009 and k2 is unchanged from Eq. (3).

It was found that for the lift/cruise engines, both Eqs.(4) and (7) predicted the lift loss quite well, 4.0 and 3.4

per cent, respectively. With the peripherally located

lift engines at the wing tip, Eq. (4) failed to produce

a satisfactory result, again predicting 4.0 per cent

(as would be expected) compared with 2.4 per cent for

Eq. (7).

4. Transition out of ground effect

The transition from hover to wing-borne flight is a

very important design consideration for STOVL air-craft. Fig. 7 shows the principle aerodynamic phenom-

ena affecting a STOVL aircraft in transition out of

ground effect [56]. During the transition from hovering

to conventional flight, the efflux from the lifting jet(s) is

swept back by the freestream and rolled up into pairs of

vortices. These vortices, along with the entrainment

action and blockage effect of the jet(s), induce suction

pressures on the bottom of the configuration beside and

behind the jet(s) and a smaller region of positive

pressure ahead of the jet(s). In most cases, these induced

pressures result in a nose-up pitching moment and a loss

of lift on the aircraft.

4.1. The free jet in a cross-flow

The jet in cross-flow has received considerable

research attention because it is a fundamental fluid

dynamic phenomena with many applications such as

smoke plumes from chimneys, the dispersal of liquids in

streams, jet engine combustors and reaction control jets

on rockets and missiles. The most widely studied, and

reviewed, application, however, is related to STOVL

aircraft [57,58].

A jet exhausting into a cross-flow generates a complex

flow-field with several distinguishable features. When

the jet efflux exits the nozzle it is deflected by the

freestream to follow a curved path downstream while its

cross-section changes. For the case of a circular jet, there

are stagnation points upstream and downstream and

minimum pressures at the lateral edges. As a conse-

quence, the jet spreads laterally into an oval shape. At

the same time the cross-flow shears the jet fluid along the

lateral edges downstream to form a kidney-shaped

cross-section. At increasing distances along the jet path

this shearing folds the downstream face over itself to

form a vortex pair which dominates the flow. Associated

with the vortex pair is the flow induced into the wake

region of the jet from the freestream. Fig. 8 shows the

topology of a jet in cross-flow [58].

4.1.1. Correlation parameters

Early jet in cross-flow investigations usually charac-

terised the test condition by the ratio of freestream

velocity to jet exit velocity, V1=Vj. It was soonrecognised, however, that this ratio was not appropriate

for hot jets. Williams and Wood observed that non-

dimensional increments in lift, drag and pitching

moment were primarily, but not solely, a function of

jet to freestream velocity ratio [35]. The influence of

jet Reynolds number (based on nozzle diameter) and

model Reynolds number (based on wing chord) on jet

Viscous effects onwing and controlsurfaces

Intake mass flow

Jet blockage and entrainment

Fig. 7. The flow field about a STOVL aircraft in transition out

of ground effect[56].



8/17

interference effects were secondary. They showed that

the influence of temperature and compressibility effects

could better be accounted for by the use of an effective

velocity ratio (Eq. (9)), which is still widely used.

Ve

V1

Vnffiffiffiffiffirarn

r

M1

Mnffiffiffiffiffipa

pnr

. (9)

For STOVL applications the magnitude of Ve for

transition to wing-borne flight depends on the pressure

ratio of the lifting propulsion device. For high-pressure

ratio jet engines, transition will occur with Veo0:2; forthe Harrier, transition will occur with Veo0:3; and forlift fans with low pressure ratios, transition will occur

with Veo0:5[58].

4.1.2. Jet trajectory

One property of jets in cross-flow which has received a

lot of research attention is the jet trajectory[5963]. The

most common parameter for defining the trajectory is

the locus of maximum velocity; however, other defini-

tions are sometimes used depending on the preference of

the researcher. These trajectory definitions include: the

locus of maximum stagnation pressure; the locus of

maximum dynamic pressure; the locus of maximum

stagnation temperature and the line of maximum

vorticity. Since none of these definitions will give the

same trajectory, it is necessary to exercise great care

when comparing data from different sources.

Nearly all the work done to establish a jet trajectory

has used a subsonic jet in a subsonic cross-flow. A

widely used empirical equation for the jet centreline

trajectory is [58],

x

dn aVne

z

dn m

z

dn b cotdn. (10)

Thez and x origins are taken as the centre of the nozzle

exit. When the nozzle vector angle is 90 (i.e. perpendi-

cular to the cross-flow), the second term in Eq. (10)

becomes zero. The empirical approaches correlate the

experimental data using similarity laws to obtain the jet

trajectory. Table 2 summarises the coefficients and

exponents for jets with a 90 vector angle [6467] and

for jets with varying vector angles[6870].

Eq. (10) has been plotted using an effective velocity

ratio,Ve of 0.25 which covers the range of applicability

for most of the researchers. Fig. 9 shows the jettrajectories for a nozzle vector angle of 90 while

Fig. 10shows the trajectories for a vector angle of 60.

The equations derived by the researchers show good

agreement with each other. There are slight variations in

the initial depth of penetration and the rate of jet

deflection. These can probably be accounted for by

variations in the initial jet and cross-flow conditions in

the experiments from which the empirical equations are

derived. In particular the turbulence intensities of the jet

and cross-flow, which will affect the mixing rate between

the two, will have a strong influence on the jet

Table 2

Jet trajectory equation terms and applicability

Author(s) a n m b Ve dj (deg)

Bradbury [64] 1.08 2.70 3.00 n/a 0.080.43 90

Kamotani and Greber[65] 1.38 2.61 2.78 n/a 0.130.26 90

Chaissaing et al. [66] 0:59 Ve2:6 2.60 2.60 n/a 0.160.42 90

Fearn and Weston[67] 1.08 2.68 2.95 n/a 0.100.33 90

Ivanov[68] 1.00 2.60 3.00 1.00 0.030.71 60120

Shandorov [69] 1.00 2.00 2.55 1 Ve2 0.210.71 4590

Margason[70] 1=4sin2dj 2.00 3.00 1.00 0.100.85 30180

V

Jet CL

Vortex curve

Separation line

Vortex pair

Horseshoe vortex

Wake vortex street

Fig. 8. Sketch of the vortex systems associated with a jet in

cross-flow[58].



9/17

penetration and curvature. Zhang and Hurst [71] have

observed that with supersonic jets, where the turbulence

intensity in the jet may be high (due to the breakdown of

the shock structure), the jet centreline trajectory will

ultimately lead to less penetration than for a subsonic

jet.

Integral methods, e.g. [7277] for jet in cross-flow

research have also been developed which consider the

deflecting mechanisms of a jet by taking into account

either a pressure force or cross-flow momentum

entrainment or both. More recently the use of computa-

tional fluid dynamics (CFD) has given researchers anaddition numerical tool, e.g. [7882].

4.2. Jet-induced interference effects

In general, the jet-induced interference effects in

transition are characterised by a loss in lift and a nose-

up pitching moment.Fig. 11(a) illustrates typical trends

for a tail-off configuration[83]and shows the lift divided

by the thrust as a function of angle of attack. Fig. 11(b)

shows pitching moment as a function of angle of attack.

In hover, the jets produce a lift which is equal to the net

thrust. With forward velocity the wing also develops lift

and, in the absence of interference effects, lift from these

two sources could be added together to produce the

solid curve. The jet-induced effects, however, cause a

loss of lift and the actual lift measured in a wind tunnel

is shown by the symbols. The difference between the

calculated and measured curves is the interference lift

loss, which is generally independent of angle of attack.

The rolling up of the jet wakes into contra-rotating

vortices is the primary cause of the interference effects in

transitional flight[83]. The vortices change the flow field

in the near-field and induce additional suction pressures

on the lower surface of the fuselage and wing of the

aircraft.

The interaction between the jet and the cross-flow

means that positive pressures are generated on surfaces

ahead of the jet and negative pressures behind [84].

Spreeman[85]measured negative pressures as high as 3

to 4 times the freestream dynamic pressure. Thepressures diminished with distance from the jet but

extended 10 to 15 jet diameters downstream and 5 to 10

(deg.)

L/T

-10 0 10 20 300.0

0.5

1.0

1.5

Calculated

Measured

Jet lift

Power-off lift

Interference lift loss (L)

(deg.)

M

-10 0 10 20 30

0

Interference moment (M)

Measured power-on moment

Power-off moment

(a)

(b)

Fig. 11. Jet interference in transition flight (tail off) [83]. (a)

Lift, (b) pitching moment.

x/dn

z/dn

0 5 10 15 200

5

10

Ivanov [68]

Shandorov [69]

Margason [70]

Fig. 10. Comparison of predicted jet in cross-flow trajectories

dj 60; Ve 0:25.

x/dn

z/dn

0 5 10 15 200

5

10

Kamotani and Greber [65]Chaissang et.al [66]

Fearn and Weston [67]

Bradbury [64]

Fig. 9. Comparison of predicted jet in cross-flow trajectories

dn 90; Ve 0:25.



10/17

diameters to each side of the jet[85].The combination of

positive pressures ahead of the jet and negative pressures

behind gives a nose-up pitching moment.

The following sections aim to describe parameters

which have been observed to influence the jet-induced

interference effects. Due to the complex nature of the

flow fields surrounding STOVL aircraft in transition, it

is difficult to isolate the effects which different para-

meters may have on any particular configuration and as

such only general trends can be identified.

4.2.1. The effect of velocity ratio, VeThe lift loss on a typical STOVL aircraft in hover has

already been discussed and is of the order of 2 to 3 per

cent of the installed thrust. Early experiments conducted

by Williams and Wood [35] on a simple rectangular-

wing model with a centrally located jet established that

the lift increment, DL=T falls steadily below its static

value as the effective velocity ratio Ve is raised fromzero. The lift loss was accompanied by the expected

nose-up pitching moments, again increasing as Ve was

raised. The observed interference effects are due partly

to the steady growth of the downward load from jet

interference on the lower surface behind the jet and

partly from the rearward movement of the centre of this

jet interference load. Additionally, the normal compo-

nent of thrust will be reduced because of the jet

deflection.

As the velocity ratio is increased further, the lift loss

reduces but the nose-up pitching moment continues to

increase. This may be due to two main factors; firstly,

the configuration being tested may be generating

aerodynamic lift which will offset the lift loss, and

secondly, the very high jet deflection angles involved

may put the jet interference close to the trailing edge of

the wing. The jets may then act as a crude jet-flap on the

wing lower surface and thereby generate extra lift by

super-circulation.

For practical STOVL aircraft, effective velocity ratios

below about 0.3 are of primary concern during

transition with jet-lift schemes although values as high

as 0.5 could be of importance with high by-pass ratio

engines or lift fan arrangements. Fig. 12 shows typical

results for a four-jet configuration[37].

4.2.2. The effect of area ratio, An=SThe interference lift loss is strongly sensitive to the

ratio of nozzle area to planform area, An=S. With theconfiguration mentioned above, Williams and Wood

observed that the lift loss increases as the area ratio is

decreased and the relationship is approximately linear

for a simple wing/jet geometry [35]. A larger planform

provides more area over which the interference pressures

can act thereby increasing the lift loss. At very low area

ratios, An=So0:0016, L=T actually fell to zero. For

more practical area ratios (between 0.01 and 0.04 for a

typical STOVL aircraft) the lift loss and accompanying

nose-up moments are less severe, a maximum lift loss of

25 per cent was recorded with an area ratio of 0.01.

4.2.3. The effect of nozzle geometry

Extensive studies of the effect of nozzle geometry were

carried out by Volger[86].A VTOL model was equipped

with various arrangements of interchangeable single or

multiple, round or slotted jets, with and without jet

deflection. All configurations showed interference lift

losses that increased with velocity ratio. For most jetarrangements, nose-up pitching moments due to jet

interference occurred and increased with velocity ratio.

The configurations which suffered the least interference

effect on the lift and pitching moment were the single

longitudinal slot jets, which was thought to be due to

their more streamlined shape in cross-section and the

smaller planform area behind the jet.

An ejector powered STOVL model, equipped with

various arrangements of slot nozzles, was tested by

Volger and Goodson [87]. The aerodynamic lift of the

fuselage, which was almost square in cross-section, at 0

angle of attack was zero. As velocity ratio was increased

Ve

M/Tdne

0 0.1 0.2 0.30

0.2

0.4

Ve

L/T

0 0.1 0.2 0.3-0.4

-0.2

0

(a)

(b)

Fig. 12. The effect of wing planform on lift and pitching

moment S=An 38 [37]. (a) Pitching moment, (b) lift.



11/17

from 0 (hover) to 0.4, all configurations showed a

reduction in the combined aerodynamic lift plus thrust.

Moving the slots outward reduced the affected model

area directly behind the jets, thereby reducing the

interference effects between the model, jets and free-

stream. Yawing the slots either inward or outward at the

forward end resulted in a reduction in lift and an

increase in nose-up pitching moments compared with

the unyawed slots. Yawing the slots outward was less

detrimental than inward. The effect of splaying the jet

20 away from the plane of symmetry produced some lift

improvement and a more nose-down pitching moment

as a result of less interference between the freestream

and the outward spreading jets. Any indicated benefit

from splaying the jet would, however, be partially offset

by the 6 per cent lift loss resulting from the inclination of

the thrust axis. A combination of 10 outward yaw of

the slots and 10 outward splay gave less lift loss than

configurations with or without angular deflections. Yawappeared to affect the results more than inclination.

4.2.4. The relative location of wing and jet

As regards jet position relative to the wing, rearward

location of the jet exit naturally tends to alleviate the lift

loss and the nose-up pitching moments arising from jet

interference, since the surface extent aft of the jet exit is

reduced. Williams and Wood found evidence to suggest

that with multiple jets the velocity ratio range over

which the lift loss occurs can be much reduced and the

subsequent lift augmentation much increased by the

adoption of a span-wise row arrangement towards therear of the wing instead of a close cluster [35].

A generalised study of jet positions from several wing

chord lengths ahead, to several wing chord lengths

behind, an unswept wing was reported by Hammond

and McLemore [88]. In this investigation, an aspect

ratio 6, unswept, untapered wing-fuselage model

equipped with a 30 per cent chord slotted Fowler flap

was used. Two jets, one on either side of the fuselage,

were positioned at about 25 per cent semi-span and at

the various longitudinal and vertical positions shown by

the plus marks in Fig. 13. The jets were mounted

independently of the wing so that only the aerodynamic

forces and interference effects were measured on the

wing. The data show that with the jet exits on the wing

chord plane, considerable jet interference was experi-

enced even with the jet as far as four wing chords ahead

of the wing. Favourable interference effects, however,

were encountered with the jets beneath and behind the

50 per cent chord point of the wing and the interference

effects are most favourable for positions closest to the

flap. These favourable interference increments were

believed to be due to the action of the jet in helping

the flap achieve its full lift potential. Another slightly

different configuration [88], this time with jets operat-ing simultaneously both in front of and behind the

wing, indicated an overall favourable interference lift

effect, again indicating the importance of configuration

geometry on the jet interference lift and moment

characteristics. These findings agree well with those of

Margason and Gentry[112], Schultz and Viehweger[89]

and Nangia[90].

Winston et al. [91] report on a configuration which

was tested to determine the optimum jet exit location

from an induced lift standpoint. The model was tested

with the lift/cruise jets in three longitudinal positions as

shown in Fig. 14 and illustrate the lift improvement

obtained by successive aft movement of the jet exits

towards the wing trailing edge.

The Harrier II, with its new wing and large slotted

trailing edge flap, initially suffered from interference

problems whereby the forward nozzle flow interacted

x/chord

L/T

-4 -3 -2 -1 0 1 2 3-0.6

-0.4

-0.2

0

0.2

+ + +++ + ++ + + + +

+ + + + + +

+ + + ++++ + +

n = 90 Ve = 0.18

Fig. 13. The effect of varying chordwise jet location relative to a nearby wing on the induced lift [88].



12/17

with the wing undersurface and inboard pylon at

transitional forward speeds causing a flow separation

upstream of the flap. Removal of the scarf directed the

forward jet to a position further below the wing and

produced a lift coefficient increment of approximately

0.2[50].

4.2.5. The effect of nozzle vector angle,dnWind tunnel tests carried out on a one-sixth scale

model of the Kestrel (XV-6A) measured the effect of

vectoring the nozzles on the incremental lift and pitching

moment at a range of velocity ratios and angles of attack

[92]. At 0 angle of attack, increasing nozzle vector angle

to a more vertical position, increased the lift loss and

nose-up pitching moment. This correlates well with

similar works[93,94].

4.2.6. Lift improvement devices (LIDs)

Spreeman [85] noted that in one full-scale flight

investigation, deflecting the trailing-edge flaps reduced

the losses in lift and nose-up pitching moments. The

beneficial effect of the flaps on this configuration can be

attributed to positive pressures being built up in front ofthe flap on the lower surface.

The alleviation of unfavourable jet interference effects

was shown possible by Williams and Wood with the

addition of stream-wise fences to the lower surface of

the airframe, along the sides of the jet exit [35]. The

geometry of the fence was found to be important. The

depth of the fence should be at least one jet diameter and

the stream-wise length at least two jet diameters so as to

extend forward of the jet exit as well as bounding the

sides. Flow visualisation suggested that the fences

reduced the initial curvature of the jet, thus increasing

the penetration of the jet plume and delaying the growth

of the trailing vortex flows. This was consistent with the

apparent negligible effect of the fences at low velocity

ratios, where the rate of deflection of the jet is inherently

small. The effect of the fences on a simple fuselage

model was excellent, reducing maximum transitional lift

loss from 27% to 7%.

4.3. Jet and intake-induced interference effects

During wind tunnel testing of jet-lift STOVL aircraft,

it is usual to simulate the jet efflux but not the intake

flows. The intakes, which are commonly faired over or

are unpowered, are generally tested in separate wind

tunnel experiments. This may lead to a lack of

integration between intake [95,96] and nozzle design

[97]. Separate jet and intake testing ignores any mutual

interferences between them and airframe, but is adopted

due to the complexity of building a fully powered, metric

STOVL model[49,98]. This is in addition to the complextunnel interference problems encountered during transi-

tion testing [99] necessitating careful planning of test

procedures [100,101].

Early research using simple wind tunnel models

showed only minor interference effects due to intake

flows [45,112,102,103],which were easily accounted for.

This lead researchers to conclude that when testing

models in the transition phase of flight the intake

interference effects were generally not sufficiently

important to warrant modelling the intake flows [64].

Nevertheless some research projects have shown sig-

nificant jet/intake interference effects. An investigation

carried out by Mineck and Margason[103], and Mineck

and Schwendemann [104]determined some interference

effects between the jets and intakes on a STOVL

aircraft. Comparison of the force and moment data

for the intake open and intake closed (using elliptical

plugs) configurations, indicated that closing the intakes

decreased the aerodynamic angle of attack by approxi-

mately 2. Data for the VAK 191B described by

Haftmann [105] showed a consistent discrepancy be-

tween the lift loss measured in flight and the lift loss

measured on a wind tunnel model. The full-size aircraft

had a transitional lift loss 410% greater than the wind

tunnel model and a nose-up pitching moment 58%higher. This was probably due, in part, to the use of a

free flow intake on the model with no suction simulated.

As more complex STOVL aircraft designs were

developed, it became increasingly apparent that the

argument for separate jet and intake testing on STOVL

aircraft was not valid. The term close-coupled aero-

dynamics has become increasingly popular in describing

the engine/airframe designs and operating conditions in

which the air intake and nozzle system flows create

significant interferences with respect to each other and/

or which, when combined, create interferences to

airframe surfaces differing from the sum of the

Ve

L/T

0 0.1 0.2 0.3 0.4 0.50

1

2

3Calculated (no interference)

Fig. 14. The effect of chordwise location of deflected lift/cruise

jets relative to the wing trailing edge on the total lift for a

supersonic combat lift plus lift/cruise V/STOL aircraft config-

uration[91].



13/17

individual interferences. Harris et al. [106] suggest that

aircraft designs, in which the distance from the intake

face to the front nozzle relative to the diameter of the

compressor face is less than 3, will undoubtedly feature

close-coupled aerodynamics. Whereas for designs in

which this ratio is greater than 8 the aerodynamics can

generally be considered to be uncoupled.

Recent research has concluded that during low speed

transition Veo0:1 mutual jet/intake interference ef-fects do exist for close-coupled propulsion systems

[107109]. A generic jet-lift STOVL aircraft equipped

with powered intakes was tested in the RMCS low-speed

wind tunnel (Fig. 15). The model was not fully metric

and so airloads were inferred from static pressure

tappings on the wing. The model was tested with jet

and intakes powered separately and simultaneously

the difference between the airloads giving an indication

of the mutual interference effect. This is shown in

Fig. 16, which illustrates the mutual interference as anincrement of wing root sectional lift coefficient for the

rearmost nozzle position.

One would expect that the solution to the complexities

of building a STOVL wind tunnel model would be to use

computational fluid dynamics (CFD) tools. The combi-

nation of viscous interactions, broad range of flow

velocities and extensive regions of separated flow has

meant that CFD methods tried so far have been unable

to adequately predict overall airframe forces and

moments [110]. Although CFD is undoubtedly useful

in some aspects of STOVL aircraft development,

designers are still reliant on exhaustive wind tunnel

tests backed up by large historical databases. Even in

the 21st Century semi-empirical correlations are still

being developed for propulsion-induced interference

effects [111].

5. Conclusions

This paper has provided a review of the out-of-

ground-effect propulsion-induced interference on the

aerodynamics of jet-lift short take-off and vertical

landing (STOVL) aircraft. Two main operational

environments encountered by STOVL aircraft have

been discussed: hovering out of ground effect and

transition out of ground effect. The former has included

the fluid mechanics of an isolated free jet and the

interference effects caused by such a jet when combined

with simple and more complex fuselage/wing arrange-

ments. On the subject of transition out of ground effect,

the discussion has followed a similar format. Where

possible, an engineering assessment is made of the effect

of jet and/or airframe configuration parameters on the

observed interference effects.

Although some parametric trends are observed in

simplified experiments, jet-induced interference effects

for a complete airframe are difficult to predict and must

therefore be assessed through rigourous tests of the

chosen configuration. The evaluation process is still

predominantly experimental since computational fluid

dynamics models of complete STOVL aircraft do not yetexist with sufficient fidelity to provide a reliable

prediction of overall airframe forces and moments.

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