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1 © 2012 Lockheed Martin Corporation

Unmanned Aircraft and the Applicability of Military Flying Qualities Standards

J. Conor Bingaman Georgia Institute of Technology and Lockheed Martin Aeronautics Company1, Fort Worth, TX 76101

Unmanned aerial vehicles (UAVs) have a growing role in military aviation, but are

not addressed in the flying qualities standard for military aircraft, MIL-STD-1797B. This

deficiency is gradually being recognized, but there is little published work assessing MIL-

STD-1797B as a standard for unpiloted (or remotely piloted) aircraft. The simulations

performed in this study serve as test cases to demonstrate the need for revisions of the

standard that specifically address UAVs. The study was limited to lateral-directional flying

qualities assessments, and a gust during landing approach was used as the stressing case.

Four different sizes of UAVs were simulated responding to a discrete gust at approach

speeds, 100 feet above the field. The results showed that roll-mode time constant

requirements for gust recovery are strongly dependent on the size of the UAV and can be

larger or smaller than the 1797B standard. They also showed that roll control effectiveness

requirements could be significantly relaxed (assuming low-altitude gust recovery is the most

stressing lateral control task). Additionally, it was found that a revised classification system

is necessary in order to apply requirements to a diverse group of military UAVs. This

research reveals the need for a new or revised standard. Further research and accumulation

of flight test data will be required to create a flying qualities standard that is applicable to all

UAVs.

1 Graduate Student, School of Aerospace Engineering and Aeronautical Engineer, Lockheed Martin Aeronautics Company, AIAA Member


Nomenclature

α = angle of attack CYβ = coefficient of side force due to sideslip Clβ = lateral stability derivative (dihedral effect) Cnβ = directional stability derivative Cmδe = pitch control derivative CLδe = coefficient of lift due to elevator deflection Clδa = roll control derivative Cnδa = coefficient of yawing moment due to aileron deflection Clp = roll damping dm = distance aircraft travels from initiation to completion of 1-cosine gust δa = aileron deflection Ixx = x-axis moment of inertia Iyy = y-axis moment of inertia Izz = z-axis moment of inertia φ = bank angle φt = bank angle target for time-to-bank TR = roll mode time constant vm = maximum magnitude of 1-cosine gust 6DOF = six-degree-of-freedom AOA = angle of attack AGL = above ground level CAP = control anticipation parameter CPR = control power required simulation tool DOE = design of experiments MAC = mean aerodynamic chord PID = proportional-integral-differential


I. Introduction

The department of defense standard “Flying Qualities of Piloted Aircraft” or MIL-STD-1797B1 is accepted

universally for military aircraft development; however, the title carries a very important qualifier: “Piloted.”

Unpiloted aircraft are a growing part of military procurement, yet they lack any dedicated military standard. MIL-

STD-1797B1 has been refined through 60 years of research and serves as the recognized standard by which military

aircraft are designed and evaluated. Unmanned aircraft can be guided by this standard, but research is needed to

develop a comprehensive standard that addresses UAVs specifically. This report focuses on lateral-directional flying

qualities during final approach to landing. Though this is a very specific part of the standard, the comparison is

intended to validate the more general need for requirements that are tailored to Unmanned Aerial Vehicles or UAVs.

To test the standard laid out in 1797B1 different UAVs were simulated responding to a gust on final

approach. Different lateral control parameters were simulated to determine the characteristics required to meet the

established recovery criteria. The idea was to configure a vehicle that could successfully respond to the gust and

then compare it against the requirements in the standard. The low speed, low control authority flight condition on

final approach can be an important factor in sizing a manned vehicle’s control power requirements. As such, this

task seemed a natural choice to assess applicability of the standard for UAVs. It is hoped that the basic

recommendations resulting from this test will provide the starting point for follow-on research designed to

systematically address all aspects of MIL-STD-1797B for UAVs.

Four differently sized UAV models were chosen to span the range of current military UAVs. All models

used the same aerodynamic properties derived from wind tunnel data for a tailless, flying-wing type UAV. The size

and mass properties of the vehicles were adjusted to simulate a 20 lb, 200 lb, 2,000 lb, and 20,000 lb airframe. Six-

Degree-of-Freedom (6DOF) simulations were performed using a Lockheed Martin Proprietary simulation and

control system design tool called “Control Power Required” or CPR. All simulations began at 100 ft altitude, on a

standard day, in steady level flight. Two parts of the Lateral-Directional Flying Qualities Requirements were

addressed: Roll Mode Time Constant (MIL-STD-1797B sec. 5.2.3.1.1) and Roll Control Effectiveness (MIL-STD-

1797B sec. 5.2.3.5)1.

The results showed that the classification system laid out in 1797B is insufficient to accommodate the

diversity in size of current UAVs. Roll mode time constant for small-scale UAVs should be shorter than what is

recommended in 1797B, to allow rapid response to gusts. For the larger UAVs, the gust-response task allowed a


longer roll mode time constant than what is recommended in 1797B, suggesting that this task may not be the

requirement driver for this size of aircraft. Roll Control Effectiveness (measured in time to reach a specified bank

angle) was shown to be overly restrictive for all of the UAVs tested.

The classification system used by 1797B is addressed first, and then the lateral-directional flying qualities

requirements in 1797B are reviewed in more detail. This is followed by description of the experiments performed, a

discussion of the results obtained, and finally, recommendations for UAV flying qualities standards.

II. UAV Classifications

To provide requirements for different aircraft 1797B1 divides all aircraft into the four classes shown in

Table 1. These classes are defined in terms of gross weight and maximum load factor as seen in Figure 1. The

categories overlap because the classification is intended to be somewhat qualitative based on the mission of the

aircraft.

Table 1 MIL-STD-1797B Air Vehicle Classes1


Figure 1 MIL-STD-1797B Classification of Air Vehicles1

UAVs present a problem for this classification system because the range of airframe sizes is much larger

than conventional aircraft. Some UAVs are the size of traditional piloted vehicles, while others are carried in a back-

pack and launched by hand. Micro-UAVs are smaller still and even harder to classify. When applying the 1797B

classifications to the four UAVs studied in this report the larger 2,000 lb and 20,000 lb UAVs fall in Class I and

Class II respectively, while the smaller two UAVs do not fit on the chart as shown by the overlay in Figure 2. The

only choice when applying current standard would be to put all small UAVs into Class I which are described in

Table 1 as “light utility, primary trainer, or light observation.” The description of Class I aircraft is unlikely to

support reasonable requirements for a 20 lb, back-pack carried UAV. Clearly, the existing classification system

needs to be expanded to accommodate UAVs.


Figure 2 1797B Classification of Air Vehicles with overlaid UAVs

M. Christopher Cotting in his report “Evolution of Flying Qualities Analysis: Problems for a New

Generation of Aircraft” discusses in detail the classification problem UAVs face.2 He outlines the different

government classifications for UAVs that have been created over the years, none of which have been universally

acceptable enough to be useful as a flying qualities classification. Cotting proposes a new classification system,

which uses gross weight and Reynolds number as the defining parameters. The use of Reynolds number involves a

reference length, and scale is an important factor when attempting to differentiate UAVs of similar configurations

but greatly different dimensions. Cotting’s classification system is shown by the pink zones in Figure 3 (compared

to a government UAV standard shown by blue lines). The UAVs studied in this report have been plotted with blue

diamonds. Figure 3 shows the impact Reynolds number has of grouping the UAVs with their similarly scaled peers.

The four UAVs in this report fit neatly into Cotting’s Class II, Class III, Class IV and Class V (Class 0 and I are

reserved for micro-UAVs). The use of Reynolds number can create a problem; because of there is no standard way

of defining reference length. However, 1797B1 also requires some qualitative assessment by the user, to apply the

appropriate classification. The purpose of this report is not to suggest a final classification system, but to

demonstrate how the existing classifications used in 1797B1 are insufficient to classify unmanned aircraft.


Figure 3 Cotting UAV Classification System with overlaid UAVs2

III. Review of the Existing Standard

The purpose of MIL-STD-1797B1 is clearly laid out in its opening pages. It states that it “contains the

requirements for handling qualities, in flight and on the ground of US military piloted air vehicles. It is intended to

assure flying qualities that provide adequate mission performance and flight safety regardless of design

implementation or flight control system mechanization.”1 It is to be applied “during the design, construction, testing,

and acceptance of the subject air vehicle.” 1 The value of this standard lies in its broad applicability and widespread

acceptance. It provides an aircraft design team with a basic set of requirements to design from, and the customer

with a basic set of acceptance criteria on which to evaluate. This standard makes the acquisition process more

efficient for both contractor and customer. A flying qualities standard that is applicable to UAVs will benefit all

those involved in aircraft development and the creation of such a standard should be encouraged by all stakeholders.

20,000 lb UAV2,000 lb UAV

200 lb UAV

20 lb UAV


Cotting traces the birth of the flying qualities standard to the beginning of the Korean War.2 The great leaps

in aircraft design that occurred during the early jet age were necessarily accompanied by a great focus on flying

qualities research. This led to the creation of the first military flying qualities standard MIL-F-8785 in 1953, which

by 1980 had been revised to MIL-F-8785C3. This document was used as a basis for MIL-STD-1797 in 1987 which

“reflected a change from specific requirement to design guidance allowing requirements to be customized for each

aircraft.”2 This document was revised in 2000 and 2006 to become the current standard we know today: MIL-STD-

1797B1.

The lateral-directional flying qualities characteristics addressed in 1797B are immense, and it is not the

intention of this research to explore the full breadth of the standard. Instead two important characteristics were

chosen for evaluation: Roll Mode Time Constant and Roll Control Effectiveness.

A. Roll Mode Time Constant Recommendations in 1797B

Roll mode time constant (TR) determines the speed at which an aircraft reaches its commanded roll rate, as

illustrated in Figure 4. It will determine how responsive the roll control feels to the pilot. The 1797B recommended

values are shown in Table 2 sorted by aircraft classification and flight phase category. Flight phase Category A

represents aggressive maneuvering mission segments such as air-to-air combat or in-flight refueling. Category B

represents gradual maneuvering phases of flight such as cruise and loiter, and category C represents terminal fight

phases such as takeoff and landing. All simulations for this report were conducted on landing approach, so category

C is the applicable flight phase. The “-L” and “-C” refer to land based and carrier based aircraft respectively.

Figure 4 Normal aircraft roll rate response following a step aileron input1


Table 2 MIL-STD-1797B recommended values for roll mode time constant1

The basis of the roll mode time constant recommendation seems to come from AFFDL-TR-65-138 “A

Study of Conventional Airplane Handling Qualities Requirements,”4 published in 1965, which assimilates pilot

ratings data to determine basic requirements. It shows that pilots will begin to rate an aircraft as unsatisfactory if the

roll mode time constant increases beyond 1.5 seconds. It also notes that pilot ratings do not improve as the time

constant drops below 1 second. If the time constant was shortened below 0.5 seconds the acceleration imparted on

the pilot was great enough to affect his ability to maintain the stick input and could produce effects such as roll-

ratcheting. 4 These roll mode time constant recommendations are based on the limits of a human operator. A flight

computer can be designed to tolerate much greater levels of roll acceleration than a human and does not suffer from

roll-ratcheting. Similarly, a flight computer can be designed to accommodate a longer TR than would be rated

acceptable by a pilot. The human operator needs to feel the physical feedback from his commands to close the loop

on his control action while the flight computer does not.

B. Control Effectiveness Recommendations in 1797B

Control Effectiveness is measured by the time required for an aircraft to reach a specified bank angle and is

a way of measuring the control power an aircraft can bring to bear in changing its roll attitude. The 1797B

recommended values are shown in Table 3 and Table 4, and once again Class C represents the mission segment

studied in this report. The “-L” for Class II aircraft specifies it as a land-based requirement. Roll requirements are

much stricter for carrier-based aircraft and the success criteria for gust response used in this report assumes an

approach to a land-based airfield.


Table 3 MIL-STD-1797B recommended time-to-bank values for Class I air vehicles

Table 4 MIL-STD-1797B recommended time-to-bank values for Class II-L vehicles

1797B1 does not make it clear what research was most conclusive in defining the recommended values; it

simply states that the requirements are “coalitions of the various roll control effectiveness sections of MIL-F-8785C.

Considerable flight test data and pilot commentary are available on a larger number of modern aircraft.”1 Ref. 4,

cited previously as the justification for roll mode time constant requirements, provides some interesting guidance on

roll control effectiveness requirements. It states that “The maximum rolling moments obtainable with full aileron

displacement or maximum pilot force must in general be sufficient to: (1) Balance the airplane under all conditions

of aerodynamic, inertial, or power-plant asymmetries, (2) Maintain attitude in steady side winds or deliberate

sideslips, (3) Maintain or quickly recover attitude in gusty air, (4) Permit rapid recovery from spins, (5) Permit

crosswind landings and takeoffs, (6) Perform required maneuvers consistent with the airplane’s effective

utilization.” The report goes on to discuss how the open-ended item (6) is often the driving requirement for control


power, and must be determined by pilot ratings during the specific task. For an aircraft engaged in missions that do

not require aggressive maneuvering such as an Intelligence, Surveillance and Reconnaissance (ISR) UAV, one could

assume that the roll power requirements to meet item (6) would be minimal. Item (3) then becomes of particular

interest as a requirement driver, and is the focus of this report.

IV. Approach

All simulations were performed during powered approach and analysis was limited to the lateral-directional

response. The effects of landing gear were not simulated or included in the aerodynamics model. The flight

condition was chosen at 100 ft on a standard day. The approach Mach number and airspeed was adjusted for each

UAV to generate a 10° angle-of-attack (AOA) trim condition. The gust was a discrete, 1-cosine gust of the

magnitude calculated per 1797B1 (Sec. 5.1.2). More details on the gust model are given later in this report.

A. CPR Tool

Control Power Required or CPR is a Lockheed Martin developed proprietary tool used for conceptual

design and simulation analysis. It is a FORTRAN based code that performs 6DOF time integration of the nonlinear

equations of motion. The program has several modes, one of which operates as a basic simulation package. The user

specifies vehicle properties and control inputs and CPR will trim the vehicle and produce a time-history output. A

dynamic inversion control system is used to calculate real time control gains. The desired closed loop parameters

that can be set for dynamic inversion are Control Anticipation Parameter (CAP) or Short-Period Frequency, Short-

Period Damping, Dutch Roll Frequency, Dutch Roll Damping and Roll-Mode Time Constant. The dynamic

inversion controller was useful because the desired roll mode time constant could be set by the user and the control

gains were adjusted automatically based on the bare airframe characteristics. CPR also has the capability for discrete

gust and random turbulence generation. The frequency and magnitude of the gusts are set as an input. For control

inputs, CPR reads in a pilot file which allows specified stick commands to be sent at specified times in the

simulation. Additionally, the pilot file has a bank-angle-capture mode in which it uses a proportional gain with a

lead-lag network to capture and maintain a specified bank angle. This capability was used to simulate the UAV’s


autopilot attempting to maintain a wings-level attitude. The basic pilot model was used to create step inputs to

measure time-to-bank capability.

B. Aircraft Models

Four different sized aircraft models were used, but they had identical aerodynamic properties. The

aerodynamic and stability properties were based on wind tunnel data from a tailless flying wing UAV concept. The

wind tunnel data surveyed a range of AOAs; however only the data at 10° α is shown in Table 5 since all aircraft

were trimmed at this condition to begin the simulation. The gross weight of the aircraft was scaled logarithmically,

with aircraft of 20 lbs, 200 lbs, 2,000 lbs and 20,000 lbs. Aircraft span was then determined using equation (1),

based on parametric data relating span with aircraft weight.

2.2977 ∗ (𝐺𝑟𝑜𝑠𝑠 𝑊𝑒𝑖𝑔ℎ𝑡)0.2935 (1)

The remaining geometry of the vehicle was scaled proportionally to the span. The moments of inertia were

then scaled based on the gross weight, and associated geometry (span for Ixx, length for Iyy, and span plus length for

Izz). The resulting aircraft parameters are shown in Table 6.

Table 5 Aerodynamic and Stability derivatives at 10° angle of attack

CYβ -0.00135

Clβ -0.00024

Cnβ -0.0006

Cmδe -0.0011

CLδe 0.0033

Clδa -0.0014

Cnδa 0.0003

Clp -0.25


Table 6 Aircraft weight, geometry and moments of inertia

Gross Weight

(lbs)

Scale (%)

Span (ft)

Length (ft)

Ixx (sl-ft2)

Iyy (sl-ft2)

Izz (sl-ft2)

Sref (ft2)

MAC (ft)

MAC Leading

Edge X-Loc

(in)

Aircraft Fuselage Station Data

Ref. (in)

Aircraft Waterline Data Ref.

(in)

20 10 5.50 3.86 0.3 0.4 1.0 10.3 2.38 11.7 20.4 13.1

200 30 10.9 7.66 13.4 15.2 37.8 40.4 4.71 23.2 40.3 26.0

2000 50 21.4 15.03 517 586 1456 155.8 9.25 45.5 79.2 51.0

20000 100 42.0 29.50 19914 22580 56095 600.0 18.16 89.3 155.4 100.0

Standard values were chosen for the desired dynamic inversion parameters and are shown in Table 7. All

were held constant throughout except the desired roll mode time constant which was adjusted to achieve satisfactory

gust response. The parameters of the lead-lag controller, which acted as the roll attitude autopilot, had to be set

differently for each aircraft. The system uses a proportional gain (K) and a lag time constant (t1) and lead time

constant (t2). Figure 5 shows the block diagram of the autopilot which acts on bank angle feedback. The values of K,

t1, and t2 were determined using a design of experiments (DOE). The aircraft was trimmed wings level, and then

commanded to capture 30° of bank. The time in which the aircraft could capture 30° within a 10% tolerance band

was measured and used as the design parameter. The DOE involved surveying the space of possible control values

and then using a MATLAB command that applies a quadratic curve fit to the data. This allowed for the

identification of controller parameters that captured bank angle in the minimum time. The resulting autopilot

parameters for each vehicle are shown in Table 8. During the course of the gust response experiments, the roll-mode

time constant had to be adjusted. Large adjustments in the roll-mode time constant required a re-tuning of the

autopilot, and so an iterative approach was sometimes required. Table 8 shows the final control parameters that were

used.

Table 7 Desired characteristics for dynamic inversion

Desired Short Period Frequency 0.7 Desired Dutch Roll Damping Ratio 0.7 Desired Dutch Roll Frequency 1.5

Desired Roll-Mode Time Constant varies


Figure 5 Autopilot Block Diagram

Table 8 Roll attitude autopilot parameters

Gross Weight (lbs)

Proportional Gain, K

Lag Time Constant, t1

Lead Time Constant, t2

20 16 0.02 0.6 200 15 0.03 0.72

2000 14 0.04 0.85

20000 13 0.05 0.98

C. Simulation Procedure

The first step in the process was to calculate the gust parameters for each vehicle. 1797B1 calls for a “low-

altitude atmospheric disturbance model” to be used for all terminal flight phases and flight below 2000 ft above

ground level (AGL). One part of that model is a discrete gust. The intention is to generate a gust that could be

created by turbulence, but to tune its magnitude and frequency to be the worst case for the given aircraft and flight

condition. 1797B specifies that “Discrete gusts have the ‘1-cosine’ shape,” as shown in Figure 6. The vm and dm

terms specify the magnitude of the gust, and the rate at which the gust reaches its maximum (specified in terms of

the distance the aircraft travels as it flies through the gust). CPR can generate exactly this type of gust; however

instead of setting dm, the user specifies the aircraft frequency to be excited by the gust. CPR converts this into a time

for the 1-cosine function to reach maximum. This makes tuning the gust simpler because dm is calculated

independently for every vehicle based on its airspeed, while the gust frequency can be held constant for all four

vehicles. The gust frequency was targeted at closed loop Dutch Roll frequency, so as to generate the maximum

lateral-directional disturbance. The value for dm was then calculated for each aircraft and used in determining the

gust magnitude (vm). The calculation steps required to calculate vm are listed in detail in 1797B1 and are a function of

K1 + t2*s

1 + t1*s

1

1 + t1*sSimulation

Roll StickCommandError

Bank AngleBank Angle

Command + -

Roll Attitude Autopilot


altitude above terrain, mean wind speed at 20 ft above ground level and dm. The 20 ft mean wind speed was taken

from the 1797B statistical data shown in Figure 7. “Extraordinary” wind conditions were used to size the gust for all

but the 20 lb UAV, which could not maintain control under a gust of that magnitude. “Uncommon” wind conditions

were used to scale the 20 lb UAV gust. The resulting gust magnitudes (vm), dm, and the mean wind speed, and

airspeed for each UAV are shown in Table 9.

Figure 6 "1-cosine" shape for a discrete gust


Figure 7 Probability of exceeding mean wind speed at 20 ft1


Table 9 Gust parameters for each UAV

UAV Gross Weight (lbs)

Airspeed (kts)

Mean Wind at

20 ft (kts) Probability dm (ft) vm (kts) 20 33.0 30 Uncommon 222.9 10.3

200 52.8 45 Extrordinary 356.6 17.8 2000 85.8 45 Extrordinary 579.6 19.0 20000 142.0 45 Extrordinary 958.8 19.4

After sizing the gust, the UAV was simulated responding to the gust with the autopilot active and standard

values for roll mode time constant. If the aircraft did not meet the success criteria for gust response (success criteria

will be discussed in detail later in this section), then roll mode time constant was reduced until the aircraft could

meet the criteria. Changes to control parameters other than roll mode time constant had little effect on the response

time, confirming results in Ref. 4 which show that TR is an important factor in time-to-recover from a gust. If the

initial run showed the UAV to be exceeding the success criteria, roll mode time constant was lengthened to

determine the maximum value that still produced a successful recovery. The control power used by the autopilot was

measured during the successful recovery in terms of horizontal stick force applied. Finally, this configuration and

associated control power was tested in a calm environment to determine the time-to-bank for the aircraft. This

method produced both a roll mode time constant and a time-to-bank requirement that allowed the vehicle to respond

successfully to a worst-case gust. These parameters were compared to the 1797B values as a test of its applicability.

D. Assumptions

The 1797B1 “low-altitude atmospheric disturbance model” calls for testing discrete gusts, random

turbulence, and wind shear. The gusts tested were limited to discrete gusts; random turbulence and wind shear were

not used to drive the roll control requirements. Test runs were performed with random turbulence, but generated

lower control forces from the autopilot and their zero-sum nature meant that the aircraft was rarely disturbed beyond

5° wings level for any significant period of time. Wind shear was not tested though it is assumed that wind shear

would generate a smaller rolling moment than a discrete gust. The discrete gust represents a worst case turbulent

gust and therefore was the best test. The discrete gust was generated in the X and Y directions simultaneously,

several combinations of gusts in all three axes were tested and the X and Y gust generated the most aircraft roll-off.


The altitude of 100 ft was chosen because it is often the minimum decision altitude for an aircraft on

approach. An aircraft needs to be able to maintain a level pitch attitude below this altitude or else wave off the

landing. A 10° AOA approach was chosen as a reasonable value for a typical aircraft. It did not seem likely that the

type of day would have any impact on the performance; therefore, all tests were performed under Standard Day

conditions.

E. Success Criteria

Defining a “successful” gust recovery is a difficult task and is not specified in 1797B1. However, in Ref. 4

gust recovery time is put forward as an important factor in determining if a pilot will rate an aircraft’s gust recovery

as adequate. The report cites the fact that heavier, higher-inertia aircraft with much slower time-to-bank capabilities,

do not receive lower pilot ratings in gusty air because their higher inertia prevents them from responding as severely

to the gust. The report concludes that “it appears to the writer that a more straightforward and perhaps more

instructive approach is to require recovery from a design gust in a given time.” The report points out that an aircraft

carrying stores, thereby reducing its roll performance, does not see a corresponding drop in pilot rating. Though it

takes longer to roll the aircraft, the roll-off produced by a gust is smaller. Ref. 4 suggests that “it appears necessary

to recover from an impulsive-type maximum gust upsets in less than about 3 seconds for land-based airplanes or less

than about 1.35 seconds for carrier-landing aircraft.” The 3 second recovery is not put forward as a firm

requirement, but is a suggestion based on the pilot rating data available. For the purposes of this research, a recovery

was defined as wings within 5° of level. It was assumed that a touchdown could normally be performed with a roll

attitude in this range. Keeping roll attitude excursions beyond 5° to less than 3 seconds was the success criteria

defined based on the suggestion in Ref. 4. This is not a standard success criterion and represents a significant

assumption; however some definition must be chosen. The results obtained here could be shifted by changing this

assumption, but the trends of the data would remain the same.


V. Results and Discussion

Testing a single task at a single point in the approach path for one type of UAV does not constitute enough

data to form a new standard, however the results in this section intend to identify where the standard is most lacking

when applied to UAVs, and where research needs to be focused to improve flying qualities standards.

A. Gust Response

Before analyzing recoveries it is useful to first examine how much roll-off is generated during a gust with

neutral controls. Table 10 shows the maximum bank angle during the course of the gust response. It should be noted

that the 20 lb aircraft is responding to a lower magnitude gust because, as stated earlier, a more severe gust caused a

total loss of control. This illustrates an important result: small, light UAVs must be subject to wind limitations and

cannot be expected to have the same all-weather capability of piloted aircraft. New ideas such as flexible wings or

hinged wing roots may improve the small UAV’s robustness to gusts; however it is clear that a small aircraft at low

airspeeds (in this case 33 knots) are not capable of operating in extremely high wind conditions. Excluding the 20 lb

aircraft the results are as expected: as inertia increases, the maximum bank angle reached decreases.

Table 10 Maximum bank angle reached during gust

Gross

Weight (lbs)

Maximum Bank (deg)

20 9 200 9.5

2000 9 20000 7.2


B. Roll Mode Time Constant

Figure 8 shows an example of a gust recovery maneuver executed by the autopilot for the 200 lb aircraft.

The gust begins 2 seconds after simulation start. TR was adjusted to meet the success criteria of returning the wings

to within 5° of level within 3 seconds.

Figure 8 Example gust recovery for 200 lb aircraft

Table 11 Roll mode time constant results compared to standard

Gross Weight (lbs)

Roll Mode Time

Constant, TR (sec)

1797B Recommended

TR (sec) 20 0.2 1.0

200 0.3 1.0 2000 0.9 1.0 20000 2.3 1.4

The resulting TR values are shown in Table 11 along with the Level 1 recommendation provided by 1797B.

The results show that for the 20 lb and 200 lb aircraft, roll-mode time constant must be significantly lower than the

standard recommends. These values are below what would be acceptable for a piloted aircraft as discussed in Ref. 4

but could be reasonable for a small-scale unmanned aircraft. If the roll mode time constant is lengthened to the

standard recommended values, these small aircraft roll-off to a much higher bank angle before the autopilot can

reverse the roll rate. The short roll mode time constant is required to stay on top of the roll-off and perform a

successful recovery. The 2,000 lb aircraft fits very well within the existing standard. The 20,000 lb aircraft was able

to recover within 3 seconds with TR values higher than recommended in the standard. The 2.3 second roll mode time

-12.0

-8.0

-4.0

0.0

4.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time (sec)

φ, deg


constant would place it in the Level 2 handling qualities category according to the standard. It is possible that other

mission tasks would drive the roll mode time constant requirement for this type of aircraft; however, it may be

acceptable to allow longer roll mode time constants for large UAVs, than what would be tolerated in piloted aircraft

of similar size. The flight computer’s ability to constantly provide control input without having to sacrifice attention

to other tasks may allow it to cope with a response that would be considered “sluggish” by a pilot. This is an area

that warrants further research. The great difference in TR requirements derived for the smallest versus the largest

UAVs shows the importance of developing a new classification system. Using the Cotting classification2 all four

aircraft would be in different classes and roll mode time constant standards could be defined differently for each.

C. Control Effectiveness

Control effectiveness requirements are strongly dependent on roll mode time constant. A longer time

constant does not allow the vehicle to respond as quickly to a gust and therefore requires a larger autopilot input to

return to wings-level. The control deflection and power used for each vehicle are shown in Table 12 along with the

time-to-bank of the UAVs compared with the 1797B standard. All results in Table 12 were obtained using the roll

mode time constant values from Table 11.

Table 12 Control power and time-to-bank results compared to standard

Gross Weight (lbs)

Control Deflection, δa (deg)

Control Power (ΔCl),

Clδa*δa Time-to-bank, φt = 30° (sec)

1797B Recommended Time-to-bank, φt = 30° (sec)

20 0.564 0.000883 6.0 1.3 200 0.733 0.00123 4.3 1.3

2000 0.133 0.000219 5.2 1.3 20000 0.302 0.000452 6.6 1.8

The 20 lb aircraft is responding to a smaller gust magnitude than the other three aircraft which makes it

difficult to use when determining trends in the data. For the three larger aircraft, the time-to-bank capability

necessary for a successful recovery loosens with aircraft size. If the 20 lb UAV could respond to the full magnitude

gust, it is reasonable to assume it would continue this trend. There are no recognizable trends in the control power

(Clδa*δa) data; however, in general, it is expected that control power required would decrease with larger aircraft.


This is not the case for the 20 lb aircraft because of the lower magnitude gust it encounters. This trend is also broken

by the 20,000 lb UAV which is likely due to its long roll mode time constant. To generate a roll mode time constant

longer than that of the bare airframe, the dynamic inversion controller actually reverses the command of the

autopilot during part of the maneuver. This effect can be seen in Figure 9 which compares the normal aileron

commands used by the 200 lb UAV to the aileron command of the 20,000 lb UAV which shows the unusual

reversal. Besides roll mode time constant, bare airframe Clβ also plays a role in the control power used, and only one

value for Clβ was simulated. The dynamic inversion control-law maintains the same wing-roll-off with different bare

airframe stability characteristics, but an aircraft with a larger magnitude Clβ will drive the control system to use more

control power to achieve this. Due to all of these effects, the control power data presented in Table 12 is not

particularly useful; however the time-to-bank data is relevant as a comparison to the standard.

Figure 9 Comparison of aileron commands for 200 lb and 20,000 lb UAVs

All of the time-to-bank results in Table 12 are higher than the 1797B recommended values. This seems to

validate the idea presented earlier that time-to-bank requirements are ultimately set by item (6): “Perform required

maneuvers consistent with the airplane’s effective utilization.”4 It may be realistic to define lower control

effectiveness requirements for UAVs, especially if they only require gentle maneuvering during their mission (such

as high-altitude ISR aircraft). However, lateral-directional control requirements could be driven by trim

-1.0-0.8-0.6-0.4-0.20.00.20.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Aileron Command - 200 lb UAV, deg

-1.0-0.8-0.6-0.4-0.20.00.20.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Aileron Command - 20,000 lb UAV, deg


requirements during a crosswind or engine out landing. As discussed earlier, the bare airframe characteristics also

have an impact, so the configuration of the aircraft is another important factor. All of this reinforces the idea that a

UAVs control effectiveness requirements will be tightly coupled with its mission utilization, and a classification

system that accounts for this will be important to establish an applicable standard.

D. Conclusion

All four simulated UAVs proved capable of meeting the success criteria within wind limitations and with

the correct values of TR. Comparing the results to 1797B1 revealed key areas where its recommendations were

insufficient for UAVs. Roll mode time (TR) constant requirements vary greatly depending on UAV size, and the

1797B classification system does not accommodate this. Small UAVs require very short roll mode time constants

while larger UAVs can tolerate relatively long ones. Control effectiveness requirements for UAVs to execute a

successful gust recovery are much lower than what is required in the standard. It is likely that the mission a UAV is

designed for will define its roll control effectiveness requirement and it is reasonable to assume that most UAVs will

not require the high levels of effectiveness recommended by the piloted vehicle standard.

Further research and flight test experience is required to define a new standard. Many sizes of UAVs must

be tested as well as different types. The four UAVs tested spanned the breadth of UAV sizes, but are still an

inadequate sample size to fully define new standards. This research only examined one configuration of aircraft, and

studies must be done that encompass all UAV configurations currently in service. Other drivers of control power

should be examined such as trimming for aerodynamic or propulsion asymmetries and crosswind landings. In

addition, mission segments besides landing will need to be analyzed. For instance, sensor pointing during Category

A or Category B flight phases could easily become a design driver.

UAV requirements may be much more dependent on their mission that their piloted predecessors. In some

sense piloted aircraft are united by the common mission of returning a human payload safely to the ground, while

UAVs do not share this requirement. Some new UAVs may never even perform a landing as they are being designed

to stay aloft for their entire service life while others are small enough to be flown into a net for recovery. In this way

unmanned aircraft flying qualities requirements are more driven by their mission than aircraft that have come before

them. The most important step to building a new standard will be the flight experience that can only be gained over

time. The assimilation of data from many successful UAV programs will begin to form the foundations of a new


standard. UAV developers must remain dedicated to collecting flying qualities information and incorporating it into

written standards. The goal for a new generation of flying qualities research should be to build a standard for UAVs

that can be as universally accepted as MIL-STD-1797B has been for piloted aircraft.

Acknowledgments

I would like to thank Ken Dorsett who first suggested this research topic and made himself constantly

available for consultation. My thanks to Dr. Eric Johnson who advised me during the research of this topic, and

who’s graduate Flight Mechanics course reminded me why I became an aerospace engineer. I would like to thank

my wife who encouraged me through nights of solitary studying. I would not have been able to complete this effort

without her, and she has supported me every step of the way. Finally I must acknowledge God, for I know I have all

things through him, and I rely on his support to accomplish anything.

References

1anon, “Flying Qualities of Piloted Aircraft,” Department of Defense Interface Standard MIL-STD-1797B, Department of

Defense, Wright Patterson AFB, OH, February 15 2006.

2Cotting, M. C., “Evolution of Flying Qualities Analysis: Problems for a New Generation of Aircraft,” PhD. Dissertation,

Aerospace Engineering Dept., Virginia Polytechnic Institute and State Univ., Blacksburg, VA, 2010.

3anon, U.S. Military Specification MIL-F-8785C. Department of Defense, 5 November 1980.

4Ashkenas, I. L., “A Study of Conventional Airplane Handling Qualities Part 1: Roll Handling Qualities,” Air Force Flight

Dynamics Lab., Rept. AFFDL-TR-65-138, Hawthorne, CA, 1965.

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