Zam2_Flight Dynamic Requirements for UAVs - Do They Really Exist

7/23/2019 Zam2_Flight Dynamic Requirements for UAVs - Do They Really Exist

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American Institute of Aeronautics and Astronautics

1

Flight Dynamic Requirements for UAVs - Do They Really

Exist?

Richard Colgren1

University of Kansas, Lawrence, KS 66045

Lance Holly2

Viking Aerospace, Lawrence, KS 66049

There exist decades of experience in developing and documenting flight dynamic and

handling qualities requirements for manned aircraft. Several requirements documents with

associated user guides have been generated and are in general use. Such documents only

apply to UAVs in a general and limited sense. Attempts have been made to generate similar

documents for UAVs. These either apply to a particular platform, specify requirements only

based on sensor performance or mission maneuvers, or impose inappropriate constraints on

the vehicles flight dynamics. Requirements imposed on the operator are very different

depending on if the pilot is flying in traditional R/C mode, flying from a simulator,operating the vehicle in an augmented mode, or acting as a systems supervisor under

completely autonomous operations. Further, previous training on one system or in another

mode of flight operation can adversely prepare the pilot or operator for problems when

flying in another mode or on a different platform. Examples of differences between flight

dynamics requirements and handling qualities, and between different operating modes, are

also discussed within this paper.

Nomenclature

C-H scale= Cooper-Harper scale

FCS = Flight Control System

n = Load Factor

p = Roll Rate (rad/sec)q = Pitch Rate (rad/sec)

r = Yaw Rate (rad/sec)

R/C = Remote Control

UAV = Unmanned Air Vehicle

V = Velocity

= Pitch Attitude (rad/sec)

= Roll Attitude (rad/sec)

= Heading (rad/sec)

= Deflection (%)

I.

Introduction

here exist several decades of experience in developing flight dynamic and handling qualities requirements formanned aircraft, both fixed wing and rotorcraft. Several requirements documents with associated user guides

have been generated and are in general use. A key document for use in describing and evaluating flying qualities of

manned fixed wing aircraft is Ref. 1. Several excellent supporting documents are given in its list of references. An

equivalent document for use in describing and evaluating the flying qualities of rotary wing aircraft is given as Ref.

1 Associate Professor, Aerospace Engineering Department, 2120D Learned Hall, Lawrence, KS 66045, AIAA

Associate Fellow.2 Managing Partner, Avionics and Flight Control Systems, Viking Aerospace, 100 Riverfront Road, Suite B,

Lawrence, KS 66049.

T

AIAA Atmospheric Flight Mechanics Conference0 - 13 August 2009, Chicago, Illinois

AIAA 2009-632

Copyright 2009 by Richard Colgren and Lance Holly. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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2. Multiple useful supporting documents are also given in its list of references. Both in the case of fixed wing

aircraft and in the case of rotorcraft the C-H scale and pilot evaluation are the gold standard for the evaluation of

flying qualities. Differences are seen between the tasks used for evaluating fixed wing aircraft versus rotorcraft. For

example, coupling is allowed between axes on rotorcraft to a degree not allowed on fixed wing vehicles. As Ref. 1

and Ref. 2 were developed specifically for manned aircraft, such documents only apply to UAVs in a general and

limited sense. Attempts have been made to generate similar documents for UAVs. Some of these apply only to a

particular platform. These were developed by a particular manufacture along with their customer to support their

aerial platform. They also provide some guidance on developing flying qualities based on their specific experiences

for use on other aerial vehicle programs. Other documents specify requirements only based on sensor performance

or on mission maneuvers, or impose inappropriate constraints on the vehicles flight dynamics. Requirements

imposed on the operator are very different depending on if the pilot is flying in traditional R/C mode, flying from a

simulator, operating the vehicle in an augmented mode, or acting as a sys tems supervisor with the vehicle flying

under completely autonomous operations. Previous training while operating one system can adversely prepare the

pilot or operator for problems encountered while flying a different aerial platform. Further, previous training while

operating in one mode can adversely prepare the pilot or operator for problems encountered while flying in another

mode. Examples of the differences between flight dynamics requirements and handling qualities requirements, and

between the different operating flight modes, are also discussed within this paper.

II.

Flying Qualities and their Assessment

In evaluating the handling qualities of piloted aerial vehicles great reliance is made on the Cooper-Harper (C-H)scale3and on pilot subjective assessment of their ability (including the required workload) to accomplish specific

tasks.1, 2A wide variety of closed-loop high-gain (high-bandwidth) tasks, many requiring precise tracking, have been

developed for the evaluation of aircraft flying qualities. The recommended tasks for the evaluation of these flying

qualities are based on the Flight Phase Category. The achieved performance is stated in terms of Levels. The

familiar C-H scale used is shown in Figure 1 3. It is necessary to use equivalent definitions between the Cooper-

Harper scale shown in Figure 13and the Level definitions. Level 1 is Satisfactory, Level 2 is Acceptable, and Level

3 is Controllable. Typically, a Cooper-Harper pilot rating of 1 to 3 defines Level 1, a C-H rating from 4 through 6

defines Level 2, and a C-H rating from 7 through 9 defines Level 3. Note that Level 3 is not necessarily defined as

safe. This is consistent with the C-H scale. Cooper-Harper ratings of 8 and 9 are defined as cases where

controllability is in question. In a few cases the boundaries are further modified for specific considerations. The

bounds are sensitive to pilot workload. Examples of recommended tasks for evaluating flying qualities for Flight

Phase Category A include air-to-air tracking, air-to-ground tracking, aerial refueling, close formation flying, and

captures. No recommended tasks are given for Flight Phase Category B because this category consists of low-gain

(low-bandwidth) tasks. Recommended tasks for the evaluation of flying qualities for Flight Phase Category C are

again tracking tasks, including close formation flying, precision landings (with and without vertical and lateral

offsets), takeoffs, and captures. Note that, in many cases, the queues the authors of Ref. 1 and Ref. 2 expected to be

experienced by the pilot are not experienced by the UAV pilots or operators.

There are no recommended tasks for Flight Phase Category B because this Flight Phase Category generally

consists of low-gain (low-bandwidth) tasks. It is expected that flying qualities problems encountered while operating

within this Flight Phase Category will normally be exposed in the more demanding tasks used for evaluating Flight

Phase Categories A and C, or by normal operations during the flight test program. Thus, for piloted air vehicles,

special tasks for this Flight Phase Category are normally not considered necessary. However, Flight Phase Category

B tasks are normally of much longer duration than the tasks in the other Categories. It can also be argued that UAVs

will spend a greater percent of their flight time in the Flight Phase Category B than piloted aircraft do. Often the

UAV pilot or operator is not as tightly within the control loop as is the pilot of the manned aircraft. The UAV pilot

also does not get the variety of queues that the manned aircraft pilot does. This causes concerns on theappropriateness and timeliness of the pilots reaction. Pilot fatigue may become a significant factor in certain

mission critical Category B tasks, even in the case of the UAV operator. In this case an evaluation of this kind of

task might be required.

Flight envelopes are described by a speed (V) and load factor (n) graph as shown in Figure 2 1. It is used to define

the Operational and Service Envelopes. Also, it represents the conditions where the requirements apply at a given

altitude. However, load factor is rarely a flying qualities queue for UAV operations. In all cases the load factor

denotes air vehicle maneuverability without regard to the available thrust. Note that flying qualities specifications


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place no requirements on load-factor capability in constant-speed level flight. These flight envelopes are defined at

various altitudes corresponding to the Flight Phases.1, 2

Figure 1. Cooper-Harper Flying Qualities Scale

Figure 2. Flight Envelope for Fixed Wing Aircraft


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Proof of compliance when accomplishing these demonstration tasks consists of pilot comments and C-H ratings.

For Level 1, pilot comments must indicate satisfaction with the aircrafts flying qualities. This includesno worse

than mildly unpleasantdeficiencies. The median C-H ratings must be no worse than 3.5 in calm air or in light

atmospheric disturbances. Note again that many of the queues used in the evaluation of piloted aircraft are different

or not even available to the UAV pilot or operator. For Level 2, pilot comments must indicate that whatever

deficiencies may exist, aircraft flying qualities are still acceptable. Median C-H ratings must be no worse than 6.5 in

calm air or light atmospheric disturbances. For Level 3, pilot comments must indicate that the aircraft is at least

controllable despite the deficiencies in flying qualities. The median C-H ratings must be no worse than 9.5 in calm

air or light atmospheric disturbances. In moderate to severe atmospheric disturbances pilot comments and C-H

ratings must comply with requirements on the relationship between Levels and qualitative degrees of suitability.The

degrees of suitability are defined in Table 1.

Table 1. Suitability of Flying Qualities

Satisfactory Flying qualities clearly adequate for the mission Flight Phase. Desired performance is

achievable with no more than minimal pilot compensation.

Acceptable Flying qualities adequate to accomplish the mission Flight Phase, but some increase in

pilot workload or degradation in mission effectiveness, or both, exist.

Controllable Flying qualities such that the aircraft can be controlled in the context of the missionFlight Phase, even though pilot workload is excessive or mission effectiveness is

inadequate, or both. The pilot can transition from Category A Flight Phase tasks to

Category B or C Flight Phases, and Category B and C Flight Phase tasks can be

completed.

Actual task performance is not recommended for use as proof of compliance because it is even more subject to

pilot variability than pilot comments and C-H ratings. The performance objectives suggested in the evaluation tasks

are developed for use with the C-H scale, which was not developed with UAVs in mind. Specific definitions of

desired and adequate performance objectives attempt to reduce pilot variability by insuring that all of the evaluation

pilots attempt to achieve the same level of performance. In these performance objectives, adequate performance is

set at a level sufficient to successfully perform similar tasks within the operational service. Desired performance is

set at a more demanding level to insure that system deficiencies are exposed. Although task performance is not

recommended as proof of compliance, task performance should be recorded and analyzed by the flight test engineersto insure that pilot ratings are reasonably consistent with the level of performance achieved and that all pilots are, to

a close approximation, achieving the same level of performance.

The evaluation of aircraft flying qualities is basically a subjective science, and human variability makes analysis

of the results a difficult proposition. The decreased number of queues and the increased delays experienced by the

UAV pilot or operator experiencing these queues makes this an even more difficult proposition. Nevertheless, there

are steps that can be taken to reduce the variability in the results. It is absolutely necessary that multiple evaluations

be conducted at each test condition. Studies of C-H rating variability have indicated that three pilots is the minimum

number of pilots for an adequate evaluation.4, 5However, Ref. 5 further demonstrated that the point of diminishing

returns was reached after including about six pilots within an evaluation. Therefore, the recommended number of

pilots per test condition is three to six. Careful selection of the evaluation pilots is extremely important in reducing

the variability in the results.

In order to insure that all of the pilots attempt to achieve the same level of performance, it is extremely important

to explicitly define the desired and adequate levels of task performance. This insures consistency and reduces the

effects of pilot variability.1 Best results are achieved with task performance defined in terms of quantifiable

objectives which the pilot can readily observe in real time. This can pose a great difficulty when evaluating UAV

flying qualities, where the pilot or operator can be spatially and temporally removed from immediate motion

queues.6, 7Furthermore, consideration must be given to defining objectives that can be adequately recorded so that a

flight test engineer can confirm that pilot ratings are reasonably consistent with task performance. Some UAV

systems, especially those for smaller vehicles, can provide few if any recorded flight parameters. Defining


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quantifiable and recordable task performance objectives and setting appropriate levels of desired and adequate

performance is the most difficult part of planning flying qualities evaluations.

A method used to reduce the effects of variability is the long-look evaluation technique. In this method, the

pilot continues or repeats the task until becoming confident in the evaluation and assigning a C-H rating. The long-

lookapproach allows the pilot a more extensive appraisal of the test condition. It eliminates the effects of unique

events encountered during a single run. It allows the pilot to get over the learning curve. The problem is that in

repeating the task the pilot or operator can get overly familiar with the task and give it an improved C-H. The

concern is that with decreased task queues under UAV operations this improvement in C-H rating (with familiarity)

is magnified. The normal evaluation procedure is to specify a minimum number of runs to be performed before a

rating can be given, and then allow the pilot to make additional runs as necessary.

Pilot comments should be considered the most important data. A C-H rating is only a summary of observed

flying qualities characteristics reduced into a single number. Pilot comments identity the specific deficiencies, if

any, that must be corrected. However, these comments are flavoredby the interface with the UAV. Furthermore,

the long-look technique filters the effects of deficiencies on the C-H rating because, over several runs, the pilot

learns to compensate for some deficiencies. When pilot comments are given for every run, the comments will

hopefully identity all observed deficiencies, even those which can be compensated for. Comments on succeeding

runs provide guidance on the pilot's ability to compensate for the deficiencies. The final C-H rating should indicate

the relative significance of these deficiencies. Therefore, pilot comments must be recorded and analyzed for every

test run.

Time and cost constraints prohibit piloted evaluation of every task in every possible aircraft configuration at

every possible point within the flight envelope. The conditions that must be evaluated are the most common

operating conditions, operating conditions critical to the mission of the aircraft, and the worst case conditions. They

emphasize those where the quantitative, open-loop flying qualities requirements are violated. As most air vehicles

have multiple flight control modes, all mode transitions should be evaluated at common, mission critical, and worst

case conditions. Of specific concern are mode switches which are done automatically. Furthermore, the degradation

due to atmospheric disturbances should be demonstrated by evaluation within the different disturbance levels.

Evaluation of the effect of severe atmospheric disturbances may be performed in ground simulation. When using a

simulation to predict the degradation of flying qualities due to severe atmospheric disturbances, it is necessary to

correlate C-H ratings gathered from the simulation sessions in light to moderate turbulence with C-H ratings

obtained from flight test in light to moderate turbulence for the same task. Simulations for UAVs operating under

remote control often provide poor visual queues, making the evaluations less certain.

Any closed-loop task, performed aggressively, offers the possibility of being used to evaluate an air vehicle's

handling qualities and PIO characteristics.1When developing a specification for a particular program, Ref. 1 and

Ref. 2 provide guidance in constructing these tasks. However, the appropriateness of these tasks and the queues that

thepilots and operators use to evaluate the vehicles performance at accomplishing these tasks can be significantly

deferent for UAVs. In fact, they may differ significantly between UAV programs. This due to the various pilot to

UAV interfaces, the variety of UAV mission profiles, and the different levels of autonomy. These differences, along

with the greater uncertainty with respect to appropriate evaluation queues, have prevented the development and use

of general flying qualities guidelines as have been developed for piloted air vehicles, as demonstrated by Ref. 1 and

Ref. 2.

III. UAV Flight Testing Examples

As a published example of UAV flying qualities analysis, Ref. 7 documents the flight test of a half-scale radio-controlled model of the Pioneer unmanned air vehicle (UAV). In this testing, static longitudinal and lateral-

directional stability-and-control characteristics were measured. The air vehicle was instrumented to measure control-

surface deflections, angle of attack, sideslip angle, and airspeed. Telemetry was used to downlink the data to a

ground recorder. A wind-tunnel test of a 0.4-scale Pioneer vehicle model at full-scale Reynolds numbers was also

carried out. Finally, a CFD numerical analysis was conducted using a low-order panel method. The flight-test

determination of the neutral point was used to insure that the static margin was sufficient to conduct tests throughout

the flight envelope. Crosswind limits were determined from steady-sideslip maneuvers. The flight data was shown to

compare very favorably with other data sources. This study was interesting in the use of a variety of different scaled


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vehicles and test articles. Other examples of the use of scaled UAVs for the prediction of the flight dynamics of

piloted vehicles also exist and are of interest.6

The flight testing of multiple fixed wing and rotary wing UAVs have been conducted by the authors. The

following portion of this section provides a couple recent examples of issues that can arise with such flying qualities

assessments. This includes Case 1 where confusion over which operating mode the UAV is in and previous training

when operating under different modes causes difficulties. It also includes Case 2 where adverse training due to

flying other vehicles and previous operations under fully manually modes causes problems. These cases pertain to a

small 50 V electric UAV, the Aggressor II, shown in Figure 3.

Figure 3. Aggressor II Rotorcraft UAV

Figures 4 through 6 are plots of flight data taken from the Aggressor II UAV helicoptersflight control system

(FCS). Figure 4 gives the Euler angle attitudes for the air vehicle in radians. Time is the number of seconds into the

flight. Figure 5 gives the body fixed Euler axis angular rates in rad/sec. Figure 6 gives the pilot inputs as a fraction

of the maximum, with the maximum in one direction plotted as +1 and the maximum in the opposing direction as -1.The pilot is not a test pilot. The pilot is a seasoned R/C helicopter pilot who is very capable of flying this aircraft in a

purely manual mode, with direct control over the actuators. In this particular case, the ground control station

operator has caused the autopilot and its augmentation system to be disabled. The ground control station operator

then tells the pilot that the autopilot is engaged and requests that the pilot perform a takeoff in Assisted Mode. The

pilot does not confirm that the Assisted Mode is engaged and thus does not recognize which mode of operation the

UAV is in. The three flight control system modes are shown in Figure 7. Assisted Mode, the middle level of

automation, works like a three dimensional cruise control. The Assisted Mode display on the laptop ground control

display is shown in Figure 8. For operations in the Assisted Mode, the pilot has been trained to provide velocity


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commands to the FCS, while the FCS is to perform stability augmentation, attitude control, and reference velocity

tracking. Without the autopilot engaged the vehicle remains in a purely manual R/C mode, without stability

augmentation and with the commands fed directly to the control servos. The result is that the aircraft does not

respond to the pilots commands in the way that the pilot expects. Even though the pilot should be able to easily

control this helicopter in this mode, because he does not adapt quickly enough to the actual behavior of the

helicopter the vehicle goes into a divergent oscillation.

Figure 4. Helicopter Attitude - Autopilot Off

Figure 5. Helicopter Angular Rates- Autopilot Off


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Figure 6. Pilot Commands- Autopilot Off

Figure 7. Autopilot Modes


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Figure 8. Assisted Mode

The second Aggressor II flight example (Case 2) shows a portion of a flight in the Augmented Mode. In this

mode the R/C controller is operating as a velocity controller. The pilot is not a test pilot, but is an R/C helicopter

pilot who is capable of flying this aircraft in a purely manual mode. The pilot is commanding velocity, but is used to

using visual attitude feedback to generate R/C commands. Using attitude queues to generate velocity commands

creates inappropriate pilot actions which might even lead to a PIO. From visual flight observation and from

inspection of the Euler angle time trace plots, the operator reports an approximately 0.5 Hz oscillation. Upon further

examination of the flight data, it is seen that these oscillations are actually commanded responses based upon the

velocity commands given by the external pilot. The first command starts at about 410 seconds. The pilot sees theattitude change and pulls back on the command from 412 to 413 seconds. The pilot then repeats this process,

commanding velocity from 413 to 415 seconds then opposing this command after the 415 seconds point based on

the increase in vehicle attitude. Note that the Aggressor II is a flybar-less helicopter UAV. Because this vehicle has a

nearly rigid rotor system with no flybar, it requires a larger change in pitch attitude angle to accelerate and jerk a

given amount than does a helicopter UAV with a Bell-Hiller flybar. In this case, the R/C trained pilot, who is used to

visual control based on vehicle attitudes, tends to pay more attention to the attitude changes than the velocity

changes. When in visual range helicopter UAVs change attitude more quickly and in a more noticeable way than

velocity changes can be recognized. Because of this, pilots tend to let attitude changes effect the velocity command

they give to the helicopter. This is more apparent in transition training for UAV helicopters without a flybar than in

transition training for UAV helicopters with a flybar.

Two FCS modifications could be used to correct the oscillation seen in this case. One possibility is that the

acceleration and jerk limiters could be reduced for the case of external pilot reference commands. This option isdisliked by many pilots as this can make the pilot feel disconnected from the vehicle. It effectively introducing more

lag into the system. The advantage to this approach is that the pilot, once familiar with the delay and vehicle

response, only has to learn one mode or feel for the UAV. A second option is to introduce a dual rate switch on

the external pilot controller which allows the pilot to select between high and low speed settings. The effect is that

the pilot has effectively no tendency to PIO the system when in the low setting. The pilot has finer control over the

speed reference command and any oscillations due to a pilot command will be much more apparent. The high setting

can then be used only for fast forward flight in which the pilot command is often a more discrete command, such

as a step increase in speed. The low setting is used again when finely positioning the vehicle. Such switching does

need to take into account flying qualities during transitions, and pilot mode awareness.1


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Figure 9. Helicopter VelocitiesAutopilot in Assisted Mode

Figure 10. Helicopter Attitude - Autopilot in Assisted Mode

IV.

Summary and Conclusions

There exist years of experience in developing and documenting flight dynamic and handling qualities

requirements for manned aircraft. Several requirements documents with associated user guides are in general use.

However, the appropriateness of some of these piloted vehicle tasks and their associated motion queues for use in

UAV requirements is questionable. The tasks and queues that the pilots and operators use to evaluate a UAVs

performance can be significantly different for other UAVs. Such piloted vehicle requirements documents only applyto UAVs in a general and limited sense. Any closed-loop task, performed aggressively, does offer the possibility of

being used to evaluate an air vehicle's handling qualities and its PIO tendencies. In fact, the tasks and requirements

may differ significantly between UAV programs. This due to the various pilot to UAV interfaces, the variety of

UAV mission profiles, and the different levels of autonomy. These differences, along with the greater uncertainty

with respect to appropriate evaluation queues, have prevented the development and use of general UAV flying

qualities guidelines. Further, previous training on one system or within another mode of operation can adversely

prepare the pilot/operator for problems when flying in another mode or on a different platform. Examples of the

differences between flight dynamics requirements and handling qualities, and between these different operating

modes, were covered within this paper.


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References1anon, Flying Qualities of Piloted Aircraft, MIL-STD-1797A, 30 Jan. 1990.2Baskett, Barry J., et. al., Handling Qualities Requirements for Military Rotorcraft, ADS-33E, US Army Aviation

Directorate, Redstone Arsenal, AL, 21 Mar. 2000.3Cooper, G. E. and Harper, R. P., Jr., The Use of Pilot Rating in the Evaluation of Aircraft Handling Qualities, NASA-TN-

D-5153, Apr. 1969.4Kidd, E. A. and Bull, G. Handling Qualities Requirements as Influenced by Prior Evaluation Time and Sample Size,

Cornell Aero Lab TB-1444-F-I, Feb. 1963.5Dukes, Theodore A., Guidelines for Designing Flying Qualities Experiments,NADC-85130-60, Jun. 1985.6Foster, Tyler and Bowman, Jerry, Dynamic Stability and Handling Qualities of Small Unmanned Aerial Vehicles, AIAA-

2005-1023, 43rdAIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 10-13 Jan. 2005.7R. M., Howard, R. M., Bray, D. F., Lyons, Fl ying-qualities Analysis of an Unmanned Air Vehicle, Journal of Aircraft,

Vol. 33, No. 2, pp. 331-336, Mar.-Apr. 1996.

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Zam2_Flight Dynamic Requirements for UAVs - Do They Really Exist