The role and use of simulation as a design tool in developing integrated systems where design criteria is largely unavailable is well known. This paper addresses additional simulation needs for the development of Integrated Flight/Propulsion Control Systems (IFPCS) which will improve the probability of properly interpreting simulation results. These needs are based on recent experience with power approach flying qualities evaluations of an advanced fighter configuration which incorporated Short Takeoff and Landing (STOL) technologies and earlier experiences with power approach flying qualities evaluations on the AFTI/F-16 program. The use of motion base platforms with axial and normal degrees of freedom will help in evaluating pilot coupling and workload in the presence of high frequency low amplitude axial accelerations produced by high bandwidth airspeed controllers in a gusty environment

Anderson, D. C.

Sanders, K. E.

Watson, J. H.

English

NASA Technical Reports Server

STOL SIMULATION REQUIREMENTS FOR 
DEVELOPMENT OF INTEGRATED FLIGHT/PROPULSION CONTROL SYSTEMS 
by 
K. E. Sanders, Senior Engineer 
Dr. D. C. Anderson, Senior Engineering Specialist 
J. H. Watson, Engineering Chief 
Flight Control Systems Section 
General Dynamics Fort Worth Division 
ABSTRACT 
The role and use of simulation as a design tool in developing integrated systems where 
design criteria is largely unavailable is well known. This paper addresses additional 
simulation needs for the development of Integrated Flight/Propulsion Control Systems 
(IFPCS) which will improve the probability of properly interpreting simulation results. 
These needs are based on recent experience with power approach flying qualities 
evaluations of an advanced fighter configuration which incorporated Short Takeoff and 
Landing (STOL) technologies and earlier experiences with power approach flying qualities 
evaluations on the AFTI/F-16 program. Specific topics addressed in this paper are: 
(1) The use of motion base platforms with axial and normal degrees of freedom will 
help in evaluating pilot coupling and workload in the presence of high frequency 
low amplitude axial accelerations produced by high bandwidth airspeed 
controllers in a gusty environment. rhis would also help quantify the airspeed 
controller bandwidth ~ecessary for adequate STOL performance. 
(2) The use of high resolution visual scenes or helmet mounted displays capable of 
providing better depth perception, HUD symbology, and simulated FLIR 
imagery will help in evaluating precision (no flare) all weather landing 
techniques. 
(3) The use of higher computational capability to adequately model and execute 
more complete visual display, landing gear, and engine models will help in 
evaluation of high speed roll out dynamics. 
These needs can be met with unique government simulation facilities such as the NASA 
Ames Research Development Center (NARDC) which have special capabilities. 
Copyright 01984 by General Dynamics Corporation 
All rights reserved 
202 
https://ntrs.nasa.gov/search.jsp?R=19850006190 2020-03-22T17:48:19+00:00Z
INTRODUCTION 
Development of a STOL integrated flight control system will require extensive manned 
simulation because design criteria and guidelines are incompletely developed for the STOL 
task (References 1 and 5). Therefore, the requirements on ground-based simulation 
equipment to aid in the development of a STOL control system become very important to 
developers of STOL aircraft. Specifically, simulations must be capable of supporting 
evaluations in the following areas: 
1. Evaluations of normal axis and axial axis gust sensitivity effects on pilot 
workload .and control effectiveness 
2. Cockpit constraint system evaluations 
3. Control gradient evaluations 
4. PIO susceptability and cross control axis coupling 
5. Crew station human factors evaluations 
6. Safety evaluations 
7. High speed roll-out and ground handling evaluations 
8. Hydraulic flow demand evaluations 
9. Evaluation of more complex landing gear and engine/nozzle/reverser operations 
10. Low altitude ground effects and flying qualities evaluations with good visual 
peripheral cues and depth perception. . 
Realistically, pilot workload and effectiveness in precision STOL control tasks cannot be 
fully measured without these ~valuations. 
MOTION CUES 
Recent STOL studies (Reference 1) and IRAD results (Reference 2) indicate that landing 
precision may be obtained to the required level by providing high-bandwidth pitch rate 
control for flight path adjustments in combination with tight, high bandwidth regulation of 
aircraft airspeed. This combination of control features is readily implemented on a STOL 
configuration which utilizes the thrust reversing feature of a 2-dimensional thrust 
vectoring/thrust reversing (2-D TV/TR) nozzle to achieve more then 0.2 g acceleration 
capability axially (fore and aft) and 0.5 rps2 pitch acceleration. The high bandwidths 
achievable with this nozzle permit pinpoint control precision in piloted simUlations of 
STOL landings in fixed base simulations, but aircraft gust sensitivity is high in the normal 
and axial axes due to the high system control loop gain associated with the powerful 
control forces available from the nozzle. For instance, from Reference 2, a generic STOL 
longitudinal axis control law was designed to provide decoupled pitch rate/airspeed 
control. The desired bandwidth of the pitch rate contr .... ller was well defined from the 
simulation results, but the desired bandwidth of the airspeed controller was not as clearly 
indicated. The airspeed controller was designed to provide the maximum decoupling 
purity between pitch rate and airspeed while maintaining a critically damped airspeed 
response to an incremental step command. The resulting design demonstrated the 
capability of providing very precise airspeed control, as shown in the left column of 
203 
Figure 1, even in the presence of 1.2 FPS RMS (l-Sigma, Dryden Spectrum) random 
atmospheric gusts. However, it is not apparent what effect the small amplitude, high-
frequency gust-generated axial accelerations will have on pilot performance. It is also 
clear that the desired engine actuator requirements will have a direct impact on the 
bandwidth of the airspeed controller. A first-order-Iag filter was placed in the airspeed 
feedback path in the studies of Reference 2 to evaluate the capability to reduce axial gust 
responses. As shown in the right column of Figure 1, a .02 filter time constant 
significantly reduced the axial acceleration activity. With the control system gain levels 
used in this study, a filter with time constants as large as .2 seconds could be used without 
adversely affecting system performance or stability. Therefore, there is a large range of 
airspeed control bandwidths which appear to be acceptable to pilots on a fixed base 
simulator. Figure 2 illustrates nozzle control activity during a typical approach in a gusty 
environment with the 0.2 second time constant airspeed feedback filter. While the 
illustrated control activity is not unreasonable, the actuators were occasionally operating 
near their assumed maximum rate. Figure 3 shows the relationship between nozzle 
control activity and axial acceleration. Since the degree of airspeed augmentation 
provided by the controller can have an impact on other aircraft systems such as the 
hydraulics, pilot vehicle interface, and engine control, it is important to determine the 
pilot acceptance of high frequency axial accelerations and how these accelerations are 
coupled through the pilot into axial control and into other axes. One example of pilot 
coupling experienced in flight but not experienced during simulation evaluations is the 
pilot coupled oscillations encountered during the AFTI/F-16 flight tests (Reference 3). 
Gust sensitivity in the normal axis can be evaluated, to some degree, based on common 
pilot experience, however, notable failures in evaluation of normal axis gust sensitivity 
have been experienced (e.g., Reference 4) on fixed base simulation equipment in the 
AFTI/F-16 program. Gust sensitivity effects on pilot workload in the axial axis will be 
difficult to evaluate with fixed base simulation equipment since pilots have not previously 
experienced the combination of high axial acceleration levels and bandwidths whtch are 
possible on a STOL aircraft. A study of this type can be accomplished on a moving base 
simulator with axial and normal degrees of freedom similar to capabilities on NASA-Ames 
moving base simulator facilities. . 
VISUAL CUES 
Important primary visual cues used during a landing approach are associated with depth 
and peripheral vision. In the simulation of a STOL approach, the use of a limited area 
projection type of visual system does not provide the best result such as a wide field of 
view and good depth perception would provide. Also, because the scene is projected out in 
front of the pilot, landing biases can occur causing the pilot to land short of the intended 
touchdown point. Our recent experience (Reference 2) points to the need for a wrap-
around virtual image type visual system which is mounted closer to the pilot. The use of a 
vertically collimated raster display utilizing simple solid color pastels to form a cartoon-
like picture could significantly increase resolution near the ground. The wrap around 
feature would improve peripheral vision. Peripheral vision provides the pilot with sink 
rate information he cannot obtain very well over the nose at STOL approach angles of 
attack. This reinforces the pilot's perception of descent through the visual-motion system 
and thus increases his stress level. Since pilot gain is strongly influenced by stress level, 
real pilot workload could be more accurately determined with improvements in the visual 
system. The pilot must subconsciously feel t~at he is in real danger if the landing 
maneuver is not successfully executed for best evaluation results. 
Helmet mounted displays have a significant application to a STOL approach and landing. 
They enhance peripheral and depth perception in simulation applications but also provide 
HUD information and simulated FLIR imagery in actual aircraft applications to perforta 
204 
precIsion all weather ~nd night landings. Operational use of Helmet Mounted 
Sight/Displays (HMSD) allow the pilot to view the landing scene under poor visual 
conditions and safely land the aircraft with a minimum of additional workload. This 
technology. can give the pilot night vision, allowing the pilot to look anywhere in the 
forward quadrant through the aircraft to locate the landing field by merely directing his 
line-of-sight (LOS) to the desired area. The pilot's line of sight (LOS) commands the FLIR 
to follow his helmet (head) movements thus providing a large field of view (FOV) for 
landing the aircraft at large crab angles and high angles of attack. Symbology to aid the 
pilot in landing with minimum dispersion is superimposed on the FLIR video and projected 
onto the pilot's visor by a miniature CRT mounted on the helmet. The aircraft becomes 
"transparent" and t:te experiences a true kinetic sense of where the landing field is, 
relative to the aircraft, thus enabling him to land the aircraft using the scene on his visor. 
Proof of application and operational readiness will first have to be shown in a realistic 
simulation environment before deployment in the field. 
COMPUTATIONAL CAPABILITY 
Computer power may be the most easily attainable, yet least definable, quantity in a 
development simulator. Computers .are constantly being improved from the standpoint of 
speed and memory capabilities. What is difficult to define is how the computing power is 
to be assembled to provide engineering flexibility, growth, and eventual hot-bench 
support. A STOL development simulator must provide capabilities in several key areas. 
First, adequate input/output (I/O) capability is important to support visual scene and 
motion base drives, advanced cockpit development, output data recording (both analog and 
digital), and eventual flight control and avionics hardware-in-the-Ioop simulation. 
Secondly, several simulation models which have traditionally been kept simple in their 
implementation such as engine, actuator, and landing gear models must be made more 
complete in order to lower program risk by providing timely hydraulic demand, engine 
operation, and critical high speed ground roll-out information. And thirdly, the addition 
of an all new Nozzle Drive Unit (NDU) complex will further tax existing computer 
modeling computational power. In order to achieve adequate computational fidelity 
several computers, operating at different rates, must be employed in parallel. Most 
importantly the computer simulation complex architecture must be such as to not 
compromise the fidelity of the presentation of the flight characteristics to the pilot. 
SUMMARY AND CONCLUSIONS 
Based on recent experience with power approach flying qualities evaluations of an 
advanced fighter configuration which incorporated STOL technologies, general 
requirements for adequate STOL flight simulation have been developed. Specific topics 
addressed in this paper were: 
(1) The use of motion base platforms to and in evaluating pilot coupling and 
workload in the presence of high frequency low amplitude axial accelerations 
produced by high bandwidth airspeed controllers in a gusty environment. (This 
would also help quantify the airspeed controller bandwidth necessary f01" 
adequate STOL performance.) 
(2) The need for high resolution visual scenes 01" helmet mounted displays capable 
of providing better depth perception, HUD symbology, and simulated FLIR 
imagery in evaluating precision (no flare) all weather landing techniques. 
(3) The need for higher computation capability to adequately model and execute 
205 
more complete visual display, landing gear, and engine models. 
The importance of a high fidelity presentation of the flight characteristics to the pilot 
cannot be overstressed. 
REFERENCE 
1. STOL Demonstrator Design Study, R. W. Woodrey, et. al., AFWAL-TR-83-3022, Vol 
II, General Dynamics Corp., March 1983. 
2. Man-in-the-Loop General STOL Simuation, D. C. Anderson, K. E. Sanders, J. W. 
Roberts, ERR-FW-2345, General Dynamics Corp., Report in Progress. 
3. AFTI/P-16 Pilot Coupled Roll Oscillation, Merle G. Allen, AlA A Paper No. 83-2282-
CP, August 1983. 
4. The Role of Simulation in Development of the AFTIIF-16 Digital Flight Controls, D. 
C. Anderson, K. E. Sanders, AlA A Paper No. 83-2283-CP, August 1983. 
5. Design Criteria for Flightpath and Airspeed Control for the Approach and Landing 
of STOL Aircraft, J. A. Franklin, et. al., NASA TP-1911, March 1982. 
6. An Operational Evaluation of Head-Up Displays for Civil Transport Operations, J. K. 
Lauber, et. al., NASA TP-1815, August 1982. 
7. AIAA Paper No. 84-1901CP, Piloted Simulation Evaluation of Advanced Fighter 
Integrated Flight/Propulsion STOL Controls, D. C. Anderson, K. E. Sanders, AIAA 
Gc5cC Conference, Seattle, Washington, 20-22 Aug 1984. 
206 
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o 
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AXIAL 
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(Note - 1.2 KT RMS DRYDEN SPECTRUM TURBULENCE 
PER MIL - F-8785~) 
luL l' .~~""'" I" 
1 SEC 
Figure I High Bandwidth Airspeed Controller Introduces High Frequency Axial 
Accelerations During STOL Approach 
-.-
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208 
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-5 
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Accelerallon 
(Yxl(FT/SECI) 0 
-' ~'_' __ ~I 
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o 
Figure 3 Nozzle Control Activity and Axial Accelerations as a Function 
of Airspeed Error 


STOL Simulation Requirements for Development of Integrated Flight/propulsion Control Systems

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