EVALUATION MANEUVER AND GUIDELINE DEVELOPMENT FOR HIGH-ALPHA CONTROL LAW DESIGN USING PILOTED SIMULATION. Keith D. Hoffler * ViGYAN, Inc.

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1 EVALUATION MANEUVER AND GUIDELINE DEVELOPMENT FOR HIGH-ALPHA CONTROL LAW DESIGN USING PILOTED SIMULATION Keith D. Hoffler * ViGYAN, Inc. Hampton, VA Philip W. Brown, Michael R. Phillips, Robert A. Rivers *, John B. Davidson, Jr., Frederick J. Lallman, Patrick C. Murphy, and Aaron J. Ostroff NASA Langley Research Center Hampton, VA AIAA Abstract Advanced controls technology is making it possible for fighter airplanes to maneuver aggressively at high angles of attack. This capability has been shown to offer advantages in close-in air combat maneuvering. To date three research airplanes have flown with post-stall maneuvering capability in the United States (HARV, X-31, and F-16 MATV). These airplanes are being used to validate ground based research, learn more about the post-stall maneuvering environment, and demonstrate post stall maneuver capability in flight. As part of NASA's High-Alpha Technology Program, efforts are in progress to develop control system design and evaluation maneuvers for future airplanes capable of high angle-of-attack maneuvering. This paper outlines a set of for large amplitude maneuvers and piloted simulation tasks developed for design of a thrust vectoring system and control laws for NASA's High-Alpha Research Vehicle. Results are presented from a piloted simulation that employs a set of control laws to be flown on the HARV in the Spring of Design tradeoffs are discussed and pros and cons of the piloted evaluation maneuvers developed are addressed. Observations related to the large amplitude maneuver are made. Nomenclature In the following list of symbols, matrices and vectors are shown bold face and scalars are shown italicized. c mean aerodynamic cord, feet g gravity, 32.2 ft/sec 2 K feedback gain matrix M Mach number n z load-factor, g n z,c load-factor command, g * Senior Member AIAA Member AIAA Copyright 1994 by the American Instutute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royaltyfree license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights reserved by the copyright owner. 1 p body-axis roll rate, deg/sec p gain-schedule parameter pw wind-axis roll rate, deg/sec q body-axis pitch rate, deg/sec Ýq body axis pitch acceleration, deg/sec 2 r body-axis yaw rate, deg/sec Ýr body axis yaw acceleration, deg/sec 2 t φw =90 time to change wind-axis bank angle 90, sec. y cmd command from feed-forward command generator y p plant output vector α angle of attack, deg. Ý α angle-of-attack rate, deg/sec α c angle-of-attack command, deg. β sideslip angle, deg. δ sc stabilator command, deg δ sp pilot stick command, inch δ vc pitch thrust vectoring command, deg φ bank angle, deg. φw wind-axis bank angle, deg. Special symbols max os prefix signifying change in parameter subscript indicating maximum value subscript indicating overshoot Introduction Advancement of all-aspect missiles have increased the need for fighter airplanes to have rapid nose pointing capability for obtaining the first shot in an air combat engagement. With this in mind, research and development have been directed toward development of high performance airplanes that can rapidly maneuver at angles of attack beyond maximum lift, thus producing very high nose-turning rates. In simulation studies, good airplane agility and controllability in the high-angle-of-attack flight regime have been shown to provide tactical maneuver advantages during close-in air combat with all-aspect missiles and guns (refs. 1-4). Advancement of this technology has led to three research airplanes (HARV, X-31, and F-16 MATV) that are currently flying with the capability to maneuver at post-stall angles of attack. All

2 three research airplanes utilize multi-axis thrust vectoring to provide the required control power for post-stall maneuvering. These airplanes have very broad usable angle-of-attack envelopes and correspondingly very nonlinear aerodynamic characteristics. The nonlinear characteristics, numerous control effectors, and the ability to achieve agile controlled flight over a greatly expanded angle-of-attack range lead to a requirement for further development of control system design techniques and performance and handling quality design. As part of the advancement of high-α controls technology, NASA's High Alpha Technology Program (HATP) is in the process of flight validating control system design methods, and ground-based flight simulation techniques in the high-angle-of-attack flight regime (ref. 5). The control system design research conducted under this program is developing and evaluating advanced control system design methods and a control system design process from initial design through flight test. NASA's High Alpha Research Vehicle (HARV) (ref. 5) is being used in the HATP studies and is currently capable of controlled flight over a broad angle-of-attack range (up to 70 ). The control law design process used is shown in figure 1 (ref. 6). Guidelines and test methods for the linear, nonlinear, and piloted simulation portions of this process were very limited when work began toward design of the HARV thrust vectoring system and associated control laws. Development of a set of large amplitude (or nonlinear), such as maximum roll and pitch performance and maximum bank angle overshoot was required. The HARV's broad angle-of-attack range includes areas where evaluation maneuvers, handling qualities, and performance have not been defined. Therefore, it was also necessary to develop a set of maneuvers including definition of the piloting tasks and the associated tolerances required to assign pilot ratings. This paper describes development of nonlinear design and piloted evaluation maneuvers used for design of the HARV's thrust vectoring system and advanced high-α control laws. The rationale for these and maneuvers are discussed as well as lessons learned during their application to HARV control law designs. Implications of the on design tradeoffs are discussed. Suggested modifications and additions to the nonlinear and evaluation maneuvers, based on observations from simulated one-versus-one engagements and the evaluation maneuvers are discussed. A brief description of the simulation used for this study is included. Description of Simulation HARV Airplane The airplane configuration used for this study is a preproduction F-18 modified to add multi-axis thrust vectoring for enhanced pitch and yaw control power. The modified configuration, known as the High Alpha Research Vehicle (HARV), is discussed in reference 5. The F-18 is a multi-role fighter/attack airplane with supersonic dash capability and, by current production airplane standards, good low-speed high-α maneuvering capability. Major dimensions and key features of the F-18 with thrust vectoring are shown in figure 2. Thrust vectoring capability was added to the F-18 by removing the secondary (divergent) nozzles and adding three thrustvectoring vanes per engine. The modified airplane is shown in figure 3. HARV Simulation The HARV simulation was developed by modifying nonlinear aerodynamic, engine, and control system models of the production F-18 obtained from McDonnell Aircraft Company (McAir). The F-18 simulation on which the HARV model is based is discussed in detail in reference 7, and HARV thrust vectoring capabilities are discussed in reference 8. The original McAir aerodynamic data base covers the α range from -10 to 90, the β range from -20 to 20, altitudes up to 60,000 feet and speeds up to Mach 2.0. The F-18 simulation was modified to account for the thrust vectoring and its effect on the configuration aerodynamics and engines. The HARV engine model, obtained from McAir (McAir designed the thrust vectoring hardware for the airplane), incorporates thrust vectoring capability and includes the effects of Mach, altitude and dynamic throttle response. Also included are effects of α and vane deflection on thrust. Gross thrust and ram drag were tabulated separately allowing thrust vectoring to act on gross thrust only. Effects of vane deflection on gross thrust and jet deflection were obtained from ground based tests (ref. 8). Aerodynamic increments were added to the database to account for the addition of thrust vectoring vanes, actuator housings, and spin chute. Jet induced effects were added to account for the change in airflow over the airframe due to thrust vectoring. This data was obtained from wind tunnel tests described in reference 9. A Research Flight Control System (RFCS) consisting of the new control law discussed herein replaced the existing F-18 control system. The thrust vectored commands from the RFCS go to a vane control system known as the "Mixer/Predictor." The Mixer/Predictor converts pitch, yaw, and roll thrust vectoring commands into equivalent commands for the six thrust vectoring vanes yielding the required jet deflections. Longitudinal Control Laws Figure 4 shows that the main components of the HARV longitudinal control system (ref. 10 and 11) are the feedforward command generator (FFCG), the command generator tracker gain (CGT), and the variable-gain output feedback controller. The FFCG has two command modes (longitudinal stick modes): 1) an nz command mode is 2

3 dominant at high speed and low α; and 2) an α command mode is dominant at low speed and high α, with a smooth transition between the two modes. Commands from the FFCG go to the feedback controller that uses α, q, and nz for the feedback measurements. Control commands from the feedback controller are split between the stabilator and the pitch thrust vectoring actuators. Variable-gain output feedback (refs. 12 and 13) is the methodology used for designing the feedback controller. This methodology is used to generate a gain functional where the gain matrix is updated each iteration as a function of measured gain-schedule parameters. This gain matrix is then partitioned and used in a dynamic feedback controller. Lateral/Directional Control Laws In the lateral-directional control law, lateral stick commands stability-axis roll rate and pedal commands sideslip angle. Figure 5 shows a functional diagram of the lateral/directional control law. Pilot inputs are shaped and modified before being multiplied by command gains and summed with feedback commands. The feedback commands consist of measurements that have been passed through structural filters and multiplied by feedback gains. The sum of pilot commands and feedback commands produce stability-axis roll and yaw acceleration commands. These acceleration commands are distributed into the nearoptimum blend of control deflections. Thrust vectoring controls are engaged according to their effectiveness relative to that of the aerodynamic controls. The controls being used are aileron, rudder, differential stabilator, and yaw thrust vectoring. Two separate design tools, Control Power, Robustness, Agility, and Flying Qualities Tradeoffs (CRAFT) (ref. 14) and Pseudo Controls (ref. 15) are being integrated to synthesize the lateral-directional control law. This combined CRAFT/Pseudo Controls design technique is a hybrid technique that combines both linear and nonlinear design methods. The CRAFT design process is a linear design approach based on Direct Eigenspace Assignment (ref. 16) for determining measurement feedback gains. Pseudo Controls is a nonlinear control blending strategy for distributing control system commands in a near-optimal fashion to the appropriate control effectors. Piloted Simulator The piloted simulations were conducted using the simulation model and control system described above in NASA Langley Research Center's (LaRC) Differential Maneuvering Simulator (DMS). The DMS is a fixed-base simulator capable of simulating two airplanes as they maneuver relative to each other and the earth. Two 40- foot-diameter projection spheres providing a wide-angle visual display for each pilot make up the DMS. Each sphere encloses a cockpit, airplane-image (target) projection system, and Computer Generated Image Sky- 3 Earth-Sun projection system (fig. 6). Reference 17 contains a detailed, although not current, description of the DMS. A photograph of one of the cockpits and target visual display is shown in figure 7. Each cockpit incorporates three CRT head-down displays and a head-up display (HUD) representative of current fighter airplane equipment. For this study, a fixed reticle projected on the heads-up display was used for target acquisition and tracking. The displays provided to the pilot are similar to F-18 displays with some minor modifications to facilitate some of the maneuvers and the tracking tasks. A movable center stick and rudder pedals were provided to the pilot. The center stick and pedal force, breakout, and dynamics were set up on the DMS McFadden Universal Variable Cockpit Control Force Loading System model 392A to closely represent the F-18 HARV airplane. The DMS is driven by a real-time digital simulation system built around a CONVEX 3800 series computer. The dynamics of the airplane and control system are calculated using six-degree-of-freedom rigid body equations of motion with an 80Hz frame rate. Communications between the computer, cockpit, and displays have a 40Hz frame rate. Overall transport delay of the system is around 110 milliseconds, stick to scene motion. Nonlinear Guidelines Results and Discussion Nonlinear, used to describe characteristics for large amplitude maneuvers, were developed. These were based on previous experience from simulations at NASA LaRC combined with knowledge of HARV characteristics (ref. 18). Therefore, the are a blend of HARV design goals and preliminary roll and pitch agility requirements for an airplane capable of poststall maneuvering. In the following discussion some of the nonlinear are summarized, and some direction on making tradeoffs between them is given. Simulation results are shown comparing HARV and F-18 performance relative to selected. Pitch Axis Guidelines Nonlinear used for the longitudinal control law design were maximum pitch rate (q) and acceleration ( Ýq ). The were specified as a function of α at 1- g trim airspeed throughout the α range and for greater than 1-g conditions at Mach 0.6 from low α to α = 35. Only the 1-g data are given herein. Maximum elapsed time from when stick motion started to reaching the guideline was also specified. The time to achieve peak rate and acceleration requirements were met in all cases. The q and Ýq for 1-g trim are shown in figures 8 and 9. The F-18 does not meet these at low α due in part to control law design and beginning at moderate

4 α's due to lack of control power. The HARV met the up through post-stall α's, but falls short of the guideline at high α due to lack of sufficient control power. There are also tradeoffs to be made when control power is limited. High rates make the configuration less predictable due to excessive overshoot after the aft stick input is reduced to make a capture. The overshoots result from limited control power available to counter pitch inertia. Also, very high rates can simply make it difficult for a pilot to anticipate a capture even with reasonable overshoots. Roll Axis Guidelines Nonlinear used for the roll axis included maximum wind axis roll rate (p w ) during a roll through φw=90, time to bank through φw = 90 (t φw=90 ), wind axis bank angle overshoot φwos, and maximum α and β excursions (coupling). Values for 1-g trim airspeed throughout the α range were defined as well as values for Mach 0.6 from low α to α = 35 (only 1-g data are given herein). The lateral/directional coupling for full-lateral stick with longitudinal stick fixed were defined as shown in Table 1. Maximum Excursion From φ w Target Condition α β 90 ±6 7 adverse 1 proverse ±10 7 adverse 1 proverse Table 1. Lateral/Directional coupling. The t φw = 90 and pwmax and performance achieved for the F-18 and the HARV are shown in figures 10 and 11. The HARV met both of these through α = 35 and the pwmax guideline was nearly met throughout the α range. The F-18 met both through only around α = 15 It fell short of the guideline due to insufficient directional control power required to coordinate rolls above α 8. The HARV t φw = 90 performance fell outside the guideline above α = 35 primarily due to lack of available control power. However, time to bank could be improved for α > 20 if all available performance was used without regard to controllability and predictability. Wind-axis bank-angle overshoot is a guideline developed during the HARV control law design effort that directly addresses lateral/directional predictability at moderate to high α. Wind-axis bank-angle overshoot is defined here as the amount of φw used to stop a maximum performance roll with a full stick reversal applied when passing through 90 wind-axis bank-angle change. If the overshoot is 4 excessive pilots will have difficulty judging the lead required to capture the desired bank or heading angle. The result would be poor predictability and a tendency to overshoot or undershoot the target. At low α where roll rates are high and rolling (body axis rolls) alone does not significantly change the nose pointing angle, bank angle overshoots are not as critical. Also, pilots generally refrain from using maximum performance rolls when trying to make precise bank angle captures with the very high rates available at low α. However, available peak roll rates are much lower at moderate and high α. Therefore, pilots tend to use maximum lateral stick rolls at higher α's to make acquisitions. If the control law always allows maximum achievable rolls at these conditions, bank angle captures would be very difficult to predict reliably due to control power changes throughout the α range. The control power changes would yield different achievable roll rates as well as widely varying bank-angle overshoots. Large bankangle overshoot variations with α would lead to poor predictability. Bank-angle overshoot can be estimated via a one dimensional analysis assuming that the rolls are coordinated (β = 0 ) and that the yaw axis is limiting, that is, body-axis yaw control power saturates before body-axis roll control power. Then, for coordinated rolls and ignoring gravity effects (fig. 12): r = p tan α. (1) Therefore, as angle of attack increases more yaw rate is required for coordination. Doing a one dimensional analysis (yaw axis) yields the relationships below. φw = p w 2 sin α 2Ý r or pw = 2Ý r ( φ w ) sin α Because maximum yaw acceleration Ýr capability is known for each angle of attack, if a fixed allowable bank-angle overshoot ( φw) is assumed, maximum allowable windaxis roll rate can be estimated using this relationship. Using the control surface yaw acceleration capability and ignoring the damping contribution to make the first estimates of commanded maximum roll rates worked well. Figure 13 shows the wind-axis bank-angle overshoot criteria and the achieved values obtained after a few iterations from the original estimates. Because the available control power is fixed, there is a direct trade off between bank-angle overshoot and maximum roll rate. The bank-angle overshoot as defined herein equates to pilot lead requirement necessary to make a capture within desired tolerances. Maximum lead of around 40 to 45 was considered acceptable by the pilots in this design effort. Although the pilots could learn to consistently predict different lead requirements ( 45 ) at each α, in air combat maneuvering, α is not always known by the pilot and constantly changes. Therefore, it was considered important to have fairly consistent overshoots throughout (2)

5 the moderate- to high-α range making predictability consistent anytime lateral stick inputs produced significant amounts of yaw rate. High body yaw rates as opposed to body roll rates serve as a cue to the pilot that α is high without having to refer to the α display. With fairly consistent overshoots throughout the moderate- to high-α range, predictability was consistent. Piloted Evaluation Maneuvers Pilot-in-the-loop evaluations were conducted using NASA Langley's Differential Maneuvering Simulator and the nonlinear HARV simulation model described earlier. The evaluations used a series of piloted tasks designed to test the longitudinal and lateral/directional control systems throughout the HARV flight envelope. The tasks were designed for this effort with the intent of having broad applicability because no concise set of maneuvers existed to evaluate high-α capable configurations in the moderateto high-α regions of their envelopes. The piloted evaluation maneuvers included α captures, n z captures, heading captures, large amplitude rolls with bank angle captures, and target gross acquisition and tracking tasks. Departure resistance was also assessed using crossed controls and other aggravated inputs. The piloted maneuver set was developed with goals of allowing evaluation of each axis of the control law individually where possible, using consistent maneuvers and task across the flight envelope when possible, and obtaining piloted evaluations in a short time. The intent was to adjust the control law to get good ratings from a series of pilots for all maneuvers and have no new problems appear when the airplane and control law were flown freely or in Air Combat Maneuvering (ACM). The maneuvers and associated Cooper-Harper (ref. 19) task tolerance are briefly discussed herein including piloting technique, associated task criteria, and pros and cons of the maneuvers. The task tolerance were intentionally restrictive to make pilot gains high, thus aggravating any pilot induced oscillation (PIO) tendencies that might exist. The very tight criteria also tends to produce less favorable Cooper-Harper ratings (CHR's) which was considered acceptable. The maneuvers were conducted at altitudes ranging from 15,000 to 40,000 feet but only results from 25,000 feet are presented herein. Five NASA test pilots were involved in this study. Two have extensive air combat training and experience and all have high performance airplane experience. Three of the pilots have at least three years experience with simulated high-α airplanes. One pilot has extensive experience in simulated as well as actual high-α capable airplanes. These evaluations were the first experience with an airplane capable of agile and precise maneuvering at highα for one of the five pilots. Four of the five pilots have experience with the use of simulated within-visual-range air-combat scenarios using high-α airplanes against one or two conventional airplanes. The evaluations were conducted as follows: Initially each task was flown repeatedly for the pilot to become familiar with the required piloting techniques, flight condition, and configuration. Once the pilot felt proficient, the task was repeated a few times to rate it. Ratings based upon a single good or bad attempt were prevented with this procedure as were ratings based on first impressions with an unfamiliar system or maneuver. The drawback is that a task can be learned when flying the same task at the same condition repeatedly. Then, when flying the configuration in air combat, many different flight conditions and tasks are randomly encountered requiring proper task execution on the first try. If lead requirements (for example) change significantly from one flight condition to another predictability could suffer, but that may not show up in the pilot ratings obtained using this "long look" method. Therefore, after the evaluation maneuvers were completed with all pilots, limited one versus one engagements against an F-18 were simulated in the DMS. The one-versus-one results are not presented herein, but control system related pilot comments and observations from the engagements are briefly discussed. Results from use of piloted simulation maneuvers are presented in terms of Cooper-Harper Ratings (CHR), pilot comments, and observations concerning usefulness to the control law engineer. The CHR scale is a numerical scale from 1 to 10 with 1 being the best rating and 10 the worst (fig. 14). In practice CHR's from 1 through 3 are referred to as level one, ratings from 4 through 6 are labeled level two, and ratings from 7 to 9 are considered level three. CHR's 4 indicate desired performance was achieved. Herein, the average of the five individual pilot ratings are shown for each maneuver instead of individual ratings for space considerations. Single Airplane Maneuvers Seven single airplane maneuver sets (meaning maneuvers where no target airplane was involved) were developed. Early in the design process these maneuvers were used almost exclusively because they could be completed very quickly and there was only one dynamic system (the airplane) affecting the result. This made assessment and correction of any problems more straight-forward than if a target airplane was involved. The tasks fell into two categories: 1) maneuvers that primarily evaluated the longitudinal axis; and 2) maneuvers that concentrated on the lateral/directional axis but also addressed inertial and kinematic coupling. This set of tasks make up the elements of any maneuver likely be conducted in ACM. Longitudinal Maneuvers α-captures: The low dynamic pressures associated with high-α and/or low speed results in slow flight path response to commands. Therefore, at high-α and low air speeds rapid pitch changes result from rapid α changes rather that turning the flight path. Rapidly pointing the 5

6 nose requires the ability to rapidly capture a new α at low speed and high α. With this in mind an appropriate longitudinal evaluation maneuver for post-stall capable airplanes at high α is an α capture. Both nose-up and nose-down α captures were evaluated. The Cooper-Harper task tolerance criteria for all α captures required making the captures within ±4 of the target α for desired and ±7 for adequate with no overshoots or undershoots. The pilots gave their ratings based on the capture task and gave comments on pitch rate and acceleration and time to make the capture. Maximum longitudinal stick inputs were not required but an aggressive capture was required. When piloted evaluations began on early versions of the control law, objectionable characteristics were noted for some captures due to control law mode transitions from n z to α command. Therefore, the primary longitudinal control law modifications resulting from piloted simulation were in mode transitions between dominate n z and α command. The transition problems did not show up during evaluations with programmed inputs because, the stick motion was exact resulting in good captures for the original system. However, pilot initial stick inputs were not based on an exact predetermined stick position but on initial response and the amount of α change needed for the particular task. In the original control system, stick gains changed significantly with mode transitions. Nose-up α captures: Nose-up α captures were conducted from two initial conditions: (1) 1-g trim at Mach 0.6; and (2) 1-g trim at 20 α. The captures from Mach 0.6 produced high q and Ý α during pull-ups and assessed the effects of mode transition from dominant n z to dominant α command. The captures from 20 α generated lower q and Ý α values and the control system remained in the α command mode for the entire maneuver. For each nose-up α capture the configuration was trimmed at the initial condition. When the simulation began, the pilot selected maximum afterburner (A/B) (maximum thrust), rolled to approximately 45 bank angle (φ), waited two seconds for the engine to reach maximum power, then pulled aggressively to capture the target α. From the time maximum A/B was applied to the beginning of the maneuver Mach number typically increased A non-zero φ was used during the maneuvers to prevent excessive energy loss and the possibility of entering a tail slide during the captures from high speeds to large α's. For consistency, the non-zero bank angle was also used for the lower speed initial condition and smaller α changes. Average CHR's for the α captures are shown in figure 15. The captures where α change was less than approximately 30 were conducted with less than maximum aft stick to meter the pitch rates for these relatively small α changes. With full stick inputs it was not possible to stop without an 6 overshoot outside the desired criterion. The pilots considered the use of less than maximum aft stick a natural technique that produced good pitch rates and capture times for these "small" α changes. For the Mach 0.6 to 45 α case one of the pilots considered the rates he had to use to obtain desired criterion "sluggish". When the pilot was more aggressive he could only do the capture within ±5 yielding a CHR of 5 (adequate criterion met). At 60 α there is very little pitch control power remaining and the high rates obtained from the Mach 0.6 pull-up made the desired criterion difficult to meet. The limited control power here resulted in sluggish response during the capture. A slight overshoot was generally followed by an undershoot because control power was insufficient to prevent it. However, desired criterion was consistently met from either initial condition by most pilots. Nose-down α capture: A nose-down α capture was also used to assess controllability and predictability when recovering from deep post stall conditions, as well as transition from α command to n z command. This maneuver began trimmed at 60 α. When the run began, the pilot pushed over aggressively to momentarily capture 10 α and then recover to 1-g level flight at 250 KCAS. The ratings primarily reflect the 10 α capture. Recovery to 1-g level flight was done to assure no problems occurred with the α to n z mode transition. This maneuver was designed to evaluate nose-down pointing capability from a post-stall initial condition. It was not intended to address minimum altitude loss. With the original control system, this task received CHR's of 7 or worse from all pilots due to mode transition problems that were not detected using programmed inputs to the simulation. Small errors in initial stick position during the capture attempt set up large amplitude oscillations. The oscillations were induced by mode transitions caused by even small correcting stick motions during the capture attempt and were clearly unacceptable. The control system discussed herein and in reference 11 solved this problem and yielded CHR's of 3 or 4 except for an 8 from one pilot (fig. 15). All pilots still approached the task with caution because the very high nose-down rates available were difficult to arrest within the desired criterion. The technique used by most pilots was to put in slightly less than maximum forward stick making the capture more predictable. The resulting work load was high, but the task was repeatable with the intentionally reduced nose-down rate. However, the pilots all liked the nose-down rate capability and felt they could regulate the rate as needed. They liked the reassurance that α can be rapidly decreased for a quick recovery from post stall if necessary. Nose down maximum pitch rate and accelerations were -40 deg/sec and -45 deg/sec 2 respectively. This compares to pitch rate and acceleration nose-down of -24 deg/sec and deg/sec 2 given in reference 18.

7 g/heading Capture Task: Maneuvers were also conducted to evaluate the n z command mode of the control system. At high speed and low α, nose pointing is primarily achieved by turning the flight path, as is typical for current airplanes. The g/heading capture maneuvers started with the configuration trimmed at 1-g and Mach 0.6. When the simulation began, the pilot rolled to an appropriate bank angle (around 60 for 2 g's and around 70 for 3 g's) and pulled to capture the target n z. The g and Mach were held for 90 of heading change, followed by a heading capture. The pilots gave separate ratings for the longitudinal and lateral/directional sub-tasks. Cooper-Harper longitudinal task tolerance criteria required holding g within ±0.2 g's for desired and ±0.3 g's for adequate. The g capture criteria was left up to pilot's discretion in terms of overshoot and undershoot. The heading capture desired criteria was ±2 and adequate was ±6. The average CHR's for both the 2-g and 3-g captures indicate desired criterion was met by most pilots (fig. 16). Most pilots had generally favorable comments about the g captures with a slight overshoot tendency noted. One pilot commented that there was a tendency to overshoot and to drift away from the target g. He did not consider this a big problem, just a little annoying. The overshoot and drift tendency could be less in the g environment of the real airplane as opposed to the fixed base simulator. The lateral/directional task was found to be difficult because of the task as opposed to the control law. Correct timing was involved when attempting to roll out on a particular heading at even these small α's (<20 ). To make the capture, the pilot continued the turn until the heading was about α past the target heading (velocity vector approximately on the heading) before the rolling back to wings level for the heading capture (fig. 12). The proper timing was found to be very sensitive to Mach number and roll out rate. Lateral/Directional Maneuvers Single airplane lateral/directional evaluation maneuvers consisted of large amplitude rolls to capture target bank angles at various flight conditions. Angle-of-attack control was required during the rolls because α changes during rolls can significantly alter airplane roll performance and energy state as well as lead to departures. Also during gross acquisition tasks (pointing) at moderate and high α, both the lateral/directional and longitudinal axes directly affect the capture due to the coning motion (fig. 12). To augment target acquisition and prevent departures, α must be controllable while rolling. Therefore during the roll tasks the pilots were required to give CHR's on two subtasks, α regulation and the φ capture. The pilots also gave comments on roll rate. The CHR criteria required maintaining the target α within ±2 for desired and ±6 for adequate during all roll tasks. The φ capture criteria were within ±10 for desired and ±20 for adequate with no overshoots or undershoots, for all roll tasks. The task tolerance were intentionally tight to keep the pilot's gain high. The longitudinal and lateral/directional sub-tasks were separated by concentrating on α control during the rolls. When the pilot's concentration transferred to the φ capture for rating purposes the α task was complete. However, some α control was still required during the φ capture to assure the task was done near the target α. 1-g 360 rolls: A complete 360 roll was used because it gave the pilots a convenient φ to capture, wings level with the horizon, thus simplifying the task both from a piloting and data analysis point of view. It is recognized that this is an extreme maneuver, beyond what would normally be done in ACM (rolls beyond 180 are not expected in ACM). The 1-g 360 roll maneuvers started from 1-g trim at the target α. If not already at maximum thrust (α < 35 ), maximum thrust was applied, time was allowed for the engine to reach maximum thrust, then maximum lateral stick was applied. This task exposed roll coordination and coupling problems and lateral/directional predictability problems and PIO tendencies in the control system. Average CHR's for the 1- g rolls from simulation are shown in figure 17 for the set of control laws to be evaluated in flight. The majority of the ratings were in the level one region with a few results edging into the level two handling qualities region. The roll rating at α = 5 was level two primarily because of the task. Maximum stick rolls at low α produce very high roll rates making φ captures difficult. The high rates made predictability difficult, and given the simulation transport delay the results at this α may be unreliable. With less than maximum lateral stick input the maneuver could be done within the desired parameters by all pilots. Maximum roll rate at low α's would only be used defensively, i.e., for offensive maneuvers maximum roll rate would not likely be achieved at this flight condition. At α = 15 the desired criterion was met by most pilots in both axes. But φ capture difficulty was still seen due to high roll rates. At α = 15 the longitudinal axis is in the n z mode; therefore, as speed changes when the airplane rolls, α tends to decrease making it difficult to maintain α within ±2. Average ratings were level one and desired criterion was met by all pilots from 25 through 65 α with the exception of the roll rating at α = 65. Here the yaw vectoring control power is limited and predictability was reduced (see fig. 13). Loaded rolls: Loaded rolls were conducted at Mach 0.4 and 0.6 and various α's. These maneuvers started with the airplane trimmed at 1-g and typically faster than the target 7

8 Mach number. The pilot rolled to a φ of around 60 and pulled to the target α, then waited to decelerate to the target Mach number. At the target Mach number, maximum lateral stick input was applied to roll back through wings level and capture φ = 90. This gave a φ 150 for the task. The loaded rolls had the same purpose as the 1-g 360 rolls but did not roll as far because the energy loss during such a roll would result in a capture at or near 1-g. Average CHR's from the five pilots for the loaded rolls are shown in figure 18. The ratings are similar to the 1-g roll ratings and generally on the level one-level two boundary. The roll ratings were originally worse than the 1-g roll ratings due to oscillations and poor predictability when making the φ capture. The oscillations resulted from β excursions after the initial φ capture and predictability was also a problem with early versions of the control law. These problems were solved through control system changes that reduced the β excursions and the φ os. The lateral-directional control law was designed to use knowledge of the available control power at any given flight condition to command maximum coordinated roll rate/acceleration within the φos constraints at 1-g. The dynamic pressure decrease during the loaded rolls made control power available at the beginning of the roll higher than at the end. This resulted in initially higher roll rates that yielded large φ w os at the at the end of the roll. The large overshoots caused poor predictability. This problem was solved simply by reducing the roll rate command for loaded rolls. These tasks were significantly more difficult to carry out than the 1-g roll tasks because of the set up required to get to the initial condition. Due to rapidly decreasing Mach number at the higher α's, to conduct the maneuver at the target Mach number, the target α had to be captured quickly. Target Tracking and Acquisition Tasks After good ratings were achieved from the single airplane maneuvers, target acquisition and tracking tasks were conducted. These tasks are essential to the evaluation of high performance airplanes because they are intend to acquire and track target airplanes. However, they are considerably more complicated to analyze for cause and effect when problems occur than the single airplane maneuvers. Moderate-α/elevated-g Tracking Tasks: Moderateα/elevated-g tracking tasks were developed to evaluate tracking in the 15 to 25 α range under greater than 1-g conditions. Two tasks were used, both tracking a target maneuvering at 3-g's, one with a Mach range from 0.55 to 0.65 (M 0.6 task, approx. 15 to 20 α) and one from Mach 0.4 to 0.5 (M 0.45 task, approx. 20 to 25 α). Initial range to the target was 600 feet and a maximum range of feet was allowed during the task. The target during these tasks rolled into a left 3-g turn and held it for 30 seconds; at 30, 40, and 50 seconds elapsed time the turn direction was reversed with a smooth moderate rate reversal. The total task time was 70 seconds. The target trajectories used were recorded and therefore perfectly repeatable. Pilots gave both longitudinal and lateral/directional ratings for each maneuver. The Cooper-Harper task tolerance used for these tasks required keeping the target within a 12.5 milliradian reticle (±0.36 of aim point) 50% of the time for desired performance and 10% of the time for adequate performance. The reticle depression angle was 35 milliradians. Tracking time during the reversals was not counted. This is a precision tracking task. Average CHR's from the five pilots showed desired performance was generally achievable in the roll axis and frequently achievable in the pitch axis (fig. 19). The pilots considered the control law to have good tracking characteristics. However, the control law exhibited a slight tendency for small amplitude PIO in the pitch axis and some "wandering" in the roll axis. The pitch control law originally had significant problems during the Mach 0.45 task due to n z to α mode transitions. With the newer version of the control law the pilots could not detect the transitions. The lateral/directional axis originally had a PIO tendency and then a wander tendency. These problems were eliminated by a series of changes including gain changes and stick shaping changes. High-α Tracking and Acquisition Tasks: These tasks were developed by McDonnell Douglas Corporation under task order contract to NASA LaRC and used during this control law development. Detailed task descriptions can be found in references 20 and 21. The tasks were used to evaluate longitudinal and lateral/directional target acquisition and tracking characteristics. Details of the tasks are not given herein, but a general description of the maneuvers follows. All tasks started with the target positioned in front of the HARV in straight and level flight. The target then rolled into a nose low descending right turn at a specified α, airspeed, and power setting. This was done to put the target in the proper position for the HARV to acquire or track the target at the designated α. Target trajectories were recorded in advance for play back during the evaluations. Tracking: For the tracking tasks the HARV selected maximum A/B and rolled in behind the target. The HARV delayed a pitch toward the target so that pulling to the target would result in a HARV α near the task-designated α to begin tracking. Both 30 and 45 α tasks were used. When the α required to track the target was more than 5 from the task-designated α the task was terminated. Desired criteria for these tasks required keeping the pipper within ±5 milliradians of the aim point 50% of the time and within ±25 milliradians of the aim point the rest of the time with no objectionable PIO.

9 Adequate criteria required keeping the pipper within ±5 milliradians of the target 10% of the time and within ±25 milliradians the remainder of the task. For both tasks concentric 12.5 and 50 milliradian diameter reticles depressed 80 milliradians were provided to the pilot. Average lateral-directional and longitudinal ratings from the five pilots are shown in figure 20 for both α's. All longitudinal ratings were level two with some pilots achieving the desired criterion. All pilots commented on a small-amplitude pitch PIO tendency whenever making "large corrections" in tracking error and a tendency to have uncommanded nose motion relative to the target. A solution to this problem is currently being investigated but is not part of this paper. The lateral-directional ratings were mostly level two. Desired criteria were met by all but one pilot, whose ratings were not far from the others. A difficulty in tracking at these conditions is that lateraldirectional and longitudinal motions couple in a way foreign to pilots not experienced with high-α configurations. At these (extreme by current airplane capabilities) α's, a roll input significantly affects the longitudinal tracking due to the nose moving around the "cone" (fig. 12), i.e., lateral-directional tracking corrections couple with longitudinal tracking errors more as α increases. Also of interest here is that the longitudinal control law was getting much better ratings early in the lateral-directional control law development. The longitudinal PIO tendency did not show up until the lateraldirectional work load decreased to the level associated with the ratings shown herein. Acquisition: The targets for the high-α acquisition tasks flew similar paths to those used for tracking. For the longitudinal acquisitions the HARV selected maximum A/B, rolled to put the target in the HARV's vertical plane, waited for the proper lag, and pulled to acquire the target. The lag required was dependent on the HARV's initial and task-designated α as well as the target's turn rate and the HARV's flight path changes during the acquisition. The lateral-directional acquisitions required the HARV to pull to the task-designated α then, when the target reached the proper position, roll at that α to capture the target. These tasks were similarly dependent on target turn rate and HARV motion and for the 45 α tasks, the target was out of view below the nose during part of the maneuver. The tasks required precise timing in order to make the acquisitions at the target α. With practice, the pilots were consistent and the tasks worked well. The Cooper-Harper desired performance criteria for the 30 α acquisition tasks required aggressively acquiring the target within ±25 milliradians of the pipper with no overshoot and in a desirable time to accomplish the task. Adequate criteria allowed one overshoot/undershoot. A 50 milliradian reticle depressed 35 milliradians was provided to the pilot for these tasks. The Cooper-Harper task tolerance for the 45 α acquisition tasks were similar except the criterion was within ±40 milliradians of 9 the aim point. An 80 milliradian reticle depressed 80 milliradians was provided to the pilot for these tasks. The average CHR's from the five pilots for both the longitudinal and lateral-directional acquisitions at both α's are shown in figure 21. Most ratings for all these tasks were level two with some level one ratings for the 45 α longitudinal acquisition. Predictability relative to the capture criteria was a problem in both axes. Pitch rate available combined with the available stopping power made overshoots likely if near maximum rate was used. The 45 longitudinal acquisition task allowed more time to anticipate the capture than the 30 task, which is the likely reason for its better ratings. One pilot gave significantly less favorable ratings than the other four pilots for the 30 pitch and 45 roll acquisition tasks. For all pilots in both axes, typically one overshoot was seen followed by a good acquisition unless timing was precise. Summary and Comments Development of nonlinear design and piloted evaluation maneuvers used for design of the HARV's thrust vectoring system and advanced high-α control laws have been described. The rationale for these and maneuvers were discussed as well as lessons learned during their application to HARV control law designs. Implications of the on design tradeoffs have been discussed. Suggested refinements and additions to the nonlinear and evaluation maneuvers are discussed below. Also, an assessment of the suitability of the maneuvers for control system design is made. There were 50 CHR's given by each of the five pilots including both the pitch and roll ratings. The standard deviation in the ratings for each maneuver (across five pilots) was less than 1.0 for 44 of the 50 ratings and less than 1.5 for 49 of the 50 ratings. This small rating spread implies the tasks are well defined (ref. 19) and gives a high level of confidence in the ratings given. Therefore, a limited number of pilots can be used allowing rapid evaluation of the control laws during the design process. While all pilots in this study had extensive relevant experience, one had not previously flown (in flight or simulation) airplanes capable of agile maneuvering at nearor post-stall α's. This pilot had a limited time to become comfortable with and conduct these evaluations. For the single airplane maneuvers his ratings were similar to the other pilots. However, for a significant number of the high-α tracking and acquisition tasks his ratings were significantly different from the other pilots. This, along with other similar experiences, implies that while all tasks in the near- and post-stall flight regime are new (particularly lateral/directional), tasks involving target tracking and acquisition present the most difficulty in learning. It also implies that single airplane (or open-loop) maneuver ratings may be more reliable in this flight regime until pilots have extensive experience using high-α against targets. The dynamics of high-α target acquisition and

10 tracking are very different from those seen at low and more moderate α's. After good Cooper-Harper ratings were achieved for all maneuvers, the pilots conducted free form maneuvering including attempts to make the configuration depart from controlled flight. Then limited one-versus-one engagements against an F-18 were simulated. No new problems with the control laws were found indicating that the evaluation maneuvers tested the control law sufficiently. However, the slight PIO tendency seen in the tracking tasks was a bigger problem in air combat maneuvering than anticipated from the tracking tasks. This is because the targets used in the tracking tasks maneuvered repetitively, smoothly, and at moderate rate. Whereas, the targets in air combat maneuvering naturally did not. Therefore, additional tracking tasks should be added to the evaluation maneuvers. These tasks should have targets that maneuver aggressively, requiring rapid acquisitions closely followed by settling down to a tracking solution before the target maneuvers again. The predictability problems seen in the high-α acquisition tasks did not seem to be a significant problem during the ACM engagements. No problems were found with predictability and controllability during rapidly changing flight conditions. This indicates that the "long look" evaluation method is valid. It also indicates that one overshoot of some as yet undetermined size (larger than the desired criteria specified by the high angle-of-attack acquisition tasks) is probably acceptable as desired criteria. Use of the single airplane maneuvers worked well for uncovering most problems with the control system. Analysis of problems with this type of maneuver was easier than when using the tracking and acquisition tasks. Not many new problems were found when going to the tracking and acquisition tasks from the single airplane maneuvers other than the small amplitude PIO tendency in the pitch axis. The desired α control criterion of ±2 for the rolls was intentionally very tight. However, after conducting these tests numerous times and considering the ACM results and pilot comments, ±4 for desired and ±7 for adequate (same as α captures) were considered more reasonable. If these criteria were used, α regulation ratings for all rolls would likely be level one. On the basis of pilot comments from the maneuvers and simulated one-versus-one engagements, the nonlinear seem to be a reasonable first attempt at design for high-α capable airplanes. Higher pitch and roll rates than those specified herein at α's above approximately 20 would likely be beneficial. This is especially true in the lateral/directional axis. However, the pilots generally agree that more roll rate with less predictability would not be beneficial. Therefore, more control power would be required. Acknowledgments Our sincere appreciation goes to: Susan Carzoo of Unisys Corporation and Mike Messina of Lockheed Sciences and Engineering Corporation for setup and operation of the piloted simulation. Without them and others, this work would not have been possible. References 1. Doane, P.M.; Gay, C.H.; Fligg, J.A.; et al: Multi- System Integrated Control (MuSIC) Program. WRDC-TR , June Ogburn, M.E.; Nguyen, L.T.; Wunschel, A.J.; Brown, P.W.; and Carzoo, S.W.: Simulation Study of Flight Dynamics of a Fighter Configuration With Thrust- Vectoring Controls at Low Speeds and High Angles of Attack. NASA TP-2750, March Lynch, U.H.P.; Ettinger, R.C.; Palt, J.V.; and Skow, A.M.: Tactical Evaluation of the Air-To-Air Combat Effectiveness of Supermaneuverability. WRDC-TR , June Hoffler, K.D.; Ogburn, M.E.; Nguyen, L.T.; Brown, P.W.; and Phillips, Lt.Col.M.R.: Utilization and Benefits of Advanced Aerodynamic and Propulsive Controls: A Simulator Study. NASA CP-3150 Volume II, October 1990, pp Gilbert, W.P.; and Gatlin, D.H: Review of the NASA High-Alpha Technology Program. NASA CP-3149 Volume I Part 1, October 1990, pp Davidson, J.B.; Foster, J.V.; Ostroff, A.J.; Lallman, F.R.; Murphy, P.C.; Hoffler, K.D.; and Messina, M.D.: Development of a Control Law Design Process Utilizing Advanced Synthesis Methods with Application to the NASA F-18 HARV. NASA CP 3137 Volume 4, April 1992, pp Buttrill, C.S.; Arbuckle, P.D.; and Hoffler, K.D.: Simulation Model of a Twin-Tail, High Performance Airplane. NASA TM , July, Mason, M.L.; Capone F.J.; and Asbury, S.C.: A Static Investigation of the F/A-18 High-Alpha Research Vehicle Thrust Vectoring System. NASA TM-4359, June Bowers, A.H., Noffz, G.K., Grafton, S.B, Mason, M.L., and Peron, L.R.: Multiaxis Thrust Vectoring Using Axisymmetric Nozzles and Postexit Vanes on an F/A-18 Configuration Vehicle. NASA CP-3149 Volume 1 Part 2, October Ostroff, Aaron J.; and Proffitt, Melissa S.: Longitudinal-Control Design Approach for High- Angle-of-Attack Aircraft. NASA TP-3302,

11 11. Ostroff, Aaron J.; Hoffler, Keith D.; and Proffitt, Melissa S.: High-Angle-of-Attack Research Vehicle (HARV) Longitudinal Controller: Design, Analyses, and Simulation. NASA TP-3446, Halyo, Nesim; Moerder, Daniel D.; Broussard, John R.; and Taylor, Deborah B.: A Variable-Gain Output Feedback Control Design Methodology. NASA CR- 4226, Control law architecture definition Linear Control synthesis & analysis Airplane model Nonlinear batch simulation Non-linear Refinement using updated model and lessons learned Nonlinear piloted simulation Task performance Flight test evaluation / validation 13. Ostroff, Aaron J.: High-Alpha Application of Variable-Gain Output Feedback. Control. J. Guid., Control, & Dyn. vol. 15, Mar.-Apr. 1992, pp Fig. 1 Control law design process. Methodology validation Lessons learned 14. Murphy, P.C., Davidson, J.B., Control Design for Future Agile Fighters. Presented at AIAA Atmospheric Flight Mechanics Conference. AIAA Paper No August Lallman, F. J., Relative Control Effectiveness Technique With Application to Airplane Control Coordination, NASA TP 2416, April, Davidson, J. B. and Schmidt, D. K.: Flight Control Synthesis For Flexible Aircraft Using Eigenspace Assignment. NASA CR , June Ashworth, B.R.; and Kahlbaum, W.M.Jr.: Description and Performance of the Langley Differential Maneuvering Simulator. NASA TN D- 7304, Foster, J.V.; Bundick, W.T.; and Pahle, J.W.: Controls for Agility Research in the NASA High- Alpha Technology Program. SAE Paper No September Cooper G.E.; and Harper R.P.Jr.: The Use of Pilot Rating in the Evaluation of Aircraft Handling Qualities. NASA TN D-5253, April Krekeler, G., Wilson, D., and Riley, D.: High Angleof-Attack Flying Qualities Criteria. AIAA January Wilson, D., and Riley, D.; Flying Qualities Criteria Development Through Manned Simulation for 45 Angle of Attack - Final Report. Volumes 1 and 2. NASA CR April Fig. 2. Diagram of HARV airplane showing major dimensions in feet. Fig. 3. Photograph of HARV airplane. 11

12 FFCG δ sp n z - command α - command Mode selection logic y cmd CGT Feedback controller PIF structure with thrust vectoring washout filter K(p) Gain Functional Disturbances & Noise δ sc δ vc Aircraftt & Sensors Aircraft outputs y p α Air data Flap schedule Leading-edge flap command Trailing-edge flap command p Parameter calculation Fig. 4. Block diagram of HARV longitudinal control system. Lateral Stick Pedals Body Roll Rate Body Yaw Rate Lateral Accel. Sideslip Rate Fig. 5 Command Shaping Structural Filters Stick and Pedal Gains Feedback Gains Pilot and Feedback Commands Lateral Command Directional Command Body Angular Rates Flight Condition Information Inertial Coupling Compensation Control Interconnect and Distribution Thrust Vectoring Management Pseudo Controls Aileron Rudder Diff Stabilator Yaw Vectoring Block diagram of HARV lateral-directional control system. Fig. 7. q max, deg/ sec Photograph of DMS cockpit. Area meeting F-18 HARV Maximum rate obtained within 1.0 sec from initiation of full aft stick input (Full aft input within 0.5 sec) Initial α, deg Fig. 8. Maximum pitch rate guideline with maximum aft stick pull up from 1-g at 25,000 feet (Max A/B). F-18 Fig. 6. Cut-away drawing of DMS. q Ý max, deg/ sec 2 Area meeting HARV Maximum acceleration obtained within 1.0 sec from initiation of full aft stick input (Full aft input within 0.5 sec) Initial α, deg Fig. 9. Maximum pitch acceleration guideline with maximum aft stick pull up from 1-g at 25,000 feet (Max A/B). 12