Dynamic stability and handling qualities tests on a highly augmented, statically unstable airplane

NASA Technical Memorandum 88297

Dynamic Stability and Handling Qualities

Tests on a Highly Augmented, Statically

Unstable Airplane

Joseph Gera and John T. Bosworth

(NASA-2'1-88293) D I I i A ffIC S f i B I L I f T ABD B87-26920

B A B D L f l G QUALITIES ZBSTS 01 4 816ELY AUGCIEZIHCED. SPASICALLY UUSTABLB ALIPPLILEIB

( P A S A ) 16 p Arafl: IOTIS HC 1102/LIP A 0 1 Unclas

CSCL O X 63/08 6092572 n August 1987 a National Aeronautics and Space Administration NASA Technical Memorandum 88297 ~ ~

Dynamic Stability and Handling Qualities

Tests on a Highly Augmented, Statically

Unstable Airplane

Joseph Gera and John T. Bosworth Ames Research Center, Dryden Flight Research Facility, Edwards, California .

National Aeronautics and Space Administration Ames Research Center Dryden FI ig h t Research Faci Iity Ed wards, Gal i forn ia 93523-5000 DYNAMIC STABILlTY AND HANDLING QUALITIES TESTS ON A HIGHLY AUGMENTED, STATICAILY UNSTABLE AIRPLANE Joseph Gera* and John T. Bosworth* NASA Ames Research Center Dryden Flight Research Facility Edwards, California output vector Y output sequence for open-loop roll Initial envelope clearance and subsequent flight YR testing of a new, fufly augmented airplane with an ex- frequency response tremely high degree of static instability can place un- usual demands on the flight test approach. Previous output sequence f o r open-loop yaw YY flight test experience with these kinds of airplanes is frequencyresponse very limited or nonexistent. The safe and efficient flight testing may be further complicated by a multi- U control vector plicity of control effectors that may be present on this class of airplanes. This paper describes some novel flight test and analysis techniques in the flight dynam- ics and handling qualities area. These techniques were The X-29A airplane, which began the initial utilized during the initial flight envelope clearance of flight tests in late 1984, is a fascinating example of a the X-29A airplane and were largely responsible for the statically unstable, highly augmented, multisurface completion of the flight controls clearance program airplane. Its evolution, design, development, and without any incidents or significant delays.

initial flight test results are documented in Refs. 1 to 11. This paper describes those flight test techniques Nomenclature that were used for the fmt time at NASA Ames Re- search Center, Dryden Flight Research Facility (Ames- matrices defining the linearized A, B, C, D Dryden), in the flight dynamics and flying qualities mathematical model of the test area and contributed considerably to the safe and effi- airplSme of the airplane.

cient flight testing ARI aih-to-ncdda intacoMect gain The most significant of these techniques, from the point of view of ensuring that adequate levels of FFT fast Fourier transform dynamic stability existed at each test point, is the near- real-time computation of the open-loop frequency re- RAI rudder-to-aileron interconnect gain sponse of the X-29A in the pitch axis from pilot-gen- erated frequency sweeps. The frequency sweeps had to X state vector be performed with all the feedback loops left intact be- cause of the e x m m e static instability of the unaug- time derivative of state vector X m n t e d airframe. The open-loop frequency response yielded the actual gain and phase margins immediately, input sequence for open-loop roll allowing a quick comparison with precomputed XR frequency response stability margins stored for each test point. Any un- expected nonlinearities, such as position or rate or time delays associated with input sequence for open-loop yaw saturation of actuators XY I data conversion between the analog and digital ele- frequency response ments o f the airplane, were reflected in reduced stability margins. Gain scheduling errors due to unmodeled air data characteristics also became obvious long before m any indications were given by conventional flight test *Aerospace Engineer. Member A I M .

techniques. The utilization of the open-loop frequency response turned out to be a highly successful endeavor The abstract of this paper was submitted by the authors in other respects as well: It resulted in a roughly 30- to the Society of Flight Test Engineers and was accepted percent reduction in the time allotted for initial enve- by the Society for presentation at its 18th Annual Sym- lope clearance.

posium, Amsterdam, Sept. 28 - Oct. 2, 1987.

Although not related directly to safety-of-flight gain and phase margins, respectively. As the final issues, the closed-loop frequency responses of the pitch control system design evolved, however, it was found and roll axes were also computed by the same trans- that these requirements were not met in the longitudi- form algorithms. This computation, followed by nal axis when the analysis included higher order dy- finding the lower order equivalent system dynamics, namics. Since the requirements are usually applied to allowed a rigorous application of current military han- airplanes in series production, they were relaxed for rhe dling qualities specifications to the X-29A aircraft dur- X-29A airplane, an experimental aircraft built for ing the early phases of the flight test program. flight research. The relaxed requirements are shown on a typical open-loop frequency response plot for the Recent improvements in the computational ca- pitch axis in Fig. 2. This plot is obtained from the pabilities at Ames-Dryden allow many routine data linear transfer function model of the augmented air- processing tasks to be performed in real time because plane.

of the utilization of extremely fast parallel processing of the data. One of the first utilizations of this capa- After encouraging results from simulation and bility is the comparison of X-29A flight data with the postflight analysis of the data from the initial flights, output of the linearized simulation in real time. the procedure for measuring frequency responses, shown in Fig. 3, evolved and was used with consider- Test Obctives able success throughout the envelope clearance pro- gram. The procedure involves the computation of the open-loop frequency response while all feedback loops The main objective in the flight controls disci- remain intact. The computation relies on a fast Fouri- pline was the demonstration of the design dynamic er transform (FFT) algorithm, which is executed in a stability levels during 1-g flight in each of the control high-speed parallel processor on the ground using system modes that could be selected easily by the pi- telemetered data from the test aircraft as input. As lot. In addition to the normal digital mode, the pilot- shown in Fig. 3, the input and output sequences to selectable modes include a digital and an analog rever- the FFT, designated as X and Y, respectively, are the sion mode. The verification of the design margins error and feedback signals in the pitch axis control during I-g trimmed flight was felt to be sufficient to loop. Excitation of the loop is provided either by the extrapolate to higher load factors by using either ana- pilot through the command shaping path or by an up- lytical or simulator results.

linked signal that is summed with the pilot command.

Although the latter approach results in more precise Since the determination of stability levels re- control of the excitation signal, it was found in prac- veals little about how the airplane flies as far as the tice that the pilot-generated frequency sweeps were en- pilot is concerned, a parallel test objective was to es- tirely satisfactory. The use of the uplinked signal be- tabiish the handling qualities of the X-29A airplane came necessary for computing the frequency response with the assumption that the requirements for high- of the roll and yaw axes whenever roll-to-yaw or yaw- maneuverability airplanes apply.

to-roll interconnects are employed in the control sys- tem. Figure 4 is a schematic of such a system, which Flight Test A m is similar to the X-29A lateral-directional stability augmentation system. It can be seen from the figure Flight testing of the X-29A airplane was differ- that the total error and feedback signal cannot be used ent from routine evaluation of the airplane in flight in either axis to define the open-loop frequency re- since many new technologies were incorporated into sponse through a Fourier transform since the pilot- the design. From the flight controls point of view the generated frequency sweep excites the control system of these new technologies were the most significant through more than a single location. Although several approximately 35-percent static instability of the air- possibilities exist for obtaining the open-loop roll and frame, the digital fly-by-wire primary flight control yaw axis frequency responses, the approach used for the system, and three kinds of pitch control surfaces: ca- X-29A testing involved an uplinked frequency sweep nards, symmetric flaps, and strake flaps. The general summed into the aileron and rudder signal directly arrangement of these surfaces is shown in Fig. 1. The actuator commands. This procedure amounted to approach to the initial flight tests and envelope clear- mathematically breaking either the roll or yaw axis ance was influenced to a significant extent by the new feedback loop at the actuator while keeping the other technologies incorporated in the flight control system.

loop closed. It is important to keep in mind, however, that no feedback loop was opened physically, thus Originally, the design criterion for dynamic sta- flight safety was not compromised by the frequency bility waq the usual requirement of 6 dB and 45" of response tests.

In addition to monitoring dynamic stability in quiet in flight. Transitions among the flight control the frequency domain in near real time, the availability system modes and between the ground and air were of the linearized mathematical model of the test air- smooth.

plane at each test condition and the high-speed data processing capability on the ground ma& it possible to compare the response of the airplane with that of the The computation of the open-loop frequency re- !icexized simu!atien ! c Identical pi!ot inputs in real sponses turned out io be a surprisingly uoubie-free time. This procedure is shown conceptually in Fig. 5.

operation. For the longitudinal axis, the computation In practice the procedure involved the computation of was performed as soon as enough data were accumulat- the linear equatons of motion at each test condition ed for the FFT algorithm. For the particular FFT al- immediately before flight from an all-FORTRAN, gorithm in question, this was the case after accumulat- nonlinear simulation of the airplane. To facilitate the ing 2048 data points. This required 52 sec of trimmed real-time solution of the linear differential equations, flight during which the pilot performed not only a the state transition matrix was also computed for the pitch stick frequency sweep but also a series of longi- sampling interval of the flight control computers, so tudinal pulses and doublets. The execution of the that during flight only the solution of the difference FFT algorithm required a negligible amount of com- equations was required. The comparison is made be- puter time, and a video display of the frequency re- tween the linear and measured time histories of the sponse and the associated stability margins was pro- principal motion variables, such as pitch rate, angle of duced in less than 3 sec. Figure 6 shows a typical attack, and normal acceleration. The initial or aim pitch axis open-loop frequency response plot obtained values are subtracted from subsequent flight measure- during flight. Also shown is the frequency response ments in order to make the flight data directly compa- predicted by linear analysis at the same flight condi- rable to the linear solutions.

tion. A remarkably close fit between the flight data and the prediction may be noted in the rigid-body fre- The open-loop frequency domain analysis was quency range. Generally, this was the case below and also extended to include the computation of the closed- above transonic Mach numbers, indicating where the loop frequency response of the airplane between mathematical modeling of the airplane was most suc- longitudinal stick and pitch rate, and between lateral cessful. It should be noted that the flight data were stick and roll rate. In contrast with the open-loop fre- consistent and repeatable everywhere in the flight quency response and the time history comparison, the envelope. In fact, the frequency response determined closed-loop frequency responses were computed post- from flight data was of sufficiently high quality that it flight. The objective of this work was to obtain a was possible t o make changes in the pitch axis control quantitative measure of handling qualities. The closed- system loop gain based solely on the frequency re- loop frequency responses were approximated in the fre- sponse results. An example of this is shown in Fig.

quency range of 0.3 to 10 rad/sec by a lower order 7. The initial determination of the open-loop frequen- system that also contained a pure time delay or trans- cy response clearly shows that the value of the loop port lag term. The approximation yielded the equiva- gain is too high by approximately 2.5 dB, resulting in lent modal response characteristics and the associated inadequate stability m a r g i n s . Reducing the loop gain time delays. The lower order system characteristics by this amount by simply changing the flight control could then be compared with the requirements for high- system software restored the stability margins to maneuverability airplanes. Data from this comparison nearly optimal values. This gain change was the only and the pilot ratings and associated comments during major control law change that affected stability, and it standard handling qualities tasks revealed whether the was accomplished during scheduled airplane mainte- requirements were applicable to airplanes with the un- nance without any delay in the envelope clearance usual characteristics of the X-29A.

program. With this change in place, the longitudinal dynamic stability exceeds the minimum margin requirements throughout the flight envelope. This example demonstrates the utility of the on-line frequency domain analysis of flight data; namely, the The envelope clearance of the X-29A airplane direct information on the overall system stability and in all selectable flight control sys- was accomplished the ability to make control system adjustments with- tem modes without any unusual occurrences such as out the precise knowledge of conventional stability and control surface oscillations, limit cycles, or unfore- control derivatives. In previous flight test programs, seen interactions between the flight control system and of similar adjustments required a considerable amount the structure. The airplane appeared to the pilot to be time for postflight data reduction, analysis, and gain well damped in all axes, and the control surfaces were correction.

Since it is the pitch axis in which the X-29A that the amplitudes, frequencies, and damping levels m airplane exhibits highly relaxed static stability charac- close enough to predictions that flight safety is not teristics, efforts to obtain stability margins during compromised.

flight tests concentrated on the pitch axis. Attempts are being made to perform similar computations for the roll and yaw axes. Although no flight results have Closed-loop frequency response characteristics been obtained to date, simulator results are encourag- were obtained for both the longitudinal and the lateral- ing. Figures 8 and 9 show the comparison of linear directional axes. For the longitudinal axis, the fre- predictions with nonlinear simulator results. As can quency response was computed between the pitch stick be seen from these figures, the augmented X-29A air- displacement and pitch rate; for the lateral-directional plane exhibits very generous stability margins in the axes, frequency response was computed between the lateral-directional axes; these margins are reasonably roll stick displacement and roll rate. The pitch and roll close to the predicted values obtained from linear frequency responses were then fitted in the mean square analysis.

sense with a pure time delay term in conjunction with second- and fmt-order transfer functions, respectively.

Figures 12 and 13 are examples of closed-loop frequency responses along with the results of the fit- Similar data were obtained throughout ting procedure.

To enhance flight safety during the initial enve- the flight envelope of the test airplane. According to lope clearance, monitoring the stability margins was the data, the airplane appeared to the pilot as a well- augmented by comparing time histories obtained dur- damped system in all axes.

ing stick and rudder pulses and doublets in flight with time histories generated by the linearized simulation of the test airplane in response to identical pilot inputs. In the pitch axis at some flight conditions the equivalent short-period damping ratios were above the These inputs were obtained from the telemetry data and maximum recommended values for high-maneuver- were used in real time to generate time histones that ability airplanes. At the highest dynamic pressures at were paired with the corresponding flight data and dis- which the airplane has been tested to date, the equiva- played in the identical coordinate systems. A typical lent short-period frequencies were below the recom- comparison plot is shown in Fig. 10 for pitch axis mended values. The equivalent time delay in the pitch variables. For the pitch axis, due to the extreme in- stability of the unaugmented airframe, the linear math- axis at all flight conditions was found to be slightly above the recommended values, being mostly in the ematical model included not only the rigid airframe 110- to 140-msec range.

equations of motion but also the entire flight control system including such details as sensor dynamics, For the roll axis, the equivalent system results transport lag, notch filters, and antialiasing filters. For indicate that the roll mode time constant is in the the longitudinal normal mode, this resulted in a 48th- order linear system. The processing of this sytem in neighborhood of 0.3 sec throughout the flight enve- lope, well within the range of recommended values.

real time was well within the capability of the array As in the pitch axis, the equivalent time delay is processor that was utilized for the ground computation.

slightly longer than the recommended 100 msec or The use of linear simulation for comparison with less, being in the 120- to 150-msec range.

flight data is not without certain advantages over a complete nonlinear simulation. These advantages in- A limited amount of testing was devoted to clude the increased speed of computation necessary for evaluating the handling qualities of the baseline flight real-time comparison and the ease of detecting unex- control system during air-to-air tracking. In this con- pected nonlinearities by the test personnel.

text the t e r m baseline means that no modifications or tuning of the control system had been made for the For the lateral-directional axes, the comparison specific purpose of improving the handling qualities of required considerably less computation. Since the test the test airplane. The tasks used for the evaluation are airplane does not have highly unstable modes in its the simulated lateral-directional characteristics, there was no need to shown in Fig. 14. In each task except terrain following, the lead T-38 airplane was perform- model the entire flight control system. It was suffi- cient to consider the rigid airplane lateral-directional ing turn reversals either at the request of the X-29A equations of motion. excited directly by the aerody- pilot or randomly while gradually increasing the load namic surface positions, which were available from factor up to 3 g. The simulated terrain following was a pure pitch axis task in which the lead airplane per- telemetry data. A typical comparison plot is shown in formed mild pushover-pullup sequences at load factors Fig. 1 1 . Although in the fine details of the motion varying between 0.5 and 2.5 g. Since this task proved the Comparison is not as close as might be expected, it to be the most difficult for the X-29A airplane, the yields adequate information to ascertain in real time task was also performed by each pilot in another T-38 airplane. Figure 15 summarizes the pilot ratings. The the data allowed the verification of the linear results show that the airplane has satisfactory charac- mathematical model of the test airplane and were used teristics for the tasks, with some minor deficiencies for redesigning the pitch axis loop gain at transonic, that warrant some flight control system improvements. low-altitude flight conditions.

Before discussing the deficiencies, it should be noted that the airplane handled remarkably well considering The availability of a linear mathematical mod- the number of new technologies incorporated in its el of the test airplane d b w e d the real-time computa- design and the fact that no control law changes had tion of predicted time histories using the pilot inputs been made for improving the handling characteristics.

from telemetry. These time histories were compared I t is worth noting that one pilot found the air-to-air with flight data, also in real time. The comparison not tracking characteristics to be excellent.

only allowed an immediate assessment of frequencies and damping levels at new test points but also gave a clear indication of any nonlinear behavior of the air- Pilot comments indicated that they would have plane, which could result from rate or position satura- preferred - slightly faster initial pitch response, tion of any component of the flight control system.

- better stick geometry since pitch stick travel is too large in comparison with lateral stick Postflight closed-loop frequency response data travel, and were obtained from pilot-generated frequency sweeps.

- lower lateral sensitivity for small stick in- These were fitted numerically with a lower order puts, faster roll rate for large stick inputs.

equivalent system, which yielded the equivalent time &lay and modal response characteristics. The results In general, it was found that the current handling indicate that the current handling qualities requirements qualities requirements are applicable to the X-29A air- for high-maneuverability airplanes are generally be required t o show plane, although more testing will applicable t o the X-29A airplane.

whether the equivalent time delays indicated by the analysis can be better reconciled with pilot comments.

References lKrone, N.J., Jr., "Forward Swept Wing Flight Demonstrator," AIAA-80- 1882, Aug. 1980.

The initial envelope clearance of a statically unstable, highly augmented airplane required a signifi- 2Spacht, G., "The Forward Swept Wing: a Unique cant amount of on-line data processing. Examples Design Challenge," AIAA-80-1885, Aug. 1980.

from the X-29A flight tests illustrate that the open- loop frequency response of an airplane with highly re- 3Whitaker, A., and Chin, J., "X-29 Digital Flight laxed static stability can be successfully computed on Control System Design," AGARD-CP-384, Active the ground from telemetry data. The data were obtained

Control Systems - Review, Evaluation, and Projec-

while all feedback loops remained intact, so the process tions, Oct. 1984.

did not compromise flight safety. The required compu- tation and graphical display of the results, which in- 41shmael, Stephen D., and Wierzbanowski, Ted, "X- cluded the gain and phase margins, were performed in SETP 29th Sympo- 29 Initial Flight Test Results," less than 3 sec. In the pitch axis where the flight sium Proceedings, 1985, pp. 95-113.

control system is essentially of a single-input, single- output type, the frequency sweep required to excite the 5Zislin, A., Laurie, E., Wilkinson, K., and Gold- system was performed manually by the pilot. In the stein, R., "X-29A Aeroservoelastic Analysis and lateral-directional axes where the stability augmenta- Ground Test Validation Procedures," AIAA-85-3091, tion is accomplished by a simple example of a multi- Oct. 1985.

input, multioutput system, the required frequency sweep will be uplinked from the ground directly to the aileron and rudder actuation system.

%efic, Walter J., and Cutler, William, "X-29A Advanced Technology Demonstrator Program The on-line procedure that utilizes a fast Fourier Overview," AIAA-86-9727, Apt. 1986.

transform algorithm yielded open-loop frequency re- sponse data that were consistent and repeatable throughout the flight envelope of the X-29A test air- 7Smith, Rogers E., and Schroeder, Kurt C., "Flight plane. The data were used as the principal means of Testing the X-29," SETP 30th Symposium Proceed- monitoring the level of pitch axis dynamic stability ings, 1986, pp. 116-134.

throughout the envelope clearance flights. In addition, parison of X-29A Flight Data and Simulation Data," kiera, Joseph. "Dynamics and Controls Flight Test- AIAA-87-0344, Jan. 1987.

ing the X-29A Airplane," NASA TM-86803. 1986.

l2 Military Specification - Flight Control Systems - oBosworth, J.T., and West, J.C., "Real-Time Open- Design, Installation and Test of Piloted Aircraft, Gen- Loop Frequency Response Analysis of Flight Test eral Specification for - MIL-F-9490D. June 1975.

Data," AIAA-86-9738, Apr. 1986.

(Supersedes MIL-F-9490C. Mar. 1964.)

losmith, R.E.. and Sarrafian, S.K., "Effects of

l 3 Military Specification - Flying Qualities of Piloted

Time Delay on Flying Qualities: An Update," AIAA- Airplanes. MIL-F-8785C, Nov. 1980. (Supersedes 86-2202, Aug. 1986.

MIL-F-8785B, Aug. 1969.)

Bauer, Jeffrey E., Crawford, David B., Gera, Joseph, and Andrisani, Dominick, "Real-Time Com- 27 11 2.44 in - a f t 1 in

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2. Government Accession No. 3. Recipient's Catalog No.

1 Report No.

N A S A TM-88297 4 Title and Subtitle 5. R q m i Date D Y N A M I C STABILITY AND HANDLING QUALITIES TESTS A u g u s t 1 9 8 7 O N A H I G H L Y AUGMENTED, STATICALLY UNSTABLE 6. Performing Organization Code R [RPLANE 8. Performing Organization Report No.

7 Authorb) .Joseph Gera and John T. Bosworth H-1422 10. Work Unit No.

RTOP 533-02-81 9 Performing Organization Name and Address N A S A Ames R e s e a r c h C e n t e r 11. Contract or Grant No.

Dryden F l i g h t Research F a c i l i t y P.O. Box 273 Edwards, CA 93523-5000 13. Type of Report and Period Covered T e c h n i c a l Memorandum 2. Sponsoring Agency Name and Address N a t i o n a l A e r o n a u t i c s a n d S p a c e A d m i n i s t r a t i o n 14. Sponsoring Agmcy Coda DC 20546 W a s h i n g t o n , I 5 Supplementary Notes P r e p a r e d a s AIAA-87-2258-CP f o r p r e s e n t a t i o n a t the A I M G u i d a n c e , N a v i g a t i o n a n d C o n t r o l C o n f e r e n c e , Monterey, C a l i f o r n i a , Aug. 1 7 - 1 9 , 1987, a n d f o r p r e s e n t a t i o n a t t h e SFTE 1 8 t h Annual Symposium, S e p t . 28 - O c t . 2, 1987, Amsterdam.

- 6 Abstract I n i t i a l e n v e l o p e c l e a r a n c e a n d s u b s e q u e n t f l i g h t t e s t i n g o f a new, f u l l y augmented a i r p l a n e w i t h a n e x t r e m e l y high d e g r e e o f s t a t i c i n s t a b i l i t y c a n p l a c e u n u s u a l demands o n the f l i g h t test a p p r o a c h .

P r e v i o u s f l i g h t test e x p e r i e n c e w i t h t h e s e k i n d s o f a i r p l a n e s i s v e r y l i m i t e d or n o n e x i s t e n t . The s a f e a n d e f f i c i e n t f l i g h t t e s t i n g may b e f u r t h e r c o m p l i c a t e d by a m u l t i p l i c i t y o f c o n t r o l e f f e c t o r s t h a t may be p r e s e n t o n t h i s class of a i r p l a n e s . This paper d e s c r i b e s some novel f l i g h t test and a n a l y s i s t e c h n i q u e s i n t h e f l i g h t d y n a m i c s a n d h a n d l i n g q u a l i t i e s area. These t e c h n i q u e s w e r e u t i l i z e d d u r i n g t h e i n i t i a l f l i g h t e n v e l o p e c l e a r a n c e o f the X-29A a i r p l a n e a n d were l a r g e l y r e s p o n s i b l e f o r t h e c o m p l e t i o n of the f l i g h t c o n t r o l s clear- a n c e program w i t h o u t a n y i n c i d e n t s or s i g n i f i c a n t d e l a y s .

7 K e y Words (Suggested by Authorls)) 18. Distribution Statement H a n d l i n g q u a l i t i e s U n c l a s s i f i e d - U n l i m i t e d Lower o r d e r e q u i v a l e n t s y s t e m s R e l a x e d s t a t i c s t a b i l i t y S t a b i l i t y m a r g i n s X-29A S u b j e c t c a t e g o r y 08 22. Rice' 21. NO. of pages 19. Security Classif. (of this report) 20. Security Classif. (of this page) U n c l a s s i f i e d U n c l a s s i f i e d 1 4 A02 *For s a l e by t h e Nationdl Technical Information S e r v i c e , S p r i n g f i e l d , V i r g i n i d 22161.