FLYING QUALITIES CRITERIA FOR PRECISE LANDING OF A STOL FIGHTER David J. Moorhouse* David B. Leggett** Kenneth A. Feeser*** AF Wright Research and Development Center Wright-Patterson AFB Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 Abstract The STOL and Maneuver Technology Demonstrator (S/MTD) program was defined to develop technologies that would enable a supersonic fighter to land on a 1500 x 50 ft usable strip of runway. A short ground roll is achieved using thrust reversing plus control laws to minimize touchdown dispersion. The final /h4TD configuration includes a speed hold system (i.e. augmented airspeed stability) and pitch rate command from the stick. Pilot ratings in a piloted simulation were Level 1 with the speed hold and Level 2 without it. An additional piloted simulation experiment was conducted to investigate the trade-off of pitch axis bandwidth and speed stability. The 03jectives of this paper are to, first, document the rationale and SIMTD approach to precision landing flying qualities and second, to discuss more general design criteria developed from the piloted simulation. Introduction The S/MTD program was structured to investigate, develop and validate through analysis, experiment and flight test, four specific technologies related to providing current and future high performance fighters with both STOL capability and enhanced combat mission performance. These technologies are: * * * * Two-dimensional thrust vectoring and reversing exhaust nozzle Integrated flight/propulsion control (IFPC)system Pilot Vehicle Interface (PVI) Roughlsoft field landing gear These technologies have been incorporated into a two-place F-15B together with all-moving canard surfaces (see Figure 1). The contract was written in terms of performance requirements, as much as possible, and those pertaining to landing are: * * * * Minimum operating strip of 1500 ft x 50 ft Crosswinds up to 30 kts (disturbances as defined by MIL-F-8785C, MODERATE levels) Wethcy runway surface conditions No ground-based landing guidance aids *Chief Engineer, S/MTD ADPO, Senior Member A1AA **Flying Qualities Engineer ***Simulation Test Manager, Member AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Integrated Fllghff ProDuislon Control- Crow Statlon Modified Landing Gear Figure 1. S/MTD Configuration Field length allowances for flare and touchdown dispersion can be on the order of 1,000 ft which is obviously not compatible with the above requirements. The specified field length is an implicit requirement that landing flying qualities in pitch be designed for precise touchdown and not allowed to "fall out". Similarly, the 50 ft width and 30 kt crosswind drive the lateral/directional flying qualities development. Another factor in the solution to these requirements is that the approach speed of 119 kts is not significantly less than the production F-15's. The short landing distance is achieved by reverse thrust rather than reduced touchdown speed. The ovemding requirement of the IFPC system was stated to be "capable of functionally integrating all aspects of flight, engine, and nozzle control including aerodynamic control surfaces, engine thrust, thrust vectoring, thrust reversing and differential efflux modulation, control and stability augmentation, high lift system, steering and braking". The intent was to convey the understanding that integration was an objective of the demonstration program, not just a means to .achieve mission requirements. The IEPC system was required to provide "good inner-loop stability and positive manual control in all axes of the air vehicle throughout its intended operating envelope both in flight and on the ground (satisfying the intent of ME-F-8785C)". This subjective requirement was intended to convey that we were seeking good flying qualities over the whole envelope guided more by the intent than the letter of the specification. This recognizes that, while the intent is to provide flying qualities clearly adequate for the mission, the letter of the specification is no guarantee. In addition, the requirement for "positive manual contrql" was intended to preclude consideration of automatic landing systems, for instance. Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 The objective of this paper is, first, to document the control law approach that allows touchdown in a "box" that is 60 ft long by 20 ft wide. The second objective is to document results of a piloted simulation experiment to develop pitch axis design criteria. At the start of the S/MTD program there were questions as to whether Navy experience could be used. The Navy has stringent requirements for touchdown accuracy in the case of landings on aircraft carriers. Four arresting wires are spaced within a distance of 150 ft, so that touchdown must be made to this accuracy or a go-around has to be executed. The requirements are met with carrier-based guidance, and either a manual or coupled approach using a flight control system known as an Approach Power Compensator System (APCS). Reference 1 analyzes and documents development problems with APCS: "Trial and error is the essence of the design procedure now employed in APCS development. While such a procedure is usually to be avoided and certainly not condoned, it has proven necessary and reasonably effective under the circumstance". Each new aircraft is the subject of intense development to meet the operational requirements. The current APCS implementations are angle-of-attack based which implies coupled airspeed and flight path responses. Quoting from Reference 1: "The basic manual control technique, involving operation of both stick and throttle, emphasizes the necessity of using thrust plus attitude as the combined means of maintaining reference airspeed and angle of attack on the glide slope. The (old) notion of using throttle to control altitude and nose attitude or stick input to control airspeed is pointed out as not being wholly valid in all situations along the approach". Conversely, in discussing requirements for a coupled approach, "He must depend on the APCS to provide the proper thrust inputs; and quite often this means he must change to a more suitable piloting technique to obtain reasonably acceptable APCS action". Thus, Navy pilots typically must learn (and train) to fly two different landing techniques, manual and coupled, and possibly different from one aircraft to another. The purpose of this section is not to criticize the Navy system, the Air Force typically puts little effort into landing flying qualities, but rather to provide background for the selection of the S / M T D design approach. There are also significant differences between the landing tasks of the two services. In normal operation, the carrier steams into the wind producing a turbulence environment which tends to be deterministic and repeatable, although definitely severe. The S/MTD configuration was designed to land in winds gusting to 30 knots from either direction, turbulence as a function of altitude and both magnitude and vector wind shears. Further, in a carrier landing the pilot either catches the wire or goes around. Conversely, the S/MTD has to touch down with heading under control to remain within the 50 ft width during the rollout in 30 kt crosswinds, on wet or dry surfaces and using reverse thrust. Lastly, low pilot workload was required with only conventional (Air Force) pilot techniques. The S/MTD landing requirements are believed to be unique and not achievable by any other aircraft. They led to the development of a special SLAND control mode. The Landing Control Laws An overall requirement of the S/MTD development was to integrate all the control effectors to satisfy the requirements with conventional piloting techniques. The short landing distances are facilitated by providing maximum reverse thrust immediately after touching down. To achieve this, the final approach is made with the engine spooled up to 100% RPM and the exhaust passing through vanes which are controllable from 45 degrees aft to 45 degrees forward of normal. These fast-acting vanes also provide for high-bandwidth control of airspeed because there is no influence of engine spool-up time. Design of the SLAND longitudinal control laws (pitch and thrust axes) was accomplished using multivariable control techniques. The complete design requirements are presented in Figure 2, showing the required form of the pitch and airspeed responses. There are also requirements to decouple the two axes, so that there is neither airspeed response to a stick input nor pitch rate response to throttle input. The final form of the control laws is given in Figure 3. Two features are to be noted -- decoupling is achieved by sending both pitch and thrust commands to the upper and lower reverser vanes, and the form of the response in each axis is defined by a command prefilter. Now, in terms of pilot action, the feedback of airspeed to vane angle holds airspeed constant until the throttle is moved. The pilot retains control because throttle movement commands a new airspeed and consistent flightpath change. The stick commands pitch rate directly but, with speed held constant, it is effectively a flightpath rate command. On approach, the pilot sets airspeed with throttle and, once the correct airspeed is acquired, flightpath is controlled solely with stick input. The lateral/directional control laws are conventional with the addition of direct sideforce control to satisfy the crosswind requirement. This is achieved with differential canard plus rudder deflections commanded by the rudder pedals during the approach and by lateral stick after touchdown. This integration of direct sideforce is totally natural in terms of pilot technique, giving the appearance of a crosswind half the actual magnitude. In the piloted simulations, the lateral touchdown requirement was easier to meet than the longitudinal (pitch) requirement. The development effort (and this paper), therefore, concentrated on the pitch and thrust axes. The final S/MTD configuration does meet the requirements and is rated Level 1 in the piloted simulation. Existing Criteria The S/MTD contract was written to the "intent" of MIL-F-8785C in general, but a change was directed for landing characteristics. Precise landing was designated a Category A tracking task rather than a Category C Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 Figure 2 . SLAND Mode Design Requirements Figure 3 . SLAND Mode A r c h i t e c t u r e Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 terminal area task. The requirement and the effect of the change are shown in Figure 4. The minimum allowable short period frequency is increased, which will tend to increase pitch attitude bandwidth, all other things being equal. Explicit requirements for bandwidth form an alternate criterion suggested in MIL-STD-1797 (which has replaced MIL-F-8785C as the Air Force flying qualities specification), as shown in Figure 5. It is worth noting that the difference between the two categories is far more pronounced in Figure 5 than in Figure 4. The various requirements for precision landing flying qualities were discussed in Reference 2 before the SIMTD contract award. A minimum pitch attitude bandwidth of 3.5 radsec was suggested because the proposed MIL-Standard did not differentiate between "conventional" and STOL or precision landings. Figure 4. Modified MIL-F-8785C Requirements Designing to a given bandwidth poses a problem of exactly how to increase the value of a deficient response. In designing the fix to an up-and-away tracking problem during the S/MTD development (Reference 3), both the numerator time constant and short period frequency were increased equally to increase pitch bandwidth without introducing other problems. Conversely, the required change in Figure 4 would imply increasing short period frequency while keeping a fixed (natural airframe) value of numerator time constant. This raises the question of whether the optimum increase in bandwidth depends on the relation between numerator time constant and short period frequency. Another question was raised by the final configuration of the SLAND mode. The SIMTD pitch attitude bandwidth is approximately 2.6 radsec with a phase delay of 0.075 seconds. The Category A requirements of Figure 5 indicate that Level 1 cannot be achieved because of the magnitude of the phase delay. The Category C requirements suggest a bandwidth between 2.5 and 4.2 rad/sec is required for Level 1. The S/MTD simulation pilot ratings were Level 1 with the speed hold and Level 2 without it. A piloted Figure 5. Bandwidth Requirements in MIL-STD-1797 simulation experiment to address the above questions was therefore set-up in the Flight Dynamics Laboratory. Simulation Experiment Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 Simulator Descri~tion The study was conducted in the Large Amplitude Multi-mode Aerospace Research Simulator (LAMARS) at the Flight Dynamics Laboratory, Wright Patterson Air Force Base. The LAMARS consists of a single-seat cockpit installed in a 20 ft diameter dome attached to the end of a 30 ft beam. The combined beamldome movements result in a five degree-of-freedom motion system which can generate angular velocities of up to 60 degrees per second, linear velocities up to 13 feet per second, and instantaneous accelerations of up to 3 g's. The LAMARS cockpit represented to the extent possible the forward crew station of the S/MTD aircraft (Figure 6). An F-16 LANTIRN Wide Field of View (WF,OV) Head-Up Display (HUD) and two 6 inch square Multi-Purpose Displays (MPD's) were the pilot's primary flying instruments. The S/MTD center stick and rudder characteristics were modeled by a McFadden three axis feel system. Toe brakes with the standard F-15 force-feel characteristics and S/MTD throttle quadrant with a spring loaded thrust reversing region were also provided. Task Descri~tion The experiment consisted of an approach task to a STOL landing on a 50 x 1500 ft Minimum Operating S e p . The simulation was initialized at 150 kts calibrated airspeed (KCAS), 1000 ft above ground level (AGL), approximately two nautical miles from touchdown, with flaps and gear down in the SLAND control mode. The task was for the pilot to follow the command steering guidance on the HUD (Figure 7) and achieve a landing speed of 119 KCAS to the touchdown. Atmospheric disturbances consisting of light turbulence and a head-to-tail wind shear were given to keep the pilot in the loop at all times during the task. Also, the command steering cues were initially offset 200 ft from the touchdown area both in the longitudinal and lateral direction. At approximately 200 ft AGL, these cues were then corrected to the touchdown area on the landing strip. This offset provided the pilot with a gross acquistion task which was a valuable aid in his evaluation of the various test conditions. The pilot gave Cooper-Harper (CH) ratings only for the approach task which was from initialization to approximately 50 ft AGL. The CH ratings were based on adequate and desired criteria that had been determined from earlier evaluations. Adequate performance was defined as keeping the velocity vector "boxed" by the command steering bars (i.e., keeping the flight path marker within the length of the steering bars though not necessarily on them) and hold the airspeed within 10 kts of the 119 kt landing speed. Desired performance was defined as keeping the velocity vector within half the length of the command steering bars and hold the airspeed within 3 kts of the 119 kt landing speed. Test Mamx At the appropriate value of phase delay, Reference 4 indicates that pitch attitude bandwidth for a landing task should be between 2.5 and 4.5 rad/sec (see Figure 5). Neal-Smith (Reference 5) indicates the Level 1 range to be 1.6 to 3.1 rad/sec. The S/MTD pitch attitude response to stick input was nominally designed for a bandwidth of 2.75 rad/sec, i.e. apparently Level 1. Because of the mixed results noted previously, an experimental text matrix was set up to investigate bandwidths from 1.5 to 4.5 rad/sec. These changes were achieved by varying appropriate parameters in the stick command prefilter (Figure 3) which defines the response characteristics. The bandwidth changes were produced two different ways consistent with the previous discussion, by varying short period frequency, o only (the A configurations) and also by varying bc% as and the numerator time constant, L,, (the B config&tions) as shown in Figure 8. Figure 6. S/MTD Cockpit Displays on LAMARS The influence of speed stability was investigated by changing the gain on airspeed feedback to the reverser vane angle. With the gain at its nominal value ("with speed hold") the speed response would be independent of pitch inputs, as in the S/MTD. With this gain zeroed out ("without speed hold") speed response would be more like that of a normal F-15, which is stable, but allows the speed and angle of attack to change with pitch changes. The complete experimental mamx was - Calibrated Alrspeed 16- 21 -16 21 za Aircraft Waterline Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 Barornetrlc Altlfude Angle-of-Attack Thrust Reversing Vane Advisory IFPC Mod8 Autobrake Advisory Mlnlmurn Operable Strip 'E Bracket Radar Altitude Vertical Velocity Velocity Vector Autl S t e e r l n ~ Flare Cue 1 Figure 7. SLAND Mode HUD Display With SDeed Hold Pilot ratings as a function of bandwidth are shown in Figure 9 for the configurations with speed hold. The height of the bars indicates the range of ratings received for each bandwidth. The horizontal bars show the average ratings. The numbers in parentheses at the top of the chart indicate the number of ratings given for each bandwidth. Figure 8. Test Configurations to fly the landing task at each value of both series of pitch bandwidths both with and without speed hold. Simulation Results A Confinurations vs. B Confinurations It is difficult to draw any conclusions when only one pilot has flown the cases where we changed bandwidth by changing both L, and wsp (the B configurations). However, comparison of his results with the results from changing only w indicate the same comments as those given by the othg pilots for each bandwidth. All of his ratings for the B configurations fell within the ranges of those given for the A configurations and they followed the same trends. At this point, over this range of bandwidths (1.5 to 4.5 rad/sec), we have seen no difference between changing bandwidth by changing only w and changing bandwidth by changing both L, andPws Because of the similarity between A configuratiogi and B configurations,we have combined the results for both cases in the discussion that follows. Comments for the 1.5 rad/sec case describe it as sluggish and slow to respond and the stick forces felt heavy. One pilot said "it felt more like a bomber than a fighter". The pilots felt like they had to overdrive it to achieve desired performance and, consequently, there was a tendency to overshoot. All of the pilots felt that they could achieve desired performance but it took moderate workload to compensate for the slow response. Pilot ratings were borderline Level 1/Level2, mostly Level 2. Comments for the 2.0 radlsec case were mixed. One of the pilots had comments similar to those for the 1.5 rad/sec case. The other pilots felt that it was quite good, though sometimes they said it was a little sluggish, but not too bad. The pilot who did not like it gave CH 4's because of the sluggish response. The other pilots' ratings were CH 2's and 3's. For them the turbulence and t!e slightly sluggish response, when they did perceive it, were minor annoyances. All of the pilots agreed that they could achieve desired performance, the difference of opinion was how much workload was involved. There was no such difference of opinion for the next two cases. AU of the pilots liked both the 2.6 and the 3.0 rad/sec cases and the comments and ratings were very similar for both. Speed of response, precision, and stick forces were all satisfactory. The pilots felt that they could put the flight path marker exactly where they wanted to and hold it, but occasionally they sensed a slight sensitivity when trying to hold it on. The Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 1 I 1 1 1 I I I 1.5 2.0 2.6 3.0 3.5 4.0 4.5 BANDWlDTH (radlsec) Figure 9. Pilot Ratings With Speed Hold sensitivity was not regarded as annoying, they mentioned it only as an observation. Workload was a factor only because of the turbulence, which forced them to stay in the loop. Pilot ratings for both of these cases were all Level 1. Comments for the 3.5 and 4.0 rad/sec cases were also very similar. Response was described as quick and crisp, but there was a slight tendency for pilot-induced oscillations (PIO). For the most part the pilots felt that the PI0 tendency was no more than mildly unpleasant. They could track within desired criteria, they just had a little more trouble holding the flight path marker precisely on target. Ratings were slightly more diverse, ranging from CH 2 to 4. The 3.5 rad/sec case received only one CH 4, all of the other ratings were Level 1. The 4.0 radlsec case received all Level 1 ratings. In the 4.5 rad/sec case the PI0 tendency became more pronounced. Though the pilots still described it as a "slight" PI0 tendency, it graduated from mildly unpleasant to annoying. For the most part the pilots were still able to achieve desired performance, but they had to compensate more to avoid overcorrecting. In some cases they felt that they could do no better than adequate. One pilot felt that the PI0 tendency was insignificant and rated this case CH 3. The other pilot's found it more annoying and their ratings ranged from CH 3 to 5, mostly Level 2. From these results it would seem that for a phase delay of .075 the Level 1 boundaries for bandwidth would go from about 2 rad/sec to around 4 rad/sec, which corresponds very closely with the criteria in MIL-STD-1797. Below 2 rad/sec the aircraft response is too sluggish. Above 4 rad/sec the response is too sensitive, at least for the stick gradient we used. Without S ~ e e dHold Unfortunately, at this point we only have one pilot's evaluation of the configurations without speed hold. But his comments and ratings show significantly different trends from those configurations with speed hold. The pilot ratings for the cases without speed hold are shown in Figure 10. Comments and ratings for 1.5, 2.0, and 2.5 rad/sec were all similar. The pilot said that the pitch response was too slow and also somewhat unpredictable. He described it as a tendency to "balloon". He said this tendency made it very difficult to put the flight path marker precisely where he wanted to. It felt like a low-frequency PIO. He also mentioned that he had to devote considerably more attention to speed control and that this aggravated his problems in pitch and flight path control. It was impossible to achieve desired performance and it was difficult to keep it in the adequate range in both speed and flight path control. He rated the 1.5 and 2.0 rad/sec cases CH 6, and the 2.5 rad/sec case CH 5. He had similar comments for the 3.0 rad/sec case except that it was much more predictable. It was still a little sluggish and, consequently, there was a tendency to overshoot when he med to force it, but he did not get into the low-frequency PIO. He still had to devote considerable attention to airspeed, and this kept his workload high, but he was able to achieve desired performance. He rated this case a CH 4. Comments and ratings for the 3.5, 4.0, and 4.5 Downloaded by UNIVERSITY OF NEW SOUTH WALES (UNSW) on October 27, 2017 | http://arc.aiaa.org | DOI: 10.2514/6.1989-3390 I I 1.5 2.0 I I t 2.6 3.0 3.5 BANDWIDTH (radlsec) 1 1 4.0 4.5 Figure 10. Pilot Ratings Without Speed Hold rad/sec cases were all similar to each other as well. For these cases, speed of response and predictability were good. There was no tendency to overshoot or PIO. Speed control was easier and, as a result, interfered less with his attention to flight path control. The pilot achieved desired performance with minimal workload. He rated all three of these cases CH 3. The minor deficiency was the workload involved in controlling both speed and flight path in the turbulence. He commented that he had definitely preferred the speed hold cases, because it left him free to concentrate solely on flight path control. With only one pilot evaluation so far it would be a mistake to try to draw any conclusions about boundaries for the case without speed hold, but we do feel that these results indicate that the pilot prefers a higher bandwidth without speed hold than he does with speed hold. His comments also indicate a definite preference for speed hold, regardless of bandwidth. Conclusions Control laws have been designed to enable an F-15B to achieve very precise touchdowns in stringent environmental conditions. These control laws feature augmented airspeed stability (speed hold) and decoupled airspeed and pitch rate responses. Piloted simulation verified the predicted Level 1 pilot ratings, but only with the speed hold feature. A piloted simulation was conducted to define the required pitch attitude bandwidth with and without speed hold. The Level 1 requirements on pitch attitude bandwidth in MIL-STD-1797 are verified with the speed hold, i.e. with a high level of airspeed stability and with minimal coupling between the speed and pitch responses. Preliminary results indicate that higher values of pitch bandwidth are required if there is coupling between the axes or airspeed stability is low. References 1. Craig, S. J.; Ringland, R. F.; and Ashkenas, I. L., "An Analysis of Navy Approach Power Compensator Problems and Requirements," Systems Technology, Inc., ST1 Technical Report No. 197-1, March 1971. 2. Moorhouse, D. J., "STOL Flying Qualities and the Impact of Control Integration," NAECON 83, May 1983. 3. Bland, M. P.; Citurs, K. D.; Shirk, F. J.; and Moorhouse, D. J., "Alternative Design Guidelines for Pitch Tracking," AIAA Paper 87-2289, August 1987. MIL-STD-1797(USAF), "Flying 4. Piloted Vehicles' ', March 1987. Qualities of 5. Neal, T. P., and R. E. Smith, "An In-flight Investigation to Develop Control System Design Criteria for Fighter Airplanes", AFFDL-TR-70-74, December 1970.