One of the delights of visiting Edwards Air Force Base, and the USAF Test Pilot School, was the opportunity to fly in a variety of unusual aircraft. One of these was the Avionics System Test Training Aircraft (ASTTA) operated by a specialized flight test support company, Arvin Calspan of Buffalo, New York. Basically the ASTTA was an in-flight simulator. Starting life as a piston-engine Convair airliner this beast had been upgraded to a Convair 580 and now had turboprop engines. However in now had extra side force controller fins mounted part-way out along the wings, and a prominent proboscis containing a fighter radar, and a fly-by-wire flight control system.
However the most radical modifications to the aircraft were only to be found on climbing aboard. In addition to the regular cockpit up front, the ASTTA was equipped with an extra cockpit inside the cabin. Here the students from the USAF Test Pilot School could familiarize themselves with the new systems such as infra-red targeting devices and various types of radar.
There was more. Back in the blacked-out cabin the student also had a sidestick controller similar to that used in the F-16. Using these controls, coupled to a sophisticated fly-by-wire control system through the ASTTAs computers, and with a head-up display, he could fly the big transport which would respond just like the normal fighter.
For my flight Calspan pilot John Ball, up front in the regular cockpit, took off in the normal way and set us up for cruise before engaging the fly-by-wire system and turning control of the machine over to the student in the back. Thereafter Ball acted as safety pilot, and monitored the panel of lights signifying the operation of the flight control system. Ball could override the system if things started getting out of hand.
We flew on a brilliant autumn day, operating against a T-38 jet which was scheduled to carry out a series of head-on passes against us so that the students in the cabin could in turn practice acquiring targets on the radar and IR scopes in the cabin. From the cockpit I could hear the reports of target range and distance coming from the cabin. As the range closed a tiny dot ahead of us would grow rapidly into the T-38 which would flash past us, then turn to set up another run.
With the air-to-air portion of the mission completed, we flew over to one of the ground ranges in the desert to assess problems of acquiring ground targets using the IR and radar systems. It was absorbing work, and the safety pilot and the relatively slow cruising speed of 180 knots gave more time for the students to appreciate what was happening throughout each run, and was also safer than if the students had been trying to fly an F-16 at 400 knots while assessing the system. By the end of the flight I was thoroughly convinced that this was a cost-effective way of training pilots to carry out systems assessment.
However Systems testing was only a part of the syllabus at the Test Pilot School. The classic test pilot role of stick and rudder work to assess aircraft handling was a different matter. Training test pilots in the assessment of aircraft handling qualities is a difficult task. Student Test Pilots need to be exposed to a variety of aircraft, but this only provides experience on aircraft which have already been through a flight test development program and had most of the bugs ironed out. The problems can be covered in the classroom, but teaching the pilot to recognize the handling deficiency in flight and then to deal with it in the appropriate manner is not simple. If the aircraft turns out to be unstable, it could be downright dangerous. Ground-based simulators can go part of the way to allow a test pilot to assess the behavior of an aircraft prior to flight, but the subtleties of the flight environment make it desirable to use an in-flight simulator to achieve a realistic cockpit environment.
In order to provide a safe demonstration of the various aspects of aircraft handling qualities in flight, Calspan had modified a Learjet 24 with a variable stability flight control system. This Learjet was operated under contract to the USAF Test Pilot School at Edwards Air Force Base and also flew for the Navy Test Pilot School at Patuxent River, Maryland.
Calspan and the Air Force invited me to fly the Learjet at Edwards AFB.
This workload, in addition to other Calspan flight commitments, kept the Calspan test pilots busy, with the three pilots rotating between the Learjet, ASTTA and a variable-stability
NT-33A. As a result they spent a lot of time on the road, completing a two-week stint at each location before moving to their next assignment. It took some time before our schedules were able to mesh. I took the opportunity to read the Pilot Manual for this very unique aircraft.
Under contract to the USAF and USN Test Pilot Schools, the Learjet is based at Edwards or Patuxent River to fly the student test pilots and Flight Test Engineers, and to demonstrate aircraft handling.
This Learjet 24, N101VS was converted into a variable stability aircraft by the Calspan Flight Research Department under joint funding from the USAF and Navy Test Pilot Schools. The evaluation pilot’s station at the right hand seat was fitted with a fly-by-wire flight control system which coupled on-board computers to electro-hydraulic servo-actuators on the control surfaces. Meanwhile the left seat safety pilot’s control system remained directly connected to the aircraft control surfaces and reflected the commanded surface movements during simulation.
The system could be manually disengaged by either pilot at any time, when the aircraft reverted to the basic Learjet handling for operation by the safety pilot. A safety-trip system was incorporated so that whenever pre-set limits were exceeded, the simulation system would trip out, with the aircraft reverting to the control of the safety pilot.
So here we had the ability to simulate hundreds of different aircraft, with their handling quirks and idiosyncracies, together with the ability to fly any future aircraft that could be dreamed up by an aircraft designer. And to do it in safety. In all there were 128 pre-programmed configurations readily accessed in flight together with another 128 variations that could be punched in during flight.
This Learjet had an awesome capability.
We briefed for the flight at the Test Pilot School at Edwards Air Force Base. Calspan pilot Jim Baker was a former Navy test pilot whose qualifications included a medical degree which led to him spending a tour at the Institute of Aviation Medicine at Farnborough. However, Baker said with a smile, flying was more fun than doctoring, so here he was flying the Learjet and the other Calspan research aircraft.
Our afternoon demonstration flight would be Baker’s third flight that day, TPS flights having started early in the morning to ensure the calmest air for test work. During our pre-flight walk-around the sleek red and white jet, Baker pointed out the two extra angle-of-attack sensors on the nose which characterizeds 101VS from the standard Lear 24. These sensors, together with a sideslip vane under the nose of the aircraft, fed the computers for the Variable Stability System (VSS) whose electronics filled the rear part of the cabin. Baker would be in the left hand seat and I would occupy the right hand seat as the evaluation pilot.
Once we were in our seats, with the cabin door left open for the moment to keep some air circulating in the ninety-degree desert heat, Baker pointed out the cockpit differences from a standard Learjet. The VSS was controlled by two panels, one being a gain-change panel and keyboard located on a console between the pilot’s seats, the other being on the left hand cockpit side wall. The main instrument panel additionally had meters displaying pitch, roll and yaw angles. Red sectors on the meters showed the limits at which the system would trip. If things got out of hand, either pilot could disconnect the system independently. The trip warnings and VSS reset buttons were in front of Jim Baker. On my side of the cockpit, the panel was that of a standard Learjet, with just the addition of a meter showing stick-force per g. While on his side of the cockpit, Baker retained the control yoke which operated the standard Lear cable-operated flight control system, I had a fighter-type stick operated by the VSS which was connected solely by electronics to the control surfaces.
The cabin door was then closed and we were ready to go. Once both engines were running, Jim Baker checked the VSS and the fly-by-wire(FBW) systems to ensure that control could be maintained from the right seat in case of safety pilot incapacitation. As always, the greatest danger was from birdstrike, the Lear being limited to 306 knots below 14,000 feet because of windshield bird strike considerations. Having had to avoid the local turkey vultures around Edwards on previous occasions , I practiced the actions required of me in the event of Baker being put out of action by a birdstrike. I could re-engage the system by simply hitting the FBW switch and then the three FEEL, PRESS and ENGAGE buttons on the instrument panel to regain control.
Having checked that the VSS was serviceable, Baker engaged nosewheel steering and got us moving. The nose dipped as he checked the brakes. With callsign COBRA 37 we were cleared to Runway 22. We taxied down the Edwards ramp past the T-38s, A-7s and F-4s of the TPS fleet and past the B-1s and their chase F-111s. Once lined up on Runway 22, Baker increased power, carried out a power check at 80 per cent and released the brakes. Acceleration was rapid and we rotated at 125 knots. Once established in the climb, Baker pulled the power back to ninety-five per cent and engaged the VSS.
We climbed out over Lake Isabella and got clearance to enter the military airspace known as Eddie 2, north-west of Edwards over the Sierra Nevada. We set up in the cruise at 240 knots and 16,000 feet. As we whistled on over the rugged mountains, I played the role of a student test pilot. Jim Baker started our demonstration of longitudinal handling with the aircraft set up with positive static stability, as in the standard Learjet. Initially we looked at the long-period oscillation or phugoid. This standard flight test maneuver was entered by pulling the nose up to twenty degrees, then releasing the stick, at the same time starting the stopwatch on the panel. The aircraft arced up into the sky, trading speed for height. As the Lear’s nose came down to the horizon, speed had dropped to 190 knots. Then the nose continued to drop and we headed down towards the mountains. In an act of faith we kept our hands on our knees and away from the controls.
After a few seconds the positive stability of the Learjet caused the aircraft to self-recover from the dive and the maneuver bottomed out at 14,800 feet. As we passed through 16,000 feet after having completed one full cycle of this oscillation, seventy seconds had elapsed. The subsequent oscillations, with no pilot inputs, diminished gradually in amplitude and the Learjet gradually regained level flight as the motion damped out. It was a graphic example of textbook behavior.
The short period mode, a potentially more risky maneuver, was next to be demonstrated. Jim Baker had briefed me that we would start by carrying out a frequency sweep in pitch, initially moving the stick slowly fore-and-aft and gradually speeding up the frequency. At the resonant frequency the aircraft would exhibit the maximum response. A sharp fore-and-aft input on the control stick would excite the Short Period Oscillation. Sure enough, when the stick was pulsed, the aircraft nose rapidly oscillated up and down twice, and then the motion damped out, confirming the positive damping of the basic Learjet.
O.K. so far, and next to see how this behavior translated into the operational world, in this case a ground attack maneuver, we would attempt to acquire and track a ground target. With a grin Baker pointed out “our Calspan five-cent bombsights”, tape crosses on our respective windshield panels. Picking a spot on the mountains below, Baker banked the Learjet past the vertical, pulling back on the yoke to roll into a typical ground attack maneuver, then demonstrating the technique of changing his aim point to different targets during the dive. It was apparent that the basic Learjet’s response was sluggish compared with a regular attack aircraft as he attempted to track four separate targets during the dive. When it was my turn, I appreciated the difficulty of shifting the target in the dive with this aircraft designed as a stable business jet.
We climbed back to level flight at 16,000 feet and Jim Baker reduced the damping ratio for me. The aircraft started to behave in a squirrelly fashion. Pitch disturbances after the fore-and aft stick pulse now took five oscillations to damp out. As we headed downhill for my attack maneuver, it was noticeably harder to track the target, with the nose overshooting in pitch each time. It felt like driving an old car with ineffective shock absorbers. During this oscillatory maneuver I hit the lower limit of .15g and the VSS system tripped out, illuminating the panel in a blaze of warning lights.
Once back at 16,000 feet and with the VSS once more engaged, Baker then reduced the damping to zero. The aircraft was still stable but with the lack of damping the handling was disastrous. Not only was it virtually impossible to aim the aircraft, but it was all too easy to excite a Pilot Induced Oscillation (PIO); I had a wholesome respect for PIOs, having seen film of an F-4 Phantom come apart during a high-speed pass when a PIO developed. But all was not lost. Even though the motion was violent, Baker demonstrated how the aircraft could be recovered from the PIO by catching the oscillation at the top or bottom of the maneuver.
Baker now increased the damping ratio. This changed the character of the motion completely, the heavily damped motion giving no overshoots but resulting in a sluggish feel to the aircraft.
“ Like trying to carry out ground attacks in a B-52” one Edwards test pilot, Colonel Bob Behler, subsequently described flying this configuration.
Having reached the northern end of our allotted airspace, we turned south-west of Owens Lake and headed south again over the rugged Sierra Nevada. The next phase of the assessment was to show the effect of varying the center of gravity of the aircraft. This was not done by shifting weight around, but done electronically by feeding in an angle of attack value from those extra sensors on the nose. With the pitch damping restored to the standard Lear value and the c.g now artificially shifted forward, the normal stick force gradient of 8lb/g had doubled to 16 lb/g. When I pulsed the stick fore and aft to excite the short period oscillation, the frequency of the motion increased and the damping went down, giving five oscillations of the nose before we were back in level flight. This felt pretty weird. Here we had drastically changed the damping of the motion simply by moving the c.g forward. Baker explained that the task of the test pilot at this point was to observe, not to try and analyze the motion which could have a number of parameters varying due to one specific change.
Baker then adjusted the VSS to artificially move the c.g rearwards to the aft limit. I tried flying this configuration. Stick forces were lighter, although the other flying qualities were basically unaffected. Moving the center of gravity further aft resulted in marked changes to the flight characteristics. As I pulled the stick back, tracking was still possible but there was no speed stability or speed cues. In this configuration, as I pulled back on the stick the nose just kept pitching up at a constant rate.
With the c.g further aft still, my aft deflection of the stick resulted in the Learjet pitching up to a constant 1.8g. The stick deflection was now translating into a pitch acceleration and the stick forces had decreased to zero. This felt most uncomfortable…like balancing on the head of a pin.
I suddenly remembered, in the first World War pilots flying the Sopwith Camel biplane described it in the same way. With an aft c.g and virtually no stability, it could turn like a flash in combat to overcome its enemies, but killed many of its own unwary pilots in doing so. Seventy years on, flying this time machine, I was learning what flying a Camel must have been like.
Flying cautiously, I determined that it was just possible to fly and maneuver the Lear in this condition, but it was ever-so-easy to get it into a PIO. As I deliberately initiated larger inputs to simulate a landing approach the aircraft started to oscillate in pitch. The motion became divergent, with the nose plunging below the horizon , then rearing up to the sky. I was too late in clamping the stick and the VSS tripped out at zero-g during the oscillation and I was jammed up against my seat harness, floating clear of the seat for a few seconds. Luckily Calspan does not carry paying passengers during these flights.
This was a challenge. Licking my lips, when the VSS had been reset I repeated the sequence and this time after I had let the PIO develop I caught it at the top of the oscillation, timing my correction at the top of our arc through the sky by applying steady back pressure each time the nose started to drop. This was successful in damping the motion and demonstrated the advantage of Baker being able to demonstrate and teach recovery from what would have been a dangerous situation if inadvertently encountered in test flying. Flushed with success I repeated the exercise and deliberately initiated a PIO, this time trying to catch it at the bottom of the oscillation. This proved more difficult. As the nose dropped, mountain peaks filled the windshield and again the VSS tripped before I could arrest the motion. I was doubtful that the aircraft could even be landed in this configuration.
Baker then returned the system to the basic Lear characteristics and then demonstrated the effect of varying the stick force gradient from the 8lb/g of the basic Lear, over a range between a bomber like 25lb/g(only pilots with Popeye-like forearms need apply) and a gradient of 2lb/g (almost like the aerobatic Pitts Special). Again I found tracking was difficult at the heavy end, when during maneuvers it was necessary to pull with a 50lb force with one hand, and uncomfortably responsive at the light end. It was a strange sensation one moment to be flying a lumbering heavy aircraft, then the next moment to be flying a sensitive aerobatic mount.
“Have you flown at F-16 yet?” Baker asked. I shook my head and admitted that opportunity had not yet come my way.
Baker punched in another combination of gains on his panel, explaining as he did so,” This next configuration gives the fixed stick that we flew on the early F-16s. It was sensitive to pressure only. See how pilot performance is affected, It’s very sensitive.”
That was an understatement. In this configuration my stick was locked and I found the Lear responded briskly to the lightest pressures on the stick. This was the way the flight control system on the F-16 was set up for initial flying. I had seen the film of the prototype getting into a roll-axis PIO during what was supposed to be a high – speed taxi run. After one horizontal tail had scraped the ground and the aircraft had departed off the side of the runway the pilot had chosen to pull it off the ground to resolve the situation, resulting in an inadvertent first flight. Sure enough as I cautiously tried my tracking maneuvers on a puffy cumulus cloud, this led to some bobbling in pitch and roll as I got used to this ultra-sensitive system.
The CALSPAN Learjet N101VS was the first to be fitted with the Variable Stability System that enabled it to reproduce the handling of a huge range of aircraft, past, present or future.
Baker punched in a small degree of motion to the stick. This greatly improved the response and was in fact the solution adopted on later F-16s. Then he added a few more changes to further increase the stick deflection. My comment now was that the handling felt like a sailplane, where the resulting control was loose, adequate for thermaling, but not good enough for a tracking task. Baker then wound up the stick friction electronically. I found this awkward for tracking. Baker pre-loaded the stick back to neutral with a simulated centering spring to oppose the friction. This improved tracking considerably.
We were now ready for some lateral-directional work. I slowed the Learjet to 170 knots for this because of sideslip limitations. We started with the baseline aircraft, which has positive dihedral. I pushed on the right rudder pedal. The nose swung to the right and the left wing rose because of the dihedral effect. Just a normal airplane. Positive dihedral was designed in and tended to level the wings if the aircraft was disturbed by a gust. Nothing unusual to report so far.
Baker punched a few more buttons to give me an aircraft with zero dihedral. Now when I pushed the rudder, the wings stayed level throughout the sideslip. This felt strange. I had flown aerobatic aircraft like this. More button pushing. Negative dihedral was now set up. As I pushed on the right rudder the left wing started to drop. This was a most uncomfortable feeling. I had never experienced this in flight before, having only seen reference to it in textbooks on stability and control. It was an undesirable and potentially unsafe phenomenon, and something to be avoided.
Baker reset the VSS to the baseline aircraft again. At his bidding I lined up my tape cross on a distant cloud, then initiated a dutch roll at 170 knots by pushing on the left rudder pedal, then the right. This started a classic dutch roll oscillation, with the tape cross moving in an ellipse against the cloud as the basic yawing motion was accompanied by a rolling motion as each wing dropped in turn. After a couple of oscillations the motion died away. I had seen this characteristic on tactical fighters. It looked conventional enough so far.
Jim Baker now started varying the parameters as he had done with the longitudinal motion, challenging me to identify what parameters he was varying. Correctly assessing the situation for the first couple of changes, I was caught out the third time. As I kicked the rudder to the left, then the right, the nose swung left, then to the right. But instead of the motion damping out, the nose wandered off to the left again, hesitated and then came back.
“What have I done?” asked Baker innocently. “Decreased the damping?” I said tentatively. Baker grinned. “Sure, the damping has changed. But you are now flying an aircraft with negative directional stability. The damping has decreased, but as a by-product of the instability. "
In my previous experience with tactical fighters, I had met up with this type of phenomenon, but only at supersonic speeds. This behavior meant that the vertical fin was not big enough to restrain the sideslip. Many years previously, Chuck Yeager had barely escaped with his life in the rocket-powered Bell X-1 when reduced directional stability at supersonic speeds had caused the aircraft to go out of control. Here it was, demonstrated at low speed in a normally stable jet. It was as if we had suddenly lost the vertical tail. We were still wallowing on through the sky in a persistent dutch roll, albeit a very untidy maneuver. There was little the pilot could do to retrieve this situation (Way back in 1985 when a JAL 747 had lost its vertical tail due to a structural failure it had continued flying for over half an hour, with video taken from the ground showing it wallowing in just this way. It finally crashed with the loss of all on board.)
“Try applying a bit of sideslip,” said Jim Baker. “We still have rudder control.”
I cautiously pressed on the right rudder pedal. The nose lurched to the right, with the sideslip needle on the panel oscillating between six and eight degrees. It was very uncomfortable . We seemed to be flying sideways and the lateral g-force was squashing me into the corner of my seat. An extreme example of what could be demonstrated.
With fuel now down to 2,000lb, we had completed our planned mission. I brought the Learjet back to Edwards, descending to follow an F-15E on an approach over Rogers dry lake. Jim Baker disengaged the VSS and took over for a landing on Runway 22 after just over two hours airborne. The wind had increased during our flight and was now gusting thirty five knots, with the wind buffeting our aircraft during the lengthy crosswind taxi back to the ramp. We were marshalled efficiently into our parking slot and Baker shut down the engines. As the jets whined down into silence, I reflected that this Learjet provided an efficient platform for assessing the handling qualities of a vast range of aircraft, even to the extent of safely demonstrating some characteristics which would be too dangerous to be demonstrated in a fixed-stability aircraft.
The capability was awesome, giving the pilot the ability to fly maybe twenty different aircraft within a single sortie. It was an invaluable tool for this task of teaching student test pilots. All aircraft designers should have access to such an aircraft before finalizing their designs.
Every pilot should have one.