The Future of Variable-Stability Airplanes

The engineers atNASA, Calspan, DERA, the Canadian NRC, Princeton University, and other European and Asian laboratories who had so much to do with the development of variable-stability airplanes can point to impressive accomplishments using these devices. Variable-stability airplanes shed light on many critical issues, such as the role of roll-to-yaw ratios on required Dutch roll damping, permissible levels of spiral divergence, and the effect of longitudinal flying qualities on instrument landing system (ILS) landing approaches. Variable-stability airplanes have also provided a preliminary look at the flying qualities of radical new airplanes such as the Convair B-58 Hustler; the Rockwell X-15, XB-70, B-1, and Space Shuttle Orbiter; the Lockheed A-12 and F-117A; the Grumman X-29A; various lifting body projects; and the Anglo-French Concorde before those new airplanes flew.

The TIFS machine, based on a reengined Convair C-131B transport, has had a particularly productive career (Figure 3.12). Calspan engineers provided the TIFS with the ability to add aerodynamic forces and moments to all 6 degrees of freedom. Flight tests are carried out from an evaluation cockpit built into the airplane’s nose, while a safety crew controls the airplane from the normal cockpit. Some 30 research programs have been run on this airplane. The majority of them were general flying qualities research; ten programs were on specific airplanes. A T-33 variable-stability airplane also had a very productive career, with more than 8,000 flying hours to date. A new application of variable-stability airplanes has been reported from the DLR, in which the ATTAS in-flight simulator investigated manual flight control laws for a future 110-seat Airbus transport airplane.

In spite of this impressive record, there are reasons to look for limitations in the future use of variable-stability airplanes in the engineering development of new aircraft. A sig­nificant obstacle is the practical difficulty in updating and maintaining the vast computer data bases needed to represent the mathematical models of complex digital flight con­trol and display systems and nonlinear, multivariable aerodynamic data bases. Maintaining current data bases should be inherently easier for locally controlled ground-based simula­tors, as compared with variable-stability airplanes operated by another agency at a remote site.

Another limitation to the future use of variable-stability airplanes in the engineering development of specific airplanes has to do with the cockpit environment. Correctly detailed controls, displays, and window arrangements, important for a faithful stability and control simulation, may be difficult to provide on a general-purpose variable-stability airplane. Correct matching of accelerations felt by the pilot is also desirable. Although variable – stability airplanes do provide the pilot with both acceleration and visual cues, both cannot be represented exactly, along with airplane motions, unless the variable-stability machine flies at the same velocity as the airplane being simulated and unless the pilot is at the same distance from the airplane’s center of gravity in both cases.

Those conditions are rarely satisfied, except in some landing approach simulations. For example, the Princeton University VRA, flying at 105 knots, has been used to simulate the Space Shuttle Orbiter flying at a Mach number of 1.5. Pilot acceleration cues can be retained under a velocity mismatch of this kind by a transformation of variable-stability airplane outputs that amounts to using a much higher yaw rate (Stengel, 1979). Likewise, pilot location mismatch is conveniently corrected for by a transformation on the sideslip angle. If these transformations are applied to correct pilot acceleration cues, visual cues will be made incorrect. An alternative scheme to provide correct pilot acceleration cues relies on the direct side and normal force capabilities of advanced machines such as the TIFS.

In general, the cockpit environment of a new airplane can be represented fairly readily in a ground-based simulator. Correct visual cues can be provided as well, although there are often troubling lags in projection systems. The major loss in fidelity for ground simulators, as compared with variable-stability airplanes, comes from the compromises or actual losses in pilot motion cues. When these are provided by servo-driven cabs, accelerations must be washed out. That is, to avoid unreasonably large simulator cockpit cab motions, only acceleration onsets can be represented. Sustained accelerations must be tapered off smoothly and quickly in the ground-based systems, or they must be simulated by pressures applied to the pilot’s bodies with servo-controlled pressure suits. Belsley (1963) provided an early summary paper in this area. Later on, Ashkenas (1985) and Barnes (1988) reviewed the utility and fidelity of ground-based simulators in flying qualities work.

There is a debatable size problem involved with the use of variable-stability airplanes. W. H. Phillips points out that in Robert Gilruth’s original handling qualities studies, contrary to the expectationsof many people, pilotswere satisfied with much lower valuesof maximum rolling velocity on large airplanes than on small ones. This finding is reflected in the pb/2V criterion of acceptability, which allows half as much maximum rolling velocity when the wing span is doubled at the same airspeed.

Again, pilots of small airplanes choose lower control forces than do pilots of large air­planes. Phillips concludes that pilots adapt to airplanes of different sizes and that erroneous results may be obtained if this adaptable characteristic of the human pilot is not accounted for. This might be the case when a large airplane is simulated with a much smaller variable – stability airplane, or vice versa.

A counterargument is that two fundamental airplane dynamics properties affecting air­plane feel vary systematically with airplane size, giving the pilot a cue to the size of the airplane, even if all that the pilot sees of the airplane is the cockpit and the forward view out of the windshield. Short-period pitch natural frequency shows a systematic trend downward with increasing airplane weight and size. The roll time constant, the time required for an airplane to attain final rolling velocity after step aileron inputs, shows a systematic trend upward with increasing airplane size.

Thus, a small variable-stability airplane whose dynamics match those of a large airplane may well feel like the large one to the pilot. W. O. Breuhaus (1991) reports that this seems to be the case:

the pilot must be able to convince himself that he is flying the assigned mission in the airplane being simulated… one of the variable-stability B-26’s was used to simulate the roll characteristics of the much larger C-5A before the latter airplane was built. The results of those tests showed a less stringent roll requirement for the C-5A than was being specified for the airplane, and these results were verified when the C-5A flew.

The relative meritsof variable-stability airplanesascompared with ground-based simula­tors for representing airplane flying qualities are still being debated; each has its proponents. However, it is a fact that sophisticated ground-based simulators are now absolutely inte­gral to the development of new aircraft types, such as the Northrop B-2 and the Boeing 777. Typically, ground-based simulators handy to the engineering staff are in constant use during airplane design development. At the same time, variable-stability airplanes remain important tools for design validation and for the development of generalized flying qualities requirements.

The question of when variable-stability airplane simulation is really necessary is taken up by Gawron and Reynolds (1995). They provide a table of ten flight conditions that seem to require in-flight simulation, together with evidence for each condition. An example condition is a high gain task. Evidence for this is the space shuttle approach and landing and other instances such as YF-16 and YF-17 landings.

The Air Force operates the new VISTA/F-16D variable-stability airplane (Figure 3.13) and the Europeans are running impressive programs of their own. However, in-flight simulation was not considered for the Jaguar fly-by-wire, the EAP (Experimental Aircraft Programme), or for the Eurofighter. Shafer (1993) provides a history of variable-stability airplane operations at the NASA Dryden Flight Research Center, with an extensive bibliography.