We know about multirole fighters, but this seems ridiculous. So how about an airplane that just thinks it can change itself in flight, and that makes its pilot think that too? The Variable Stability In-Flight Simulator Test Aircraft can replicate the handling qualities of all the aircraft mentioned above, plus many others, for a virtually unlimited number of types. Software makes it possible to change from one to the other merely by pushing buttons.

For example, if a test pilot is working with control laws simulating the F-23 but wishes to compare them to something he remembers from flying the F-22, all he has to do is ask his backseater to tell the computer. Or he might say, "I once flew an airplane that did such and such. Can this machine do that?" The answer will almost always be yes.

The VISTA's true identity is a modified, Block 30 F-16D based on the airframe design of the Israel Air Force version, which incorporates a dorsal fairing running the length of the fuselage aft of the canopy and a heavyweight landing gear derived from the Block 40 F-16C/D. The fairing houses most of the variable-stability equipment and test instrumentation. The heavyweight gear will permit simulation of aircraft with higher landing sink rates than a standard F-16.

The aircraft is minus the gun system and some of the F-16's standard military electronics. It retains the advanced APG-68 radar and incorporates the Block 40 version's digital flight control computer and supporting avionics suite.

The cockpit arrangement is unique among F-16s. The front seat is the simulation cockpit, to be occupied by the pilot flying the test. The rear cockpit is the primary flight control station. Its control priority will allow the backseat driver to take command and return the aircraft to normal F-16 behavior if the front-seat "experimental" pilot encounters problems.

Pilots of existing variable-stability aircraft call it "punching off," as opposed to punching out, when the rear-seat test conductor - also called the safety pilot - takes control.

The rear pilot also controls the variable-stability equipment and initiates its engagement or software selection during flight. The front cockpit is equipped with both a standard sidestick and a removable, variable-feel center stick, for use depending on the type of aircraft being simulated. The front cockpit is equipped with a programmable HUD and other variable displays to further increase the realism of the simulation.

The variable-stability equipment suite is a complex mix of digital and analog electronics. "It's like a sophisticated autopilot, because it takes control of the airplane like an autopilot would, except that the pilot still makes manual inputs," explained Dr. Philip A. Reynolds, program manager for the variable-stability system that goes into the VISTA/ F-16. Reynolds works at Arvin Calspan Corporation in Buffalo, New York, where the system is being developed.

The heart of the variable-stability system is three high-speed, flight-rated, 32-bit Hawk 32 computers manufactured by Rolm. The key to the aircraft's unusual capability is a process called response-feedback stability augmentation, which changes the static and dynamic flight characteristics.

"All of this gear will measure how an F-16 would fly and force it to fly like the airplane being simulated," Reynolds said, making a complex chain of interfaces and safety interlocks sound simple.

Expressing the VISTA capabilities another way, he compared the VISTA/F-16 to "a ground simulator in the sky that produces all the required motions with none of the washout that you get in a motion-based ground simulator." (Motion washout or dampened response is notoriously unpopular with fighter pilots, which is one of the reasons General Dynamics chose visual-scene-only simulators for its engineering flight simulator laboratory.)

The VISTA/F-16's brain will be able to hold 200 different conventional aircraft types and ten experimental or "test" airplanes, as well as the X-29 or X-30. The VISTA platform will be used in a variety of test scenarios: training for pilots at the Air Force and Navy test pilot schools, evaluation of control laws, preflight testing of developmental aircraft, correction of faults found in flight tests, and others.

General Dynamics is the prime and integrating contractor for the VISTA/F-16. We are currently completing assembly of the basic airframe and working on the necessary modifications. An engineering ground simulator is also being built. Most of the variable-stability equipment has been delivered and integrated in the simulator "hot bench" and cockpits in the division's Flight Simulation Facility, where software development and preflight tests will he completed.

Rollout of the aircraft is expected during the second quarter of 1991, with first flight planned in late summer.

Calspan currently operates three variable-stability aircraft - a company-owned Lear 24, the NT-33A, and the Convair NC-131H. The VISTA/F-16 will replace the Air Force NT-33A, which has been in service since 1957 and has one computer with about one-tenth the capacity of the VISTA's. The one-of-a-kind NT-33A, a Lockheed T-33 derivative, was used recently to develop and evaluate the fix for flight control problems. The problems caused the crash of the first Swedish Gripen prototype in early 1989. The NT-33A was also used in YF-22 development.

The NC-131H Total In-Flight Simulator (TIFS) is a converted turboprop airplane that has been used to simulate a variety of aircraft for more than twenty years. Major programs that employed the TIFS's services recently included preflight evaluations of the YF-23, B-2, and X-29.

The Air Force and Navy test pilot schools use the variable-stability airplanes extensively in familiarizing students with the handling qualities of different aircraft types. All test pilot school graduates in the last three decades have flown the aircraft.

Pilot Joe Sweeney flew variable-stability aircraft at the US Navy Test Pilot School and for Calspan before joining General Dynamics. He said variable-stability aircraft are unequaled for teaching stability and control laws, developing flight control laws, and evaluating handling qualities.

"You can literally give a pilot or an engineer a physical look at an enormous amount of textbook-type material in a two-hour light," he said. "Even more importantly, you can expose both student and project test pilots to potentially uncontrollable flying characteristics in a completely safe, exploratory manner.

"VISTA promises to greatly expand the performance envelope that the NT-33 presently offers for this instruction, research, and development," Sweeney said. "It will allow designers to be that much more sure of a new aircraft's handling qualities before the prototype or developmental version ever lifts off the ground."