Ramon Lopez/WASHINGTON DC
A NASA-led research team has taken what it calls the "first step" towards achieving full control of an aircraft using propulsion alone for all phases of flight and not just emergency situations.
The landmark was achieved during Phase III of the F-15 ACTIVE (Advanced Control Technology for Integrated Vehicles) programme which involves Boeing, the US Air Force and Pratt & Whitney.
During this latest phase, due to have ended around 15 December, the aircraft's two pitch yaw balance beam, thrust vectoring nozzles were integrated with the testbed aircraft's fly-by-wire flight control system (FCS) to achieve "inter-loop" thrust vectoring control.
The initial phase of 14 flights was completed using the reversionary mode of the FCS "-so if any problems cropped up, or we exceeded the limits of the envelope, it would automatically turn itself off", says NASA ACTIVE programme chief project engineer Gerard Schkolnik.
The integrated system "performed flawlessly" in all tested areas of the envelope. This included operations at 20,000ft (6,100m) and 0.6 Mach, 37,000ft and 0.75 Mach, angle-of-attack limits of 30° at 20,000ft and 30,000ft and a top speed of 1.2 Mach at 30,000ft. "We did more than double what we set out to do," says Schkolnik.
"Some of the most exciting stuff was achieved when we performed vectoring only tasks," he adds. The ACTIVE F-15 is fitted with two FCS gain sets that can be used to turn off all longitudinal inputs except for thrust vectoring, and all lateral inputs in the same way. "We turned off the longitudinal (pitch) and did advanced manoeuvering and tracking in vectoring mode only. The pilot gave it between one and two on the Cooper-Harper rating scale [one is best]," adds Schkolnik.
In standard formation-keeping flight and other tasks, all three project test pilots unanimously rated it with a one on the Cooper-Harper scale. "All three had never given any aircraft that rating before in their lives, so we are extremely pleased with the way it worked out."
Schkolnik describes ACTIVE's integrated control system as the "next logical step beyond PCA [propulsion controlled aircraft]", the get-you-home emergency flight control system developed by NASA in the wake of the Sioux City McDonnell Douglas DC-10 crash.
Further tests of the integrated system, this time in non-reversionary mode, are planned for later in 1999. "In that mode, we will fly beyond the limits of the baseline controller, for example, doing take-offs and landings using vectoring only. We will also fly high to unlimited angles-of-attack and to speeds of Mach 2," he says.
Other landmarks achieved during the initial phase included the first application of the versatile control augmentation system (VCAS) on a manned aircraft. The VCAS was first tested on the McDonnell Douglas DC-X andX-36 projects, both of which were remotely controlled.
The test flights also marked the first use of pitch vectoring only control in an operational, rather than an emergency, environment. They also marked the first use of a sophisticated software writing tool, the Matrix X autocoder. This effectively analyses the aircraft's control system and converts it to Ada code. "Once the code is generated it is completely untouched," says Schkolnik.
Source: Flight International
