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Aviation History
1994
1994 - 0027.PDF
HIGH AoA FLIGHT bility and agility at low-to-moderate airspeeds — less than 300kt (550/km/h) — tactical utility for close-in combat and operability of the turbofan engine at high angle of attack (AoA). Phase I of the programme covered airworthiness, engine operability, airstarts and departure recoveries. A second phase covered envelope expansion and soon astonished even the most optimistic of the team members. In the first ten weeks, the demonstrator progressed from first flight at Lockheed Fort Worth, Texas, on 2 July, 1993, to an unlimited AoA and restric tion-free flight envelope. Some "peculiar aerodynamic instability at high AoA [between 35° and 50°]" was countered initially with changes to the software in the DFCS. Two nose-mounted chines were added at the end of December for further stability by reducing asymmet ric forebody vortex generation. Towards the end of December, the NF-16D had been flown for more than 120h across 90 flights. A flight-test enve lope, ranging from a safety ceiling of 20,000ft (6,000m) up to 40,000ft, and from 0-435kt, had been cleared without restriction. A maximum stabilised AoA of 83° had been obtained and maximum manoeuvring AoAs of +/-1800 had been demonstrated during unprecedented agile flight manoeuvres. Thrust vectoring had worked well throughout, "with no problems with the engine", according to Maj Mike Gerzanics of the 416th Test Squadron at Edwards AFB, California. The engine did not suffer any afterburner blowouts, or "pop stalls", and the afterburner was successfully re-lit at 70°, zero airspeed and 50°/s yaw rates. More than 20 pilots had flown the air craft, including three guest pilots from other high-AOA research programmes, such as the McDonnell Douglas F-18 HARV (high-alpha research vehicle) and the X-31 (see panel). Phase three moved to the vital opera tional assessment, in which military- evaluation flights were conducted with and against aggressor pilots from the 422nd Test and Evaluation squadron at Nellis AFB, Nevada. The squadron's Capt Jim Henderson says: "In a one-versus-two engagement, this jet allows you to actually be offensive instead of defensive like a normal jet would be. It gives you an opportunity to quickly kill one bandit and then engage the other in a one-versus-one. Once again, the gun was by far easiest to employ." The squadron assessors did warn that the pilot had to be careful "...not to use the high AoA manoeuvring capability too soon, too often, in case it gets you into low-speed flight too soon and suddenly you're a sitting target". This is certainly not the intended thrust of the project. CI • Flight-testing of NASA's McDonnell Douglas F-18 High Angle-of-attack Re search Vehicle (HARV) was to have resumed on 21 December after installa tion of an inlet-mounted pressure rake. The star-shaped rake will be used to measure intake flow-distortion during the next phase of high AoA flight-testing with the F-18, which has thrust-vectoring and is capable of being manoeuvred at up to 75° AoA. The six-month "Phase 2B" flight-test effort will complete high-AoA flying- qualities research and gather data on tail buffet, says NASA F-18 HARV pro gramme manager Denis Bassette. Phase 2B introduces a new version of the F-18's research quadruplex digital flight-control system (RFCS), which, with programmable Ada software, is used above 15,000ft (4,600m) to operate the F-18's aerodynamic control-surfaces and thrust-vector exhaust paddles. The next phase of flight testing, sched uled to begin in October, will introduce vortex flow-control nose strakes. These are conformal, 1.2m-long x 0.15m-wide devices, hinged along their lower edges, which are rotated outwards and down wards by modified F-18 aileron actuators to control the formation of nose vortices. Vortex flow-control is expected to provide good yaw control at high AoA, when the rudders are masked. This phase of flight testing will also evaluate another version of the RFCS, Bassette says. This version will use "inversion" control laws to eliminate non-linearities in aircraft handling automatically, he says. A further version of the RFCS, offering variable aircraft stability, will be tested by McDonnell Douglas in an F-18 simulator, Bassette says. • NASA is flight-testing a helmet- mounted display (HMD) on the Rock well/Deutsche Aerospace X-31 Enhanced Fighter Manoeuvrability (EFM) technol ogy demonstrator. After initial flights in December, the GEC-Marconi Avionics visor-projected HMD will be used in the tactical-utility evaluation now under way to quantify the benefits of thrust- vectoring in air combat. The evaluation involves close-in air-to- air engagements between the X-31 — which is equipped with thrust-vectoring paddles — and a standard McDonnell Douglas F-18. The second of the two X-31 As has been equipped with the HMD, says NASA programme director Garry Trippensee. The monocular HMD presents basic flight information, such as airspeed, alti tude, angle of attack (AoA) and g, he says, and allows the X-31 pilot to keep his head "out of the cockpit" during high-AoA exercises. The X-31 is capable of being manoeuvred at up to 70° AoA. Initial HMD flights "...worked out very well", says Trippensee. The X-31 EFM programme involves NASA, the US Advanced Research Pro jects Agency, the US Air Force and Navy and the German defence ministry. The tactical-utility phase is being conducted at NASA Dryden Flight Research Center, Edwards AFB, California, where a minia ture air-combatmanoeuvring range has been created, says Trippensee. Both the X-31s and the F-18s are instrumented and tracked by radar dur ing the unscripted engagements. Each sortie involves four to five engagements, each lasting 1.5min or less. Various data are collected, but the principal aim of the evaluation is to see whether the thrust- vectoring X-31 can win more air-to-air engagements than can standard F-18s. The tactical-utility evaluation is to be completed by February 1994, when NASA plans to begin "pseudo-tailless" flight tests to determine whether thrust- vectoring will allow reduced tail size. • Funding shortages have stalled US Air Force plans to flight-test forebody-vortex control technology on a Lockheed F-16. The flight tests would follow analytical and windtunnel work to be completed in early 1994 by California-based Eidetics International. Forebody-vortex manipulation can provide yaw control at high AoA equal to, or greater than, that provided by the rudder at zero AoA, says Eidetics re searcher Brian Kramer. "If the vortices are affected near the nose, very little force is needed," he says. Yaw control is provided by altering the vortices to create a sideforce on the nose. Eidetics is windtunnel-testing a variety of forebody-vortex control techniques on the F-16, including blowing jets and slots and rotating nose-boom strakes. The company is also windtunnel-testing simi lar devices on the F-18. Forebody-vortex control was flight- tested on the Grumman X-29 forward- swept wing research aircraft in the form of nose-mounted pneumatic jets. Later this year, NASA plans to flight-test an other technique, moveable nose strakes can be deployed into the airstream to deflect the vortices, on its F-18 high-AoA research vehicle. Kramer says that rotating nose-boom strakes were selected instead of blowing for flight-testing on the F-16. These consist of two small vanes mounted at the tip of the nose and able to be rotated left or right to control forebody-vortex creation. The angle between the vanes remains fixed as they rotate through up to 360°. A similar technique has proved effective on the F-18 windtunnel model. FLIGHT INTERNATIONAL 5-11 January, 1994 25
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