GUY NORRIS / LOS ANGELES
USAF focuses on structural solution ahead of start of operational test and evaluation
The US Air Force is considering several options to solve fin buffeting problems that have the potential to exceed design margins on the Lockheed Martin/Boeing F/A-22 Raptor stealth fighter.
Buffet problems were first noticed in July 1999, but are only now fully understood. They are caused by interference from a highly energetic vortex generated at the inboard junction of the inlet and fuselage. The effect is strengthened by another vortex at wing root leading-edge which guides the first vortex onto the tail where it "releases a tremendous amount of energy in turbulent flow", Maj Colin Miller, F/A-22 test pilot, told the Society of Experimental Test Pilots symposium in Los Angeles last week.
To gather more data on the problem the flight test team has developed a new 'Zorch' manoeuvre to gather up to 45s of test data per run. To conduct each run, the pilot climbs up to 15,250m (50,000ft) in full afterburner and enters a split-S to rapidly set flight path angle. The pilot then rolls to set the lift vector and angle-of-attack while beginning a constant roll to achieve the desired flight path angle.
Data shows that the problem occurs in crucial areas of the F/A-22's flight envelope above 18¼ angle-of-attack; below 6,100m and between Mach 0.5 and 0.6; and between 1,500m and 7,000m at Mach 0.6 to 0.9. At lower speed the aircraft suffers buffet in the 30Hz region with large vertical and horizontal stabiliser hinge moments, high actuator loads, and strain on the fin rear spar. At higher speed, buffeting in the 40 to 60Hz range puts strain on the fin rear spar.
Several aerodynamic options have been considered, including the addition of fences on top of the inlets to disrupt the vortex, but this could affect radar cross-section and require structural strengthening. Other potential solutions include changes in leading-edge flap scheduling, rudder toe-in or rudder toe-out modifications, and side weapon bay door opening during fin buffet, but all have drawbacks.
The USAF is pursuing two structural solutions, focused on adding a rudder 'blow back' actuator to act as a dampener and reduce loads, and strengthening the rudder with titanium (Flight International, 24-30 September).
Lockheed Martin has completed modifications of Raptor 08 in readiness to begin USAF operational test and evaluation next year.Source: Flight International
