The US Air Force Lockheed Martin/Boeing F-22 combined test force (CTF) at Edwards AFB, California, is investigating unexpected tail buffet and vibration met during test flights that could require the strengthening of future airframes. The problem was not predicted by pre-flight aerodynamic analysis.

Tests are being conducted on aircraft 4001, or Raptor 01, which has been fitted with tufts to help visualise the airflow patterns emanating from the engine inlets, chines and wing root before impinging on the twin-fin tail. Flight tests will involve wind-up turns and other manoeuvres at transonic speeds and angles of attack between 20° and 30°, with an emphasis on 26° where most of the severest buffeting is reported.

The CTF says it has resorted to sticking tufts to the F-22 after analysis using computational fluid dynamics (CFD) failed to predict the non-linear airflow patterns and resulting high loads and vibration around the vertical tails.

Flight test data will be fed back into the CFD models for further analysis to improve the design tool and to evaluate if any structural strengthening will be required for production F-22s.

Lockheed Martin/Boeing believes that, despite the buffet investigation, further structural changes are unlikely to be needed to aircraft beyond 4003, which are a stronger "build standard 2" configuration.

The flight tests are expected to be concluded next month, assuming the flow visualisation gives a clear picture of the strong vortex patterns that are believed to be hitting the fins at high angles of attack.

The buffet problem was revealed publicly for the first time during Congressional testimony earlier this year.

The trials come as the first avionics test airframe, 4004, is readied for flight at Lockheed Martin's F-22 assembly site in Georgia.

Source: Flight International