USAF Test Pilot School Research Articles

Modeling the Effects of Underwing Missile Canards on F-16 Limit Cycle Oscillations

This research evaluated various methods using ZONA’s Euler Unsteady Aerodynamic Solver against flight tests previously flown at USAF Test Pilot School, using underwing air-to-air missiles with removable canards, which allows the aerodynamics of the store configuration to be changed while maintaining identical mass properties. ZONA’s Euler Unsteady Aerodynamic Solver was employed with various linear flutter and nonlinear time-domain analyses. ZONA’s Euler Unsteady Aerodynamic Solver correctly predicted that removing the canards would delay limit cycle oscillation onset. However, all of the methods generally predicted onset earlier than flight test. Consistent with the results found by previous researchers, the nonlinear simulation still predicted divergent classical flutter rather than limit cycle oscillation until nonlinear structural damping was added and tuned to produce limit cycle oscillation. Without known structural damping characteristics, the nonlinear structural damping model needed to be arbitrarily tuned to match flight-test data. This research demonstrated a different tuning method from that had been used by previous researchers. Nonlinear structural damping plays an important role in limit cycle oscillation, but a better understanding of the actual nonlinear aerodynamic and structural effects is needed before reliable limit cycle oscillation predictions can be made for all configurations and throughout the flight envelope.

Controlled focused sonic booms from maneuvering aircraft

In April 1994, the USAF Armstrong Laboratory, in cooperation with USAF Test Pilot School, conducted an experimental study of controlled focus boom generated by supersonic maneuvers. The objective of this study was to collect focus and postfocus booms and to assess the ability of aircrews to control the placement of the focal region during basic maneuvers. Forty-nine supersonic passes were flown and included level linear acceleration, level turn, accelerating dives, and climbout-pushover maneuvers. These flights were flown under calm and turbulent atmospheric conditions. Turbulent conditions had a defocusing effect which caused distortions in the focus region and resulted in smaller maximum overpressures. Sonic booms were collected by up to 25 boom event analyzer recorders (BEARs) placed in a 13 000-ft linear array. The BEAR units were spaced 500–2000 ft apart with the denser spacing at the expected focal region. This spacing was chosen to evaluate the thickness of both the focal and postfocal regions. The target location varied from 2000–5000 from the uptrack end of the array. Of the 49 flights, a focus boom was placed within the array 37 times and within ±3000 feet of the target point 27 times, demonstrating the ability to place controlled focus booms.

USAF flight test investigation of focused sonic booms

In April 1994, the USAF Armstrong Laboratory in cooperation with USAF Test Pilot School conducted a measurement study of controlled focus booms generated by supersonic maneuvers. The objective of this study was to collect focus and post‐focus booms and to assess the ability of aircrews to control the placement of the focal region during basic maneuvers. Forty‐nine flights were performed and included level linear acceleration, level turn, accelerating dives, and climbout/pushover maneuvers. Sonic booms were collected by up to 25 boom event analyzer recorders (BEARs) placed in a 13 000‐ft linear array. The BEAR units were spaced 500 to 2000 ft apart with the denser spacing at the expected focal region. This spacing was chosen to evaluate the thickness of both the focal and post‐focal regions. Of the 49 flights, a focus boom was successfully placed within the array 35 times which demonstrated the ability to place controlled focus booms. This ability of the aircrew can be employed to avoid collateral damage to noise sensitive receptors. Along with capturing focus U waves, complex N–U signatures were recorded at distances away from the foci. Turbulent conditions had a defocusing effect, resulting in smaller maximum overpressures.