Flight-Loads Effects on Horizontal Tail Free-Play-Induced Limit Cycle Oscillation

Abstract

The purpose of this paper is to study the impact of the flight-loads effects on the free-play-induced limit cycle oscillation using a direct simulation approach. This is achieved by computing the transient responses of an F-16 aeroelastic model with various free-play angles in its horizontal tails under various trim conditions, while subjected to external excitations due to gust or pilot input command. It is found that for the restrained structures, limit cycle oscillation always occurs, even with a very small free-play limit, as long as the horizontal tail is unloaded. However, once the horizontal tail is loaded at a trim condition, limit cycle oscillation disappears, even with a large free-play limit up to ±0.2 degrees. In contrast, for the unrestrained structures, even with large free-play limits up to ±0.1 degrees, limit cycle oscillation occurs only within a short period of time when the horizontal tail is unloaded. This is because for a free-free aircraft, when the horizontal tail is unloaded, the aircraft is untrimmed and then a transient maneuver will be developed and, consequently, the horizontal tail is rapidly loaded. In addition, it is unlikely that the F-16 can maintain an unloaded horizontal tail for a sufficiently long time until the limit cycle oscillation is fully developed. This suggests that the military specification for control surface free-play limit (±0.017 degrees) is probably too stringent for the F-16 and can be relaxed to reduce the manufacturing and maintenance costs.