Effects of thrust line offset on neutral point determination in flight testing

Abstract

On aircraft with high or low thrust lines, conventional stability flight test methods result in shifted neutral points, which do not correspond to the actual pitch stability neutral points of the aircraft. Specifically, e.g., an elevator-position neutral point extrapolated from flight test data of an aircraft with a high thrust line, may be significantly behind the actual stick-fixed neutral point, causing a potential hazard. This implies that slopes of elevator position and stick force vs velocity diagrams do not necessarily mean that the aircraft is stable in pitch.