Effect of large amplitude pitch-up motions on un-commanded roll behavior of low swept wings-slender body model

Abstract

Desire for super maneuverability and agility for modern air vehicles has resulted in high angles of attack flights. Different dynamic instability phenomenon may arise at high angles of attack. Wing rock, which is self excited limit cycle oscillations in roll mainly, is amongst one of these. Experiments have been carried out at sub-critical Reynolds number to investigate un-commanded free-to-roll (FTR) motions induced by forebody complex flow on a 30° swept back non-slender wings-slender body-model. Experimental investigations of FTR motions are firstly conducted for static angles of attack (0° to 90°) to understand the basic flow and aerodynamics mechanisms followed by the effects of large amplitude pitch-up motions at variable rates on these un-commanded roll motions. It has been observed that asymmetric forebody vortices (AFV) dominate and control the roll motion of the model for angles of attack > 26°. For the dynamic (pitch-up) case it has been observed that roll amplitude decreases and lag increases with increase in pitch-up rate. Increase in windward and leeward surface pressures with increase in pitch-up rate is observed. This increase in surface pressures is asymmetric on the wings leeward side in AFV region, angle of attack > 26°. Decrease in roll amplitude, increase in damping or restoring moment provided by the lower surface and increase in normal force and side force coefficients are attributed to these changes in the surface pressures due to the pitch-up effect. Roll behavior for non-dimensional pitch-up rate ≥ 1×10−2 is characterized by sinusoidal type curve, absence of roll divergence and no significant roll contribution from the wings flow.