Book Description

Since winged space vehicles perform unpowered hypersonic reentry flights at very high angle-of-attack avoiding excessive heat loads, the lateral/directional stability and controllability are the critical aerodynamic design subjects to accomplish desired trajectory and to meet cross range requirement. Over past years so-called dynamic directional stability parameter Cn(beta)dyn and lateral control divergence parameter LCDP (or AADP) have been proposed not only as preliminary but also practical aerodynamic design criteria. Although considerable discussions have been made on the lateral stability and controllability problems, these investigations are mainly based on six degree-of-freedom flight simulations using computers. There have been not enough interpretations on the lateral flight mechanism from the analytical point of view. This paper reviews the lateral stability and controllability criteria, and presents analytical considerations on the lateral control divergence mechanism using linear transfer function method. It is clarified that Cn(beta)dyn plays a role of necessary stability condition for the open-loop airframe dynamics. AADP is a necessary stability condition for the closed-loop system of bank angle control by aileron alone. If AADP is negative, the vehicle behave to have unfavorable adverse effectiveness of aileron control. This mechanism of the uncontrollability can be interpreted as follows. At high angle-of-attack, rolling motion induced by aileron control causes rapid increase of side-slip angle. If the vertical tail does not have enough restoring force about yawing axis to suppress the side-slip angle, the aileron control results in an adverse effectiveness due to the counter rolling moment caused by dihedral effect. Rudder coordination can make up for the insufficient tail restoring force. This necessary controllability condition is represented by LCDP. This paper shows that the zero loci of the multi-loop control transfer function indicate the stability of the bank angle control with rudder coordination and has equivalent meaning to that of LCDP. Further study yields an interesting result that an additional stability relation exists between Cn(beta)dyn and LCDP. This stability characteristics is examined on an actual fighter aircraft and the present analysis agrees well with the actual simulator test result. Based on the present study, a synthesis of simple lateral multi-loop controller is proposed for practical application of the reentry vehicles flying at very high angle-of-attack. Consequently flight simulations are performed in order to verify the lateral controller.