Significance of the dihedral effect in rapid fuselage-reorientation maneuvers

Abstract

A study is presented about the role the dihedral effect (rolling moment due to sideslip) can have in fuselagereorientation maneuvers that involve high angles of attack. A mathematical model for attitude maneuvers is developed, which accurately represents the High Angle-of-Attack Research Vehicle, including propulsive moments generated by thrust-vector ing. The fuselage-reorientation problem is posed as an unconstrained timeoptimal control problem, and numerical extremal trajectories are obtained. These trajectories are examined in order to determine if and when the dihedral effect contributes significantly to the maneuvers. Results indicate that for most reorientation maneuvers the dihedral effect is small, and that these minimum-time trajectories occur with small sideslip angles. ARIOUS technological advances have enabled the development of a new generation of combat aircraft, with expanded flight envelope and improved maneuvering capabilities (supermaneuverable aircraft 14). These aircraft will be more agile and have the ability to operate at extreme angles of attack. One of the enabling technologies is the concept of propulsive control-moments,5-6 generated by thrustvectoring (TV). At higher angles of attack, where the aerodynamic control surfaces are less effective, TV can be used for attitude control, while at lower angles of attack the propulsive control-moments can supplement the aerodynamic control surfaces. In order to gain a tactical advantage in a combat situation, it is desirable to perform the combat maneuvers in minimum time. Modern computers and advanced numerical techniques, along with the accurate aircraft aerodynamic models obtained from wind-tunnel measurements, facilitate numerical study of the problem of optimal tactical maneuvering. The authors have recently reported7 <s an analytical mathematical model for the High Angle-of-Attack Research Vehicle9 (HARV), developed for study of time-optimal fuselage-reorientation maneuvering problems. The model neglects the translational motion of the aircraft (therefore being valid only for rapid attitude maneuvers, during which the aircraft velocity-vector does not change significantly). Results obtained with this model can be used as a starting point for a 6-degree-of-fr eedom model and more complex tactical-maneuvering optimal control problems. In performing minimum-time reorientation maneuvers, one might expect that the use of sideslip to create rolling moment (dihedral effect) might be a useful means to reduce the maneuvering time. In this article we look at this possibility in detail, and show that for a large class of reorientation maneuvers of practical interest and tactical significance the di