An analytic criterion for the prediction and analysis of wing rock onset

Abstract

Modem fighter design of highly sweptback or delta wings is characterized by large interactions between the body and vortex dynamics at high angles of attack. This leads to a self-sustained lateral oscillation phenomenon known as Wing Rock. Prediction of the onset angle of attack of wing rock for the airframe in the preliminary design phases is of prime importance. The analysis of the onset is important as well, which provides the information needed to construct the feedback architecture for delay or suppression. Prediction methods based on unconstrained testing are very costly, and don’t provide analysis of the onset. In this paper, a simple analytic criterion is derived to predict the value of the onset angle of attack using very little computational cost. The criterion is based on only three standard derivatives, namely Cl& C,, and CYs, and thus no extra testing is required to predict the onset. Also, based on this criterion, the dihedral derivative C, is proved to be the major contributing derivative that controls the onset value. A zero value of the dihedral derivative is proved to occur at the onset. The onset criterion is applied to the E15B fighter model (with the automatic flight control system turned off) and accurately predicts the onset angle of attack. An accurate aerodynamic model for the F-15B fighter model based on high incidence wind tunnel testing together with a correlated flight test data is used to simulate wing rock at the predicted onset with an excellent agreement. A zero value of the C,*derivative is shown to occur at the onset. Also, an accurate measurement of the derivatives at high incidence proved to accurately simulate the motion in the pre-stall, stall, and post stall regions. Introduction The thin, low-aspect ratio, highly swept-back, or delta wing configurations of modem fighters were a quantum leap in the development of their transonic and supersonic aerodynamics, and hence their speed and maneuverability. However, these modem configurations were associated with lateral instability problems due to inertial and/or aerodynamic coupling. Two major lateral instability problems were distinguished; one arises when the aircraft is under lateral rate command (commonly known as “Roll Coupling”) and the other arises. when the aircraft is under hold (or displacement) command (commonly known as’ ‘Wing Rock”). Roll Coupling is primarily caused by inertial coupling3*‘5, while wing rock is a result of a complex aerodynamic/dynamic coupling at moderate-to-high angles of attack6*7. Since roll coupling was shown to be a result of inertial coupling with minor aerodynamic contribution’4, the problem of prediction and suppression was relatively easier compared with wing rock, which relies heavily upon ,the aerodynamic model. A consequent control problem may arise when the pilot attempts to compensate for the highfrequency wing rock oscillations. This problem has been known as “Pilot-Induced Oscillations or PRY’, which has led to many disastrous aircraft accidents. Lateral position precision is of prime importance for handling quality assurance of a modem fighter, and thus its rating4. Target tracking (Roll-and-Hold), air. refueling (receiver), and landing are examples of maneuvers where stable lateral position control is important. The high maneuverability and agility requirements imposed on modern fighter design required This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. * Ph.D. Candidate, Air Force Institute of Technology, WPAFB, AIAA Student Member. ’ Professor of Aerospace Engineering, Department Head, Air Force Institute of Technology, WPAFB, AIAA Senior Member.