Free-To-Roll Analysis of Abrupt Wing Stall on Military Aircraft at Transonic Speeds

LE leading edge Transonic free-to-roll and static wind tunnel tests for four military aircraft – the AV-8B, the F/A-18C, the pre-production F/A-18E, and the F-16C – have been analyzed. These tests were conducted in the NASA Langley 16-Foot Transonic Tunnel as a part of the NASA/Navy/Air Force Abrupt Wing Stall Program. The objectives were to evaluate the utility of the freeto-roll test technique as a tool for predicting areas of significant uncommanded lateral motions and for gaining insight into the wing-drop and wing-rock behavior of military aircraft at transonic conditions. The analysis indicated that the free-to-roll results had good agreement with flight data on all four models. A wide range of motions limit cycle wing rock, occasional and frequent damped wing drop/rock and wing rock divergence were observed. The analysis shows the effects that the static and dynamic lateral stability can have on the wing drop/rock behavior. In addition, a free-to-roll figure of merit was developed to assist in the interpretation of results and assessment of the severity of the motions. LERX Leading edge root extension M Mach number n geometric scale factor p roll rate V P p − free-to-roll figure of merit PID parameter identification q pitch rate q∞ freestream dynamic pressure r yaw rate Re Reynolds Number based on cref S reference surface area t time TE trailing edge TDT Transonic Dynamics Tunnel V∞ freestream velocity α angle of attack β angle of sideslip δht differential horizontal tail deflection φ roll angle φ& roll rate, time derivative of the roll angle Symbols and Abbreviations φ& & roll acceleration, time derivative of the roll rate 16-ft TT 16-Foot Transonic Tunnel a speed of sound ρ∞ freestream density AWS Abrupt Wing Stall θ pitch angle b reference wing span ω oscillatory frequency in radians per second (ω= 2πf) CL lift coefficient l C rolling moment coefficient ∞ V pb 2 reduced angular rate cref reference wing chord (mean geometric chord) ∞ V b 2 ω Strouhal number f frequency in cycles per second, Hz Subscripts FOM figure of merit a airplane FTR free-to-roll m model x I inertia about the roll axis