Computational study of the F-5A forebody emphasizing directional stability

Abstract

Computational fluid dynamics (CFD) has been used to study the F-5A forebody flowfield at low-speed high angle-of-attack conditions combined with sideslip. The classic wind-tunnel experiment demonstrating the dominant contribution of the F-5A forebody to directional stability at high angle of attack has been simulated computationally over an angle-of-attack range from 10 to 45 deg. The key wind-tunnel trend for C nft was obtained computationally using the CFL3D code to solve the Reynolds' equations employing the Baldwin-Lomax turbulence model with the Degani-Schiff modification to account for massive crossflow separation. The computations provide detailed and fascinating insights into the physics of flowfield. The results of the investigation show that CFD has reached a level of development where computational methods can be used for high angle-of-attack aerodynamic design. Nomenclature b' = wingspan based on F-5A wind-tunnel model, 52.68 in. C L = lift coefficient, \ift/q^S ref C m = pitching-moment coefficient, pitching moment/g^ S^Z/ C n = yawing-moment coefficient, yawing moment/q^S^b' C, l/3 = directional stability derivative, dC n /d/3 C p = pressure coefficient, (p -p^)/qĈ v = side-force coefficient, side force/'q^S Tef c = mean aerodynamic chord, 16.08 in. c v = sectional side force, section side force/'qF S = fuselage station / = forebody model length, 31.025 in. M x = freestream Mach number, 0.2 p = pressure p x = freestream pressure q^, = freestream dynamic pressure Re, = Reynolds number based on model length, 1.25 x 10 6 5 ref = reference area, 754.56 in. 2 u* = wall friction velocity, Vr^/p F sep = crossflow velocity magnitude at separation point (chine edge) jc, y, z = coordinate system: x positive aft on model axis, y positive to right, and z positive up y + = inner law variable, yu*lv a = angle of attack, deg j8 = angle of sideslip, deg AC P = difference between leeward and windward C p across the vertical plane of symmetry 0 = azimuthal angle, measured clockwise from windward plane at any cross section p = density T W = shear stress at the wall Presented as Paper 91-3289 at the AIAA 9th