Aerodynamic and static stability characteristics of airfoils in extreme ground effect

Abstract

The purpose of this paper is to analyze the aerodynamic and stability characteristics of airfoils in extreme ground effect, where gradients of variables are intense. Camber line of NACA4412 is modified to become S-shaped, named M4412, which turns the passage shape between the ground and the lower surface of NACA4412 from convergent to convergent–divergent. Reynolds-averaged Navier–Stokes equations with Spalart–Allmaras turbulence model have been solved by applying the finite volume approach. Lift of M4412 is less than that of NACA4412. Divergent effect normally creates a negative lift zone. Reducing the height when angle of attack is lower than a specific angle strengthens the divergent effect; however, above this angle, weakens it. A stable airfoil has a negative moment coefficient derivative with the angle of attack and a negative lift coefficient derivative with height, where both airfoils satisfy them also, the height center should be upstream from the pitch center (height stability). NACA4412 is quite height unstable, whereas the neutral curve, where the pitch and the height centers are co-located, divides the height stability plane (height vs. AOA) of M4412 into the unstable and stable regions. While reducing the height influences the height stability of NACA4412 adversely, it affects that of M4412 favorably. Raising the angle of attack varies the height stability level of the both airfoils negatively.