Authors: Mongkol Thianwiboon
Volume 9, Issue 1, Paper No. 090105
Flight in the vicinity of the ground is known to be more efficient than flight in a free air stream. However, a nose-down pitching moment created by a typical cambered airfoil generally increases due to ground effect. Thus, a larger tail for the aircraft is required to remain stable, which creates more drag and reduces the efficiency. The pitching moment in the ground effect becomes more complicated because it varies with height above the ground. Thus, the reflexed or S-shaped airfoil was introduced to overcome this effect. The addition of reflex reduces the lift of the airfoil, but it is required for improved stability.
This study applied computational fluid dynamics to investigate the aerodynamic characteristics of a reflexed airfoil, N60R, in ground effect over a range of angles of attack from 0 to 20° at a Reynolds number from 0.8Í106 to 5Í106 and ground clearance from 5% to 150% of the chord. The numerical results reveal that the boundary layer close to the ground affects the lift, drag, pitching moment coefficients, and center of pressure.
As the airfoil operates close to the ground, the lift increased due to a higher pressure build up under the airfoil. Except for a relatively low angle of attack (less than 2°), the lift decreases with a reduction in ground clearance due to loss of upper surface suction. The maximum lift-to-drag ratio, approximately 120, occurred at an angle of attack of 6 and ground clearance of 5%.
In summary, this study presents the aerodynamic characteristics of the reflexed airfoil, N60R, over a wide range of angles of attack, Reynolds numbers and ground clearance. Furthermore, regression models for each characteristic were developed and can be used to predict the coefficients of the N60R without the need for consuming time in Computational Fluid Dynamics (CFD) analysis.
Keywords: Wing-in-ground effect craft; WIG; Ground effect; N60R; CFD.090105_Mongkol