Aerodynamics an aerofoil in transonic ground effect : numerical study at full - scale Reynolds num rs

The potential positive effects of ground proximity on the aerodynamic performance of a \Ving or aerofoil have long been established, but at transonic speeds the fonnation of shock waves betvveen the body and the ground plane would have significant consequences. A numerical study of the aerodynamics of an lu\E2822 aerofoil section in ground eiiect flight was conducted at freestream Mach numbers from 0·5 to 0·9, at a range of ground clearances and angles of incidence. It \Vas found that in general the aerofoil's lifting capability was still improved with decreasing ground clearance up until the point at >,vhich a lower surface shock wave formed (most commonly at the loviest clearances). The critical Mach number for the section \Vas reached considerably earlier in ground effect than it \vould be in freest ream, and the buffet boundary vvas therefore also reached at an earlier stage. The tlowfields observed were relatively sensitive to changes in any given variable, and the lower surface shock had a destabilising effect on the pitching characteristics ofthe Viing, indicating that sudden changes in both altii11de and attitude would be experienced during sustained transonic Hight close to the ground plane. Since ground proximity hastens the lower surface shock formation, no gain in aerodynamic efficiency can be gained by flying in ground effect once that shock is present.