Aerodynamic Characteristics of a Swept Wing in Close Ground Proximity at High Subsonic Mach Numbers

The potential positive influence of close ground proximity on the aerodynamic performance of a wing has long been established; however, flight at a Mach number that causes shock waves to form between the wing and the ground plane would have significant aerodynamic consequences. A numerical study to investigate the effects of extremely low-level flight at subsonic Mach numbers up to 0.9 was conducted involving the symmetric, swept ONERA M6 wing. The aerodynamic coefficients as well as the pressure distributions and tip vortex behavior were analyzed for cases involving the angle of incidence, ground clearance, and Mach number as variables. It was found that lift was augmented with decreasing ground clearance in the majority of cases with the wing at positive incidence, although the critical Mach number for the wing, and in particular for the lower surface, was reduced at very low clearances. The earlier emergence of a lower shock was accompanied by a considerable increase in drag, although this was offset in part by a marked decrease in the strength of the tip vortex when the wing was within one chord length of the ground plane. As a result of the more pronounced shock structures and separation regions that occur above a freestream Mach number of 0.8, there is no net aerodynamic efficiency to be gained from flight with this wing in close ground proximity compared with unbounded flight at altitude. DOI: 10.1061/(ASCE)AS.1943-5525.0000169. (C) 2012 American Society of Civil Engineers.