This paper examines the capability of the Reynolds-averaged thin-layer Navier-Stokes codes to simulate the results from a two-dimensional aileron effectiveness test. This unique test was carried out in the IAR high Reynolds number wind tunnel and addressed the effects of Reynolds number, Mach number and angle-of-attack on aileron effectiveness. The test results showed a highly nonlinear variation of lift for downward trailing edge deflections. It provides a valuable database for using CFD to determine the adequacy of the corrections applied to the experimental data due to the presence of the wind tunnel walls, and for assessing the current CFD capability to model the flowfield with separation. CFD predictions are obtained by using CFL3D with the Spalart-Allmaras turbulence model and TLNS2D with the modified Johnson-King turbulence model. It will be shown that the CFD analyses reconfirm that the “Murthy” sidewall boundary layer correction method is only an approximation for transonic airfoil testing. Some success was realized in duplicating the “2-D” measured aileron effectiveness characteristics with the 2-D “free-air” CFD approach, particularly at the higher Reynolds numbers, higher Mach numbers and/or higher angle of attack. However, there are some cases where the agreement is not so good, and where the prevailing flow physics is not adequately modeled, that lead to the conclusion that much more must be done before the CFD methodology employed could be considered validated.