Alain Pelletier

The unsteady aerodynamics of slender wings and aircraft undergoing large amplitude maneuvers

Abstract Aircraft that maneuver through large angles of attack will experience large regions of flow separation over the wing and fuselage. The separated flow field is characterized by unsteadiness and strong vortical flow structures that can interact with various components of the aircraft. These complicated flow interactions are the primary cause of most flight dynamic instabilities, airload nonlinearities and flow field time lags. The aerodynamic and the vortical flow structure over simple delta wings undergoing either a pitching or rolling motion is presented. This article reviews experimental information on the flow structure over delta wings and complete aircraft configurations. First, the flow structure of leading-edge vortices and their influence on delta wing aerodynamics for stationary models is presented. This is followed by a discussion of the effect of large amplitude motion on the vortex structure and aerodynamic characteristic of pitching and rolling delta wings. The relationship between the flow structure and the unsteady airloads is reviewed. The unsteady motion of the delta wing results in a modification of the flow field. Delays in flow separation, vortex formation, vortex position and the onset of vortex breakdown are all affected by the model motion. These flow changes cause a corresponding modification in the aerodynamic loads. Data is presented which shows the importance of flow field hysteresis in either vortex position or breakdown and the influence on the aerodynamic characteristics of a maneuvering delta wing. The free-to-roll motion of a double-delta wing is also presented. The complicated flow structure over a double-delta wing gives rise to damped, chaotic and wing rock motions as the angle of attack is increased. The concept of a critical state is discussed and it is shown that crossing a critical state produces large transients in the dynamic airloads. Next, several aircraft configurations are examined to show the importance of unsteady aerodynamics on the flight dynamics of aircraft maneuvering at large angles of attack. The rolling characteristics of the F-18 and X-31 configurations are examined. The influence of the vortical flow structure on the rolling motion is established. Finally, a brief discussion of nonlinear aerodynamic modeling is presented. The importance of critical states and the transient aerodynamics associated with crossing a critical state are examined.

Effect of Endplates on Two-Dimensional Airfoil Testing at Low Reynolds Number

The presence of endplates (or sideplates ) in two-dimensional wind-tunnel force measurements on airfoils has a strong effect on lift and drag coefe cients at low Reynolds numbers. Results on an Eppler 61 airfoil indicate that the endplates are responsible for a sharp decrease in the airfoil performance. The lift coefe cient is reduced and the drag coefe cient is increased due to the interaction of the airfoil boundary layer with the sideplate boundary layer. Nomenclature b = span Cd = section proe le drag coefe cient Cl = section lift coefe cient c = chord length eQ = quantization error M = resolution of A/D converter Rec = chord Reynolds number Rex = Reynolds number based on distance x along endplate U1 = freestream velocity x = distance from leading edge of endplate ® = angle of attack ± = boundary-layer thickness

Prediction of Vortex Breakdown Location on a Banked Delta Wing

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PROBLEMS ASSOCIATED WITH NONLINEAR AERODYNAMIC PHENOMENA

An experimental investigation of nonlinear aerodynamic phenomena, such as wing rock and wing drop, was conducted. It is shown that a simple wing geometry can present different dynamic regimes, or aerodynamic phenomena, when it is free to oscillate in a flow field at low to moderate angles of attack (0° to 25°). Wing drop, wing rock and autorotation have all been observed to occur. Fine resolution force and moment data were used to detect critical states. Near the stall angle of attack, a large standard deviation was found to be a sign of a critical state.