A Study of Wings with Constant and Variable Sweep for Aerodynamic Efficiency in Inviscid Flow

Wing sweep has been studied by industry and academia since the pioneering days of aviation for both high-speed and low-speed applications. In transonic and supersonic flight regimes it serves to delay the onset of compressibility effects and decrease wave drag. In subsonic conditions, flying wing designs sweep back the main lifting surface in such a way that it can be used for longitudinal stability and control, to allow for the elimination of a traditional empenage. This is desirable because it can decrease the aerodynamic drag. Sweep can also be seen in nature in the wings of birds and fins of fish. While sweep in man-made airplanes is mostly limited to a constant sweep angle from wing root to wing tip, nature shows a curved sweep profile in the wings of birds and fins of fish. There might bean aerodynamic benefit to non-constant or variable sweep profiles. This research attempts to discover the potential aerodynamic benefits of non-constant sweep. In the present work, the theoretical background of our current understanding of swept wing aerodynamics is revisited. Inviscid numerical methods are used to investigate the lift, induced drag, and aerodynamic center position of conventional wings with constant sweep, and crescent wings with a linear sweep profile, where the local sweep increases from zero at the wing root to some finite value at the tip. A comparison between the two types is made to see whether the curved wing planforms offer a potential aerodynamic benefit over conventionally swept wings. The wings are compared at equivalent aerodynamic center position so that they will offer similar longitudinal stability. An induced drag factor that is nearly independent of lift coefficient and acts as a measure for aerodynamic efficiency, is the performance metric used in the aerodynamic comparison. A cross-over point that indicates at what aerodynamic center position or equivalent sweep angle the crescent sweep profile produces less induced drag than the constant sweep profile is found and shown as a function of aspect and taper ratio. The wing of an albatross is used to demonstrate that some more complex sweep profiles can produce less induced drag than both constant or purely linear sweep profiles in inviscid flow. A separate chapter studies the effects of viscosity on the results found in this work, by modeling the boundary layer thickness, flow transition, and laminar and turbulent skin friction using Flight Stream. It shows that when including viscous effects, the wings with constant and linear sweep show similar trends with sweep as those resulting from inviscid results. The cross-over point between wings with constant sweep and linear sweep when considering total drag coefficient is shown to not differ significantly from that of the inviscid results for induced drag, especially not at the small angles of attack considered in this research. Therefore, the findings from the inviscid study are insightful in understanding the effects of sweep type and angle on induced drag. Finally, an optimization exercise is performed to find sweep profiles that offer lower induced drag than both the constant and linear sweep profiles in a purely inviscid scenario. It is shown that there are more efficient sweep profiles, but proving that any solution is the global minimum is difficult. It is also addressed that these results would likely not perform as well in a real world setting