Effects of a Bell-Shaped Lift Distribution on an Oblique Flying Wing and its Impact on Aerodynamic Performance

Date of Award

2020

Degree Name

M.S. in Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering and Renewable and Clean Energy

Advisor/Chair

Advisor: Sidaard Gunasekaran

Abstract

Conceptually, the Oblique Flying Wing (OFW) has shown promise as an effective aircraft configuration but creates significant design challenges that must be overcome in order to realize its benefits. In part, the difficulties come from the very nature of the OFW as the configuration lacks horizontal and vertical tail surfaces. The exclusion of these surfaces makes developing a stable configuration non-trivial and the OFW has not seen many flying examples. To develop a stable OFW configuration, a bell-shaped lift distribution is used by employing a geometric twist distribution in an OFW planform to increase directional stability. The geometric twist distribution was determined using methods developed by the Horten brothers so the local lift coefficients could be determined using 2D airfoil properties. Comparing against a baseline untwisted OFW, both OFW configurations were evaluated using Euler CFD and low-speed wind tunnel testing performed at the University of Dayton Low-Speed Wind Tunnel (UD-LSWT).The inviscid CFD solutions showed that there are significant differences in the aerodynamic performance and moment coefficients between the baseline and twisted OFW configuration. For the baseline configuration, the magnitude of the rolling moment was shown to be a severe constraint on the ability of the OFW to be stable in take-off like conditions. The twisted OFW configuration was shown to alleviate the severity of the rolling moment by redistributing the lift along the span of the wing, making take-off probable. Similar reductions were seen in the yawing moment where it is expected that the twisted OFW configuration with the bell-shaped lift distribution will have the most impact. For the yawing moment, the redistribution of lift and drag along the span caused by the twist distribution was shown to reduce the magnitude of the yawing moment for the twisted OFW.A wind tunnel testing campaign was performed for the same conditions as the CFD solutions. The experimental results showed that while the aerodynamic performance trends were similar between the methods, there were many significant changes in the moment coefficients. For the rolling moment, the differences that were seen between configurations in the inviscid CFD solutions were not replicated in the wind tunnel results. Likewise, the yawing moment showed little change between configurations from the wind tunnel test. At the Reynolds number tested, the viscous effects and real flow effects appear to have a significant impact on the OFW configuration and its aerodynamic response. In addition to Reynolds number effects, it is likely that there are errors in the measurement of the combined asymmetric aerodynamic loads experienced by the OFW models.Overall, the impact of the bell-shaped lift distribution was seen primarily in the CFD results and to a lesser extent in the wind tunnel results. It is clear that in order to verify and validate inviscid CFD results with wind tunnel results, further work is required to validate the ability of the measurement equipment to measure combined asymmetric aerodynamic loads. In addition to validation of the measurement equipment, further investigation with RANS CFD is likely required to better compare the wind tunnel results with simulations to fully understand the implications of the viscous effects. In supporting the higher-fidelity CFD, surface flow visualization on both OFW configurations would be needed to verify the CFD results.

Keywords

Aerospace Engineering, OFW, Oblique Flying Wings, Bell-shaped lift distribution, CFD, Windtunnel, aerodynamics

Rights Statement

Copyright © 2020, author

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