1Therefore, in keeping with the spirit of the Wright brothers research and design accomplishments, as well as the scope of this meeting, I have elected to review some of the aerodynamic research performed at the Langley Research Center related to the application of slender wing benefits in the design of high-speed military aircraft In the context of this paper, slender wing benefits refer primarily to the supersonic performance and leading edge vortex flow associated with very highly sweptback wings Following a review of some early slender wing research, the paper presents several case histories of Langley contributions to the development of aircraft incorporating slender wing benefits and then summarizes some vortex flow technology that may contribute to future aircraft

Applying slender wing benefits to military aircraft

Induced drag is associated with the shedding of vorticity along the span of a finite wing, especially its tip region; for most subsonic aircraft configurations, induced drag constitutes about 50 percent of total aircraft drag throughout the flight envelope. NASA and the U.S. aircraft industry have aggressively studied induced-drag reduction methods. The state-of-the-art CTOL commercial aircraft wing is as a result of these efforts virtually optimal, with a total induced drag lying within a percent of the theoretical minimum. Many of the devices currently under study for induced drag reduction are added to wingtips, yielding benefits through their effects on the wake vortex as well as through forces generated in the flowfield.

Induced drag - Historical perspective

The influence of sideslip on the flow about a sharp-edged biconvex delta wing of a unit aspect ratio is investigated using flow visualization techniques as well as pressure and force balance measurements. The tests have been carried at a constant incidence of 21.1 deg and at angles of sideslip ranging from 0 to 20 deg. The freestream velocity was 44 m/s, corresponding to a Reynolds number of 2.5 x 10, based on root chord. Up to 12-deg sideslip, the asymmetry of the vortex crossflow and surface pressure distribution depends on the increasing symmetry in the strength and position of the vortices, as well as on boundary-layer transition. At layer angles of sideslip, the vortex flow and pressure distribution is additionally influenced by asymmetric bursting. The flow about the yawed wing is computed using a slender-body free-vortex-sheet method. Good agreement is obtained with experimental data on the part of the wing away from the apex and trailing edge.

Experimental and numerical investigation of vortex flow over a sideslipping delta wing

A 65 deg. delta wing has been tested in the National Transonic Facility (NTF) at mean aerodynamic chord Reynolds numbers from 6 million to 120 million at subsonic and transonic speeds. The configuration incorporated a systematic variation of the leading edge bluntness. The analysis for this paper is focused on the compressibility and bluntness effects primarily at a Reynolds number of 6 million from this data set. Emphasis is placed upon on the onset and progression of leading-edge vortex separation, and compressibility is shown to promote this separation. Comparisons with recent publications show that compressibility and Reynolds number have opposite effects on blunt leading edge vortex separation

Compressibility and Leading-Edge Bluntness Effects for a 65 Deg Delta Wing

A three-dimensional Euler aerodynamic method based on a finite-volume, multistage time-stepping algorithm is used to simulate free vortices generated by flow separation along the edges of swept, slender wings at moderate to high angles of attack. Computed results for a cropped-delta wing, an arrow wing, and a strake-wing-body configuration are correlated with experimental data and, for cropped-delta wing, with predictions of other numerical methods also. The flow is impulsively started and the vortices are automatically captured. The following two issues are specifically addressed: 1) sensitivity of the solutions to artificial viscosity and 2) effect of grid density on the results. Relatively small changes in the subsonic solutions are noticed with variations in the magnitude of artificial viscosity parameters and grid density. The correlations presented here provide an added measure of confidence in computational simulations using the Euler equations. The present investigation also raises some new issues related to vortex instabilities.

Free-vortex flow simulation using a three-dimensional Euler aerodynamic method

Free Vortex Sheet theory has been applied to a variety of configurations for the estimation of three-dimensional pressure distributions for wings developing separation-induced leading-edge vortex flows. Correlations with experiment show reasonable estimates for the effects of compressibility, side-slip, side edges, swept-wing blast-induced loads, and leading-edge vortex flaps. Theoretical studies expand upon these correlations to show general aerodynamic trends. Consideration is also given to simple, yet effective techniques which expedite convergence and therefore reduce computational expense.

N. T. Frink

Papers

Recent advances in applying Free Vortex Sheet theory to the estimation of vortex flow aerodynamics