Aerodynamic pitch-up of cranked arrow wings: estimation, trim, and configuration design

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Virginia Tech

Low aspect ratio, highly-swept cranked arrow wing planforms are often proposed for high-speed civil transports. These wing planforms offer low supersonic drag without suffering greatly from low lift/drag ratios in low-speed flight. They can, however, suffer from pitch-up at modest angles of attack (as low as 5°) during low-speed flight due to leading edge vortex influence, flow separation and vortex breakdown. The work presented here describes an investigation conducted to study past research on the longitudinal aerodynamic characteristics of highly-swept cranked wing planforms, the development of a new method to estimate pitch-up of these configurations, and the applications of this new method to the analysis of tail designs for trim at high lift coefficients. The survey of past research placed emphasis on 1) understanding the problem of pitch-up, 2) ascertaining the effects of leading and trailing edge flaps, and 3) determining the benefits and shortfalls of tail, tailless, and canard configurations. The estimation method used a vortex lattice method to calculate the inviscid flow solution. Then, the results were adjusted to account for flow separation on the outboard wing section by imposing a limit on the equivalent 2-D sectional lift coefficient. The new method offered a means of making low cost estimates of the nonlinear pitching moment characteristics of slender, cranked arrow wing configurations with increased accuracy compared to conventional linear methods. Numerous comparisons with data are included. The new method was applied to analyze the trim requirement of slender wing designs generated by an aircraft configuration optimization and design program. The effects of trailing edge flaps and horizontal tail on the trimmed lift coefficient was demonstrated. Finally, recommendations were made to the application of this new method to multidisciplinary design optimization methods.