Vortex Effects and Self-Induced Wing Rock

Self-induced wing rock on slender delta wings was observed first at NASA’s Langley Research Center in the late 1940s, in the free-flight wind tunnel. Wing rock ap­peared as a limit cycle, or undamped, roll oscillation at angles of attack below the stall. We know now that wing rock is typically associated with separated flows and time-dependent effects.

Because of interest in both supersonic transports and reentry vehicles, research activities into wing rock continued in both the United States and the United Kingdom (Ross, 1988). Attention turned later to combat airplanes, where wing rock was thought to have contributed to loss of control in high angle of attack maneuvers. Attempts to alleviate wing rock by stability augmentation have been successful, as in the case of the Grumman X-29A research airplane (Clarke, 1996).

However, attempts to correct the problem aerodynamically have been less successful because of the complex flow mechanisms involved. On the X-29A, the driving mechanism for wing rock was determined to be the interaction of vortices from the forebody with other components, as was the case for the Northrop F-5, at low airspeeds. On the other hand, a high-airspeed wing rock of the F-5 was driven by shock-induced separation on the wing. Some measurements indicate asymmetry of the vortices shed from wing leading edges as driving the motion, with vortex breakdown limiting the motion’s amplitude (Ericsson, 1993). Dr. Ericsson is a leading expert and prolific author on the effects of unsteady flows on stability and control.

It turns out that self-induced wing rock can also occur on very low-aspect-ratio rectan­gular wings, caused by vortices shed from the side edges. This is interesting but academic, since rectangular wings of aspect ratio less than 0.5 have never been considered for actual airplanes or missiles. What is decidedly not academic is the role of highly swept wing leading-edge extensions, or LEXs, in wing rock and other undesirable behavior. Wing leading-edge extensions were pioneered on the Northrop YF-17 and F-5 airplanes to in­crease maximum lift and reduce drag at high lift, by vortex interactions with the main wing surface. Wing leading-edge extensions have gone on to be used on many other modern fighter airplanes, such as the F/A-18. The highly swept side inlets of the Russian MiG-25 airplane act as leading-edge extensions, developing vortices at high angles of attack.

Wing rock is often studied in wind-tunnel tests in which a model is mounted on low – friction roll bearings and is free to roll. Forced roll oscillations can also reveal the wing rock tendency by regions of negative roll damping or positive signs of the rotary derivative Clp. A comparison between wing rock amplitude measurements on a free-to-roll model F-18 in a wind tunnel and on the F/A-18 HARV (High Angle of Attack Research Vehicle) in flight shows good agreement (Nelson and Arena, 1992). There was also a fair correlation for (reduced) frequency between wind-tunnel and flight testing. This supports the notion that flow conditions during flight wing rock are close to the single-degree-of-freedom wind – tunnel conditions.

Contradicting the reasonable correlation of F/A-18 HARV single-degree-of-freedom wind-tunnel tests and flight test measurements of wing rock is the finding on the F-4, Tornado, and RAE HIRM (High Incidence Research Model) that wing rock occurs at frequencies close to those of the classical Dutch roll. This is unusual since wing rock is a nonlinear limit cycle, while the Dutch roll is consistent with linearized equations of motion and requires roll, sideslip, and yaw degrees of freedom.

In addition to the wing rock phenomenon, vortex bursting at high angles of attack has undesirable effects, such as loss in lift and negative dihedral effects. Vortex bursting is associated with leading-edge sweep angles less than about 75 degrees. The interactions of wing leading-edge vortices with other airplane components of modern fighter airplanes is covered in a comprehensive paper by Andrew M. Skow and G. E. Ericson (1982). A more recent review was provided in 1992 by John E. Lamar, published in AGARD Report 783.

Lamar finds that the leading-edge suction analogy, first proposed by Edward C. Polhamus in 1966, providesa powerful tool for estimating aerodynamic forcesand momentsfor sharply swept wings with vortex flows. It appears that the leading-edge suction force in attached flow is reoriented in the direction of the rotating vortex, when the vortex forms. Typically, lift and moment terms using the analogy are added to linear aerodynamic and vortex lattice computer codes.

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