Transonic Pitchup

Transonic high angle of attack pitchup is an instability caused by reversal of the normal, stable wing-fuselage pitching moment variation with angle of attack. In the normal, stable angle of attack range, increases in angle of attack cause negative, or nose – down, pitching moments. At high angles of attack on sweptback wings at transonic speeds flow separation on the outboard panels reverses the wing-fuselage pitching moment from negative to positive, or nose-up. The airplane will continue to rotate in a nose-up direction,

increasing the angle of attack. This can happen even against pilot application of nose-down control.

This phenomenon was first seen in flight in August 1949, on the Douglas D-558-II Skystreak research airplane being flown by Robert Champine. While pulling 4 g at a Mach number of 0.6, the airplane suddenly pitched up to 6 g. This was not much of a surprise since wind-tunnel tests had shown wing-fuselage pitching moment reversal at high angles of attack. Outer panel vortex generators delayed slightly the onset of pitchup on the D-558-II, by about 0.05 in Mach number.

Seth B. Anderson and Richard Bray (1955) had the opportunity to analyze in detail the same transonic pitchup phenomenon as it occurred on the North American F-86 Sabre, in flight tests at Ames Aeronautical Laboratory. The key was strain gage measurements of horizontal tail load, which separate wing-fuselage instability from changes in downwash at the horizontal tail. In windup turns at a constant Mach number, load factor or g increased during the pitchup, although the tail carried an increasing, stabilizing up-load as the pitchup continued (Figure 11.13). Thus, F-86 pitchup is caused by wing instability, independently of the tail.

Transonic Pitchup

Figure 11.13 Pitchup on the North American F-86 Sabre in a turn. Normal acceleration increases after 1.5 seconds, although the elevator angle is constant or moving down. By measuring the horizontal tail load, NACA verified that the pitchup was caused by wing-fuselage instability, just as predicted in the wind-tunnel test. (From Anderson and Bray, NACA Rept. 1237, 1955)

A refinement of the Shortal-Maggin instability boundaries (Figure 11.9) was made by Joseph Weil and W. H. Gray in 1953. They showed that transonic speeds shift the bound­ary toward lower values of wing sweepback. That is, for the same wing aspect ratio, less sweepback is allowable at transonic speeds.

The Boeing B-47 has an interesting transonic pitchup case history. With a wing aspect ratio of 6.0, a quarter-chord sweepback angle of 35 degrees, and a taper ratio of 0.23, the B-47 wing falls close to the Furlong and McHugh pitchup boundary, which applies at low speeds. With the Weil-Gray shift in pitchup boundary toward less allowable wing sweep at transonic speeds, the B-47 would be expected to have transonic pitchup, and indeed it does (Cook, 1991).

As with the Douglas D-558-II, outer wing panel vortex generators reduced the pitchup instability. Vortex generators made no improvement in B-47 wind-tunnel tests, possibly because of the small scale of the generators, but Cook reports that two rows of generators actually eliminated the problem in-flight (Figure 11.14). The B-47 installation is apparently better than the one that created only a slight delay in pitchup to a higher Mach number for the D-558-II.

A design innovation of the late 1940s has made the pitchup problem relatively tractable. Where the airplane’s arrangement permits, a low horizontal tail, located below the wing chord plane extended, alleviates the problem. Wing downwash over a low horizontal tail can be counted upon to drop off, increasing tail upload and causing pitchdown, precisely when wing outer panel separation causes wing-fuselage pitchup. The influence of the vertical position of the horizontal tail was integrated into the Weil-Gray pitchup boundary in 1959 by Kenneth P Spreeman of the NACA Langley Laboratory.

The English Electric Lightning applied the low horizontal tail principle “against strong official insistence from Farnborough that the tail must be placed on top of the fin,” according to John C. Gibson. The prototype first flew in 1954. The first low horizontal tail to appear on aU. S. airplane was on the North American F-100 Super Sabre prototype a year earlier. With its high horizontal tail and low-aspect-ratio wing the Lockheed F-104 has a severe pitchup at the stall. The pilot is given a stick shaker warning of impending pitchup. A stick pusher then causes an automatic recovery. The problem of outer wing panel premature separation at transonic speeds and high angles of attack is still with designers of modern airplanes. Of course, if transonic pitchup is a problem these days, modern digitally implemented stability augmentation can correct the problem by inserting programmed countercommands or prevent it by angle of attack limitation.

The entire period when stability and control engineers grappled with the new problems brought about by flight at transonic Mach numbers is remembered by those involved as a period of great confusion and hard work. In unpublished correspondence, W. Hewitt Phillips remembers that period:

[W]ith eachnew discovery a whole generation of new airplanes appeared that solved some of the earlier problems but got into some new ones. When the dive recovery problems appeared on the fighters of World War II one of the reactions was to eliminate the tail. As a result we built the Northrop X-4 and the Vought F7U, and the British built the de Havilland DH 108. All of these airplanes were quite unsuccessful, although the Vought company got their airplane into service and gained a lot of experience on power controls.

After that came the studies of the effects of sweep and aspect ratio, and companies built planes that looked like the earlier unswept versions but with swept wings. These include airplanes like the F9F-6 and the F-84F. About this time came the discovery of notched wings, low tails, etc., and many of the pitchup problems were cured. This generation includes the F8U and the F-100.

Transonic Pitchup

Figure 11.14 Boeing B-47 wing showing the locations of the two rows of vortex generators that eliminate transonic pitchup. (From Cook, The Road to the 707, 1991)

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