For every airplane, there's a region of the flight envelope into which it dare not fly.
- By Peter Garrison
- Air & Space magazine, March 2001
(Page 3 of 5)
At the time, the tools available for the mathematical analysis of flutter were primitive, though, as has often been the case in the history of aviation, theoretical understanding was far ahead of practical application. Problems that today take barely a split-second's attention from a desktop computer then required weeks of manual computation on battalions of mechanical calculators--what physicist Richard Feynman termed the "many females" approach to mathematics. Alternatively, analog computers, in which physical objects were simulated by electrical circuits with similar resonant characteristics, were programmed with data collected by shaking airplanes with variable-speed electric motors swinging unbalanced weights. Bisplinghoff recalled pressing his head against the underside of vibrating wings to locate their "nodes," the points that remained still while the wing vibrated.
Another approach to flutter analysis is testing scale models in a wind tunnel. This is tricky, requiring duplication in the proper scale of not only the geometry of the airplane but the mass and elastic characteristics as well. Since small structures naturally tend to have higher resonant frequencies than large ones, wind tunnel models for flutter studies were originally built with wooden spars and rubber skins to lower their natural frequencies. Today, glass-fiber laminates are used because their flexural behavior can be tailored by changing the fibers' orientation.
The world's premier flutter-testing facility is the 16- by 16-foot Transonic Dynamics Tunnel at NASA's Langley Research Center in Virginia, which provides its clients with empirical data that they use to fine-tune their computer models. It also provides occasional moments of excitement when a model actually flutters. Sometimes, says TDT research engineer Don Keller, this happens only once per model, because afterwards, "you are sweeping it up into a bag." Because the tunnel can operate at up to Mach 1.2, it allows engineers to explore the critical speed range, around the speed of sound, where the unpredictable behavior of shock waves creates a "transonic bucket" in which flutter is much more likely to occur. Even with the tunnel and extremely sophisticated software, however, some flutter modes are elusive: "You can't predict them," Keller quips, "until they happen."
In the absence of detailed computer analysis or costly ground vibration testing, airplanes can be tested for flutter resistance in an ad hoc way. The late John Thorp, whose design career spanned the glory days from 1930 to 1960, called this "tickling the dragon's tail." Beginning at a low speed where the airplane was known to be flutter-free, the test pilot would accelerate by a mile or two per hour, then deliver a sharp slap or "pulse" to the control stick or the rudder pedal. He would pay careful attention to the immediate aftermath of the disturbance. Did the stick or pedal immediately return to center, and the airplane appear unperturbed? This was a "dead beat" response; it indicated that no tendency to flutter was present at that speed. The pilot would then increase speed by a small amount and repeat the test.
The seat-of-the-pants approach to flutter testing is possible for the same reason that an automobile tire that vibrates most severely at 60 mph starts shaking at 55 and stops at 65 or 70. The tendency to flutter does not usually rise instantly to a maximum when one parameter or another--say, airplane speed--reaches a critical value. It normally ramps up gradually enough for speed increments of one or two mph to give the pilot warning of impending trouble. The pilot relies on the feel of the stick to warn him of diminished damping. The dead beat response softens; the stick wobbles once or twice before returning to center. Sometimes a control surface actually flutters with a low intensity, so the structure flexes but does not fail and the pilot, by slowing the airplane down, can arrest the flutter.
Still, "usually" and "normally" must be added. On occasion, catastrophic flutter occurs at a speed only very slightly above a speed that was considered "safe," and sometimes even below it. Today, except in the lowest-budget flight test programs, sensitive electronic motion sensors and strain gauges measure and record vibrations that are triggered by a mechanical shaker rather than from the pilot's hand or foot. The slightest decrease in damping is instantly detected. Nevertheless, testing for flight flutter is no test pilot's favorite activity.
Until after World War II, structures were built as lightly as possible, and balanced and stiffened later as needed. Flutter problems were mainly attacked with empirical methods, often crude and sometimes quite drastic. Before the National Air Races in 1934, Steve Wittman's midget racer Chief Oshkosh repeatedly encountered severe but non-destructive flutter of a wingtip. It happens that short beams, like short piano strings or organ pipes, have higher natural frequencies than long ones. Therefore, after each test flight on which flutter was encountered, Wittman chopped a few more inches off the wing. Eventually the span was reduced to 16 feet, and the wing area from a minimal 78 square feet to a microscopic 42. The airplane landed awfully fast, but at least its wing was free of flutter.
William R. Laidlaw, former chief of the Structural Dynamics Section at North American Aviation, describes in The Revolution in Structural Dynamics a similar approach to controlling stabilizer flutter in the 1950s. During a test flight, an FJ-4 Fury, a carrier-based derivative of the F-86 Sabre Jet, lost more than half of both horizontal stabilizers to a flutter incident (the pilot managed to land safely). Engineers mounted a Fury tail assembly on a rocket sled at the Navy's test facility at China Lake, California, and the flutter was duplicated. North American engineers considered various modifications to solve the problem, but finally settled on the crudest one: They sawed 12 inches off each end of the tail. A few months later, however, an FJ-4 shed its tail during a pullout from a high-speed dive, killing the pilot. The experts went back to work. They then discovered that putting a heavy load on the tail reduced its flutter margins. Another amputation, this time of a mere six inches, solved the problem for good.