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 2 of 5)
One of the most famous and spectacular cases of destructive flutter befell not an airplane but a bridge. When the Tacoma Narrows Bridge, then the third longest suspension bridge in the world, opened to traffic in the fall of 1940, it had already acquired the nickname Galloping Gertie because it heaved rhythmically and visibly when the wind blew. In fact, people repeatedly crossed the bridge just to enjoy its roller-coaster-like undulations, which were considered harmless. On November 7, only six weeks after the bridge opened, a steady 42-mph wind was blowing along Puget Sound. The slender span began its dance. Then a cable near mid-span snapped, creating an unbalanced condition. Soon the bridge was performing twisting, heaving, and swinging motions of an incredible magnitude. These continued for more than half an hour before the center span fell into the water--long enough for an amateur filmmaker to record for posterity the astonishing spectacle of the giant bridge writhing like a wounded snake as a terrified motorist abandoned his car and ran for his life.
Even today the exact mechanism of the flutter of the Tacoma Narrows Bridge is disputed. The fact that half a century of reflection and analysis has not settled the question gives some indication of the abstruse nature of flutter itself. The case of the Lockheed Electra might have remained similarly mysterious--the Civil Aeronautics Board, precursor of today's National Transportation Safety Board, was ready to throw in the towel and label the crash "unexplained"--had not a second accident, almost a carbon copy of the first, occurred. This time it was an Electra flying a Northwest Orient route from Chicago to Miami in March 1960 that broke up in flight over Indiana, killing 63. The flight was known to have been operating in an area of severe turbulence, and the failure might have been attributed to structural overload had the damage signatures around one engine nacelle--this time the right outboard engine rather than the left--and the distribution of parts in the debris field not been so similar to those in the previous accident.
General Motors' Allison Division, manufacturer of the Electra's engines, dismantled and minutely examined all eight engines from both aircraft. NASA weighed in with detailed analyses of flutter modes that might occur if various structural failures had gone undetected in a wing or nacelle. Every path came to a dead end. All analyses found that the structure incorporated large margins of safety. Flutter, the only possible explanation, seemed impossible.
Then Lockheed structural dynamicist J. Ford Johnston had the idea of investigating the hitherto neglected contribution that might be made by small yawing deflections of the propeller. As pilots of propeller aircraft know from experience, the center of thrust of a climbing airplane's propeller shifts to the side of the downgoing blade. This phenomenon, colloquially called P Factor, occurs mainly because a component of the airplane's forward velocity is added to the speed of a downgoing blade and subtracted from that of an upgoing one. A similar phenomenon, rotated 90 degrees, naturally occurs when the engine swings to one side. Shifting the center of thrust flexes the engine mount, creating a new shift in the center of thrust and a new direction of flexure. As a result, a propeller and nacelle can vibrate continuously in a circular motion called "whirl mode". Johnston suggested that whirl mode vibration might have initiated an unsuspected flutter mechanism.
As early as 1938, a study on powerplant vibrations had raised the possibility of propeller whirl inducing structural flutter. But the relative weights of engines and propellers, the stiffness of propeller shafts, and the engine power outputs that were typical in the late 1930s made it a practical impossibility. As Lockheed mathematician Robert Donham, who participated in the accident investigation, says today, "Probably nobody involved with the design of the Electra even knew the paper existed. Nobody thought about whirl-mode vibrations causing flutter."
Lockheed's flutter analysts reprogrammed their computer to include whirl mode, and the mechanism of the accidents began to emerge. By an unlucky coincidence, the whirl-mode frequency of the Electra's big four-blade propellers happened to match the flapping frequency of the wing. The propellers, like the child driving a swing higher by small movements of her body, had eventually caused the wing to flap so violently that in 30 seconds it broke at the root without the propeller whirl ever overloading the nacelle structures.
Microscopic examination of fractures in the wreckage of the two airplanes revealed engine mount damage that had preceded the inflight breakups. The cause of the earlier damage was uncertain--in one case a hard landing was suspected--but Lockheed redesigned the engine mounts and no Electra ever suffered from whirl-mode flutter again. Flutter is all about stiffness, not strength; even the strongest structure may fail if it flutters. In general, structures that are light and stiff vibrate more rapidly; they are said to have higher natural frequencies. Structures more massive or less stiff have lower frequencies. The usual treatment for a flutter problem is to raise the natural frequency of one structure by stiffening it, but sometimes the opposite approach is used: lowering a frequency by the careful placement of damping weights. The essential thing is to eliminate coincident frequencies in structures that can feed energy to one another. A wing that is very stiff in bending should be made "softer" in torsion, and vice versa.
Flutter specialists speak a language incomprehensible to ordinary engineers. For a long time, the designers of aircraft structures confined their attention to static loadings and ignored dynamic loading. The late Raymond Bisplinghoff, a specialist in aeroelasticity whose career included top-level roles at the Massachusetts Institute of Technology and NASA, recalled his time at Wright Field's Aircraft Laboratory during World War II in Hugh Flomenhoft's book The Revolution in Structural Dynamics: "The design-desk officers would frequently fly into a rage when told by an apple-cheeked youngster that weight or speed restrictions had to be added to their airplane to prevent aeroelastic problems. I was thrown out of their offices on an almost daily basis and frequently told "that flutter was a figment of my imagination."