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
A little before midnight on September 29, 1959, a Braniff Airways Lockheed Electra was cruising at 15,000 feet between Houston and Dallas. Some of the 26 passengers may have been reading, but most probably dozed, lulled by the throb of four big turboprops. Then came a different sound, a shaking that grew gradually until the interior of the cabin began to rattle and creak. Passengers sat up and looked around anxiously. The shaking lasted 30 seconds, becoming rapidly more violent and ending with the terrible shriek of tearing metal.
The Electra had lost its left wing. Parts of the airplane rained down over an area four miles long. Witnesses on the ground described a glow like a meteor--bright, then fainter, then bright again--a screaming sound like a jet engine, and a boom. They later identified a recording of a runaway propeller as most similar to the sustained sound they had heard.
Accident investigators focused their attention on the failed wing's outboard engine nacelle, because scratches and tears in the metal indicated that the propeller and gearbox, mounted several feet ahead of the engine and connected to it by a long power shaft, had swung as much as 35 degrees out of alignment. But the sequence of events was baffling. Lockheed's engineers had, of course, anticipated that a propeller could shed a blade, throwing the engine out of balance and causing a catastrophic failure. But they had designed the nacelle to break away from the airplane before it could overstress the wing. Yet even though the wing had separated from the fuselage and the propeller and its gearbox had separated from the engine, the nacelle had remained attached to the wing.
The first hypothesis was that the primary failure had been caused by a wing overload, and the powerplant damage had occurred during the subsequent disintegration of the airplane. But it was hard to believe that this was a case of simple overload due to turbulence or pilot action. Wing failures in airliners were extremely rare--only five had occurred in the previous 40 years--and if the Electra had a structural weakness it would have surely turned up in Lockheed's rigorous ground- and flight-test programs or in Eastern Airlines' thousands of hours of high-speed, low-level operations along the eastern seaboard. This looked more like the work of the mysterious and deadly demon called flutter. But no plausible flutter mechanism could be found to explain this accident.
Flutter has always been aviation's dirty little secret. Seldom reported and little understood, it occupies one of those dimly lit and unsafe places that decent people prefer not to visit. The idea that an airplane could shatter, disintegrate, for no reason other than its own motion through the air--better to let sleeping horrors lie. Compared with most other concepts in aeronautics, flutter is obscure and difficult to grasp, but there are examples of the phenomenon in everyday non-aeronautical life. An out-of-balance tire is one; it begins to vibrate at a certain speed as the car accelerates; at some higher speed the vibration subsides. What is happening is that when the tire's natural bounce frequency matches its rate of rotation, the wobble due to imbalance--which is always present--is amplified by the bouncing of the tire on the road. Another example is a child on a swing: the amplitude of the pendulum motion increases when the motions of the torso and legs are properly synchronized with it, but not otherwise.
We use different terms for different instances of the same underlying phenomenon. When we talk about riding a swing, we call it "pumping"; when it's a vibrating tire, we say "resonance" or "sympathetic vibration." When the subject is music, we speak of harmony or being in tune.
"Flutter" is the term used for synchronized vibration when it takes place in a flexible structure moving through a fluid medium--for instance, an airplane in flight. It occurs when two regular, rhythmic motions coincide in such a way that one feeds the other, drawing additional energy from the surrounding flow. In airplanes, there are countless combinations of vibrations that can join forces in this way.
Each component of the airplane has, like a guitar string, a natural or fundamental frequency, plus a whole family of harmonics--integral multiples of the fundamental frequency--of diminishing strength. A classic case of wing flutter might combine wing bending--a flapping motion of the entire wing--with either wing twisting (torsion) or the flapping of an aileron, which has the same lift-amplifying effect as twisting the wing does. But there are myriad other possibilities involving all sorts of combinations of bending, twisting, and flapping, each with its own fundamental and harmonic frequencies, in wings, tails, fuselages, control surfaces, and trim tabs. Out-of-balance tires seldom lead to structural failure of the car because automobile suspensions are vastly overbuilt for the loads they normally encounter. But airplanes, which must be kept as light as possible, are not superfluously stout. They are capable of failing with explosive suddenness when flutter sets in.