Bad Vibrations

We’ve all been warned to watch out whenever our airspeed enters the yellow arc, and we’ve learned to absolutely stay away from never-exceed speed, VNE. Here’s why. In addition to the g-loads exacted by turbulence, there’s the small matter of something with a misleadingly serene and delicate name: flutter. The term is evocative perhaps of a gently quivering feather, or among audiophiles perhaps suggestive of an audiotape with an inconstant speed—or if you’re a cardiologist, something worse. It is also a dangerous phenomenon encountered in flexible structures including buildings, telegraph wires, and bridges (and of course, airplanes) whenever they are subjected to aerodynamic forces.

Enhanced by the innate flexibility, and the amounts of aerodynamic loads airplanes can sustain, flutter can be the demon at the end of the airspeed spectrum after VNE. But don’t forget that even well before that red line, at the beginning of the yellow arc (VNO) or maximum structural cruising speed, 14 CFR Part 23 only requires airplanes to withstand about a 30-knot gust (50 fps), and tolerance drops off rapidly after that. The thing to remember is that while flutter may haunt the realm of excessive speed, if the air is rough enough, structural damage can occur at any speed.

What is it? Flutter occurs as a result of interactions between aerodynamic and inertial forces. For an aircraft wing, flutter may occur when the aircraft is accelerated to a speed where, when even slightly disturbed, the wings flex, and the resultant vibrations do not have sufficient damping. In fact, they may be intensified by resonance frequencies.

wing hits gust, begins upswing
wing completes upswing, twists downwards
wing goes back to lowest point, twists upwards again

Shown above is the simplest two-dimensional case of bending/torsion flutter. When a wing hits a gust it experiences an increase in lift. This causes the wing to flex upwards and due to the location of the center of pressure, bending is often combined with torsion as shown in the first image. The resulting torsion causes a change in lift and stress on the internal structure, causing the wing to swing back downwards. At the lowest point, the wing begins to twist back up again. (Depending on control balance, the trailing edge of the aileron may lag the movement of the wing — thereby increasing torsional loads. This is why high speed aircraft have ‘balanced‘ controls.) The movement is naturally occurring to an aircraft in flight, but in the case of flutter, such vibrations are divergent. Ultimately, the increasing flexure can reach the point where the wing just rips right off. A famous case of prop whirl flutter that happened to an airliner in 1959 was recently written about by Peter Garrison (in the February/March 2001 issue of Air & Space Smithsonian). It tore a wing right off the fuselage. Of course, it doesn’t have to be wings.

Many everyday things, aside from metronomes and pendulums, have resonance frequencies. The famous case of the Tacoma Narrows Bridge, tearing itself asunder in 1940, is another case of flutter and clearly illustrates the resonance factor of the equation. Everything on an airplane has an inherent vibration frequency. Go fast enough, and something will start behaving like a flag in a stiff breeze. I’m as patriotic as the next guy, but this is not a good thing. The classic case of flutter is probably the loss of control while in IMC and subsequent overspeed. How many of us can’t remember reading the usual “eyewitnesses reported seeing airplane parts raining down through the bottom of the clouds…”

In-flight flutter testing is required for certification of an aircraft. The aircraft is required to be safe up to something known as the design dive speed, or VD (which is actually about 11% higher than VNE). At that point, an outside force (like a wingtip oscillator, or even a firm tap on the controls) is used to simulate a wind gust. The force must provide sufficient excitation that, if flutter were possible at this speed, it would be induced. Accelerometers or strain gauges on the wing, tail, fuselage, and control surfaces provide data for the test engineers, and their feedback is analyzed for unstable indications. Incidentally, if there’s so much air resistance that the airplane can’t reach VD even when it’s pointed straight down, then the test pilot’s ‘top speed‘ becomes the demonstrated diving speed, known as VDF. The listed VNE can be no more than 90% of that (of which that 11% overage noted above is the reciprocal).

The yellow arc exists to warn us about the structural limitations of the airframe. When the air is dead calm and the aircraft is moving relatively slowly through it, the safety margin against divergent oscillation is great. But any sudden gusts at high speed, and you become a test pilot. Add in an imbalance in the ailerons (one loose counterweight), and you’ve just introduced a wingtip oscillator capable of introducing a divergent resonance. Other maintenance related problems, skin damage, or other asymmetries may also induce flutter.

BOTTOM LINE: A cursory outside inspection won’t always reveal that your aircraft might be a candidate for self-destruction: The imbalance doesn’t have to be on the outside. Poorly rigged control rods and slack cables can cause a very unwelcome sort of wiggle room. If you ever feel–or hear–a vibration somewhere, and you’re going downhill pretty fast, level the wings (if they aren’t already), reduce power, and gently raise the nose (to slow down some more). Airplanes are meant to go fast–but there are limits. And those limits can be closer than you think.