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 4 of 5)
Today, the practice of carrying armaments and auxiliary fuel on external pylons, and the vast variety of possible combinations of external loads, make flutter analysis of modern fighters especially difficult. On the other hand, composite structures using carefully controlled arrangements of graphite and other exotic fibers are much stiffer than aluminum or steel, and can even be made to deform under load in such a way as to reduce aerodynamic loads and therefore the chance of flutter.
Increasingly, new fighter and transport designs rely on electronics for stability and control, and in some cases for flutter prevention as well. The F-16, for example, is prone to a non-destructive wing flutter when carrying certain combinations of external loads. The wings flap out of phase--the left wing goes down while the right one goes up--imparting a rocking motion to the fuselage. Rather than modify the wing, researchers at the U.S. Air Force flight testing facility at Edwards Air Force Base in California programmed the fighter's electronic flight control system to sense the flutter and use the ailerons to oppose the wing's flexing. The fix, which will be incorporated in a flight control software upgrade scheduled for 2002, is indicative of what electronic flight controls can do. But they are not a panacea for flutter; the number of control surfaces available on an airplane's wings and tail falls far short of the number of possible flutter modes they can exhibit.
Discernible in the future are "smart" materials that expand or contract slightly in response to electrical signals. "They're like muscles," says Tom Noll, head of the aeroelasticity branch at NASA's Langley center. "They're normally in a neutral state, but they can be 'flexed' when extra stiffness or resistance to deformation is needed." Another possible weapon against flutter comes from the new field of MEMS--micro electro-mechanical systems. Thin surface overlays could raise thousands of tiny spoilers on an electrical command, disrupting airflow and preventing the aerodynamic augmentation that is fundamental to flutter.
Flutter analysis is often called a black science. Even though flutter is today well understood and largely preventable, it is still as formidable a foe as ever, and its malevolent unpredictability remains. "Some fear flutter because they do not understand it," said the famous aerodynamicist Theodore von Karman. "And some fear it," he added, "because they do."
Tuning Up for Disaster
When a musician wants to ensure that his instrument is playing at the proper pitch, he compares a note to that of a vibrating tuning fork. The tuning fork is a reliable standard because it always vibrates at exactly the same frequency and therefore hums at only one pitch, regardless of how you hold it or how hard you strike it.
When, hundreds of years ago, clockmakers needed a reliable way to measure small units of time, they turned to the pendulum. As Galileo had discovered in 1582, and as a child on a swing may notice, a pendulum of a given length always oscillates with the same frequency, regardless of the arc through which it swings.
The utility of tuning forks and pendulums as standards is due to a common physical phenomenon. A great many objects and structures have what is called a natural frequency of vibration. The natural frequency depends on the mass of the moving object and the stiffness of the "spring" that makes it oscillate. In the case of the pendulum or the swing, the spring in gravity, and in the case of the tuning fork, the elasticity of the steel. All elastic systems have this property, including the structures of airplanes. If you timed the up-and-down motions of an airliner's wingtip as it flies through rough air, you would find that its frequency is constant, regardless of the strength of the turbulence, because and airplane's wing is like a huge, very-low-frequency tuning fork.
An object is said to "resonate" when it begins to vibrate in tune with some other vibrating object. A sufficiently loud sound at 440 cycles per second, for instance, will set a tuning fork in the key of A to humming. Resonance occurs because it is easy to make things vibrate at their natural frequency, but difficult to make them vibrate at any other frequency.