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What is Flutter?

            Flutter is one of the worst things that can happen to an aircraft in flight.  It is instantly a life-threatening situation.  It takes just a second or two to break apart major airplane structures and control surfaces.  It is not uncommon for flutter damaged aircraft to break apart into pieces in the sky.  Events of flutter have a bad fatal accident record. 

            Flutter is caused by dynamic pressures finding a resonant frequency that oscillates the structure perpendicularly.  The oscillations are divergent, which is why flutter rapidly gets worse.  The up and down (or back and forth) motion gets stronger at a particular frequency until the energy is transferred into sound waves or bending of metal parts past its engineered load limit.  The structure can no longer perform.  If a control surface is lost, control is impossible.  The plane immediately rotates violently, but still carries tremendous amounts of inertia.  The airplane then has so much pressure drag at high speed, the wings or tail sections can snap off.  This triggers the rest of the in-flight breakup, which happens for the same reason. 

            Flutter happens because three factors are just right for it to start.  The factors are high dynamic pressure, structural stiffness, and inertia.[1]  Dynamic pressure is the product of one half the density of the air and the square of the velocity.  This is the formula for Bournoulli’s principle.  As you can see, dynamic pressure is directly proportional to speed and air density.  Structural stiffness is required for flutter.  Without stiffness, the structure could not store energy elastically.  This property allows large amounts of energy to be built up in between cycles.  Inertial forces is the final factor which causes flutter.  Every structure has a natural frequency which will make it oscillate.  An example would be a tree swinging in the wind. Thankfully though, engineers work out flutter problems before aircraft become certified. There have been very few cases of certified aircraft breaking apart from flutter.