Control Surface Flutter

FLUTTER:
A phenomenon whereby the elevator or aileron control surface begins to oscillate violently in flight. This can sometimes cause the surface to break away from the aircraft and cause a crash. There are many reasons for this, but the most common are excessive hinge gap or excessive "slop" in the pushrod connections and control horns.

One of the most common causes of flutter is loose linkage between the servo and control surface. This is easily corrected but often overlooked or ignored. It only takes a moment to jiggle your control surfaces by hand to see how much movement is there and determine its origin. Servo gears get worn, clevises get worn, and control horns get worn. Not if, buy when your surface flutters you will likely crash if immediate action hasn't taken place. If you ever hear a low-pitched buzzing sound, reduce throttle and land immediately, if you haven't already crashed...

Flutter can occur at any time on any control surface if the conditions are correct. All of your controls are subject to flutter but the typical pecking order is: AILERONS, ELEVATOR, then RUDDER. This isn't always the case though! If you lose a control surface because of flutter here is the likelyhood of crashing and here is your percentages.

CONRTOL SURFACE LIKELY to CRASH % NOTES
Single AILERON dismemberment: 30-50% You can fly on one aileron,  it's mushy to control but very flyable
Double AILERON dismemberment: 50-80% You can fly with no ailerons, but you must use your rudder in place of ailerons
Elevator dismemberment: 100% Crash is immenent.
Aileron single stripped servo gear 80-100% depends where the damaged aileron neutrals at, and IF it doesn't move
Aileron double stripped servo gear 100% Crash is immenent.
Elevator stripped gear 100% Crash is immenent.
Rudder stripped gear 80-100% it will likely crash but there is a slim chance of getting down

These percentages are based on "experienced" pilot skills and on "common" flutter conditions which will vary dramatically in flight. As you can see, the odds are stacked against you. So check those control surfaces.

Other causes of flutter are; Airgaps, Loose or broken hinges, and speed.

Sealing the Airgaps on your control surfaces not only minimize flutter, they also improve the efficiency of the controls. This is a good thing.

Loose or broken hinges must be repaired, as this gets perpetually worse with every flight.

Speed. If you fly your airplane beyond it's intended envelope you are rolling the dice every time. Sooner or later flutter will occur unless preventative measures were employed.

One thing about flutter is,  it makes LOTS of NOISE prior to blowing up! If you react quick enough there is a chance you can save your airplane. Immediately cut your throttle and land.  

mark@fuess.net

Copyright GSWAM 2006