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Vibration is the enemy of helicopters. It causes screws to loosen, parts to break, gyros to work badly or not at all, fuel to foam, and generally make your life as an RC pilot hell.

Any large rotating mass that is not completely in balance can cause vibration. The most common causes are:

If the main blades are not balanced, or are out of track, or if the paddles are not equal distances from the head or have unequal blade pitch, or the main mast or feathering spindle is bent, or any other part of the head is bent, badly machined or out of alignment, you will likely get '1 per rev' (once per revolution) vibration. Tracking this down is usually a matter of stripping the head down to a bare shaft, and then slowly building it back up piece by piece until the vibration reappears. I have had vibration occur when the washout to Bell-Hiller mixer linkage and Bell-Hiller mixer to blade grip linkage on each side were of different lengths; this can leave the main blades in track but add a lot of vibration as it 'pushes' on the flybar continuously. Any play in the head or mast, caused for example by a damaged ballrace can also create vibration.
The tail rotor may not be as large as the main rotor, but it rotates up to 5 times as fast, and is less well supported, so any imbalance here can cause a great deal of vibration. Some causes are the tail shaft being bent; the tail blades being out of balance, or the rotor hub being badly machined. Also check the tail case is screwed tightly together, and there is not play in the tail shaft; and that the boom stays are tightly screwed to the boom so the boom cannot move up or down, or left or right.
Reciprocating (piston) engines bring with them their own set of vibration problems. Ideally, the piston and crank counterbalance should be calibrated at the factory, but other elements such as poor ignition timing (cause, possibly, by a poor choice of fuel or glow plug in a nitro engine) can cause unnecessary vibration. The engine cooling fan and clutch can also be badly attached to the crankshaft and cause vibration.
Much less common, the can of a brushless motor can be out of balance, or the shaft slightly bent.

See also

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