Aviation Glossary - Blade Runaway

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Blade Runaway

The uncontrollable continuing increase in Rotor RPM as a result of compressibility effects on the rotor blades as the rotor blade tip speeds approach the Speed of Sound. As the rotor blade tips approach the Speed of Sound, the Center of Lift on the rotor blade moves aft on the blade and can cause the rotor blade to twist from root to tip to a lower AOA of the rotor blade airfoil. The effectively lower AOA of the blade causes further increase of RRPM to maintain autorotation, which further increases rotor blade tip speed – a continuing cycle of increasing RRPM – “runaway”. The repositioning aft location of the center of lift on the rotor blade also increases the forward cyclic stick pressure – eventually overpowering the ability of the pilot to pull the stick back at higher blade runaway speeds.

source: Glossary of Gyroplane Terms

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