Abstract: Autorotation in helicopters at low altitudes can be achieved even when the inherent design of the helicopter would prohibit successful entry into the autorotation mode. Angular kinetic energy from the main propulsion engine is stored in a flywheel which automatically transfers this energy to the main drive shaft in the event of engine failure. During normal operation, the flywheel is driven through the main drive shaft by coupling to a secondary drive shaft. The main drive shaft in turn is driven by the engine through an overrunning clutch assembly. The flywheel is maintained within an evacuated housing whereby air drag is substantially eliminated. In the event of engine failure, the overrunning clutch disengages the engine from the main drive shaft and flywheel will continue to apply energy to the main drive shaft through the secondary drive shaft to permit autorotation and a no-damage landing even at low altitude flight levels.