The NOTAR Alternative
The most vulnerable and dangerous part of a helicopter is the tail rotor. On most makes and models it is fully exposed to the environment. Even a small piece of debris in the air, hitting at high speed, can mangle the tail rotor and render the helicopter inoperable, most likely causing a crash.
Some manufacturers have adopted a partially enclosed tail rotor design where the ends of the rotor blades are protected by the body of the tail shaft, but where most of the rotor is still exposed. This lessens the probability of a debris strike, but doesn't remove the risk entirely.
MD Helicopters Inc. has exclusively developed a rotor alternative called NOTAR (it's an acronym for NO TAil Rotor). Since there is no rotor at all, a debris strike to the tail is unlikely to pose a risk. NOTAR is a system where air is jetted through holes in the end of the tail. The air intake is between the body and tail shaft and is powered by the main rotor engine. This provides essentially the same stabilization and control in flight as a tail rotor. A postive side-effect is that helicopters with NOTAR are considerably quieter than traditional rotorcraft.
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