posted on Oct, 6 2004 @ 04:11 PM
My colleauge Mr. Dreamstone is correct when he says that most of the helicopters' aural signature comes from the tail rotor, but even the
NOTAR (no tail rotor, which patented concept is owned by Boeing and licensed to MD Helicopters) and the fenestron designs (enclosed tail rotors) are
not completely quiet.
There are several ways to ameliorate tail rotor signature; the AH-64 A and D Apaches have their tail rotors offset (so that they look like an 'X'
rather than a 90-degree cross).
Remember that there are three signatures important to helicopters; of these, the most important is IR signature, since most threats today come from
shoulder-fired IR-seeking missiles. Most helicopters today have adoptedc the "black hole" technology, pioneered by the AH-64A back in the
seventies, where by the heat signature from the engines is diffuesed through an external "heat-muffler".
Radar signature or RCS is of secondary importance; even the now-defunct AH-66 Comanche had a poor (i.e., overly large) RCS, especially when its stores
carriage doors were open. since most radar "stealth" involves bouncing radar away from returning to the emitter rather than absorbing the
signal, the physical construction of the aircraft is a tremendously important consideration. Unfortunately, with the rotors constantly moving, the
helicopter simply cannot keep from bouncing back a signal to the emitter, and its RCS, therefore, is inherently large.
Aural signature for a military helicopter is probably the least important "stealth" aspect, since most of the new "fire and forget" air-to-ground
missiles have an effective range far beyond the distance that the sound of the helicopter would carry. For civilian helicopters, aural signature is
more important, since these thyically overfly cities and other built-up areas, and are subject to FAA and other regulations regarding noise pollution.