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On Fri, 4 Aug 2006 13:01:50 -0400, mark_digital© <xxx976r@comcast.com>
wrote:
[color=blue]
>OK, there's two identical helicopters. One has no tail rotor, and uses ram
>jets on the tip of it's blades. The other helicopter uses a piston engine
>and has a tail rotor. Both of them set out to do the same travel distance
>and lifting while stationary in the air. They both return. Which one used
>less fuel?
>Yes of course this is a trick question.[/color]

Well, for starters they don't sound "identical" at all, so you'll
have to clarify in what ways they are "identical". Dry weight,
payload, fuel load, etc.

In conventional aircraft jet engines (turbo-prop or turbo-shaft for
a heckicopter, or full jet) usually aren't as economical to operate as
a piston engine for equivalent work, unless you make the blade to
housing tolerances in the jet very tight and run them really hot
(pushing close to the melting point of the alloys used) to get the
best thermal efficiency out of them - which invites a disaster if the
engine comes apart in flight. You sacrifice some efficiency in the
name of reliability and safety.

Plus, ramjets on the tips of the rotor won't work for high speed
travel - the ramjets on the leading edge of the blade go supersonic,
and as soon as they go around to the trailing edge the airflow slows
to an effective stall. That's not going to make for a very efficient
ramjet rotor, unless you restrict it to low speeds - and then you
can't sell it.

And if you are using ramjets on the main rotor, where are you
getting the thrust to run the blower for the tail rotor? You need
something to counter the reactive forces of the main rotor turning.

There is one glimmer of truth hidden in there - Hughes has proven
that the NoTaR design works for small helicopters, they duct the
engine exhaust and a ducted fan though a nozzle on the tail boom for
vectored thrust. That imitates a tail rotor with no big exposed
moving parts.

--<< Bruce >>--