Subject:
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Re: Human Powered Helicopter
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Newsgroups:
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lugnet.org.ca.rtltoronto
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Date:
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Tue, 10 Aug 2004 02:15:31 GMT
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Viewed:
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566 times
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In lugnet.org.ca.rtltoronto, Ivan Louch wrote:
> “The coaxial counter-rotating rotor system negates the need for a tail rotor and
> its drive system. Because of this, this aircraft is unaffected by wind strength
> and direction, has an unlimited hovering turn rate, and gives a smaller profile
> and acoustic signature, while allowing a 10-15% greater power margin”
I think I hear my cue... ;]
Those claims, out of context, are misleading/exaggerated. The aircraft
is certainly affected by winds, just not in the same way or with the
same unique problems as a conventional tail-rotored aircraft (although
it probably has its own unique problems). Ditto for hovering turn rate.
And there's not enough details given with "greater power margin" for it
to be meaningful---where, in hover? At max speed? At max rate-of-climb?
Compared to what? Etc.
> The typical deafening helicopter
> “thumping” noise is nearly non-existent
To be fair, that sound is generally only characteristics of 2-bladed rotors,
particularly any of the aircraft from Bell (Huey, Jet Ranger, etc.). Rotors
with more than 2 blades and with the associated higher rotor speed tend
to have much different sounds. As well, much development has gone into
noise signature reduction, particularly for military birds. It's just that
the civilian market continues to be stuffed full of 2-bladed Bell aircraft,
so that's the sound most poeple associated with helis.
> All of this said, would anyone actually happen to know why this technology seems
> to be widely used only by the “Soviets” and only for military applications? Or
> at least am I not aware of any commercial co-axial rotor designs.
Kamov (Russian) made a pile of designs using coaxial rotors. (I recall
Sikorsky started out with a similar concept before heading in a different
direction, but maybe I'm thinking of someone else). Kamov's are beginning
to see civil service now outside of Russia, as people begin buying them
for unique roles (for example, heli-logging).
Kaman has an inter-meshed twin rotor design that is similar ("K-MAX"):
http://www.kamanaerospace.com/photo/index.htm
Thought not coaxial, tandem rotor aircraft (Chinook, Sea Knight) and
twin rotor aircraft (Osprey) are also a similar concept, that is to
try to cancel any rotation tendencies with the main rotor system itself.
Among the down-sides of these arrangements are higher drag due to the
larger/messier rotor hub, which ultimately limits forward speed. The
tall rotor mast is not good from a stuctural point of view, so life
of the mast is an issue. The design is also complex mechanically (but
what heli isn't?!), which creates other issues. I have also been told
that some manoeuvring response can be slower than a conventional
tail-rotored heli, unless there is a sophisticated control system in
place (=extra cost and complexity).
Among the advantages quoted are a smaller airframe and safer ground
ops (don't need to worry about people walking into the tail rotor, which
happens more often that you'd think!). But few coaxial helis have been
able to get away with a short tail in practise, because you still need
something back there for directional stability (to keep the noise
pointed forward) in forward flight. Some of the smallest Kamov aircraft
have short little tails but huge barn-door vertical stabilizers for
directional stability.
Ultimately, all helis are hugely compromised designs. Since much of
their characteristics are still black magic even to we engineers, the
debates continue to rage about which configuration is best for which
purpose..., fuelled by the marketing hype of the different manufacturers.
More often than not, companies have to build something different because
everything else has been patented...
KDJ
______________________________
LUGNETer #203, Ontario, Canada
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Message is in Reply To:
 | | Re: Human Powered Helicopter
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| (...) Even though he only has to spin the rotors at 6.2 rpm and the total weight of all four blades is only 55.4kg, the sheer size of the “wing-span” is indeed daunting! (...) Ohh the co-axial rotors are just beyond sweet! I doubt I can sum it up (...) (20 years ago, 8-Aug-04, to lugnet.org.ca.rtltoronto)
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