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Re: [ST] Emile - Coaxial Rotors



Thanks for that Emile. Mostly it is the explanation of the terms to describe whats happening. I have a better understanding now.
 
Would I pull your leg????
 
The coriolis effect is present as it is on any spinning object. In this case as the rotors spin and become a "disc" the coriolois effect, effects the way the air slides off the disc. I'm not sure how much of an effect it is and how much a helo pilot would have be concerned about it, but it is mentioned in the helo basic flight handbook.
 
But as you say its off topic and not really relevent to the counter rotating brakes discussion.
 
Brett.
 
-----Original Message-----
From: Emile Nossin <Emile@xxxxxxxxxx>
To: ST@xxxxxxxxxxxxxx
Sent: Sat, 22 Apr 2006 00:03:08 +0200
Subject: Re: [ST] Emile - Coaxial Rotors


>From: BrettWilson21@xxxxxxx
>How does it change direction with only one control movement?

Getting a bit too much off topic perhaps.. I used the heli analogy
to try and see a logic between canceling out precession. I'm not a
helicopter expert, let alone on one with coaxial rotors.
I can guess some logical answers though. What you mean is what
they mean with this sentence:

"For translational flight along any axis, the movement of only ONE
control is required"

What that means is that a transition along one axis (any axis), so
not a change of direction but a change of speed or rate along the
same axis, for example going from hover to forward flying, doesn't
require any control cross-coupling. A conventional helicopter is
hard to fly because whenever you increase the RPM or blade angle
(or both) of the main rotor, a torque reaction will take place
which will require "rudder" input to control the tail rotor to
counteract the resulting yaw motion. Also when converting to
forward or sideways flight, there will be a precession effect,
requiring more collective and rudder input to counteract that.

With a coaxial rotor system you can just increase the power 
simultaneously on both rotors and go up, without needing any
rudder input (transition along vertical axis). When you tilt
both rotors forward or sideways you will also be able to do
that with one control, no cross-coupling with the rudder is
needed. It makes it lighter and MUCH easier to control, on
top of that you can put a lot more power on the rotorsystem
without adverse effects or risks. Of course the complexity of
the double rotor system (maintenance costs) probably didn't
help the development, but I believe the US airforce / army
uses coaxial rotors on their unmanned, remote controlled
surveillance helicopters.

>What happens to the coriolis effect from two rotors spinning in
>opposite directions so close to each other?

Not sure you're pulling my leg here :-). Read up on your coriolos
effect knowledge: http://www.answers.com/Coriolis%20effect
To save you the time, the coriolos effect is the effect of the
earth turning as seen from an observer on that rotating planet.
There's no coriolis effect involved with this discussion at all.

>From the picture, it looks like the traditional collective & cyclic have been 
>done away with, so what is there to control the pitch of both rotors and rpm?
>Do both discs tilt to change direction?

I think indeed both rotors increase RPM and / or blade angle at the same
time for vertical changes. To yaw left or right they probably change the
blade angle and / or RPM on one rotor to induce a torque reaction. To
"change direction" you can tilt the whole rotor unit (I think, but I
haven't dug into this heli design very much yet).

>In traditional helicopter design there is a 'do not exceed speed' as the 
>retreating blade can stall as the forward speed matches the retreating blade 
>speed. How has this been overcome, or is it no longer relevent due to the 
nature
>of two opposing spinning blades?

The same would apply here, but since you have two rotors you can reduce
the size of each rotor for the same lift / power. Reducing the diameter
means that the tips of the rotorblade will have a lower speed as well.
And that speed at the tip of a rotorblade is often a critical factor
(I guess) since the rotational speed plus forward speed could reach
the speed of sound, which can cause a destructive supersonic wave on
one side of the rotor. Decrease the blade length, reduce the tip speed.

>Please keep your answers short, concise and easy to understand ;-))

Oh sorry... ehmm.. in that case the answer is:
1. No torque and precession effect = no rudder control needed
2. Coriolis has nothing to do with this subject
3. Same as conventional heli
4. Two rotors means less diameter means less tip speed means
   you can increase RPM to reach speed of sounds on tips means
   you can go faster

Better, Brett? :-)

Emile
www.piloot.com

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