Re: [Flightgear-devel] Chinook Was: Re: Helicopter: First Impressions

[Josh B. <jba...@at...>]

Joacim Persson wrote:
> On Tue, 27 Jun 2006, Maik Justus wrote:
> 
>> I'm working now on simulating the rotor on several points along the
>> rotor blades (with twist).
> 
> I suspect that twist to vary with load. It's the tips that has a 12° higher
> incidence (sic!) than the root, when the rotor is stopped that is. I guess
> that's a pre-tensioning of the blade; that the lift moment on the blade (it
> has some naca profile if I remember it right) will twist it back to a more
> straight shape under load or perhaps even twist it the other way.
> 

But you don't *want* the rotor to be straight. Assuming a constant
airfoil section along the length of the blade, you would want a constant
aoa so that you can get the maximum lift out of the blade. Otherwise,
some sections of the blade would stall earlier simply due to too much
aoa, which is a waste. (ok, in reality the lift distribution is probably
not flat, ant the roots are probably made to stall first, but it has to
be relatively close or you are wasting blade)

The air column passing down through the disc is pretty much all moving
at the same speed, but the tips and roots of the blades are moving at
extremely different speeds. If you think about the velocity vectors
relative to the air mass, the tips have to be a lot flatter than the
roots to have the same aoa. It's the same with a propeller, they have
something like 45deg of twist to them.

I believe that it is possible to make rotor blades that resist torsion
quite well. Older blades used to use a rectangular metal spar, and these
are very stiff torsionally. Modern blades can be even better with the
addition of composite fibers oriented to resist torsion. It is possible
to see in many photos of helos that even at takeoff, where there is a
great deal of torque, the blades still retain basically all of their twist.

Also, the load bearing part of a rotor blade (spar) is located almost at
the leading edge, far ahead of the center of lift. This creates a
torsional moment opposite of that of most airfoil sections. A reasonable
engineer would put the spar in a spot that makes the two cancel out
completely under normal flight conditions.

Josh