A big deal has been made about the V22 Osprey 47 degree blade twist
and how it affects the Vortex Ring State. Unlike a helicopter
high blade twist is necessary in a tilt rotor to get good
propeller efficiency at high speed. What the V22 critics don't
realize is an off the shelf variable twist propeller blade is already
on the market and has been proven to work. Namely the IVO. We are not
affected
to the same extent as the V22 is to VRS as we have only one rotor axis.
However blade
twist negatively affects the efficiency of the rotor in the vertical
take off mode.

I am going to check with IVO and see if they can build us a 6.5 foot
long variable twist blade. http://www.ivoprop.com/

Paul Lamar ...No rotor no motor.


Paul,

I don't think the blade twist has anything to do with vortex ring state on
the V-22.  I think the reason it gets into that is that it has a very high
disk loading and a lot of vortex energy to get it going.  Another term to
describe this phenomena is "settling with power".  Almost any helicopter can
get into this situation but it is aggravated by disk loading.   That IVO
prop idea is interesting.  Are you going to put some cone angle in the hub?

Steve


There is a whole group of people that disagree.
There is no doubt a twisted blade with 47 degrees of twist, same as the V22
Osprey has, is less efficient in the helicopter mode. Check out:
http://www.rotaryeng.net/VTOL.html as I updated it today.

Paul Lamar ...No rotor no motor.

Paul,

That's an interesting article, I'll go through the whole thing at work
tomorrow when I need something mentally stimulating.  In the introduction,
the author pretty much agrees with what I said.  That picture does a good
job of describing what happens.  It looks like a toroidal tornado with the
center near the rotor tips.  The aircraft is most susceptible at low speed
when the vortices can build on each other and in or near ground effect
height.

I didn't comment about the V-22's efficiency in hover, but as a rule, the
higher the disc loading, the worse it gets.  Looking at the V-22 in hover, I
would think that the inboard third or so of the blade is stalled so it's
taking extra horsepower to turn it.  On a helicopter, the inboard third of
the blade really doesn't do that much, but it's nice to have it faired to
reduce horsepower required and preserve energy in autorotation.  That's what
would be interesting about your IVO prop idea--you could take some of the
twist out in hover.

Steve


Are you saying the blade twist is the reason the V22 won't auto rotate?

Paul Lamar ...No rotor no motor.


I was standing around looking at the twist on a helicopter blade one day
trying to visualize the airflow going up through the disk, as in
autorotation, and I concluded that the correct twist for the airflow going
up is opposite that required for the airflow going down.  Most of the
autogyros I've seen have no twist.  There was an article that mentioned this
subject in Experimental Helo magazine a few months ago that agreed with my
conclusion .

http://www.experimentalhelo.com/index.htm

When you think about it, in autorotation, the inboard portion of that blade
on the V-22 is probably 90 degrees to the airflow.  That can't help
autorotation.  On top of that, you have its enormous disk loading--I haven't
seen any numbers or bothered to calculate it myself, but I would guess that
it is higher than anything in the inventory.  I talked to the Air Force test
pilot for the thing and quizzed him pretty good.  He said the thing will
autorotate, but the descent is around 5000 feet per minute and I didn't ask
him if that was at gross or pilot only and low fuel.  To make it even worse,
the blades are relatively low inertia so there is very little energy to
flare, much less cushion the "landing".  He told me that in the event of
total or significant power failure you bring it in like a fixed wing with
the nacelles rotated up about 30 degrees (I think) so the blades don't hit
the ground.  The blades are all carbon and they are designed to shred, but
stay sort of together and not throw pieces around--there are no weights in
the tips to fly off.

I went through that article you posted and I can see how the blade twist can
exacerbate that Vortex Ring State.  If you visualize that rotating mass of
air, the additional twist allows the inboard portion of the blade to add
energy to the thing.  I haven't finished digesting that article, eigenvalues
give me a headache.

Steve

The disk loading of the V22 Osprey is about 166% as comparable helicopters.
About 15 pounds per square foot. Most helicopters are 9 pounds per square
foot.
I am using the same 15 pounds per square foot disk loading.
Here is a chart I have on the web site. http://www.rotaryeng.net/VTOL.html

The temporary solution to VRS under power was to limit the rate of decent
to 1000 FPM. I think a variable twist blade will expand the envelope
downward.
They need to equip all the Marines with 24 foot diameter air bags in case
they run out of gas or both engines stop.

I hope Boeing is reading this newsletter :)