Autorotation Aerodynamics

[inventorguy]

There used to be a book "Basic Helicopter Handbook"

 

It had very good explanations of all that kind of stuff.

 

Published by FAA or HAI or someone like that.

 

Try this.

 

http://www.geocities.com/flyingmouse1/Chap...2.html#fig%2023

 

Took me 10 minutes to find -please send check for $100

 

 

Hey Shark',

 

Your lucre (in pennies) is winging its way to your location by passenger pigeon as we speak. If it doesn't arrive, the cause is out of my hands, and may be a result of the pigeon having to switch to autorotation mode due to exhaustion before reaching your co-ordinates. Sorry.

 

I will check out this link.

 

Thanks lots.

[Over-Talk]

Yes, helicopter blades do have less pitch out at the tips.

This is sometimes called "washout", and as you suspected, this is to compensate for the relatively faster airspeed over the tips of the blades, compared to the roots.

[Phil Croucher]

Sorry, I should have said that the middle part has no drag (those single malts again):

 

At the tip, the TR lies behind the axis of rotation - in the driving region, it is on the axis, and it is in front at the root. At the tip, therefore, you get a small portion of drag, just as you would get in normal flight. In the driving region, all you get is rotor thrust, and at the root, the horizontal part of TR assists rotation and tries to accelerate the blade (i.e. it becomes autorotative force). At constant rotor RPM, rotor drag = autorotative force so there is zero torque at the hub if you ignore hub bearing friction.

 

Thus, the outer part is always providing lift.

 

Phil

[cap]

Phil Croucher ------- you just gotta watch those Single Malts buddy. d*mn, you should know that by now and you an ex-military man and all. Shame on you and wipe that silly grin off your mug too. Grinning about this is not allowed.

 

Now, concerning the subject at hand. Although I've personally had the 204 in the air on many occasions, I know for a fact that it cannot fly. I know that because when the US Army first trained me on said beast, they showed my class a movie about tests that had been done on the 204. This was to further advance our confidence in said a/c. In part of this movie it showed the 204 on a large tarmacked area and hooked by it's belly hook to an embedded hook anchored in cement. The a/c gradually pulled power to max. As this was taking place and finally when it reached that point in time, I cannot possibly describe to you what those M/R baldes were doing.They reminded me of elastic rubber bands and not anything remotely to do with metal of any type or anything to do with keeping or getting anything into the air. It inspired no confidence whatsoever and how it gets into the air, remains in the air or attempts anything to do with an autorotation is totally impossible based on my experience and observations.

 

Now if you would like to continue further with your highly testicle.......I meant technical description concerning autorotations, you just go right ahead...........but NO Single Malts before doing so........and don't give me any 'horse-poop' about it being because of the heat there either.

[inventorguy]

Sorry, I should have said that the middle part has no drag (those single malts again):

 

At the tip, the TR lies behind the axis of rotation - in the driving region, it is on the axis, and it is in front at the root. At the tip, therefore, you get a small portion of drag, just as you would get in normal flight. In the driving region, all you get is rotor thrust, and at the root, the horizontal part of TR assists rotation and tries to accelerate the blade (i.e. it becomes autorotative force). At constant rotor RPM, rotor drag = autorotative force so there is zero torque at the hub if you ignore hub bearing friction.

 

Thus, the outer part is always providing lift.

 

Phil

 

Phil,

 

I happened yesterday to be visiting with one of my neighbours who used to be a helicopter mechanic. He told me one piece of valuable information of which I wasn't aware. And that is that not ALL helicopter blades are designed with a twist to them. But, knowing that for a helicopter to get approved to fly it must be able to autorotate, I adjusted my perspective on this whole issue slightly.

 

If a helicopter blade with no twist can autrorotate, then the varying angle of the lift vector along its length is due entirely to the varying angle of attack of the apparent wind. And that varying angle of attack is due mostly to the increased component of the apparent wind caused by the increased velocity of the blade the farther along that blade that one is from the hub. Ergo, it would seem that the twist designed into helicopter blades is to assist in their function during powered operation, and said twist actually INTERFERES with autorotation; but apparently not at a level that is critically debilitating.

 

This understanding is the most incisive that I have been able to distill (strictly information distilling I'm talking about here) from all of the various perspectives that I have encountered so far.

 

Thanks for your input.

 

 

[Phil Croucher]

hey cap - I'm in Spain right now and had to bring my own single malts with me Wouldn't mind looking at those movies myself - are they on utube?

 

Inventorguy - good points, and it is true that washout is not perfect, in fact proper washout would be so inefficient that they have to compromise at about 10 degs or so so that we can fly the machine. The end result is that, although washout is an attempt to even out lift along the blade, the downwash at the tips is still more simply because the blade is going faster there.

 

I'm told that the blades on the Wessex were put into a large vice and twisted!

 

cheers

 

Phil

[windslapper]

I found Phils answers as usual to be great.

One must remember that relativity has a large part in anything moving. As the helicopter is descending at a high rate, you must look at the blade pitch angle never as in relation to the plane of rotation but always as the cord pitch relative to the airflow at any given segment of the blade. With a high rate of descent it is impossible for any portion of the blade to be in negative pitch.

 

The fact that it will not autorotate straight down as all the blades segments are stalled nor if we have the pitch set high are the same problem. There is a narrow band of speed that you have to maintain to keep the balance of driving and driven.

 

Next is the formula for lift that is just as important in the the part of velocity squared. Speed of the blade greatly changes the angle of attack as we all experiment with while training for stuck pedal etc. (the m/r and t/r are mechanically linked at ratios that have the t/r spinning much faster. Any reduction of m/r speed sees a much larger reduction in t/r speed resulting in a far less efficient t/r and therefore a different 'spin' of the helicopter at the same collective setting)

 

Back to the blades, the vary reason for twist was to address the problem of velocity as did taper did with area (another important component of lift) all were trying to get lift more evenly spread across the blade. This could only be done though to the extent that is did not reduce autorotational abilities.

 

clear as mud?

[cap]

Phil -------- Those movies I mentioned were property of 'Uncle Sam' at the time, but I've been told by some reliable old engineer friends that they'd seen the same movies at Bell in Ft. Worth. Seems a lot of a/c manufacturers have such films in their repositories. Don't know where you might find the Bell one.

 

First time I ran into 'wash in-wash out' was in F/W. On a prop-driven F/W the chord of one wing is always thicker than the other wing to prevent the a/c from doing a 360 around the rotating prop. Da*n Newton and his Third law. Hey Phil, did I ever tell you what a bunch of buggars those Englishmen are?

 

What the Sam Hill you doing in a country where you have to import your own Single Malt anyway? Sad...really sad. Oh how the mighty have fallen.

[Phil Croucher]

Hmmm - Single malt - at least I'm not in Ontario where they add corn liquor to it so there is "local content"! Well, they do it with most drinks but I believe the whisky manufacturers told them to rearrange a couple of words into a well-known phrase or saying!

 

I'm down here opening up a satellite base for my JAA school, now that the oil companies appear to have decided that the JAA/EASA one will be the licence of choice, so I am informed.

 

Now let me see - Laphraoig today, I think.......

 

Phil

[Freefall]

I heard Russian blades dont auto rotate well at all (they all have two stoves) but they lift much better. Do they have much more washout or something?