Autorotation sprag and centrifugal clutch drag

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Autorotation sprag and centrifugal clutch drag

Postby seneca2e » Wed Jan 09, 2013 10:27 pm

I was talking to a highly experienced Brantly instructor the other day whom I have the utmost respect for and he says he likes to take the engine rpm down below the top of the centrifugal clutch full lock up speed(1700rpm) to about 1500 rpm in autorotations to simulate more real world glide conditions. I can't really see this as when the sprag clutch splits it shouldn't matter what the engine rpm is really. But he has thousands of Brantly autos and he says you can tell the difference despite the theory. If this is true the rotor must be getting a little residual thrust somehow but I really can't see how it would once the over running clutch spit the needles. Now maybe there's some residual torque with the engine still locked up to the centrifugal clutch that he could be shedding when he gets it below 1700 rpm-I can see that but that would never make it to the rotor to drive it if the sprag clutch has split the needles. Any comments please weigh in!
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Re: Autorotation sprag and centrifugal clutch drag

Postby Hillberg » Wed Jan 09, 2013 10:55 pm

Once the sprage goes off line the power to the rotors is nill, piston helicopters will autorotate exactly as it will at idle &With a dead engine, A turbine helicopter will behave differently as the jet exhaust adds some distance in an auto (14 lbs of thrust out of the bell 206 exhaust). Too low an idle can stop an engine as the fly wheel has less inerta helping the engine run. (Like the 300)
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Re: Autorotation sprag and centrifugal clutch drag

Postby bryancobb » Thu Jan 10, 2013 7:19 am

First, if there's not a clearly visible split in the needles, you are not autorotating.

Second, if I remember correctly, there was a Service Letter, (Don't know why that's how the info got distributed) that cautioned against full-idle autorotation practice. It suggested a practice of rolling off throttle to just below the needle split, and I think it mentioned 1500 RPM. It went on to say that this should put the throttle in just about the right position so that when a power recovery was done, the correlator would do the job of managing power well.

I always used 1500. I don't remember having to mess with the throttle very much when ending an auto in a 5' hover. I only ever done 2 autos to the ground in my Brantly. They came out well but because I had no insurance and in my eyes, I had too much to lose, I was content doing power recoveries with almost zero groundspeed, to a 5' hover.
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Re: Autorotation sprag and centrifugal clutch drag

Postby seneca2e » Thu Jan 10, 2013 11:44 am

Well the flight manual itself says "The throttle should not be cut back to idle as it might result in engine stoppage. An engine speed of 2000 RPM is recommended. Abrupt throttle changes should be avoided." My old Brantly Hynes flight manual recommends 2000-2200 rpm so that is not in question. This instructor is well aware of that and has actually had them quit on autos so again not in question. He is not advocating idle autorotations! He still maintains there is a difference you can feel in having it at say 1500 rpm vrs 2000 rpm. As soon as I "prove" mine a little more I'll try it and see if I can tell a difference. Maybe Ron can say something on the subject as he is flying a lot in the training environment.

Hillberg that was my understanding as well. But when someone has the kind of experience in a specific machine as this gentleman it's hard not to believe it!
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Re: Autorotation sprag and centrifugal clutch drag

Postby Ron Spiker » Thu Jan 10, 2013 2:19 pm

The flight manual says to use 2000 and that is what I/we use for all autos in the Brantly. The needles are clearly split by several hundred rpm at that point. I also think (agreeing with you guys) that once the formsprag has disengaged the engine from the transmission, what makes the difference whether the centrifugal clutch is still engaged or not? The engine isn't driving the transmission/rotor at that point. At 1500 the centrifugal clutch isn't fully disengaged anyhow. Guess I'd have to see/feel this one to know why there's a difference between 2000 rpm and 1500 rpm.
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Re: Autorotation sprag and centrifugal clutch drag

Postby bryancobb » Thu Jan 10, 2013 2:56 pm

When the RPM gets down there, the spring & banana tube assembly "S N A P S" very hard back to its disengaged position, of course, but I don't see what that would do to Rotor RPM?
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Re: Autorotation sprag and centrifugal clutch drag

Postby Ron Spiker » Thu Jan 10, 2013 4:27 pm

But at 1500 rpm, it is still partially engaged. It starts engaging at about 1300. Maybe someone who knows how/why this effects rotor rpm will chime in.
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Re: drag

Postby 9121u » Fri Jan 11, 2013 8:57 am

I believe that the only way that rolling throttle off to where you have to disengage the centrifugal clutch that it would pick up the rpm better.that would tell me that the sprag is not working properly.. maybe a lack of lubrication.wrong type grease used..????
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Re: Autorotation sprag and centrifugal clutch drag

Postby bryancobb » Fri Jan 11, 2013 10:14 am

+1 on wrong grease.

That Aero Lubriplate is about the consistency of mayonaise.

Any kind of thicker grease would cause drag.
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Re: Autorotation sprag and centrifugal clutch drag

Postby 9121u » Fri Jan 11, 2013 10:42 am

BRYAN you definitely no how a sprag works.funny how a small thing like lubricant can play a role on how something works and last...
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