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Got Jet Fuel? Turbine Helicopters > turbine engines & gyroscopic precession
 
 
ProFormNW
Heliman
Location: Western Oregon

Greetings,

Turbines are cool.. no doubt, but I have a question that I havn't be able to satisify... The rotating portion of a turbine has to have some rather significant gyroscopic effects which I see causing 2 problems.

First, and least significant... a horizontaly mounted turbine will create some pitch(elevator)/yaw reactions during yawing manouvers... nose up one yaw direction and nose down the other depending on the direction of engine rotation, similary elevator to yaw... has anyone noticed this?? (for that matter, this might be seen in turbine planks).

Secondly and more importantly... if turbines are going to be the future engine of choice for sport flying, will the engines hold together if subjected to relativly high yaw rates. Unless the engine is mounted verticaly, the rotating portion of the engine is going to create some hefty bearing loads when yawed.

I have very limited exposure to RC turbines, but one of the few I've seen crashed (and burned) at the TOC a number of years ago after losing power during a "spin" (more of a lazy spiraling dive). I thought at the time that it was related to precessional forces, but have never really asked about it. This issue carries over as well to turboprops used for aggressive aerobatics in fixed wing aircraft.

Maybe the masses and radii are small enough that this isn't an issue, but I've always wanted to ask someone that might have the answers.

Food for thought and maybe an overblown internet discussion
06-10-2003 Over year old.
 
 
bobj
Veteran
Location: United Kingdom

Hi

Having done a fair amount of 3D with my Nova Cuatro fitted with a SPS turbine package I can say I had no problems with this at all, I also wondered if there would be some gyroscopic effects to deal with before a I flew it but so far nothing to worry about.
I do tend to try to fly it smoothly which will help anyway, I don't do this to aviod any possible precession but just to get the most out of it as it is a big heavy model.

BTW my engine is doing 150,000 rpm at full power so I really would have expected some funny stuff going on but so far so good.

Best regards
Bob J
06-10-2003 Over year old.
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Helijet
Heliman
Location: B.C. Canada

Despite the fact that the rotor assembly ( compressor wheel, shaft, turbine wheel ) is rotating between 35 - 120 K ( example ) gyroscopic effects appear minimal. Certainly, not that is noticed whatsoever in plank flying - be it small 50 inch wing span birds or the bigger 100 inch birds that I have flown. A couple of years ago I witnessed a turbine manufacturer actually hold a running turbine and he reported no twisting ( torque ) of any significance. I think your right that the low mass of the rotating unit is significant in this regard. BTW nowadays some pretty violent manuevers are done with turbine aircraft with no effects on turbine bearing longevity.

Dean Wichmann
( lots of turbine planks )
Century Bell 47
Century 222
Vario Bell 47
Hirobo Cobra ( wet box trans. ) the old, old one
DC Labs 222
Schluter 206 Ranger
HotCat turbine heli
06-10-2003 Over year old.
 
 
Phil Cole
Veteran
Location: Redwood City CA

I've had a few people tell me that precession of the turbine shaft was going to be a problem.

I did some experiments to see if anything could be seen.

Pirouetting my with Jetcopter at over one revolution/second produces no discernable effect in either direction. I had an observer watch the tail to see if it moved vertically. He could easily see it move an inch or so up or down, but there was no correlation with the direction of the piros.

Doing the experiment this way should show something if it's there since the angular velocity is greater than anything you can do with cyclic control, and the gyro can't compensate. The teetering head would allow the engine to move enough to be seen if it really wanted to.

Since the Jetcat engine runs at constant RPM, axial torque due to accelerating the turbine can't affect anything. Also, the constant RPM mean that the tail rotor acceleration is not a factor either.

The change in tail rotor drive torque due to large rudder inputs could cause a pitching effect which might not be distinguishable from pitching caused by turbine precession. However, no consistent pitching at all was seen, so neither seems to be an issue.
06-10-2003 Over year old.
 
 
midwestpilot
Elite Veteran
Location: Crystal Lake, IL

You may not notice it....

But your turbine does....
Aggressive flying is harder on the turbine itself...specifically the bearings.
It is the old hold a bike wheel axle and spin it deal....you try to move the axle an it fights you...pretty hard I might add...well thats what the bearings feel.

I don't think there is any hard data "yet" on how long the turbine will last under harsh conditions vs. sport (scale) flying...but I would bet it does shorten the lifespan some.

In life there is no spacebar!

Rich Erikson AMA 6175
06-10-2003 Over year old.
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GrandRC . CanoMod . Futaba-RC

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Got Jet Fuel? Turbine Helicopters > turbine engines & gyroscopic precession
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