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Author Topic: Gyroscopic effects  (Read 2319 times)

Offline Glen_Alison

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Gyroscopic effects
« on: February 17, 2015, 01:46:52 PM »
I am asking for advice here please.
 I have an electric model (with MVVS 8/950) that has very bad gyroscopic precession effects. With a normal propeller (12x6 APC electric) it rolls inwards on take off but up elevator swings the nose out. (as if lead outs were too far back). However, with the same size reverse propeller the opposite happens, good line tension on take off but nose rolls in on up elevator. (crash!)
 It is the exact same design as another model that has an ic engine (RoJett 67) but that model flies very well. Have any of you experienced this extreme effect. I would add that I am an experienced competition flier (UK) but new to electric. It seems to me that the rotation mass of the motor is having a very strong effect. I am about to experiment with smaller higher revving props to reduce this torque moment, or I might try larger props at lower revs.
Thanks for any input here, Glen Alison (UK)
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Offline Igor Burger

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Re: Gyroscopic effects
« Reply #1 on: February 17, 2015, 02:28:31 PM »
Glen, you are describing 2 different effects, one is precession, that yaws your nose as result of pitchig rate (in corners), means with deflected elevator and another one is yawing because of asymetric swirl around the fuselage, means it happens without elevator deflection.

The first you can solve 1/ with lighter prop (carbon, wood, etc) 2/ with rabe ruder

The second is usually solvabele if you move lines little bit back and deflect rudder little out (for tractor prop) your RoJet probably uses larger diameter, so asymetry of fuselage is not so important, so may be it is not so sensitive, but try to compare if you have LO on teh same place like your IC model.

In any case it will make you life easier if you start with reworking rudder to be movable :- ))
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Offline Tim Wescott

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Re: Gyroscopic effects
« Reply #2 on: February 17, 2015, 02:29:22 PM »
I can't answer your question directly (I fly slime.  I only build motor controls), but I can comment on those things that I do know:

  • I think most people are using much less pitch on their props -- check the "List your setup" thread to see if I'm right or wrong.
  • APC props tend to be heavy -- but people use them here and they work fine.  Maybe it's longitude-dependent?
  • The moment of inertia of the prop is huge compared to the motor.
  • Turning things faster would increase the gyro effect from the motor.  If that makes it better, it's because the prop got smaller.

Beyond that -- I dunno.  Hopefully someone does, and will share.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Glen_Alison

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Re: Gyroscopic effects
« Reply #3 on: February 18, 2015, 12:43:21 PM »
Thanks Igor and Tim for your replies. I will try the smaller prop and modify the rudder before the season gets going again.

Regards, Glen
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Offline Igor Burger

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Re: Gyroscopic effects
« Reply #4 on: February 18, 2015, 01:30:19 PM »
May be yo ucan try LO further back, while you play with "non destructive" things :- ))

I have lines (much) more rearward then I see on your picture, but I have also deflected rudder. You can also try 1 or 2 deg motor offset - motor and rudder offset will solve that yawing after start.
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Offline Russell Bond

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Re: Gyroscopic effects
« Reply #5 on: February 18, 2015, 04:51:54 PM »
Yep! Been there, done that!  ;D

On my World's (and UK Nats) plane I'm now using an Igor 12 x 5 tractor three blade prop. (I was using an 11.5 x 6 three blade pusher.)
I had to move both lead-outs back 3/4", 2 degrees motor offset and use more rudder offset. I now use a Rabe Rudder which works perfectly.
The centre of the lead-outs is now 1.25" from the CG.
This setup is the best the plane has ever flown.

BTW, tractor seems to work better than a pusher, less yaw problems in flight etc. (Just like Igor says.  mw~)

(Oh, and down lead-out at the front also seems to work best.)
« Last Edit: February 18, 2015, 09:14:31 PM by Russell Bond »
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Offline Keith Renecle

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Re: Gyroscopic effects
« Reply #6 on: February 22, 2015, 11:34:51 PM »
Hi Glen,

I think that the correct solution is to stick it in the flight box and post it to me!  #^ Such a pretty model!

Regards,
Keith R
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Keith R

Offline phil c

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Re: Gyroscopic effects
« Reply #7 on: February 25, 2015, 09:17:33 PM »
Have you taken a close look at the landing gear alignment?  Also, even a slight rolling resistance on the left wheel, perhaps from dragging on the wheel pant as the plane start to roll and the wheels start to skid around the circle can cause this problem.

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phil Cartier

Offline Wolfgang Nieuwkamp

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Re: Gyroscopic effects
« Reply #8 on: March 13, 2015, 01:06:38 PM »
Glen,
a Rabe rudder will work, but you will have to reverse the action when using a pusher prop.
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Offline Glen_Alison

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Re: Gyroscopic effects
« Reply #9 on: March 19, 2015, 04:47:25 PM »
   Thanks again for all the input here. I have now fully repaired and modified the model. I have added some 3 degrees of motor offset and altered the fin to make it incorporate a Rabe rudder which is something I have never tried before. I have moved the lead outs back and will move the battery forward a little to adjust the cg position.  As suggested in the thread, I have been trimming based on my ic experience. I have also now fitted one of Igor’s 11x5 three blade tractor props and am looking forward to trying out now that winter is about over here.
Glen
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