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Jun 25, 2005, 11:13 PM
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FLY...Please's Avatar
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Counter rotating prop torque?


I am having a problem with the motor/prop torque and its effect on roll stability. Due to the planform, AR, and general size of the aircraft, the configuration is not that inherently stable. I was thinking about using a counter-rotating prop to neutralize this effect.
Would using a counter rotating prop neutralize the net torque of the system, or would there still be torque unaccounted for that would add to rolling moment? Also, are counter rotating props significantly less effecient that a conventional propulsion system?
If the new 3.5 g brushless motors weren't so light and powerful, I would just go direct drive, high RPM with a small prop. However, these motors have a low motor kV, so using a small prop is not possible.
Any input would be welcome.
Thanks,
John
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Jun 25, 2005, 11:41 PM
Micro Flyer, Big Poster
Pete P's Avatar
They require more weight for the GB and extra prop, but when done properly they are more effecient, have more thrust for the prop size, and do cancel all torque from the prop.
Jun 25, 2005, 11:46 PM
Down with ARFS!
indoorff's Avatar
John - I do not believe there will be any torque associated with a counter rotating prop system, or if there is, it will be negligible. If you use two seperate motors, there could be a slight difference I suppose. j

In regards to efficiency, the rear prop expieriences turbulent air from the forward prop, so people tend to run a slightly higher pitch on the rear prop for best performances. Also, if you are using a single motor with a gearbox to drive two props, there will be some losses associated with friction.

I dont know how big a motor you are thinking of using, but here is a twin motor, counter rotating gearbox system I made for pager motors.

Cheers,
Chris
Jun 26, 2005, 01:07 AM
cool,make a mini br out of it.
Jun 26, 2005, 01:08 AM
Suspended Account
Counter rotating props generally means a plane with two props spinning in opposite directions... like the P38 I think that is what indorff is talking about.
Contra-rotating props share the same axis as in the Bladerunner-Heli, or this motor, 16 grams.
https://www.rcgroups.com/gallery/sho...cat=500&page=1
the torque is cancelled by the system, But there is also the Coriolis effect to consider
jimbo

http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
Jun 26, 2005, 12:08 PM
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Ed Couch's Avatar
The contra-rotating system I presently fly has proven absolutely flawless. Many hours on two different test vehicles have proven it out and unit two will be going in a full scale Convair Pogo. Search the sites for contra-rotating props as there is a wealth of details available. Also, have found that the #2 prop of a greater pitch only adds amps to the pull and normally provides less thrust overall. In theory this should prove out and may do so at high speed but at low speed and high AOA this does not prove out. Present system produces over 14 ounces of thrust and is torque free. VTOL off and back on to the deck is a matter of practice. Normal high AOA is flown to within a foot of the deck, stabilize, decrease throttle and land on caster. Its that easy, aerobatics are simple, roll response is equal right or left. As the man said "it works for me". ed
Jun 26, 2005, 01:22 PM
Down with ARFS!
indoorff's Avatar
Quote:
Originally Posted by Ed Couch
and unit two will be going in a full scale Convair Pogo.
ed
You dont really mean FULL scale do you? That would be so cool! I am a huge fan of the convair pogo. I even managed to track down Skeets Coleman and got him to autograph a large photograph I had printed of the Pogo.

Chris
Jun 27, 2005, 11:46 AM
Registered User
Ed Couch's Avatar
The first full bodied bird is almost pure scale in outline and is constructed of blue foam and quite a few man hours expended. The second bird will be enlarged 10 percent and be molded of depron foam and should be very light. Canopy plugs are done to pull the clear canopy that will have the pilot rotate from the vertical like the real bird and should come in at under 10 ounces. We had the blue foam bird at Small Steps but haven't had a lot of time to get things finalized. Airframe still requires finish prior to paint but hopefully will be ready to fly before summer is out. Depron bird to be ready by first of year.

ed
Jun 27, 2005, 01:20 PM
Flying...Pushing Air Down
sr71fan's Avatar

Full Scale


Ed's contra-rotating system, aside from eliminating the torque, sounds VERY cool!

Ed,

I hope to see the full-body version SOON! Full scale had ME going for a minute, also.

Gary
Last edited by sr71fan; Jun 27, 2005 at 03:30 PM.


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