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Old Jan 19, 2013, 03:00 PM
kadil is offline
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unconventional :-)
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Question

coaxial propellers


I have an idea for a model and could easily arrange a couple of coaxial propellers. I was thinking a 5x3 in front of a 5x5 on a single shaft, or 2 motor counter rotating. Would this multi stage approach give good thrust characteristics for a smallish prop length?
Last edited by kadil; Jan 19, 2013 at 03:07 PM.
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Old Jan 20, 2013, 06:01 AM
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An itch?. Scratch build.
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Do you mean something like these -

Contra Rotating BL System 375W
and -
CR23L Contra Rotating BL System 1100kv

I have never tried it, but could imagine the two props would need to be carefully selected and flight tested to get the best out of such a system.
Old Jan 20, 2013, 06:16 AM
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Looks interesting, I was going to point one motor forwards and one back. I was told that using too high a pitch too fast just has a mixing effect on the air instead of thrust. So I thought that having a lower pitch prop in front could speed up the air a bit before it hits the high pitch prop.
Old Jan 20, 2013, 09:22 AM
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You mean more like the Cessna Skymaster, or Dornier Do 335, with an engine at the front and one at the rear ?

There is also the Dornier Do 18, and the more up to date Dornier Seamaster with a push-pull twin engine arrangement in a single nacelle.

Whatever type you decide on, from a model point of view I would recommend searching for what other modelers have used in a similar cofiguration, and how successful it may have been. The eventual proof will most likely be how the model eventually flies.

Sorry I can't recommend which way to go regarding props and pitches, but I'm fairly sure someone will have already done what you are thinking of. Always easier to pick someone else's brain, (design), that has had success.
Old Jan 20, 2013, 12:53 PM
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Some twin experiments....
5motor-2CameraFlights (11 min 51 sec)

And...
These done to test rudder authority. The rudder ran out of authority at about 40% power on the running motor.
The inline push-pull thingy only resulted in amusing videos of the props running. The airplane was a miserable failure.
Old Jan 21, 2013, 04:19 AM
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Quote:
You mean more like the Cessna Skymaster, or Dornier Do 335, with an engine at the front and one at the rear ?
In such a case, the rear propeller needs a little less diameter and a little more pitch. See attached image for clarification.
Old Jan 21, 2013, 06:52 PM
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Quote:
Originally Posted by Julez View Post
In such a case, the rear propeller needs a little less diameter and a little more pitch. See attached image for clarification.
Exactly what I was thinking.
Old Jan 22, 2013, 04:03 PM
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http://www.luft46.com/db/dbjager.html
One of these is being built somewhere on here.E warplanes maybe.


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