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Contra rotating propellers

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henryk

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How much is the gear unit?
-we have only 3+2 prototypes

+1 model with auter bearings (NO belt extender) ...

-iff compare ?=
 

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Geraldc

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I think I understand how it works. If right hand shaft is connected to motor and you put equal loading on output shaft and hub (propellers) then the left hand output shaft will turn the opposite direction as motor and the hub will turn in same direction but with both outputs rotating at half the input speed.
So for motor you get reduction box and extra thrust from 2 propellers and the propellers cancel out the gyroscopic precession.
Link to files .https://grabcad.com/david.menashe-1
Edited I had it the wrong way round.
 
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Mike Stewart

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Contra-rotating propellers obviously work or they wouldn't be used but I've never understood how immediately reversing a mass of air - the second prop hitting an accelerated air mass going 180 degrees from the direction of the second prop, makes any sense. Common sense (mine anyway) would say there would be a huge loss of energy in the rear prop dealing with the air mass developed by the front prop. Can anyone explain to a simple mind like mine why there wouldn't be bucket loads of drag developed in this configuration? Seeing contra-rotating props always leaves me wondering in my head how they work.
 

pictsidhe

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Contra-rotating propellers obviously work or they wouldn't be used but I've never understood how immediately reversing a mass of air - the second prop hitting an accelerated air mass going 180 degrees from the direction of the second prop, makes any sense. Common sense (mine anyway) would say there would be a huge loss of energy in the rear prop dealing with the air mass developed by the front prop. Can anyone explain to a simple mind like mine why there wouldn't be bucket loads of drag developed in this configuration? Seeing contra-rotating props always leaves me wondering in my head how they work.
A single prop swirls the air. Any swirl is a power loss. A correctly designed contra prop spits out unswirled air. The rear prop unswirls the air from the front prop.
Feeding equal torque into the props is a simple way to get a fairly straight prop blast.
 

RonL

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I think I understand how it works. If right hand shaft is connected to motor and you put equal loading on output shaft and hub (propellers) then the left hand output shaft will turn the opposite direction as motor and the hub will turn in same direction but with both outputs rotating at half the input speed.
So for motor you get reduction box and extra thrust from 2 propellers and the propellers cancel out the gyroscopic precession.
Link to files .https://grabcad.com/david.menashe-1
Edited I had it the wrong way round.
Checking the loop feature of you tube, when copy and pasted.


Didn't work :( and I can't delete.
 

henryk

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but with both outputs rotating at half the input speed.
=iff both propellers have the same drag momentum,
they rotate with 1/3 RPM of engine

f.e. = +6000 : +2000 /- 2000 ...


="auer" CR gear is similar (construction) =


but another kinematic !
 
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Dusan

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A single prop swirls the air. Any swirl is a power loss. A correctly designed contra prop spits out unswirled air. The rear prop unswirls the air from the front prop.
A second advantage for the contra-rotating props is influx of fresh air caused by first prop slipstream contraction, but that is happening at low speeds or static condition; that's why some helicopters have a coaxial rotor. At higher speeds practically all air goes trough both propellers and as swirl is low anyway, there is not too much point for using contra-props. The single reason why high powered aircraft uses it, is because it enables much more power to be absorbed by a lower diameter propeller.

In my experience you are much better by using straightening vanes that could be used as control surfaces, instead of contra-rotating complication, if your goal is to eliminate torque or to get illusory swirl advantage.
 

dog

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Dec 29, 2019
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In my experience you are much better by using straightening vanes that could be used as control surfaces, instead of contra-rotating complication, if your goal is to eliminate torque or to get illusory swirl advantag.....
Your "experience "with contra rotating propellers and "straitening vanes" is what exactly?
Ducting? or actual factual aircraft?
Bieng snarky and hoping you have built a plane with "straitening vanes" all at once.
 

Swampyankee

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It takes no more energy to remove the swirl from the forward propeller than it cost to put it there in the first place; it's a result of the downwash that results from the lift of the propeller blade. The net result is that coaxial props have a net efficiency about 1 to 3% greater than single rotation propellers for the same power, with the efficiency gains being greater at higher power loadings.

Aircraft use coaxial, counter-rotating props (CRP) for a few reasons, probably with tight diameter constraints being the primary one; this is why they were used on Griffon-engined Spitfires. There are also some handling benefits for small, highly powered aircraft like, well Griffon-engined Spitfires. They also tend to bring in a host of complications, because of extra complication in the reduction gear system and pitch change mechanism. At the very low end of propeller loadings, in helicopters (the methods of analysis are pretty much the same; I know this, as I've done both prop and rotor aero), coaxial rotors actually have two radically different reasons: reduced overall dimensions (Kamov) and truly rigid rotors for high speed flight (Sikorsky). In both cases, there is a slight net increase in efficiency (and there's a benefit in that the tail rotor can be eliminated) but a massive increase in complication, especially as helicopters don't function unless there is a mechanism for changing blade pitch.
 

henryk

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Mar 8, 2010
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Location
krakow,poland
was dissing"dusan" for
claims of "experience" of "straitening vanes" vs contra rotating propellers.
=is it possible to made quastion more understand-able for me ?

BTW=now I am preparing CR FLIGHT mode device on the 10 kW DC motor...

(with rotating stator)

=HILLER PLATFORM type...

 
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