Contra-Rotating Propellers

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Jimstix

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What is the best way to do contra-rotating propellers? Are there any off-the-shelf kits that can be used to modify an existing propeller-hub-shaft assembly?
Okay, I have to ask: Why do you want/need counter rotating propllers?
 

Brünner

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Apr 12, 2020
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Beer country
Contra-rotating propellers have become quite common these days as used on higher horsepower outboard motors. Their reliability under rough conditions has proven to be excellent. The bottom ends from these motors are available & affordable and could provide an excellent gear and bearing set. Of course a proper box to carry these internals would be required. (A milling machine and a worthy billet to get started).
This would mean adding several kg/lbs right on the aircraft's nose, and there goes your CG.
How much improvement would an additional CR prop bring, aside from less rudder?
 

Blackhawk

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AUSTRALIA
How much improvement would an additional CR prop bring, aside from less rudder?
"You will enjoy up to 30% more thrust, up to 20% less fuel consumption, up to 25% noise reduction and complete rotational torque cancellation!" as mentioned in reply #18
 

nestofdragons

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Near Antwerp, Belgium
=hard to belive...(not differential!)?

BTW=in "auer" we have symmetrical gear (little forces on axle...)

"6,000 rpm is common in automobile "

=only crankshaft bearing 6000,
athers 4000 ...
If you take the C3 variants of the SKF-bearings, you can get higher in rpm. Just a example: Bearing 6006-2RS1/C3 has a limit of 8000 rpm. To see what kind of bearing it is, here some dimensions. inner size is 30 mm. Outer size is 55 mm width is 13mm. 13,8 kN Basic dynamic load rating.

Or ...are C3 not good for this type of practice, Henryk? C3 have more play. Too much vibrations maybe. I miss the knowledge to extimate that possible problem.
 

henryk

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krakow,poland
and complete rotational torque cancellation!"

more precise=
=complet gyroscopic momentum compensation,
=rotational torque equal to crancshaft momentum (with circa 3 :+/-1 RPM reduction)

f.e.= iff Motor RPM=6000, Propellers +2000/-2000 RPM...

=NO stream swirle , hard focused =

=70 HP, 235 kG thrust force ! (NO possibility to stay 5m after propellers !!!)
 

henryk

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Bearing 6006-2RS1/C3 has a limit of 8000 rpm.
=all 4 beavers ? are fully compensated (input,autput with zero momentum,
sattelites=propellers momentumes) and are rotated with 4000 RPM...

iff crancshaft RPM=6000.

PS=100 ml oil is anoth to 100 HP motor power !

=for simplicity we install double bearings (as car wheel are),
but oil supplied + rubber segers.
 

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nestofdragons

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=all 4 beavers ? are fully compensated (input,autput with zero momentum,
sattelites=propellers momentumes) and are rotated with 4000 RPM...

iff crancshaft RPM=6000.

PS=100 ml oil is anoth to 100 HP motor power !

=for simplicity we install double bearings (as car wheel are),
but oil supplied + rubber segers.
Henryk, can you give me the rough dimensions of your bearings. I will ask our bearing-specialists what bearing they would propose for your application. I might miss the knowledge 🙈 , but ... they have it ;).
 

Cardmarc

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Jun 16, 2021
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Do a vibration analysis for sure with aircraft props mounted? Torsional harmonics could be the bane of any gearbox/crankshaft.
 

henryk

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Niels

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Aug 15, 2019
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"You will enjoy up to 30% more thrust, up to 20% less fuel consumption, up to 25% noise reduction and complete rotational torque cancellation!" as mentioned in reply #18
If You gain 30 % efficiency the original single prop was work of genious.
A wise rotating machinery guy Stanley Hooker once said that making more than 90% takes a genious as does lower than 70%
 

MACOWA

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Grapeview WA. USA.
Those marine gearboxes are tiny, and water-cooled.
I think they wouldn't last long without some form of cooling.
Most around here take 200 HP +, center shaft becomes input shaft, input shaft becomes spider. A new gear case is Definitely a machine shop project. lubrication with engine oil would keep engine and box at roughly the same temp'.
 

Vigilant1

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Most around here take 200 HP +, center shaft becomes input shaft, input shaft becomes spider. A new gear case is Definitely a machine shop project. lubrication with engine oil would keep engine and box at roughly the same temp'.
There would be a significant MMOI difference between a spinning boat prop and a spinning aircraft propeller of typical diameter and blade mass.
 

Hawk81A

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Every marine (outboard) gearbox 've ever worked on used gearlube. Motor oil would be too thin. You could possibly whip up some kind of cooler, maybe even a larger sump. Dennis
 
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