Contra-rotating PSRU

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crytes

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May 16, 2014
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If a PSRU is determined to be the most effective way to get power to the prop why not use a planetary gear based one and countra-rotate a prop. 5-15% efficiency gain over standard props is significant and the drawbacks of weight and complexity are already present in the PSRU. The net is more efficiency at the cost of more noise. I'm still researching the cause of the extra noise as I don't understand the source and suncraft claims thiers is quieter because of contra-rotation.
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SkyClimber

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Dec 26, 2014
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Grass Vally CA
Crytes, I agree completely that many engines already have a built in PRSU (simple gear to gear) and if they went to a planetary type they could add to contra-rotation capability. Imagine if CR props were an "option" built into every engine with a PRSU. How many would use that option? It is difficult to say, but I 'm sure most would agree it would be greater than zero.

SkyClimber
 

wsimpso1

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First off, let's get real on what contra rotating props do for you... The first prop takes the in-flow at about the airspeed of the airplane, and accelerates its to higher speed and starts it whirling in the direction of prop rotation. The second prop then must take the whirling accelerated flow off the first prop and further accelerate it while presumably stopping the whirl.

Now for efficiency - most of our props are already on the order of 80-85% efficient. Your second prop will not even be that efficient because it is operating in the churned up wake of the first one... I seriously doubt your efficiency will go up very much - maybe the whole thing can go to 90%.

But to get to that higher prop efficiency, you will have to turn an extra set of gears, move more lubricant/coolant over the gears, and carry the extra weight. To really do well on efficiency, both props will have to be controllable separately. Yeah, at the least, two pitch change mechanisms, so, that means that forward prop will have a shaft that goes through the hub of the after prop and extends to the back of the gearbox. To be ahead on efficiency, your gearbox will have to lose less energy (in total) than the props get for you with efficiency.

But will the prop be more efficient? Tip vortices and other 3D wing effects from the front will be ingested by the aft prop. Then there will be twice as many sets of vortices providing drag. No, if this was a winner, we would see a lot more Griffons on Unlimited birds, and contrarotating props on turboprop arliners, etc.

I suspect that the real reason for contrarotating props has always been an effort to put the horsepower to the air with a certain prop diameter.

Billski
 

addaon

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Why would contra-rotating props driven by a single power source have to have pitch controlled separately?
 

Jan Carlsson

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Sweden
As Bill is saying, the dual-rotating propeller is slightly more efficient then a single rotating propeller, but most of it is eaten up by the extra gearing, it make more noise, it vibrate more.

the positive thing with high powered planes or PPG's is the lack of torque (on the plane or poor PPG pilot) it goes straight in the take off.
resent dual-rotating propellers have the rear prop with less diameter and with more blades then the front propeller.
 

Jay Kempf

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I suspect that the real reason for contrarotating props has always been an effort to put the horsepower to the air with a certain prop diameter.

Billski
This is probably true. But wouldn't a prop fan sort of do the same, just more blades and more blade area basically?

I surmise that you are probably correct on all counts. If there was an advantage to contrarotating in terms of efficiency you would see it used inside turbine engines. Instead they just use stators that are sort of flow straighteners between stages to keep the swirl to a minimum. Mass flow through the disk is still the variable that makes efficiency. Any flow that doesn't pass through axially is lost so any flow that is still turning in the wake of the prop is lost efficiency just due to the resultant.
 

BJC

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...
the positive thing with high powered planes or PPG's is the lack of torque (on the plane or poor PPG pilot) it goes straight in the take off.
Plus they tend to cancel the gyroscopic forces, which will make maneuvering much easier for fighters and other small aircraft relative to the moment of inertia of the propeller.


BJC
 
S

SvingenB

If it came for free. No extra cost, no extra complexity, no extra maintenance, no extra down time, no added noise. Then every propeller(s) would be contra-rotating.
 
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