The Counter Rotating Props Scam

GE's newest most efficient aircraft gas turbine engine cuts the number of bladed parts by half by using counter rotating turbine stages.
Instead of having stator nozzles everything rotates one way or another.
Do the same advantages appear in the Volvo out board motor with counter rotating props?
To be sure there is some energy lost in swirl in simple single stage props but this can be recovered with stator vanes.
The counter rotating prop is kind of a scam -- unnecessary moving parts.
The GE turbine was expanding a gas with nozzles and in that situation counter rotation makes sense.
Bret Cahill
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wrote:

Counter-rotating props pass more power for their diameter than a single prop can. So if diameter is at a premium, counter-rotation is worth considering - isn't it?
Brian Whatcott Altus OK
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< Counter-rotating props pass more
< power for their diameter than a
< single prop can.
Unless there's a problem with the drive shaft snapping under the increased torque, just gear a single prop of the same diameter to run faster to move more water.
Maybe cavitation could become a problem in water at higher speeds but that's doubtful because cavitation is generally at least as big a problem with 1 - 3 rpm ship props.
< So if diameter is at a premium,
< counter-rotation is worth
< considering - isn't it?
Counter rotation makes sense in a gas turbine only because it eliminates all the stator blading which expands the gases in nozzles. That expansion can all be done with rotor blading with counter rotation.
Nothing is being expanded or compressed with props, however, so you can always design to get the same performance with fewer moving parts just by using stator vanes.
Bret Cahill
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Firstly prop diameter is at a premium for outboards, often.
Secondly
http://naca.larc.nasa.gov/reports/1939/naca-tn-689 /
Says that a contra rotating blade prop is MORE efficient than the basic 4 blade, and almost as efficient as the two blade.
I think on balance you'll find that it isn't as crazy as you imply.
Cheers
Greg Locock
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The real advantage might not have anything to do with propulsion; it's just a good way to rid the prop of sea grass.
You can't do that with stator vanes.
Bret Cahill
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< http://naca.larc.nasa.gov/reports/1939/naca-tn-689 /
I'm surprised the advantage is only a few percent. Anyway the issue was recovering the energy lost to swirl with stator blading vs counter rotating props. From propulsion considerations only there is no way to justify the extra moving parts if stator blading does just as well.
Bret Cahill
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If you need to counter torque reactions, then counter-rotating is a good way to go.
--
Harry Andreas
Engineering raconteur
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< If you need to counter torque reactions, then counter-rotating is a good way to
< go.
I read that this was a noticeable feature of some WWI planes. They could turn one way much faster than the other but this wasn't necessarily a problem.
In any event, stator blading should eliminate this as well.
A single prop w/o stator blading is imparting an angular momentum to the air. The stator vanes eliminate this by redirecting all the swirl in an axial direction.
Bret Cahill
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wrote:
...

Guess where the angular momentum goes?
:-)
Brian W
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< >A single prop w/o stator blading is imparting an angular momentum to
< >the air. The stator vanes eliminate this by redirecting all the swirl
< >in an axial direction.
< Guess where the angular momentum goes?
The torque on the stator is equal and opposite to the torque on the rotor.
Bret Cahill
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wrote:

OK. The stator torque is passed to the fuselage, or the hull, yes?
Brian
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< OK. The stator torque is passed to the
< fuselage, or the hull, yes?
The engine. Neither stator nor rotor torque gets past the engine.
Bret Cahill
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wrote:

If I said, "Angular momentum is a conserved quantity" would that cut the ice with you?
:-)
Brian W
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< "Angular momentum is a conserved quantity"
< would that cut the ice with you?
If it was a closed system.
Bret Cahill
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wrote:

Ok so far, so good. Now: if the conserved momentum is lost from the airframe, where does it go?
To the airstream, no matter how?
:-)
Brian Whatcott altus, OK
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< Now: if the conserved momentum is lost from the airframe, where does it go?
< To the airstream,
Or the ground or a flywheel on the plane.
Bret Cahill
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wrote:

OK I think we have a mutual stepping stone here - a single prop, or several props turning in the same sense, react angular momentum on the airframe, which if airborne needs to shed it (assuming that like most, it does not have a flywheel on board to spin up) This is usually accomplished by rotating the largest possible mass of air by the slowest angular rate.
However - a contra rotating prop does not present appreciable angular momentum on the airframe - and this is a small point in its favor. The same benefit is shared by twins with mirror rotations or by tractor/pushers with opposed (forward view) rotation.
Yes?
Brian W
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< This is usually accomplished by rotating the largest possible mass of air by the
< slowest angular rate.
The lift on one wing is greater than on the other side. The angular momentum imparted to the air from the wings [large mass at slow rpm] + that of the prop [small mass at large rpm] = 0.
< However - a contra rotating prop does not present appreciable angular
< momentum on the airframe
No bending moment, no torque.
< - and this is a small point in its favor.
Probably negligible compared with the safety factor already build into the airframe but I'm not an aircraft designer.
My only focus here is saving fuel.
Bret Cahill
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wrote:

Yes!
Brian W
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Actually not much of a problem in WWI planes, but a very big problem with WWII planes. Once the engines were capable of putting out 2000HP and above, you had to be very careful on your run up and take-off, because the torque could (and did) flip the plane. It was necessary to use partial power until the aircraft got up enough speed to get usable aerodynamic forces on the wing and tail surfaces. Then full power could be applied and counteracted with a little aileron trim. This made fast take-offs problematic. It was this reason that counter-rotating props were added to the late war aircraft. Full torque and power could be applied immediately with force balance and no trim needed.
This continued in postwar prop planes, too.

It's not as much of a problem in a turbojet, but is a problem in a fan jet and the fans IIRC usually have at least 2 stages rotating counter
--
Harry Andreas
Engineering raconteur
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