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

Reply to
Bret Cahill
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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

Reply to
Brian Whatcott
< 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

Reply to
Bret Cahill

"Bret Cahill" wrote in news: snipped-for-privacy@o13g2000cwo.googlegroups.com:

Firstly prop diameter is at a premium for outboards, often.

Secondly

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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

Reply to
Greg Locock

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

Reply to
Bret Cahill
<
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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

Reply to
Bret Cahill

On 17 Aug 2005 01:45:37 -0700, "Bret Cahill" wrote: [Brian]

Hmmm...if you think that increased power means increased shaft torque, it is probably not a good idea for me to be discussing power transmission with you.

Brian Whatcott Altus OK

Reply to
Brian Whatcott
< Hmmm...if you think that increased power means increased shaft torque,

All other things being equal, it is proportional. Shaft power ~ torque X rpm.

Anyway you never addressed the issue:

A single prop with stator vanes can always be designed to perform as well as counter rotating props.

Maybe counter rotating props can chop up sea weed better but that's the only possible advantage.

Bret Cahill

Reply to
Bret Cahill

If you need to counter torque reactions, then counter-rotating is a good way to go.

Reply to
Harry Andreas
< 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

Reply to
Bret Cahill

...

Guess where the angular momentum goes?

:-)

Brian W

Reply to
Brian Whatcott

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

Reply to
Harry Andreas
< >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

Reply to
Bret Cahill
< 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

At steady rpm the torque on the stator is equal and opposite to the torque on the fan rotor. They cancel each other. No swirl = no torque.

Bret Cahill

Reply to
Bret Cahill

OK. The stator torque is passed to the fuselage, or the hull, yes?

Brian

Reply to
Brian Whatcott
< 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

Reply to
Bret Cahill

Probably -

I assume a theoretically perfect prop cuts leading edge into water angled to get boat-speed- insertion, and the prop leaves the water with the water exiting the prop at just-below-cavitation-speed. The idea being to get max use of water momentum to get max transfer of force into the prop and boat. Go for the greatest water-speed enter and exit differential, and push the physical limit of what the medium (water) can deliver. So if the idea is to get the cavitating just aft of the prop - what do they do? The props have to be side by side, or else they would have to make the first prop inefficient enough to have the water well below cavitaion for an inline second prop, or do they spin the number two inline prop at multiples of the first?

If they use side-by-side counter-rotating props like the big twin-screw inboards, the boat doesn't have to have the single prop set askew of the beam line to offset motor torque, and thus it crabs less and is more efficient?

just a wild guess - I don't do boat design, I just drive 'em and replace props.

-------------

personal notes -

the Cessnas I flew all had one (the left) wing longer than the other - supposedly to correct for engine torque without adding control. They "shut down" each engine a side in training, and both to deadstick train, and there was a difference in rudder needed to not turn depending on which engine was cut - and no engine tended to turn the plane right, as I remember.

my I/O boat is overpowered (light, short, and too many HP - hey, I was young and dumb when I got it.) and before I got used to it, it rolled a bit to the right when I rammed in the throttle at start, which I attribute to motor torque. Rev the motor in the water, and it will roll. ( Now when I hit the throttle, I unconciously turn the wheel to compensate to go out straight, and its now kind of like clutching in a car.) Anyway, my boat has the prop shaft angled off the power transfer arm and the drag-reduction fairings - visibly. (And it wasn't from hitting rocks)

fwiw

Reply to
hob

What about prop efficiency on ships? Most cargo vessels are single rotation single screw. Vessel owners/operators are the cheapest people on the planet. Bunker must now account for well over 50% of operating costs. Even before $60/barrel oil there was always a problem on oil tankers with chief mates running a line from the cargo tanks and burning crude in the main engine. That's why so many oil tankers explode; Lighter factions of the crude leak out into the engine room.

It seems like even if the efficiency of the screw could be increased by a few percent with an after market stator the owners would be interested.

Bret Cahill

Reply to
Bret Cahill

I thought they all ocean vessels had to have two screws -from the little pots to the supertankers. One on each side of the keel. Not only for prop efficiency by keeping it off the central wake, but also as a maritime regulation.

Vessel owners/operators are the cheapest people

A fact not in evidence.

Anyway, that would only mean they won't pay for the second screw, if the had a choice and if they could trade fuel against the cost of money for the screw -- and if the ocean vessels don't really have twin screws, which I believe they all do.

Reply to
hob
< I thought they all ocean vessels had to have two screws -from the little < pots to the supertankers.

Just a shaft coming straight from the 2 stroke diesel -- the cheapest possible thing.

< One on each side of the keel. Not only for prop efficiency by keeping it off the < central wake, but also as a maritime regulation.

What regulation?

Vessel owners/operators are the cheapest people on the planet.

< A fact not in evidence.

Talk to any Greek sailor.

Check out Lloyds registry. Notice all the names that are missing the last letter to mean something?

That's because you change the name when you buy a vessel and the cheapest way to change the name is to paint over the last letter.

I knew one Miami based charterer of a banana ship who would store bunker in the ballast tanks when the price of fuel was going up. The ship had a pronounced list as a result. You could identify the ship on the horizon because of the tilt.

< Anyway, that would only mean they won't pay for the second screw,

Or even stator blading if it meant relocating the rudder.

< if the had a choice and if they could trade fuel against the cost of money for the < screw

The owners might be functional enough to look at the tradeoff -- constant overhead vs initial cost.

< and if the ocean vessels don't really have twin screws, which I believe they all < do.

Visit any deep water port.

Bret Cahill

Reply to
Bret Cahill

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