Contra-props.

I'm curious...if a one blade prop is more efficient than 2 blade prop and a

2 blade prop is more efficient than a 3 blade prop than surely 2-2 blade props contra-rotating is very inefficient? With airflow turbulence and energy loss in gearing what's the reason for it? mk

The message from "MK" contains these words:

Hi, as I had it explained by RAF Cosford instructors a point was reached when engines produced so much power the only way to absorb it was with massive props of 15 foot+ diameter so the next choice was contra-props but this was a compromise I think as it added a lot of weight to the aircraft just where you didnt need it, co-axial contra-props were very effective in transfering the power into thrust I believe and in the case of the Avro Shackelton fitted with Griffons it allowed the engine to be throttled back in cruise and save fuel,

regards, Terry

On Jan 29, 5:28 am, "MK" wrote:

messagenews: snipped-for-privacy@zetnet.co.uk...

You're correct is saying that as the propeller diameter increases, it becomes more efficient. The streamtube of air that goes through a propeller disk ends up rotating in the direction of rotation. The component of velocity in the tangential direction (direction of propeller rotation) is called the residual swirl and its magnitude is a function of pressure ratio across the propeller. However as far as thrust is concerned it is the axial component of velocity that matters and the residual swirl is a loss source term. In a general aviation application the pressure ratio across the propeller is small enough that one can live with this loss. However as you go up in engine power, either you have to increase the propeller diamater and if that is not possible, switch to higher blade counts and high pitch propeller blades. If it is the latter then your propeller pressure ratio will be higher which in turn means higher swirls. One way to recover this swirl is to put in a second row of propeller that rotates opposite to the front propeller. In this arrangement the second propeller ends up removing the residual swirl of the first propeller and the exit flow becomes purely axial. So to make a long story short it is true that with two rows you're less efficient (because of the increased profile drag, & reduced propulsive efficiency) but in eliminating the parasitic swirl you may end up with a more efficient propulsion system. I hope this helps.

| I'm curious...if a one blade prop is more efficient than 2 blade | prop and a 2 blade prop is more efficient than a 3 blade prop than | surely 2-2 blade props contra-rotating is very inefficient? With | airflow turbulence and energy loss in gearing what's the reason for | it?

Well, you've already been given several possible reasons for it, but they seem to have missed the big one, which is that the two props work to cancel out a large portion of the prop wash, torque, P-factor and gyroscopic effects, and yet take up only a little more space than a single prop.

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Besides which, they look GOOD...................;-)

regards, Terry

And they weigh a lot more and present much more complexity, with the attendant risk of failure. They only make sense when there's no other way to absorb a lot of horsepower. Not many aircraft use them. Their efficiency won't be as good as two separate props, because the rear prop is operating in air torn up by the front prop. Drag increases and so does noise. The gearbox must be able to withstand high torque loads for extended periods, unlike the gears used in automobiles. Gearsets from cars have been tried in many homebuilt aircraft, and in most cases the failure rate is unacceptable, as it has been in boats that tried auto gears. Autos may have engines with high HP ratings, but that HP is seldom used except in a really aggressive acceleration and so is very brief. The automakers know that and don't bother to build really strong transmissions or differentials. Vehicles used to pull heavy trailers often run into transmission problems when pulling up long, steep hills for long periods. The early Dodge diesel pickups had that problem, with long life from the Cummins engine and really short life from the transmission. Gears under pressure generate a lot of heat. But for the modeler, the challenge of creating a system that works, and flying it, is well worth the effort if that's what he wants.

Dan

have a look half way down at the Brabazon I have somewhere in my pile of books a write up of the construction of the power plants for it

Hi-Max has several brushless models, from 350-3200W, although they're still in limited release. Two inline motors with an inner/outer shaft for the two props, no gearboxes involved. Looked pretty solid at SEFF last year....

PCPhill

The message from funfly3 contains these words:

I remember reaading about them many years ago and what a nightmare, the chances of at least one engine going down per flight must have been pretty high I would imagine. On my shelf I have some old 9.5mm film of the Brabazon on takeoff and it appears it will never get off the floor then it slowly lifts its nose and climbs away like a sick duck, the whole project was years behind in organisation and technology with no hope in hell of being a commercial success,

regards, Terry

I have an old cini film somewhere of an air show in the early 50's taken by someone with a silent hand-held movie camera showing various aircraft of the time including the Supermarine S510 and also some footage of the Brabazon lifting off, problem is it is in old 9.5mm format so I cant view it since my camera went to the museum.....;-)

regards, Terry

Here is Farnborough 1950

-- Bombergu

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