contra rotating props

[Mark A]

I did a bit of work on contra-rotating props in the 80s, when prop-fans were being investigated.
We looked at the Gannet with its double Mamba engines, each driving a separate prop with it's own controls; and the Shackleton with its geared contra-rotation driven by a single engine (times 4).

Efficiency mostly is derived by recovering the swirl energy from the front prop that would normally exit as wasted kinetic energy not generating nett thrust.

The biggest issue with them is the noise mainly due to the pressure pulse from the rear prop having its lift modulated by the wake of the front prop, and to a lesser extent the rear prop's bow-wave interfering with the flow onto the front prop. Weight, complexity and maintenance also helped consign them to history.

With fuel costs rising it may be time to re-investigate the potential for efficiency, but I suspect noise issues would be considerable.

One of the noisiest twins is the Cessna 336/7 which has a similar issue with wake interaction between the props, even separated by the length of the cabin.

[Bob F]

Welshman, I know someone did fit an ex-Shackleton Griffon C/R prop to a Spitfire but I found the info below about a production model, the Seafire 46. Seems the C/R helped stop the Torque which might have been a bit of a handfull with just the one prop.

Type 388-Seafire 46
This Mark introduced the contra-rotating propeller as standard, and the new "Spiteful"-type empennage. Unfortunately, the new wing did not fold, and only 24 were built. As an illustration of the lag times to production, all these were originally ordered as Mk V Spitfires; the order was re-established as Mk 21, before redesignation as Seafire 46.

Bob F

[MikeGodsell]

During my years as Shackleton crew flying against the Russians in the cold war we had much to do with contra props. The Russians often intercepted us with their turbo-prop powered Bears, which had huge contra props. Made intersting vortices from the props when they opened the thrust levers and left us in their wake following an interception over the sea. The reason for the contra props on the Shackleton was to absorb the power of the Griffon engines without having excessively large diameter propellers. The contra props were driven quite slowly by the engine, 1500 rpm being normal for cruise. Using low rpm and high boost gave maximum endurance of around 24 hours flying. The drag from the props when put fully fine with throttles closed enabled the Shack to descend like the proverbial brick. However the contra props had one or two fatal weaknesses: A failure of the prop control rack bolts might put the rear prop fully fine while the front one would go fully coarse to compensate. If we got the engine stopped and tried to feather, only the front prop would feather and the drag of the windmilling engine would need full power on the rest to hopefully get us back home. However the Griffon was a vee 12 engine and the oil pipes to control the props ran between the banks. On the sadly quite common occasion that an engine threw a big end, it would flail through the crankcase cut the oil pipes and the prop would go to full fine and overspeed. The drag then tore the engine out of the wing usually leaving a raging fire. That is why Shackleton crews were very twitchly about odd or unusual noises from the the propellers & engines.

[Nigel Hitchman]

There is a Griffon engined Spitfire currently flying with a Contra-rotation prop. Cant remember the model. Its based in France, Dijon I think and is a regular visitor to airshows, was at Duxford Flying legends this year. It was previously restored in the UK. Sounds great and seems to have a phenominal rate of climb.

[Will Greenwood]

On a smaller scale, I remember a German company offering a contra rotating gearbox system for 912 ? The ad was in the Light Aircraft Directory book that comes out once a year, may have the name wrong, and can't find my copy........think a mate has borrowed it !!

[Mike Cross]

Given that the ducted fan is more efficient than a prop due to containment of the spanwise flow and that they still result in the airflow being sent in a helical motion rather than straight back has there been any research done on using fixed blades behind the fan to redirect the airflow straight back and thereby add thrust?

[Bill McCarthy]

As an aside, nuc submarines fly along just as an aircraft does, have trim/elevator control and all the rest and they haven't had "propellors" for over forty years. Torpedoes have contra rotating props (gearbox is tiny) to stop them "screwing" on its journey. The old diesel powered torps put 340hp through that small g/box - one trip only of course !

[Trevor Lyons]

In a rather different area, I recall that Vetus, the Dutch manufacturer of marine products, did some research; and they concluded that a single prop in a bow-thruster tunnel was more efficient than having two.