Thread: Prop Tech Question?

Might want to check your terminology... I assume you actually mean contra-rotating props?

The Russian TU-95 is probably the best known plane with them, still in service and is the fastest turboprop around. I have often wondered about the aerodynamics of them as we'll, not something we learn about in flight training. They are frightfully complicated to build though, the gearbox and oil delivery systems in particular.

If you think about a turbine engine, the air goes through a number of compressor stages, although with stators to correct the air flow angle. The air coming off a prop shouldn't be particularity turbulent as such. Yes the air off a prop does have an element of spiralling, dependant on power and speed. I guess it's a way of increasing the disk area, with having to go with bigger blades, or more blades. Interesting I don't think anyone makes contra props anymore, maybe the technology, or the idea, wasn't there back in the day to just put more blades on. Hopefully someone that knows something real might see this and educate us

Volvo Penta have been doing it with boat stern drives for years. If it works in water I would think it would work even better in air.

I am no aeronautical expert and probably will look stupid but here goes.

1 single engine planes have significant torque reaction to the prop thrust.
I have seen it reported that more Me109s were lost in takeoff accidents than in combat due to narrow landing gear because of dry thin wing.

Think of a helicopter needing a tail fan to keep from spinning or the Russian Shipboard helicopters with contra rotating main rotors and no tail rotor.

2 often to have a prop large enough to transmit the power of turbo prop engines it would have to be impractically big. The Gannet had a contra rotating prop AFAIK for that reason, and this is also a reason for the Tupelov as the props are very big even as contras..
3 Torque reaction is also present in twin engine planes but I don't know of any with contra rotation.

I think the Volvo contra leg was designed to make it small for when the boat was sailing therefore minimise drag and also to cut torque reaction.Some stern drives for inboard outboards also have contra props.
Regards Philip A

Your judgement is correct, counter-rotating cooling fans would not increase cooling efficiency because the blade tips are not moving at the high velocity encountered by the tips of aircraft propellers and helicopter rotor blades.
A major limitation in high speed flight is the transonic air flow on the propeller blades and helicopter rotors which cause high vibration levels, power divergence and noise. Air flowing off the tips of the blades form a trailing vortex. The generation of this vortex consumes a significant amount of wasted power from the aircraft’s engines.
Jet transports also have a wing tip vortex problem and so do birds. Modern jet transports add a winglet on the wing tips to reduce the generation of these vortices which adds as much as 5% greater range to the aircraft.
Ducks fly in Vee formations so that the vortex from the wing tip of the leading ducks rotate in the opposite direction of the vortex from the wing tip of the trailing duck (e.g. Right wing tip behind left wing tip and vice versa). If the ducks fly close enough, (wing tip to wing tip), which they try to do, it is estimated that it takes 30 to 50 percent less energy for a duck to fly behind the lead duck. The lead duck has to use maximum energy to fly and this is why on occasion the lead duck will change position in the formation because the work of flying lead is much greater!
Helicopter blades are much more complex than aircraft wings because they are moving more slowly near the hub and while moving backward from the direction of flight. The blade tips reach near sonic speeds when they are advancing ahead of the helicopter. You can see plots of this complex action and the tip vortices modeled by NASA at the following URL;
http://science.nas.nasa.gov/Pubs/TechSums/9293/6.html
Aircraft propeller tips also move at high radial velocity generating large trailing vortices. Counter-rotating propeller tips create vortices that rotate in opposite directions, clockwise and counter clockwise, which significantly reduces the net trailing vortices by cancellation. This increases propeller efficiency and reduces the workload on the engine, just as the trailing ducks have a reduced workload. A second benefit of counter- rotating propellers is that the net torque on the aircraft from the propellers is zeroed requiring less trim on the control surfaces to maintain level flight. This reduction in torque also makes multi-engine aircraft more controllable when engines have to be shut down while in flight.



Re: How do counter-rotating propellers work?




Well I just learned something new.




Tom.

damn,, link broken,,

Don't know if this pic is of one of the unloved F21 Spitfires, or if it's a later date 're-built' example built from another airframe and a Shackleton engine/prop combo.

The F21 was Griffon powered, only 120 or so built, a small number of them with the 6-blade contra.
They were rather noisy...

Seafires were 'marinised' Spitfires, some also had the contra-rotating props, AFAIK. This would make a lot of sense, as the take-off swing from 2,000 + hp would be....'interesting'.

Of course the Fairy Gannet operated by the RAN in the 50s and 60s was contra-rotating props from its twin mamba power plants.

[ame="http://en.wikipedia.org/wiki/Fairey_Gannet"]Fairey Gannet - Wikipedia, the free encyclopedia[/ame]

Fairey Gannet AS1/4 | Royal Australian Navy

Yes Philip, ground prangs did indeed decimate the ranks of the 109. The landing gear was - relatively - fragile as well as narrow-tracked.
So much so, that Hitler was reputedly heard chastising Willi M along the lines that landing gear on aircraft was not new and untried technology...
They were unpredictable on the ground. This coupled with lousy forward vision and pilot inexperience meant that as the war progressed and engines got better...attrition rates went up.