MadSci Network: Engineering
Query:

Re: How do counter-rotating propellers work?

Date: Wed Jun 2 12:54:28 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 928202142.Eg
Message:

Greetings:

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.

Best regards, your Mad Scientist
Adrian Popa


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