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QUESTION: Dear Mark

http://en.wikipedia.org/wiki/Helicopter_rotor
http://en.wikipedia.org/wiki/Contra-rotating
http://en.wikipedia.org/wiki/Contra-rotating_propellers
http://en.wikipedia.org/wiki/Coaxial_rotors
http://www.youtube.com/watch?v=VZ8BUNJYtOo
http://www.maxxprod.com/mpi/mpi-266.html‎
http://dspace.mit.edu/handle/1721.1/65309‎
http://dspace.mit.edu/handle/1721.1/61906


I want to know the advantages and disadvantages of implementing
Contra rotating helicopters design over the conventional  main rotor system in terms of manufacturing cost, efficiency, power, speed etc

Awaiting your reply,

Thanks & Regards,
Prashant S Akerkar

ANSWER: Well finally a question somewhat in my area... :)
With regard to contra-rotating helicopters the primary advantage(s) are higher efficiency (due to the reduction of residual swirl in the flow) and the reduction or canceling of the torque (the reason the tail rotor or boom is necessary and hence may be unnecessary in these designs)
The main disadvantage would be the cost and weight of the complex mechanism for the rotors to be co-axial, driven by the same engine and have pitch adjustment suitable for control and forward flight.

Hope this helps,
mj

---------- FOLLOW-UP ----------

QUESTION: Dear Mark

Thank you.

The Design complexity will increase if more than two rotors in opposite directions are to be driven in odd and even configuration ?.

i.e.

3,5,7 etc Odd

4,6,8 etc Even

Increasing the Number of Rotors (3,4,5,6,7,8) can affect the Helicopter speed instead of the current twin rotor configuration ?.

Will 3,5,7 etc Odd configuration also work ?

OR

Only 2,4,6,8 etc Even Configuration work ?

Awaiting your reply,

Thanks & Regards,
Prashant S Akerkar

Answer
I'm not sure if you are meaning multiple blades or multiple rotors...
Certainly multiple blades (of odd multiples) helps with the production of noise (this has been tried with counter-rotating geometries, e.g GE-UDF)
There really would be no reason for multiple rotors as the purpose of the second is to remove the swirl (or actually) whirl) of the first...  so an odd number of rotors would likely be counter-productive...

Thanx,
mj

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Mark Janus

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I can answer questions about aerodynamics (high speed and low speed), internal fluid flow, computational methods (CFD, some design, etc.)

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