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# Aeronautical Engineering/Contra / Coaxial rotating Helicopters.

Question
QUESTION: Dear Paul

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.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

Thanks & Regards,
Prashant S Akerkar

ANSWER: Prashant - The main advantages of contra-rotating rotors is the balanced torque which does not have to be countered by the tail rotor (though you still need some directional control), the added thrust from two rotors, and the fact that blades can advance on the left and right sides simultaneously. The disadvantages include flow interactions and noise from the two rotors passing close to each other (especially from one passing through the wake of the other), weight of the second rotor, and mechanical complexity of the rotor head. Depending on the mission, a co-axial rotor can be a winner, but it is probably a heavy-lift situation where noise and vibration can be tolerated.
Paul

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: Dear Paul

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 ?.

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

OR

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

Thanks & Regards,
Prashant S Akerkar

ANSWER: I'm not sure if you are talking about rotors or rotor blades. In any case, I don't know the effects of even or odd number of blades. All I can say is that machines designed for high speed have fewer rotor blades than machines designed for high lift at static conditions.

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

QUESTION: Dear Paul

Thank you.

I am referring to the Number of Rotors (2,3,4,5,6,7,8) and not the Rotor Blades.

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 ?.

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

OR

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

Thanks & Regards,
Prashant S Akerkar

Prashant - If you want to increase helicopter speed, you have to increase the airspeed through the rotor disc. Adding a third rotor, for example, may add some power at the expense of more drag, more engine power required, more vibration, and more weight in the rotor, head, and engine. The second and third downstream rotors are in a really bad flow field. At some point, the negative factors overcome the positive factors. Although, I do not know how to analyze this, I suspect that N rotors will slow the helicopter relative to N-1 rotors (N > 1). I have no idea about even and odd combinations of rotors.
Paul

p.s. I should have said N > 2
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Dear Paul Thank you. Thanks & Regards, Prashant S Akerkar

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Aeronautical Engineering

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#### Paul Soderman

##### Expertise

Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.

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38 years as research engineer at NASA

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B.S. and M.S. Aeronautical Engineering - U. of Washington, Graduate work Standford U.

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