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# Electrical Engineering/Contra / Coaxial rotating Helicopters.

Question
QUESTION: Dear Prof Cleggsan

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

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: Sorry, I am not an expert in this field.  It is an interesting question and I am sure books can be written regarding these designs but it is not in my field.

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

QUESTION: Dear Prof Cleggsan

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: My first reaction to the contra-rotating blades is the complexity of the design which yields a much higher cost to build and some maneuverabilities due to the angular momentum during take off.  The additional complexity will bring about a greater probability of mechanical failure and safety of the crew.

The cross wind between the sets of blades running in opposite angular rotations subtracts from the efficiency of the system due to the wind frictional losses between the blades.

Also, a double set of rotating blades adds to the weight of the vehicle and lessens its economy.

The conventional helicopter uses only one set of blades - which will tend to rotate the body of the vehicle in an opposite direction of the blades.  But this is easily corrected with the tail fan at the rearward of the helicopter which corrects for the rotational movement. A contra-rotational system also must deal with resonant frequency vibrations and distortions of movements which can make difficult tracking for the pilot in many cases.  I think toy size models have been made to show the seriousness of this phenomenon and has discouraged helicopter makers from designing large scale.  Hence, there are not many full size twin rotor systems in use today.

On the other hand the coaxial rotor system does not need the fan at the back of the helicopter and anti torque forces are gone in cases of accident or failure of the back fan, etc.

There is much more to say but it is no my field and it is presumptuous of me to go any further beyond my experience level.  So, I offer the above only as my opinion, not as fact.

Hope this will help.

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

QUESTION: Dear Prof Cleggsan

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

The number of blades, their proximity to each other, the wind speeds and effect on torque, strength of materials and manufacturing methods is the focus of many very sophisticated mathematical treatise.  A similar investigation is in setting the optimum number of blades, wind speed, etc. for electric windmill power generators.

One of the great hurdles in these blade design and analysis is the maximum rotational speed that can be achieved for the length of the blade and its proximity to the adjacent blade.  A limiting factor, and a serious consideration, is the strength of materials used in the composition of these long blades.  One can easily imagine that just increasing the rotational speed of the blade assembly will improve output performance.  But, as the rotational speed increases the tip of the blade reaches its destruction point and flies apart.  So, designs must be kept within the parameters compatible with the materials available at practical costs for the project design.

And, there are other concerns as well.  Just increasing the number of blades also increases the friction components and the interaction of blades on the same plane and those above.  At one point the congestion in the swirling air becomes a liability to the overall efficiency of the aerodynamic forces.

I am not an expert in these matters but there is plenty of written documentation on these matters if you google or bing deeply.

In the case of the electric windmills so popular now days through out the world you will note that they are all 3 bladed designs. Why?  The aerodynamics for multiple blades have been analyzed time after time, tests conducted and results compiled and it always favors the design base on three blades.  I think similar studies have been conducted pointing to best design for helicopters that dictate the popularity of the 4 blade approach.  But there are other factors to consider.  More blades can give more lift  -  -  but it takes a more powerful engine to power them - -  and thus more weight to lift.

And so on.

Good question.

Electrical Engineering

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#### cleggsan

##### Expertise

All technical areas of Electronics Engineering.

##### Experience

BSEE, MBA, Design, R&D, University Research.
Senior Life Member of IEEE. Life Fellow of AES.

Organizations
IEEE, Consumer Electronics Society, Audio Engineering Society.
Broad teaching experience; work experience mostly in consumer electronics and conversion from analog to digital technologies. Pioneer in digital audio at all levels.

Education/Credentials
BSEE (Equiv) BYU BSEE University of North Dakota MSBA (MBA) Illinois State University Graduate Studies in Computer Science - Bradley University Graduate Studies - Ohio University Graduate Studies - University of Missouri Kansas City DeVry Tech - Electronics