Number of rotor blades

nikhilheb

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Aug 28, 2012
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Bangalore, India
Hello fellow rotorheads!

I wanted to know what the maximum number of rotor blades a helo can have is. What are the overall differences in multi blade rotors? Considering lift required, drag, power available, aspect ratio, solidity ratio, tip speed and any other factors you feel important, and at the same time disregarding landing space available, parking space, maintenance etc. Want a purely aerodynamic opinion.

Any and all inputs are greatly appreciated!
 
The Mil Mi-26 has eight main blades, and the CH-53E has seven.
 
To keep rotor performance constant disk loading, rotor solidity and Lock number have to be constant, which essentially means blade chord scales to the number of blades. Since the blade stiffness scales to the third power of profile thickness your blades will very soon approach the "wet noodle" state. Anything you win in terms of induced velocity due to the Goldstein factor will you loose due to excessive blade motion, flapping and especially torsion. You could try to design the blades according to the rules for ribbon rotors but these are aerodynamically less efficient. There's a good reason why designers try to keep the number of blades as low as possible, the AH-64 with just four blades is testimony to that.

There is one other thought I'd like to propose, though proof of it would require using advanced theory and some heavy iron:
Since the airflow is almost parallel to the rotor disk the vortex sheets emanating from the rotor blades are spaced very closely. With multiple blades the amount of blade vortex interaction would increase and rotor performance would deteriorate.
 
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