Dave et al,
Representatives of the Kamov company have been present at the last two big UK Helicopter technical get-togethers and on both occassions have claimed that the co-axial rotor configuration, when properly designed is infact more efficient than a single main rotor and tail rotor configuration. To the tune of about 15%!
They are understandably cagey about the exact reason for it but usually say...
The wake contraction of the downwash from the main rotor causes air to be drawn in to the second rotor from the side hence increasing the effective rotor diameter and reducing induced power requirement. Furthermore, there are addition beneficial effects of the vortex wake interactions between main rotors.
The key parameter required to achieve these performance gains are the spacing between the main rotors - required to be approximately 10% of MR diameter.
The earlier post about the directional control in autorotation is also correct. The only directional control a co-axe has in autorotation is derived from the moveable tail-planes - hence very marginal directional control especially at low forward velocity.
I understand that this is quite a fluffy explaination - but they really don't give too much away!
DAVE - Which book do you refer to? 'Latest Textbook'
Cheers
CRAN
