Hi Zigge,
I had some similar thoughts on rotor blades, why use large chord on all four blades ? this will only increase the drag area and power required to move them. Should the blades be shorter than a two blade helicopter because there are four ? etc etc and it looks like you will get answers soon from Anthony.
I came to the conclusion that the chord would be less for the co-axial due to blade solidity / disc ratio and shared load. Large size chord width allows deeper cross section which seems necessary to keep long blades intact and not folding up with a heavy helicopter. Share the load with more blades and the individual blade section does not have so much lift work.
I also came to conclusion that blade length would correspondingly become less, although the reason is somewhat vague. After studying many co-axials these points started to stand out as being common aspects. Alas, the mathematics of the intricacies to define precisely what is required is beyond my ability.
Another consideration with co-axial is the un-equal work that each rotor performs and how to balance the subsequent torque.
The air deflection from the vertical by the upper blade action effectively raises the air entry speed to the lower blade. Should the lower blade be steeper pitch ? should it be a different length ? longer ? so it works in some fresh air ?
Should the upper rotor be short and strong with full chord width and the lower rotor have a "flybar" hub extension and fly in an outer annulus to the upper rotor ? both rotors having same or similar disc area ? and/or partially share some disc area ?
Many questions to ponder, I am very interested to know your thoughts on these and Anthony's as well as others.
The only piece of info I can add is from US Govt. sponsored research which came to advise the optimum separation between rotors is 17.5% of the diameter and should not be less than 12% or exceed 20%.
The fellow who built the Revolution helicopters, Fenner, now has a business making composite blades from moulds which may be twisted during the setting phase. LH and RH rotation are available as are a few different lengths all revolving around the sizes suitable for light helis.
Also, as there are four blades and the load is shared, it opens up scope to investigate suitability of fabricated blades from lighter proven single rotor helicopters such as AW-95 and Furia.
**Jens, ! we have the same conceptual thoughts re kart engines and co-axial helicopters : ) although it does not need to be low and too slow.
vertigo