Spaced
Glad it helped. As for paper do you mean A.N. Modha’s papers? If so it’ll be control which I have trouble with too!!
There is a beneficial effect with bi-planes that mean less drag is produced than doubling that from a mono but in the case of rotors there are a lot more factors. The lower rotor operates in the downwash of the upper which means it is way off optimum. Accordingly the power consumed is higher in co-axial than for other twin rotor configurations. The bi-plane effect may offset some of the drag but I doubt it would be enough to counter the above or the greatly increased hub drag. Then there is the weight of two rotor systems, two control systems, the transmission etc. Having said all that the Kamovs have been very successful but the down sides to the configuration kept western designers away from it until the Sikorsky S-69 with the Advancing Blade Concept (XH-59) which went some way to solving some of the co-axial problems.
As for co-axial analysis, if using momentum theory use the area of a single rotor. That way, you can avoid considering blades altogether. If you were going to use blade element theory, which would be tricky as you would have to consider the interference of the rotors on the induced velocity. It really depends what you’re trying to do.
Alltorque
Sure, you can PM me or email if you want.
You’ve got it, as long as you are consistent, it doesn’t matter what reference area you use. However, since practically all of the available data uses the entire disc area, that is the one to use.
As for the extrapolation, yeah, the general way to approach it, is that the rotor is in steady state i.e. a steady hover or climb, that way there is no acceleration and the thrust is equal to the weight.
Hope it all helps.
Steve