I need your opinion on a counter rotating design. 2

This is an interesting concept, can I come by and see it?

Generally the RPM is set so the tip speed is about 600 ft/sec. You want a high dynamic pressure, but you want to avoid mach induced drag. Also, the large rotor will generate most of the thrust. Putting the large rotor on the bottom creates a tough balancing problem. You should consider putting the large rotor on the top. Also, you could put twist in your blades to improve your hover performance.

Using your numbers, and assuming the same RPM for the top and bottom, untwisted blades, and 8 deg collective. For 3 blades on each rotor, 700 RPM generates 1317 lb thrust and requires 60 HP. At 650 RPM the thrust/power are 1140 lb and 48 HP. For 4 blades on each rotor, 600 RPM generates 1205 lb thrust for 52 HP. 550 RPM generates 1,013 lb thrust for 40 HP.

Here are the equations I used

Cl = a alpha (a = 5.2 / radian)
Cd = Cd0 + cd2 alpha^2 (Cd = 0.0087 + 0.4 * alpha^2)

theta_tw=twist/R

sigma = Nb c / (pi R)

Lambda = sigma a / (16 Kappa_p^2) (sqrt(1+ 64 kappa_p^2/(sigma a) theta_75 / 3) - 1)
Ct = sigma a / 2 (theta_75/3 - lambda/2)
Cpi=kappa_p Ct^1.5 / 2 (kappa_p = 1.2)
Cpo = sigma * Cdo / 8
Cpo = sigma Cdo / 8 + sigma Cd2 / 8 ( theta_75 (theta_75 + theta_tw/10) +7/240 theta_tw^2 + 2 lambda^2 - 8/3 theta_75 lambda)
Cp = Cq = Cpi + Cpo

T = Ct (rho pi R^2 (Omega R)^2)
Q = Cq (rho pi R^2 (Omega R)^2 R)
P = Omega Q
HP = P/550

Good Luck

Im not too worried about the rotor ballance but the power plant will be the kicker,fuel & lubracation systems ? and the mass & structure required......