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I need your opinion on a counter rotating design.

I need your opinion on a counter rotating design.

I need your opinion on a counter rotating design.

I am working on a counter rotating helicopter design where the engine sits between the top and bottom rotor. The bottom rotor has a 5.25 inch chord and a 10 foot span, and is mounted on a turntable. The engine is mounted to the rotor at the axis. The top rotor has a 3.4 inch chord and a 3 foot span and is mounted to crankshaft of the engine. As the top rotor rotates in one direction the engine and bottom rotor turn in the other direction. The difference in rotor lengths and chords provide a "gear reduction" effect for the main rotor. What I need to know is what kind of rpm's will I need to lift 1000 pounds with this type of set-up? or will I have to go with a larger rotor set?  Also, before you start yelling about how I am going to kill myself I am building it radio control and it's specific use is as a "skyhook". Your opinions, positive and negative will be greatly appreciated.

                        Respectfully, Willwildy

RE: I need your opinion on a counter rotating design.

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)


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

RE: I need your opinion on a counter rotating design.

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

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