Hey, i am designing a helicopter having a bit different design. (If you have any design then suggest me but remember, i have to built it by my own.) and ya i have to make it cheap and small just like a toy. I am a beginner so guide me if i am wrong and suggestion are most welcomed. Now i have done some designing which doesnt looks that nice and yes i am not good at designing on paper too. I am concerned about the thrust of the rotar placed on both the sides. There are twin blades two propeller rotating in opposite direction on same axis as shown in figure. So what formula should i use as i googled and found many. Finally i stuck at this formulae for further work: T=[(eta*p)^2*2*pie*R^2*rho]^0.3333 where, T= thrust(in newtons) eta= propeller hover efficiency =0.7-0.8 is typical for uninstalled low pitch props p=shaft power pie=3.14159 R= propeller radius(in metres) rho= air density=1.22 kg/m^3 in this equation nos of prop is not mentioned so should i doubled what my answer comes?