Coaxial Rotor UAV Helicopter Hover Performance

1. Feb 25, 2016

Usamah Abdul Latif

Hi Guys!

I'm doing a conceptual performance calculation and analysis on co-axial rotor UAV helicopter on hover. However, I'm having conflicting results and if anyone can help me, it would be very great.

The design constant of the helicopter are as follows:

MTOW = 20 kg
κ = 1.15 (Induced Power Factor)
κint = 1.281 (Induced Power Interference Factor)
σ = 0.034 (Solidity Ratio)
A = 0.257 m^2 (Main Blade Area)
ΩR = 215.356 m/s (Tip Speed)
CDo = 0.011 (Profile Drag Coefficient)

I'm using this equation, from the research paper of Aerodynamic Optimization of a Coaxial Proprotor - Leishman & Ananthan:

P(required) = P(induced) + P(profile)
P(required) = {[κint*κ*W^(3/2)]/sqrt(2*ρ*A)} + {[ρA(ΩR)^3]*(2σ*CDo/8)}

However, I found out that the induced power is only 25% of the total power and the profile power is 75%. This is really contradict with the theory. According to Wayne Johnson's book of Helicopter Theory, the induced power should be 60-65% of the total power. This also makes the Figure of Merit is only 0.139, whereas the good helicopter should have Figure of Merit ranging from 0.6 to 0.8.

Asides from that, I also found out that the required power required decreases with increasing altitude. This is contradict with the theoretical aspect in which, as helicopter altitude increases, more power required for the helicopter to maintain the same amount of thrust, as thrust = weight. When the power required is reaching the same amount the power available, then the helicopter reach its hover ceiling, as it can't go more higher.

I read other research paper on this area, but I found out most of them uses the same equation to calculate power, derived from the momentum equation.