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I'm currently doing conceptual performance analysis of a UAV helicopter in hover. I'm having some conflicting results however, and if anyone could help me out, that would be great. Here are some of the basic design constants I'm working with:

GTOW: 95kg

κ = 1.15 (induced power factor)

σ = 0.07 (solidity ratio)

A = 6.16m^2 (main blade area)

ΩR = 204.17 m/s (tip speed)

CDo = 0.01 (profile drag coefficient)

P(available) = 15,221 W

I'm using this equation, from Principles of Helicopters - Leishman:

P(required) = P(induced) + P(profile)

P(required) = {[κW^(3/2)]/sqrt(2*ρ*A)} + {[ρA(ΩR)^3]*(σ*CDo/8)}

I'm trying to find my performance ceiling in hover, which occurs at P(excess) = P(available) - P(required) = 0.

However, I'm getting that my required power

**decreases**with increasing altitude. This is contrary to helicopters with greater GTOW's. Now, from the equation, I can see that as my density decreases, my induced power term increases and my profile power decreases. This is of course, what I would expect, because of the following:

- while maintaining a constant tip speed (RPM) at a higher altitude, you need more torque and subsequently more power to maintain the same amount of thrust, as thrust=weight (in hover)

- the profile power decreases because the amount of skin friction reduces due to the lower density

Now, I'm trying to determine the maximum ceiling for my UAV. What this essentially tells me, is that, at this particular GTOW (and anything smaller than 150kg), my ceiling is infinite? Ahhh please help me make sense of this. Thanks.