# Homework Help: Finding the Mass of a Helicopter

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1. Feb 18, 2016

### dydxx

The question is:

The main rotor blades of a helicopter sweep out an area of radius 4.0m and push air downards with a speed of 15ms^(-1) away from the blades.

By tilting forwards 20 degrees, the helicopter stops rising and starts to accelerate horizontally at 3.57ms^(-2). The density of air is = 1.3kgm^(-3)..

Determine the mass of the helicopter?

We have been learning momentum and centre of mass and this was given as an extra question for homework but kinda confused how to approach it and also im confused what relevant equations I should use as well. A push in the right direction would be amazing! :)

2. Feb 18, 2016

### alw34

outline of my thinking: cool problem!

The fundamental idea is that you have an area equivalent to that of the helicopter rotors with a radius of 4 meters pushing down air at the stated density. That gives you a force up. This assumption eliminates the need to fuss with the varying rotational speed along the rotors and likely a varying radial shape along the length as well.

I think what the problem implies is that at a reduction in downward force caused by a 20% incline of the rotors, the thrust up is reduced to just offset, that is equal, the weight[mass] of the helicopter....it stops rising and starts moving horizontally. So how much is the area effectively reduced when the rotors are inclined...I'd pick a trig function reflecting that reduction.... either area or radius.....which do you think?

If you buy that description, or have another interpretation,proceed along with this:
https://en.wikipedia.org/wiki/Betz'...onservation_of_mass_.28continuity_equation.29