# Calculating lift force of a motorised hovercraft

## Main Question or Discussion Point

Hello, I need help regarding calculating the lift force for a hovercraft. I found an equations online on thrust propulsion.

Equation →
T= π/4 x D^2 x (v + Δv/2) x ρ x Δv

D- diameter of propeller
v- velocity of incoming flow
Δv- additional velocity, accelerated by propeller
ρ- density of air (1.225 kg/m^3)

I know that the (v) would be zero initially as the hovercraft is not moving. The problem I have right now is that I do not know how to calculate the additional velocity from a given motor as it Only gives the rpm and not the velocity. If I were to convert rpm to rad/s then to m/s. This would only calculate the velocity of a point mass instead which is not the linear velocity I need. Or am I wrong? Thanks

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jack action
Gold Member

Also, you are talking about «calculating the lift force» but you give an equation for thrust propulsion. You might be interested in this thread for lift force.

Hi, I'm trying to work out what size electric motor is required for my hovercraft

I have a 6.5m x 3m hovercraft that is run by hydralics and a 3.9 Rover V8 motor which requires 150 horse power to lift it's 1200kg mass.
I am wanting to convert the motors over to electric and a diesel generator but not sure on what size Kw motor to install.
The lift fan has 5 blades at a diameter of 850mm and runs at 3000rpm

any help would be appreciated

Cheers

jack action
Gold Member
1 horsepower is 0.746 kW, so you need 112 kW of power no matter the type of motor. The equation in this post can give you an estimation of what power you need just to lift the hovercraft. You still need additional power to push it forward though.

Electric motors are generally rated by their continuous power but they can reach much much higher peak rating. Check the specs carefully. The 150 hp of your V8 motor is the peak rating. The continuous power should at least greater than the power needed to lift the hovercraft.

Your electric motor needs to have a similar rpm compare to your engine or you will have to install a gear reducer to get a comparable rpm range. Gear ratio doesn't affect the power output.