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I'm in the process of building a hovercraft. In order to lift it, I'll need a motor directing the flow of air into a skirt. The specs for the motor I'm looking at say that it can turn a 15x5 dual-blade propeller at 6,000 RPM with 262W of power.

However, the motor is designed to be used by a large drone. In that case, the motor only has to work against atmospheric pressure. My hovercraft will have about .17 psi of pressure within the skirt. How do I adjust the required power for a given RPM based on the pressure below the propeller?

Mass is 87kg

1.6m x .45m

Airgrap: 12.7mm

Cushion Pressure: 1.185 kPa

Inward flow during static flight is 3,000 CFM (I believe that means flow out is also 3,000 CFM?)

These are the motors I'm looking at: https://www.getfpv.com/tiger-motor-mn4014-330kv-antigravity-2-motors.html

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    $\begingroup$ How fast does air leak out from the skirt - that basically gives your flow rate... Of course what is the total mass? How big?... $\endgroup$ – Solar Mike Mar 14 '19 at 12:07
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Alright, I think I figured out the answer. Bernoulli's equation tells us about the energy density of air. Each pascal of pressure is one joule of energy per cubic meter. If we multiply the in-skirt pressure of the hovercraft by the volume of flow per second, we should get the cost in additional power.

1,185 Pa * 1.42 m^3/s = 1,683W

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  • $\begingroup$ But not any info on whether the propeller will operate stably under those conditions. You don't want fan hunting on a hovercraft, and you would typically use a fair bit of safety factor in the fan design against hunting. This takes you in the direction of ducted axial fans, vane axial fans, and tube axial fans, in increasing back pressure capability. As a sanity check, if the added power is not offered in an off the shelf fan, it suggests the fan doesn't work well absorbing that amount of power. The trick is figuring out what will happen during takeoffs and landings when flow velocity is low. $\endgroup$ – Phil Sweet Mar 17 '19 at 3:37

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