# Hovercraft design parameters

My washing machine broke down recently, and they replaced the motor. The old motor was not broken, as far as I could tell; the sensor that told the CPU that the drum was balanced was what was out of order. At any rate now I have a perfectly decent rotor and stator, and I started thinking about how to recycle them. Apropos of not much of anything, I started thinking about a one‐man hovercraft.

What I have so far;

• The washer service manual specifies a max RPM. I assume this is completely useless because it refers to the motor and drum assembly and I’m not going to dismantle my washing machine to weigh the drum. I can call the repair guy and find out how much the drum weighs, but I'll only do so if I find that the calculation takes too long (I don't want to bother him on the weekend).
• The required lift force is equal to a > 9.8*(mass of hovercraft + mass of rider), where a is some acceleration over a time T. Where T is simply the amount of time necessary to move vertically from rest to a position above the ground equal to the height of the deployed skirt. In other words solve $Y=Y_{0}+V_{0}T+0.5aT^{2}$ for a known Y, zero initial velocity, and $a=\frac{F}{m}$.
• The friction and air resistance are negligible, however since increasing horizontal component of motion reduces vertical component, thrust should be higher than what is required to simply achieve lift by some factor.

Questions;

1. How can I evaluate the power of the motor? I have not been able to find specs online so far. Does it even make sense to talk about the power of the motor without knowing the power supply? Is there a maximum and minimum power output? Alternately, is power even a relevant parameter, or should I be concerned solely with RPM?
2. The friction and air resistance are negligible, however since increasing horizontal component of motion reduces vertical component, thrust should be higher than what is required to simply achieve lift by some factor.
3. What factor must I exceed vertical thrust by in order to move horizontally?
• Is there no other identifying information on the outside of the motor itself? It's almost certain that the motor was produced by a separate company from the company which built the washing machine. It should be fairly easy to find all of the specs if you can identify the original manufacturer and model number. Mar 28, 2015 at 17:35
• Although I do like your goal, the question could be split up to get more concise answers.
– Mast
Mar 28, 2015 at 20:01

Designing the lift system for a hovercraft isn't actually about the mass and the acceleration, it's about the pressure required to lift the hull and the rate at which the air leaks out from under the hull and needs to be replaced. Together, these determine the lift power required.

Horizontal motion is a completely separate issue, which requires knowledge about the friction of the skirt, aerodynamic losses, the slopes you want to be able to handle, the mass of the craft and the acceleration you want to be able to achieve.

While it may be possible to use a single motor for both functions, in conjunction with ducts and baffles, it's often simpler overall to use two separate motors.

• I found the equations you're referring to re pressure, thank you. The problem is, I'm considering this as a project for my college engineering club so the bar for financial feasibility is kinda low. So, if we have to buy a separate propeller, and a gearbox to convert the drive or something, we may not have the cash. But, I was thinking of those vtol thrust vector planes. Surely we could direct thrust without sacrificing too much pressure? Mar 28, 2015 at 19:31
• Ah. I misunderstood. Normally, the term "hovercraft" refers to a vehicle that stays very close to the ground and uses a skirt of some sort to trap the air. If you want something that lifts itself just by the reaction force from a jet of air, you'll need a LOT more power. Or are you thinking of something that would be more properly be called a "helicopter"? Mar 28, 2015 at 20:25
• No, your understanding was correct. It was I who was unaware that most designs incorporated a separate prop for thrust. Am I also underestimating the design challenge of using baffles and ducts as you describe? Mar 28, 2015 at 22:12
• Based on your responses so far, quite possibly :-) Diverting air from the lift system to provide horizontal propulsion could present problems with regulating both systems. Before you start a major project with your washing machine motor, you might want to spend some time building prototypes that use, say, a shop-vac as a source of air, just to get a feel for the issues you'll face. Mar 28, 2015 at 23:03