Science Buddies: "Ask an Expert"

I am currently working on a hovercraft project with my friend for a science fair.
This has been working fine until i got to the amount weight it will be able to carry.
So far iv'e got Radiata Pine Non Structural Value Ply Length: 2000 mm, Width: 1000 mm, Thick: 7 mm, Radiata Pine Non Structural Value Ply Length: 400 mm, Width: 200 mm, Thick: 7 mm and 1.3 wide x 2.3 long marine grade Vinyl.
My lift power is one 320 kph/199 mph fuel powered leaf blower, and, a drive power of up to four 18V DeWalt battery powered blowers with battery's ranging from 18V-54V
with that amount of power minus the weight of the mentioned things and a few screws about how many people should it be able to carry as we have only one trial day available at the location were trying it at
we will be trying it with 7 boys, 4 are 30-50 kg, 3 are 45-80 kg/66-110 pound and 99-176 pound


Thanks

also if some people have name ideas it would be great

Hi huntleigh,

Can you post a photo or design diagram of your hovercraft? I have a general idea of your project, but that would allow us to help you better.

You may have to experiment incrementally, as some factors may reduce the lift power in practice. I would recommend starting at a low power setting and a low load.

Best,
Charles

hello all
i have been working on this for a while, i am needing a mathematical formula to figure out how a certain amount of wind speed+ compressed air can carry how much weight
i am currently working on a hovercraft with a 320 kph/199 mph leaf blower, the hover craft will rely on compressed air at the centre of the skirt to build enough pressure to lift.
this has been going fine and in theory has 4900 pascal, 4.9 KPA and 0.710685 PSI but cant figure out how much weight this should be able to lift
it is fine if you just reply with the formula as a certain amount of my own brain work is needed and i need the formula to work this out and to show my working

huntleigh,

Thank you for your question. It sounds like an interesting project. Since you know the output of your powerplant (aka leaf blower) we can work backwards to determine how much weight it should be able to support.

eq. 1) P = F / A where,
P = Pressure in Pa (N/m^2)
F = Force in N (kg-m/s^2)
A = Area in m^2 of the combined area of the hole(s) the blower exhaust will exit, not the total area under the hovercraft (i.e. if you have 6 holes each 0.1m diameter under the hovercraft supplying the air from the blower, then your A = pi x r^2 = 3.14 x 0.0025 = 0.008 x 6 = 0.05m^2 (Approximately, I'm rounding up a lot.))
Since you know P and A you can solve for F = P x A.

eq. 2) F = M x a where,
F = Force (we just solved for that)
M = mass
a = acceleration (since we are talking vertical lift this is equal to gravity (G = 9.81 m/s^2)

solving for M = F/a, this will yield the maximum combined mass that can be lifted vertically (hovercraft + pilot). any value less than or equal to this should begin to hover.

However, keep in mind that given inefficiency in design and losses, the real max is likely to be less than the calculated, so the closer you get to your calculated M the less likely the hovercraft is to be able to support. As you build it, you can easily test how close you got to calculating the actual operational lift capacity by putting weight on top until it "bottoms out".

good luck,