Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hover craft!

  1. Aug 19, 2007 #1
    :confused: hi there, im building a hovercraft in my garage, so far it has gone perfect....
    until i get to the thrust part! for some reason i cant get the darn thing to move! for lift i have a kawasaki back pack blower pushing air down into an air mattress with holes punched in it and it works lovely. when i sit on the seat it hovers around my garage seemingly at zero friction. i thought that would be it, but i was wrong... it seems that i could use some big brain help, maybe someone out there has an equasion that could help figure exactly how many cfm's i need or rpm's or even something to put me in the right direction. my hovercraft weighs apox. 300lbs with me on it, and like i said could be real close to zero friction at least it seems like it when im hovering back and fourth effortlesly in my garage. and for the trust part so far i have a 24'' paint booth exaust fan with a 5 hp engine with ??? rpm's . the engine and cowl seem to be very promising but the fan blade doesnt seem to be pushing enough cfm's????? idont know if that is even right! but maybe someone does? please help, and remember, im no physicist so keep it kinda basic for me please!!!!!!!!!!!! thanx a million

    Attached Files:

  2. jcsd
  3. Aug 19, 2007 #2
    Weight is the enemy, remember that above all.

    It's true that with enough thrust you would need no air cushion, and like an airboat you could just overcome friction and move forward.

    I have to think that you are not balanced and that part of the hull is making contact with the floor.

    Can you set up some mirrors (too see all the way around) and video yourself on the platform to check level?

    EDIT: You could tie a string/rope to a fixed stationary point and pull yourself. I would think you could just feel where it's catching.

    How about starting with a bubble level on the floor and then on the craft while under power to see if it pitches?
    Last edited: Aug 19, 2007
  4. Aug 19, 2007 #3


    User Avatar
    Gold Member

    That's a nice looking machine, Fatdaddym.
    Remember, though, that exhaust fans are designed for exhaust, not propulsion. You might get by with the same unit if you twist the blades to a steeper pitch.
  5. Aug 19, 2007 #4
    How easy would this be to do?
  6. Aug 19, 2007 #5


    User Avatar
    Gold Member

    Extremely simple. You just clamp one of the blades in a bench vise and twist the hub to a particular angle. Use a template of some sort so that you can duplicate that angle, and repeat the process for the other blades.
  7. Aug 19, 2007 #6
    as far as drag with the skirt, it does okay. as an example, my garage has the slightest grade (for water run off) and whenever i fire up the hovercraft and sit on it i inevitably end up riding that thing out of my garage and down my driveway, and pretty fast i might add. so that is what makes me feel that im somewhat balanced. still yes sometimes if you lean too far to one side or the other it will create drag, im still working on that part. as for the string to pull the craft with, my gararge being in a slight grade doesnt help. but my prediction on what will happen when pulled is that it would continue to go in that direction until it hits something that stops it, thats judging by what ive seen it do already. its like being on ice!!!!
  8. Aug 20, 2007 #7


    User Avatar
    Gold Member

    That would be correct on a level floor, or going with the slope. Pulling across the slope, though, would result in a diagonal movement as the rope and gravity work at different angles. It would be similar to how you draw a diagonal line on an Etch-A-Sketch by turning the x and y knobs at the same time.
  9. Aug 20, 2007 #8
    The whole point of the rope would be to simulate horizontal thrust, which in a perfect world would be 90 degrees to gravitational forces. The slope in the floor kind of messes the control situation, best set it all up to go with the flow.

    If you are not catching a corner, then we have to assume there is just a lack of thrust here.
  10. Aug 20, 2007 #9


    User Avatar

    Staff: Mentor

    I wouldn't recommend screwing with the fan blades yourself. If you unbalance the fan (either physically or aerodynamically) it'll come apart and kill you. And even if you succeed in making it more powerful, you still have only a 5hp motor.

    The first thing you want to do is check the specs on the fan: find out what it's design airflow actually is. From that, you can calculate thrust via Bernoulli's equation and that will tell you if the fan is big enough. If it isn't, you'll need a new fan.

    A certain 5hp 30" tubaxial fan I pulled off Grainger has an airflow of 16,500 CFM. That's a velocity of 3400 fpm at an area of 5 square feet. That's a velocity pressure of .72", which is .026 psi. 5 square feet at .026 psi is 18 lb of thrust. That's about 1.5 ft/s/s of acceleration. That's not a lot, but I would have thought it would be noticeable. Someone can, of course, check my math...
    Last edited: Aug 20, 2007
  11. Aug 20, 2007 #10
    Let's consider that this is a 5 hp fan.

    It's electric, right?

    You could get hurt if it gets way out of balance. You might be able to balance it in the bending process.

    I suppose it just as dangerous to glue angled flaps on it to get more thrust.

    If the motor is already matched to the fan changing the pitch will strain the motor and slow it down so that you end up with less thrust.

    Most of the time if you go larger you have less pitch. If you go shorter you have more pitch. This is like comparing fans to propellers, all very general and not specific to this situation.

    Just wasting ink/electrons here.

    No way to add fan blades, right?
    Last edited: Aug 20, 2007
  12. Aug 20, 2007 #11


    User Avatar
    Gold Member

    I somehow missed the part about this being a 5 hp motor; given the fan size, I was thinking that it was a 1/2 or 3/4 hp job like we had the print shop. Given the power involved, I retract my suggestion about altering the fan.
  13. Aug 20, 2007 #12
  14. Aug 20, 2007 #13
    wasssssuppp, okay heres the deal, im running a 5 hp b&s engine belt driven to the exaust fan going from a 1 inch pulley on the engine to about a 2 inch pulley on the fan. i know that i could get more rpms if i match the pulleys or even reverse from what i have now to opposite of that but the fan seems to heavy or something, it works fine until i give it full throttle and the fan gets up to speed, then it looks like the fan is trying to spin faster than the engine or something and it bogs out!!!! my helper at work (who took lots of physics i guess) says that thats impossible. i dont understand why but anyway, my sugesstion to my self has been to just buy a lighter fan blade from a distributer i found online who lets you decide the pitch and diameter of the fan you choose. then all i have to do is unbolt and rebolt! is this a good idea or not? the fans start at around 40 bucks for a fiberglass four blade


    check out the link to the fan i speak of and let me know what ya think thanx alot guys !!
  15. Aug 21, 2007 #14
    1. Briggs and Stratton 5-10 hp engines are mathed up to 24" fans and 36" propellers all the time for hovercraft lift and thrust.

    2. You experience overloading or over reving the fan, more rpms will not equal more power. The fan can only use or absorb so much power before it just goes to waste and spins silly (or bogs out).

    3. You have three options, A; slow down the rpm (via pulley ratios) and install a slower moving propeller or fan. B; Make a cold jet with multi-staged fans with stators in between. C; Obtain 2 or 3 more matching fans and belt them togther for a multi thruster configuration.

    You have too little fan or too much power depending on how you look at it.

    You can post as a "GUEST" in a thread I've created for you at the Hoverclub of America. People who know a lot more about the tech side than myself can help you out.


    EDIT-1: FDM, Did you know that that fan will only absorb 1/2 of a HP?

    EDIT-2: FDM, what was the old paint booth fan electric motor rated at? You might want to get a much lighter gas engine from a weedwacker or leaf blower of similar power rating and hook that up to the existing fan.
    Last edited: Aug 21, 2007
  16. Aug 21, 2007 #15
    Your first reply from a HCA member:

    Here is what you do with the 5 hp B&S engine, use it for lift on a real hovercraft. A cheaper way to go is to purchase a non-working hovercraft and put new skirt, engines and fans/propellers on it.

    UH-13PT Twin

    Attached Files:

    Last edited: Aug 21, 2007
  17. Aug 22, 2007 #16

    Attached Files:

  18. Aug 24, 2007 #17

    Attached Files:

  19. Jul 31, 2009 #18
    It seems to me that you could dump the fan and the 5hp Briggs and all its associated bits. For propulsion and steering, add two more backpack gas powered leaf blowers. One on each side; pointing aft. Vary the throttles to steer and accelerate. You've already proved that one blower can lift you up, so...
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?