Page 1 of 3 123 LastLast
Results 1 to 8 of 17

Thread: Any ideas why my experimental hovercraft doesn't work?

  1. #1
    Less than 10 posts so far!
    Join Date
    Aug 2012
    Posts
    5

    Default Any ideas why my experimental hovercraft doesn't work?

    A friend and I are trying to build a hovercraft as a summer project, and we're hoping you guys can offer us some advice!

    We bought two 6.5hp ventilation fans as used by the fire brigade at a great price, and plan to use one for thrust and one for propulsion. Obviously it won't be speedy, but we just want to make a working hovercraft.

    We've put the lift engine at the front, canted down and blowing into a bag skirt, which then vents underneath the hovercraft. Once we've got this working we'll add the second engine on the back for thrust. Pictures are attached.

    We used the lift calculator here, and the skirt design was based on this and [URL="http://4wings.com.phtemp.com/tip/bag.html"]this[\URL].

    Unfortunately, the hovercraft does not produce enough lift for a person and a second engine, and even when lightly loaded and hovering, the friction is pretty considerable. We've tried adding holes to the skirt, and added holes directly into the area underneath the hovercraft from the fan, but this has not dramatically improved performance.

    From the pictures, does anyone have an idea how we can improve the performance of our hovercraft? We feel like the performance is there somewhere, but are now in danger of aimlessly making alterations!

    Many thanks for any advice you can offer.

    Click image for larger version. 

Name:	photo 1.jpg 
Views:	86 
Size:	38.5 KB 
ID:	6463
    Click image for larger version. 

Name:	photo 2.jpg 
Views:	66 
Size:	36.5 KB 
ID:	6462
    Click image for larger version. 

Name:	photo 3.jpg 
Views:	71 
Size:	32.8 KB 
ID:	6465
    Click image for larger version. 

Name:	photo 4.jpg 
Views:	61 
Size:	47.3 KB 
ID:	6464

  2. #2
    gaz
    Guest

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    welcome aboard, I am no expert and others will be along that are far more knowledgeable than me. You will also get many differing opinions; it's the nature of the beast.



    I am assuming you have used 12mm ply, so weight could be one enemy, another no doubt will be your fan choice. It may well be that you don’t have enough air pressure to gain lift. My understanding is that air volume and air pressure are two different beasts you can have plenty of one without the other. As I said earlier others with a greater understanding of these things will be along to explain things far better than me.



    Yet another enemy could be the surface you are using it on. A flat hard surface like tarmac will give best results. If you are using it on grass then it will need to be VERY short not to lose any lift through the surface.



    I take it the craft is not for use on water, with that in mind... Are those pictures taken with the lift motor running? If not then it would appear that you have a lot of structure under the craft you could lose.I would remove everything below the top sheet and attach the skirt directly to the single sheet of ply that is left.

    what ever you do please report back as it would be nice to see if your efforts bear fruit.

  3. #3
    Forum God > 400 posts Club Member
    Join Date
    Mar 2009
    Posts
    978

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    I'm surprised - it ought to work at least on the flat concrete surface you show in your pics. Is it hovering in the pics or is it at rest? It looks like the fan isn't developing enough pressure to lift the craft. 6.5bhp should be enough but it may be set up more for flow than providing pressure. Whats the tip clearance like between fan and duct? - more than say 0.5inch will loose pressure. I presume theres no air leaks between the fan and hull. How heavy is the whole craft and whats the size of the hull? - weight/cushion area defines the pressure.
    Keith

  4. #4
    Forum God > 400 posts Club Member
    Join Date
    Mar 2009
    Posts
    978

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    Just looked at the website you quoted. First time I've seen it and to my amusement this section is entirely a (nearly word perfect) lift from the HCGBs Light Hovercraft Handbook which I edited in the 70s and is now out of print. But crucially they've left out all the diagrams!
    The reason I asked about whether the craft is hovering or not is that I'm not sure you've got the skirt set up for enough hoverheight - referring to the missing fig6 attached what are your dimensions H and h ?
    Click image for larger version. 

Name:	bagskirt.jpg 
Views:	42 
Size:	15.9 KB 
ID:	6466
    Keith

  5. #5
    Less than 10 posts so far!
    Join Date
    Aug 2012
    Posts
    5

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    Thanks for all the advice! I'll try and clarify a few points:

    - Gaz
    We have used 9mm ply, two boards separated by 8cm wood planks around the edge and in the middle. The weight is certainly not insignificant, but I don't think it's excessively heavy compared to the weight of the engine.

    I think you have a good point about air volume and air pressure. When the engine is running and the hovercraft is on the ground, around the rim of the fan air is flowing back away from the fan out of the hovercraft. This doesn't happen if the hovercraft is lifted up away from the ground. We've had a look at the fan clearance, and at most the tip is ~5 mm from the fan case; we're not sure if there's a robust way of reducing this. Perhaps these fans have been designed for operating with a low pressure differential?

    We don't know much about different fan designs, but presumably as a ventilation fan this has been designed for a big volume flux over a small pressure differential. Do you know whether if we replaced the fan with something like a propellor, would it potentially support a greater pressure differential?

    The pictures are with the engine running, so the craft is hovering, but about 5 cm.

    - Keith
    In the pics, it is hovering, but not very high and it doesn't support much weight.
    Just a question about the theory: provided the skirt provides a good seal, is the pressure to support hovering by 2 inches significantly different to the pressure to hover 4 inches?

    As I mentioned to Gaz, the tip clearance is ~5 mm, but there is definitely a back flow around the rim of the fan.

    The engine weighs 43 kg, and the rest of the craft can't be much more than 40 kg. The base is an 8x4 ft (2.4x1.2 m) board and the skirt is attached 25cm from the edge.

    Good to know you edited the book! Thanks for that information then.

    I think you might be right about not enough hover height… We kinda of guessed at that and chose what seemed like some moderate dimensions. The cross section of the skirt is this:

    Click image for larger version. 

Name:	Slide1.jpg 
Views:	27 
Size:	10.2 KB 
ID:	6467

    It's a pretty primitive design! I don't know whether it's got too big a GC area and that is causing excessive drag?


    Thanks again for any advice!

  6. #6
    Forum God > 400 posts Club Member
    Join Date
    Mar 2009
    Posts
    978

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    This ought to work - its just a question of working out why it doesn't!!

    The fan tip/duct clearance of 5mm or less should be fine - many racing craft run fine at 10mm but I'm surprised you're measuring a reverse flow

    You could have got away with 2 sheets of 4mm ply rather than 9mm but the excess weight there is small and not the problem.

    The cushion area (8 * 4 ft = 32sqft) is about half that of most single seat craft thus the cushion pressure is potentially higher. Its going to need 2 engines to trim level so that gives a basic weight of 2 *43kg +40kg hull = 277lbs. Over cushion area of 32sqft that gives cushion pressure of 8.6lbs/cuft. With say a 150lb human payload that is a total weight of 427lbs and thus a cushion pressure of 13.3lbs/sqft - highish but there are plenty of craft running at that pressure.

    The skirt design suggests you are aiming for a 1.5:1 bag/cushion pressure ratio which is fine but your target hoverheight is 9cm and you're achieving 5cm so the skirt is flattening out increasing your ground contact (GC) area and thus drag. Have you got the size and no of the feed holes in the inner bag wall correct to achieve a 1.5:1 ratio? Temporarily closing some off might be informative. Testing pressure with a water manometer at different points in the airflow (behind fan, in bag, in cushion) might also give us clues.

    I'm left wondering about the fan. As you say it may be designed to shift volume rather than achieve a pressure and with the 1.5:1 ratio you need the fan to deliver 13.3*1.5 = 20lbs/sqft. Although the nominal pressure at 5cm is the same as at 9cm hoverheight if the fan is 'straining' to deliver 20 then the flow may be too low at that pressure. Moving to a prop isn't the answer, generally you need more 'solidity' to deliver pressure so more blades the better. I guess your big investment is in the engine and fan so changing that isn't optimal. As its only a test rig you might want to try increasing the craft size (using say 3mm ply) to reduce the cushion pressure.

    Just as a check I assume the engine is actually revving to its nominated speed?

    I bet others have got better ideas! Anyone used such a fan for lift?

    Keith
    Last edited by Keith Oakley; 4-Aug-12 at 03:54 PM.

  7. #7
    Extremely active > 250 posts!
    Join Date
    Mar 2009
    Posts
    259

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    My 2 pennorth is that the lift fan is so obstructed that it's efficiency is questionable.
    It's hard to actually see the fan.
    I would certaily investigate that area.

    Kip

  8. #8
    gaz
    Guest

    Default Re: Any ideas why my experimental hovercraft doesn't work?

    All helpful stuff, told you far more knowledgeable people would be along.

    Adding to Keith and Kips advise, most of the 'useful air' comes from near the blade tips (the last few cm's), try to make any transitions as smooth and as big as possible so as not to slow or ad turbulence to the air you are shifting. You mention an 8cm gap between the two sheets, are the separators restricting air flow anywhere? Are the holes in the top sheet as big as possible to aid flow. if you can feed the air directly into the bag it MAY help.

    If you are in the UK then one of the few places you will get the thin ply Keith mentions is Robins timber in Bristol (http://www.robbins.co.uk/) They are very helpfull and they have a delivery service.

Similar Threads

  1. Want to work in the hovercraft industry?
    By Fishionado in forum General Forum
    Replies: 0
    Last Post: 11-Jul-12, 06:20 PM
  2. Lift fan ideas
    By Tobin in forum General Forum
    Replies: 6
    Last Post: 19-Jun-11, 09:54 AM
  3. Ideas for some improvements.
    By sixpackpert - n/a in forum General Forum
    Replies: 41
    Last Post: 27-Oct-08, 12:12 PM
  4. Hovercraft days? / work day out
    By Mart366 - n/a in forum General Forum
    Replies: 6
    Last Post: 6-Feb-08, 06:32 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •