Hovercraft

[edomus]

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[Mike Bortfeldt]

Edomus,

I recently built a 2-person hovercraft (Universal Hovercraft 12T4) from plans and can hopefully answer some of your questions. First, your 4'x8' platform should be able to lift one person as long as you can generate sufficient pressure from your lift fan (you don't mention what your fan blade was made out of or any of it's characteristics). Universal Hovercraft (UH) sells plans for a 3'x5' beginner hovercraft (UH-6F) that utilizes a 3 to 5 hp engine for both lift and thrust that can handle a 150 lb. payload so your engines are certainly large enough. You may want to think about buying these plans as they will have good general information on building a basic craft (skirt construction, operation, steering, etc.) that you can use when building yours. Larger engines are not necessarily a plus and can in some cases add weight for no useful purpose if your lift & thrust systems aren’t designed to utilize the engine’s capabilities. However, having existing engines available does negate a lot of the downsides.
One thing I would like to stress is to make sure you place guards around both your props. These blades will be going around 3000 RPM and could cause serious injury should they break or have some other failure. I say this from experience. After about 40 minutes of run time on the 12T4, I had a failure of the thrust system that resulted in a broken crankshaft on a 10 hp engine, and a prop split into two pieces. One of these pieces was thrown about 50' right through some lightweight steel mesh (luckily, no one was in the path.). Needless to say, there will be some changes when it’s repaired. You may want to think about making a real duct for the lift & thrust props as not only will you will get better performance, but it can also serve as guarding. You should wire in something like a lanyard kill switch to stop the engines in an emergency. Being homemade, safety should be a priority.
A few other thoughts based on your drawing and previous notes… 1) you may want to lower the hover height to about 6”, as with only a 4-foot width, it might be more stable. 2) I'm not sure if you really want a raised seat, as that will make your CG higher, again affecting stability. 3) I don't see any method to steer the craft. Typically 2 or 3 rudders are placed behind the thrust prop to direct the airflow and steer the craft.
Finally, make sure the props are designed to handle the stresses of running at the speed of the engines (possibly up to 3600 RPM). The props must also be well balanced. An unbalanced prop will cause significant forces and excessive vibration on the engine and craft as well as the prop itself.
Hope some of these comments help, and if you have any more questions, please ask and I will try to answer them. Just remember that the above comments are opinions only. My background is certainly not in hovercraft or prop design!

Mike

[falconmaster]

Originally Posted by adamrp

Im one of the hovercraft builders (along with edomus) and maybe you can help us predict what exactly is going to happen when we throw this thing together... Basically, we are using a bag skirt. We have an 11HP engine that is direct-drive to a 6-blade fan with a diameter of 24" (a little bigger, really). This is a very home made craft, so it is all built on top of an 8'x4'x3/4" plywood sheet. Do you think it will hover the way it is supposed to? I think the hover height will be about 8 inches.

arp

11hp is good for about one person. Take the total weigt and divide by surface area. That will give you the static pressure you need to acheive exualibrium. let say you are 100lbs, and you have 1m square that makes 100lbs per square meter , or .1lbs per square cm of pressure you need to be able to generate to achieve "zero weight". If you can somehow build an enclose to test the static pressure of your powerplant then you can predict how much weight you can lift.
Hover hieght is dependent on your skirt height and the statci pressure you can maintain. It is an illusion though. Even though you look like you are barely leaving the ground the skirt is flexible and can flex to clear obstacles. The is the lift cushion hieght that is critical, because that is what will get hung up when you try to clear obstacles.

I had a 650 cc snowmobile engine in the hovercraft I built and I could lift 600lbs with a 6 inch hoverheight and I could travel at about 15 to 20 mph. I had one three blade prop that provided both lift and thrust. About one third of the air volume was used for lift, and two thirds for thrust. I will dig up some pics and send them to you!! Hope that helps.

[bankievr6]

[IMG]Hey there, i figured i should post on here. I am the third hovercraft group member, and the resident computer animation student/self taught maya n00b who has supplying the renders. I was the other constructor of the RC Chevy Nova along with arp ive whipped up another one with a few changes to further illustrate our physical and conceptual progress... i know there is no steering yet, but it will come as soon as we get a concrete idea of how it will work. i zoomed in on the drive engine, being the only update section of the render. ive labeled a few new components.

[Jeffrafa]

My brother built a hovercraft from scratch a few years ago when he was in college. It consisted of 4' x 6' plywood sheet, heavy duty plastic trash bag type material, some PVC for distributing the air through the skirt evenly, two frisbees bolted to the bottom to hold the skirt up in the middle, and two electric leaf blowers for lift. It didn't have any sort of propulsion or steering, but it was downright impressive for lift. We tested it by loading it up with cinder blocks, reaching over 1200 lbs before we ran out of cinder blocks and began having trouble with the duct tape and staples giving way that were holding the skirt to the plywood.

I actually have what is left of it in my garage and have meant to improve upon it since then, but I've yet to have the chance. It is definitely in my list of things to do over the summer. I'll be sure to upload some pics if I ever get it running again

If you have any questions I may not be the best resource, but I'll help with what I can.

[edomus]

Originally Posted by Jeffrafa

My brother built a hovercraft from scratch a few years ago when he was in college. It consisted of 4' x 6' plywood sheet, heavy duty plastic trash bag type material, some PVC for distributing the air through the skirt evenly, two frisbees bolted to the bottom to hold the skirt up in the middle, and two electric leaf blowers for lift. It didn't have any sort of propulsion or steering, but it was downright impressive for lift. We tested it by loading it up with cinder blocks, reaching over 1200 lbs before we ran out of cinder blocks and began having trouble with the duct tape and staples giving way that were holding the skirt to the plywood.

I actually have what is left of it in my garage and have meant to improve upon it since then, but I've yet to have the chance. It is definitely in my list of things to do over the summer. I'll be sure to upload some pics if I ever get it running again

If you have any questions I may not be the best resource, but I'll help with what I can.

Question: what did the bottom of the skirt look like? Where were the holes? How were the holes reinforced?

[Mike Bortfeldt]

I use a 6 hp vertical shaft engine for lift, which I run at about 75% full speed. This easily provides 900+ lbs of lift using a 26" diameter 4-blade fiberglassed wood prop. The prop was purchased from Universal Hovercraft "preshaped", which basically means that they are rough cut and require finish sanding, fiberglassing, and balancing before use. They also sell plans to build your own if you want to take that route (much cheaper - but more work). The setup of my lift fan is not all that different than from your picture (although I use a duct). The lift engine is in front and about 20% of the lift duct opening is directed into the bag skirt with the remaining 80% is just directed under the hovercraft. The bag skirt itself is attached on the outside (obviously) and the second attachment point is about 9"-12" in from the outside (9" or so on the sides, more like 12" front & back – the difference is most likely due to the hull design). The skirt material is typically vinyl-coated nylon, however, probably any type of material that is not too porous to air will work (tarp?). The durability will probably just be a little less.
You can also get away with only the two center rudders as the ones on either end are not going to add a significant amount of additional turning capability and it should simplify the steering system a little. While mechanical systems are not my area of expertise, I think you will want more than a 90-degree wrap angle on the engine for the belt or you may get some slippage.
I hope you can find some useful information here. It's looking pretty good.

Mike

[Jeffrafa]

Originally Posted by edomus

Question: what did the bottom of the skirt look like? Where were the holes? How were the holes reinforced?

I dug it out of the garage to take a closer look today and took a few pics to show it. The holes were reinforced on both sides with duct tape before they were cut out of the plastic. There were 5 holes total, each 2" in diameter spread out between the two Frisbees. The entire craft was 6' x 4', with the Frisbees mounted 2' in from each end, my brother did say however that the Frisbees should have been farther apart, so I'm going to do that when I rebuild it this summer.

The leaf blower is attached to the metal box via a hole in the back of the box, from there the air is distributed into the skirt via the 4 PVC pipes, helping to spread out the air and fully inflate the skirt. The four spots of duct tape on the bottom farther out were to protect from the PVC pipes cutting holes in the plastic when the hovercraft wasn't running.

The main improvement I plan to make is with how the skirt attaches to the craft along the top edge, I plan to have another strip of wood bolted or screwed down over it rather than just duct taping and stapling the plastic down. Also I plan to add feet off of the sides of the craft to keep from dragging and ruining the skirt when Its not inflated and running.

I've attached three pictures showing the top, bottom, and a close shot of the holes in the skirt so you can see in more detail how it worked.

[adamrp]

Originally Posted by Jeffrafa

The leaf blower is attached to the metal box via a hole in the back of the box, from there the air is distributed into the skirt via the 4 PVC pipes, helping to spread out the air and fully inflate the skirt. The four spots of duct tape on the bottom farther out were to protect from the PVC pipes cutting holes in the plastic when the hovercraft wasn't running.

Is it necessary to have the 4 pipes distributing the air like that? Did you first try to make the craft without distributing the air and find that it didn't work? I ask because we did not plan to do this, but we did think that it could be a possible problem. Thanks.

arp

[edomus]

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