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Hovercraft help

by hover
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hover
#1
Jan6-07, 11:11 PM
P: 344
Hi everyone
I have a few questions.

First, I am making a small hovercraft and have a fan for a source of air. How can i calculate how much air it moves??

Second, how can i calculate how much air pressure it is putting inside the hovercraft bag for the craft to lift??

Thanks for any help!!!
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ocdocd
#2
Jan7-07, 12:45 AM
P: 4
Quote Quote by hover View Post
Hi everyone
I have a few questions.

First, I am making a small hovercraft and have a fan for a source of air. How can i calculate how much air it moves??

Second, how can i calculate how much air pressure it is putting inside the hovercraft bag for the craft to lift??

Thanks for any help!!!
1. Get a 5 gallon trash bag, deflate it, and tape it behind the fan. Get a stopwatch too. Turn the fan on, start the stopwatch, and see how long it takes the bag to fill. 5gallons / that time gives you the air it moves per unit time.

2. This question is nonsensical because you can't make a hover craft with a fan and a bag of air (unless you have something lighter than air in that bag). If, instead of a bag you have a rigid structure, then you can do it. You need to make your craft less-dense than air, so you "float in air." You do that by evacuating air from the structure with the fan.

To calculate that pressure you figure out your hovercraft's volume (with you in it if you're driving in it). Then you figure out how much air weighs in that volume. If your hovercraft is viable, the air will weigh more. Then you subtract the 2. The difference is the maximum weight of the air that can be in the rigid structure. From that you can calculate its pressure using PV=nRT.
ocdocd
#3
Jan7-07, 12:52 AM
P: 4
Quote Quote by ocdocd View Post
1. Get a 5 gallon trash bag, deflate it, and tape it behind the fan. Get a stopwatch too. Turn the fan on, start the stopwatch, and see how long it takes the bag to fill. 5gallons / that time gives you the air it moves per unit time.

2. This question is nonsensical because you can't make a hover craft with a fan and a bag of air (unless you have something lighter than air in that bag). If, instead of a bag you have a rigid structure, then you can do it. You need to make your craft less-dense than air, so you "float in air." You do that by evacuating air from the structure with the fan.

To calculate that pressure you figure out your hovercraft's volume (with you in it if you're driving in it). Then you figure out how much air weighs in that volume. If your hovercraft is viable, the air will weigh more. Then you subtract the 2. The difference is the maximum weight of the air that can be in the rigid structure. From that you can calculate its pressure using PV=nRT.
Okay, I'm wrong about #2, I don't really know how hovercrafts work.

hover
#4
Jan7-07, 11:53 AM
P: 344
Hovercraft help

Quote Quote by ocdocd View Post
Okay, I'm wrong about #2, I don't really know how hovercrafts work.
Well i know how a hovercraft works. All it is is a craft that puts a bunch of air underneath the itself. If the density of the air underneath is high enough it will lift the craft and leak out. Thus frictionless movement. I understand that well.
Bob3141592
#5
Jan7-07, 10:54 PM
P: 226
Quote Quote by hover View Post
First, I am making a small hovercraft and have a fan for a source of air. How can i calculate how much air it moves??

Second, how can i calculate how much air pressure it is putting inside the hovercraft bag for the craft to lift??
By bag I presume you mean the skirt that confines the air blown downwards by the fan, correct?

A previous poster gave a way to measure the volume of air moved. But I'm not sure why you need to know that. What dos it give you? I'd expect it's the force the fan is capable of producing that's important.

A simple way to measure the force exerted by the fan would be to simply suspend it over a scale and see how much the force of the airstream deflects the scale. I'd put a paper cylinder the diameter of the fan around the fan and almost touching the scale. That gives you a force, and since force is pressure times area, and you know the area of the cylinder, you can calculate the pressure if you want.

The skirt under the hovercraft is larger than the fan, right? So the pressure will be less, proportional to the area of the skirt. But it's not pressure that you really care about (unless the skirt material is rated by the pressure it can contain), It's the force, specifically compared to the weight of the craft, since weight is a force. If the fans can't match that force, the craft won't lift and you won't get the frictionless hovering as the air spils out the bottom of the skirt.

To calculate the characteristics of the fan is complicated, but to measure them should be simple.
hover
#6
Jan8-07, 01:57 PM
P: 344
Quote Quote by Bob3141592 View Post
A previous poster gave a way to measure the volume of air moved. But I'm not sure why you need to know that. What dos it give you? I'd expect it's the force the fan is capable of producing that's important.

A simple way to measure the force exerted by the fan would be to simply suspend it over a scale and see how much the force of the airstream deflects the scale. I'd put a paper cylinder the diameter of the fan around the fan and almost touching the scale. That gives you a force, and since force is pressure times area, and you know the area of the cylinder, you can calculate the pressure if you want.
Hmm i never thought of using a scale. Sounds like it might work. I'll be back with results.
rcgldr
#7
Jan8-07, 11:31 PM
HW Helper
P: 7,107
It's more complicated than that. When a hovercraft hovers, air is escaping thorugh the skirt at an average rate. So the fan's output has to provide enough power to maintain the pressure differential at the rate that the air is escaping at. So it's a combination of thrust and volume that is required for a hovercraft to hover.
hover
#8
Jan9-07, 02:32 PM
P: 344
Quote Quote by Jeff Reid View Post
It's more complicated than that. When a hovercraft hovers, air is escaping thorugh the skirt at an average rate. So the fan's output has to provide enough power to maintain the pressure differential at the rate that the air is escaping at. So it's a combination of thrust and volume that is required for a hovercraft to hover.
I wish there was some type of easy calculation for this.
MonstersFromTheId
#9
Jan9-07, 05:12 PM
P: 144
I've built dozens of hovercraft (models mind you, nothing you could sit in and ride).

First - As far as calculations go, forget 'em. Seriously.

Unless you have access to a lot of very delicate measuring equipment, AND can claim mastery of that equipment's proper use, AND quite a bit of expertise in compressible fluid flow, "calculations" really are a complete waste of time when it comes to building a model hovercraft you can make yourself and play with.

The problem is that "close approximations" JUST - WILL - NOT - CUT IT. Hovercraft, generally speaking, operate within a VERY thin band between just sitting there (because they lack the power to create a thin cushion of air to ride on), and flipping over (because they're so over powered that wind up trying to sit on top of a roaring torrent of air that's doing its dammedest to escape anyway it can from the confines of the hovercraft's airbox).

A much better approach is, frankly, to "tinker until it works".

My first hovercraft was made by mounting an "Oh-thirty-five" two stroke model airplane motor and prop, centered in the middle of an inverted Tupperware cake pan.

It's important to mount the motor and fuel tank as close to the center of gravity of your hovercraft as possible. It's also a good idea to mount the fuel tank directly under the motor to keep the center of gravity as low as possible. If you're going to use an electric motor and battery, both are going to require one HELL of a power to weight ratio. If you want to go that route I'd suggest using the motor and battery from a cheap electric airplane or helicopter, and making the airbox out of something VERY light, like Styrofoam.

When I first tried to "fly" my hovercraft it proved to be vastly over powered. It just jumped up off the ground and flipped over.

So I drilled a gazillion "vent" holes in the top of the Tupperware "airbox".

It then just sat there. Too many vents, allowed to much air to vent from the airbox.

But that was easy to fix. I just started covering some of the vent holes with duct tape from the inside. Once enough of the vent holes were covered the hovercraft hovered quite nicely.

Next I needed to get it to power itself forward. I did this by using a Dremmel tool (a "pencil grinder"), to grind away *just a bit* of the "skirt" formed by the Tupperware box at the rear end of the "hovercraft". This allowed more than enough air to escape from the rear edge of the "skirt", than from the front or sides, to push the hovercraft forward.

To control it, (mind you at this time radio controls were WELL beyond my budget), I just tied about 20 feet of string from the front left corner of the airbox to one end of a short handle, and 20 feet of string from the left rear corner of the airbox to the other end of the short handle.

We'd start that puppy up, someone would hold it until the "pilot" got hold of the control handle and stood with the strings held taught, and... you'd let 'er rip!

The bugger was quick too.

Lastly I added a few decorative things like fins, plastic missiles, radar dishes, and other sundry things from old model parts, a paint job, and some decals, and wound up with a homemade toy that looked like something out of an old Flash Gordon flick.

After crashing and rebuilding this thing dozens of times, it eventually acquired a pusher prop and rudder arrangement for radio control, which is where I learned a valuable lesson about toy hovercraft --

SLOWER IS BETTER! And a puller prop up front is a lot easier to control than a pusher prop in the back.

Hovercraft come about as close as you can get to exhibiting Newtonian motion, i.e., once they get moving in any particular direction they don't like to stop. So... the first thing you find is... NO BRAKES!

To stop one you have to turn it 180 degrees, and then POWER it to a stop. (Which is where a PULLER prop comes in handy, try it and you'll see why).

Turning one 90 degrees to the right and hitting the gas does NOT make it turn like a car or even a jet ski. Instead it keeps going in the direction it was originally headed while slipping sideways as if you had turned all four wheels of a car 45 degrees to the right.

To get a hovercraft to turn through an arc like a car you have to turn it more like 135 degrees to the right and hit the gas.

All in all piloting a hovercraft is especially NON-intuitive. It takes a lot of practice. Which is why slower is better, it gives you the chance to get used to things before cracking up.

Best of luck!
hover
#10
Jan9-07, 05:21 PM
P: 344
Quote Quote by MonstersFromTheId View Post
I've built dozens of hovercraft (models mind you, nothing you could sit in and ride).

First - As far as calculations go, forget 'em. Seriously.

Unless you have access to a lot of very delicate measuring equipment, AND can claim mastery of that equipment's proper use, AND quite a bit of expertise in compressible fluid flow, "calculations" really are a complete waste of time when it comes to building a model hovercraft you can make yourself and play with.

The problem is that "close approximations" JUST - WILL - NOT - CUT IT. Hovercraft, generally speaking, operate within a VERY thin band between just sitting there (because they lack the power to create a thin cushion of air to ride on), and flipping over (because they're so over powered that wind up trying to sit on top of a roaring torrent of air that's doing its dammedest to escape anyway it can from the confines of the hovercraft's airbox).

A much better approach is, frankly, to "tinker until it works".

Can you take some pictures of your model hovercrafts so i can get some kind of idea?? That will easily help me. As for the equations i guess your right but i have tried tinkering with trying to get a hovercraft to work but I never could. I think it had to do with the weight fan and source of power.
russ_watters
#11
Jan9-07, 08:27 PM
Mentor
P: 22,280
I didn't read all the posts, but in a hovercraft, the pressure needed is exactly equal to the weight of the hovercraft divided by the area of the skirt. It has to be. The catch, of course, is that if the fan isn't capable of generating that much pressure, it won't lift off.

Once the hovercraft lifts off, the height will be determined by what airflow the fan generates at that static pressure (the fan will ride its fan curve to that point as the hovercraft rises).

My guess would be that a bladed fan cannot produce enough pressure to lift a hovercraft. Bladed fans don't produce much pressure. A centrifugal fan/blower is what you would need.

If you want to measure flow/pressure, manometers and pitot-static tubes are pretty cheap or you can make them yourself.
hover
#12
Jan10-07, 02:03 PM
P: 344
Quote Quote by russ_watters View Post

My guess would be that a bladed fan cannot produce enough pressure to lift a hovercraft. Bladed fans don't produce much pressure. A centrifugal fan/blower is what you would need..
What is a centrifugal fan/blower?? Anything that is low power and light is what i obviously want.
russ_watters
#13
Jan10-07, 06:07 PM
Mentor
P: 22,280
You didn't really define "small", but a leaf-blower uses a centrifugal fan. Bathroom exhaust fans usually are too. In a centrifugal fan, the fan is doughnut shaped and the air goes in perpendicular to the doughnut and exits tangential to it.

Here is another example: http://www.purdyelectronics.com/prod...an/default.cfm
That's bladed (aka vane-axial) fans on the left, and a centrifugal on the right. These are small enough to go on a model.

Home depot has a variety of bathroom exhaust fans: http://www.homedepot.com/prel80/HDUS...gll.0&MID=9876

Broan.com is a popular and inexpensive manufacturer of centrifugal fans and they have fan curves, which you can use to calculate the airflow once you have the pressure (using the procedure I mentioned). Example: http://www.broan.com/ImageLibrary/br...s/99042775.pdf

FYI, 1" of static pressure is equal to about 1/26 psi.
russ_watters
#14
Jan17-07, 09:51 AM
Mentor
P: 22,280
A lurker emailed me this link to a hovercraft calculator. It may be useful to you: http://www.hoverhawk.com/lcalc.html

It basically duplicates the calculations I discussed.
hover
#15
Jan17-07, 01:36 PM
P: 344
hey thanks, this might be of use.
cesiumfrog
#16
Jan17-07, 04:20 PM
P: 2,050
Quote Quote by ocdocd View Post
2. This question is nonsensical because you can't make a hover craft with a fan and a bag of air (unless you have something lighter than air in that bag). If, instead of a bag you have a rigid structure, then you can do it. You need to make your craft less-dense than air, so you "float in air." You do that by evacuating air from the structure with the fan.
Hovercraft, not Zeppelin.
Maximus01701
#17
Jan17-07, 09:04 PM
P: 7
any pics of this craft you are building?
Maximus01701
#18
Jan19-07, 05:55 PM
P: 7
I too am building a H.C. I was wonder what do you plan on using for the motor/engine, Lift, thrust, how you plan on mounting stuff ect...

-Thanks


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