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I need help with calculating the pressure inside my hovercraft.

by i__fate__i
Tags: hovercraft, inside, pressure
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i__fate__i
#1
Dec31-05, 11:29 AM
P: 5
The hovercraft that I have built is from these plans.

http://www.amasci.com/amateur/hovercft.html

This is a project that I am doing for my Honors Chem and I am trying to relate how the Pressure directly effects the Hovercraft. I have a Toro Super Blower that has a 210MPH and a 270CMF. I need to manipulate the pressure inside the skirt to prove that as the pressure varies it effects the hover craft, all other variables such as the weight, area and what not are the same the only variable I am changing is the air pressure by changing the setting to high and low. What I want to do is find out what the pressure is inside the skirt with the leaf blower setting on high and with the leaf blower setting on low. I can not find a way to do this or to calculate it. I would greatly appreciate the help.

~Fate
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i__fate__i
#2
Dec31-05, 01:41 PM
P: 5
my leaf blower is a very powerfull one, would I need to then increase the amount of holes on the bottom to allow for the extra air to escape?

~Fate
aquaticlife
#3
Dec31-05, 04:07 PM
P: 5
i made a VERY similar hovercraft to that (im the one who gave you the link) and im not sure what u are asking but my hovercraft holds approximately 150 pounds not including the weight of itself....also i had about 10, 1" in diameter holes around the bottom of the craft(where it tells you to put them) and it worked pretty well.....but it also made a farting noise which i determind (strictly a guess) as to little air was escaping so im going to add more and see if that helps....also,coinsidently im using the same leaf blower and so that should work pretty well....i hope this answers your question

aquaticlife
#4
Dec31-05, 04:24 PM
P: 5
I need help with calculating the pressure inside my hovercraft.

sry....i saw the link and assumed you were (eng.m.mostafaa) but you are not....good luck with the project, i wish my chem teacher would let me do a hovercraft for a project but technicaly its not chemistry....hopefully physics next year....also...im building a revised model(i have ideas to greatly inprove that design, i hope) so if you plan on doing this as a hobby we could exchange email...also now that i re-read your question, im sorry but i dont know how to answer that...but im sure someone does...
i__fate__i
#5
Dec31-05, 04:51 PM
P: 5
answer one, but not my main question, I need a way to scientificaly prove that the pressure effects the hovercraft directly either by finding the pressure of the leaf blower on high and low wich I do not know how to do, or find the pressure inside the skirt on high and low wich I do not know how to do. That is my main problem.

~Fate
i__fate__i
#6
Dec31-05, 05:35 PM
P: 5
yes my Chem teacher teaches pressure as a unit in Chemistry so that is why I am able to use it as a topic, anyone else have any ideas as to how I can solve my dilema?

~Fate
i__fate__i
#7
Jan1-06, 04:51 PM
P: 5
does anyone have any ideas?

~Fate
Danger
#8
Jan3-06, 09:33 AM
PF Gold
Danger's Avatar
P: 8,964
Hi, Fate;
Just got your PM about this, but I'm not sure how much help I can be. I have zero math ability. My approach would be to simply screw a tire valve into the system somewhere (the blower outlet tube would be a good choice, if the owner of the thing doesn't mind). Use a standard tire gauge to check the pressure on various settings. It should be equal throughout the cushion and tube, so the location doesn't really matter. Once you attain full operational lift, it won't increase until you add more load. Any excess just bleeds out. That's also your maximum 'altitude' that can be achieved. As you add weight, the pressure rises to compensate. Eventually, you'll either reach the limit of the blower's output, or pop the curtain. Keep track of what weight you put on and how much the pressure changes. When it won't hold any more off of the ground, you've reached the maximum psi of the blower. Before that, though, you'll notice the curtain bag sinking as you approach the maximum. At that point, you no longer have proper lift, and the curtain will scrape the ground if you try to move it. Your maximum operational pressure will be just before it starts to do that, when you still have full skirt inflation.
You should be able to make a graph showing the weight/pressure/altitude relationships. Sorry I can't help with the numbers.
arthuritus
#9
Aug4-06, 08:18 PM
P: 14
I'm trying to exactly the same thing, and with great difficulty. I'll tell you where I'm up to so far huh?

Total pressure = static pressure + velocity pressure.
To clarify, static pressure is what you find in a blown up balloon, and velocity pressure is what you feel when you stick your hand out the window.

The formula for velocity pressure is VP=0.6XV^2
In my case, it's .19 m/s coming out of the bottom of my hovercraft
.19^2 X .6 = .2 N/m^2

Force = pressure X area
.2 X 1.13 (area) = 0.226 N
0.226/10 = about .02 kg, or 200 grams.

But how the heck do I work out the static pressure?? I have no idea where to start. Anybody?
mcmahad
#10
Apr2-07, 08:11 PM
P: 1
People,

I know I'm about a year too late but here goes.

Static pressure will be a function of blower characteristics and is hard to know. When you start restricting the blower, the pressure and air mass will change due to increased resistance. Blowers are not designed to handle serious backpressure.

You can measure static pressure. You need to make a water filled U-tube manometer. This can be as simple as a clear piece of tubing ($10 at the hardware store). Figure out what the pressure inside will be (approximately) and determine water column height from that. Remember that if pressure inside is too great, it will shoot the water column out of tubing. You can easily make an 7 foot water column in your garage by attaching the tubing to a 2x4. Add a bit of red food coloring to the water to see it easily. You need to see through both tubes so you can measure the difference in water height. 1 PSI = 2.3067 ft for water, so a seven foot tube will get you just over 3 PSI inside the hovercraft skirt. you should only need about 0.25 PSI (~7 inches of water colum) if my calculations and assumptions below are correct.

The next thing to do is stick the tube into the air chamber under the skirt. Drill a hole in the top of the overcraft or just stick a tube under the plastic. Start up the motor and put your standard weight on the deck, then measure the water height difference. Make sure you don't point the end at any directional air vector flowing in/out or the reading will vary. You want the static pressure. You might be able to put a sponge tip over the sampling tube to break up any gusts of turbulance and average out the water level.

Once things are set up, take your measurements. Use bricks or something for the weight as people tend to be unsteady and it throws the balance off. Five or six concrete blocks or a few tool boxes should be enough. Keep someone with his thumb close to the top of the tube to cap it quickly if water squirts out, or you will have red water all over everything.

As disk diameter decreases, you should see the pressure inside go up. I think the best idea is to pick a diameter that will maximize lift for a given weight. If you assume 230 pounds is max for a grown man, you should be able to make a disk that will lift him to max height possible for a given leafblower. The problem with blowers is not so much the air velocity as it is the mass of air created. You need lower velocity and more air mass to get best effect. Ideally, the pressure coming from the blower would only be somewhat greater than skirt pressure, and that would be only somewhat greater than velocity pressure of the air escaping from underneath.

Keep in mind a few things. Assuming your hovercraft doesn't grow and shrink area due to skirt changes, the disk size is always the same. The weight on the disk is always the same. Thus pounds of weight per square inch of disk (PSI, or pressure) will always be the same. I think if you do this experiment and can measure the real static pressure under the skirt but not around the lip, you will find the pressure stays about the same on high and low settings. What changes? The hover height! More air means you can widen the gap and let more air rush out. Here is an example:

32 inch disk = 804 square inches
200 pound person = 200 pounds / 804 Sq inches = 0.25 PSI

The other thing to note is that blowers will produce much less air if there is too much backpressure. This means make the holes in your skirt generous. The skirt must inflate but it should only be slightly more pressure than what is underneath the disk. This will rob air mass and cut your height. I know it is only a fraction of millimeter or so, but more height is better.

-d


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