Lifting Hovercrafts with Thermocoal Sheet

[jack001]

At present i m working on a hovercraft. Bt the problem is thati can't lift it.. The material i used isthe lightest of all that is thermocoal sheet which weighs approx 145-175gm. So tell me watshould be the motor rpm to lift this junk.. Is there any formula 2 lift this material.. Thnx in advance

 

[Danger]

Welcome to PF, Jack.
Hovercraft are among my favourite subjects, but I'm by no means an expert. My amateur opinion is that if it doesn't lift, there's a design flaw rather than a lack of power. Skirt leakage would be my first suspect. There really isn't much airflow required to get one off of the ground. I was hugely surprised to realize that getting my (designed but not built) 6-seater off of the ground requires something like 4 horsepower... and it's designed around a 400 horsepower engine. (Oh well, I guess that the excess will have to go into propulsion.)
As for formulae, try a forums search. Russ Watters posted a very involved set of equations for specifically this purpose a few years back. They will be very helpful to you.

 

[jack001]

Thnx for ur reply.. And bro i m not getting formulae post.. Can u link here please.. And i checked skirt as it is perfect nd no leakage ws found.

 

[Danger]

Thnx for ur reply.. And bro i m not getting formulae post.. Can u link here please.. And i checked skirt as it is perfect nd no leakage ws found.

I'll try to find it. In the meantime, please try to type in English. That "text-speak" or whatever it's called is very annoying to those of us who can read.

 

[Danger]

Well, good news and bad news. I found the thread with the link, but when I checked it out it shows the server as not responding. Anyhow, here's a copy of the post in question:

Aug18-08, 08:41 PM #4
russ_watters



russ_watters is Offline:
Posts: 17,158
Re: hovercraft help 2
Still not enough information. We also need the physical size of the hovercraft and how high off the ground you want it to go. Try playing with this calculator for a bit: http://www.olshove.com/HoverHome/hovcalc.html
__________________
www.russsscope.net

 

[jack001]

I checked the link but mine too the same error.. Server down.. Anyway i will check later.. Thanks buddy for your help..

 

[Danger]

Any time, Jack. My usefulness is severely limited, but I'll help whenever I can.

 

[john.phillip]

The homepage is now available at:

http://hovercrafthomepage.blogspot.com/p/hovercraft-design-calculator.html

 

[sid_raptor]

what is your skirt design like and what kind of fan are you using?

I have build model hovercrafts before and i found the simplest to build is a circular 'donut' type skirt powered with a fast cpu fan and a couple of 9v batteries. But it is not able to lift up a lot of weight and is a painful nightmare to control

 

[jack001]

Ya sid. Same here. I made donut shaped skirt and don't think it has any problem in it. Any ways i tried a motor of 1400rpm with 9 volt battery but dint come up with the result.

 

[sid_raptor]

i sort of solved the control problem by making a design using 3 smaller donuts arranged as an isosceles triangle which somehow i found easier to steer but it also meant increased weight.
the best way to solve the power problem is to use lightweight batteries in series. I would recommend one from a mobile phone since they can be pretty lightweight but i don't think they will last as long as regular 9vs but that's the trade off you will have to make for lighter weight.

 

[sid_raptor]

another thing you might want to consider is a thrust augmenter. i think in a hovercraft with a skirt you are not looking to move air very fast but rather a large volume of air. To do this a simple modification to you fan might help a lot. first of all you need to put a duct on your fan so it looks more like a cpu cooler fan inside a tube. This will help you to channel as much air into your skirt as possible. Then you would need to put something called a Bernoulli thrust augmenter. This is a little devise that is used to increase the mass flow rate of air out of a pulse jet and it should work here too.

To put it simply it is a set of holes cut into your air duct. the fast moving air inside the duct is at a lower pressure then the stationary air outside and it sucks more air in. The result is a greater mass flow of air into your skirt at a lower velocity.

I tried it on my model and i felt it improved the performance a bit but i would recommend you read a little more into thrust augmenters before you design one.
[URL]http://img404.imageshack.us/i/augmenter.jpg/[/URL]
I have made a (rather bad) drawing to try to explain what i am talking about.

http://img404.imageshack.us/i/augmenter.jpg/"

click this link if the image above does not show

 

[jack001]

Bro i like your answer but i am having a doubt regarding suction of air through holes as fluid flows from higher pressure to lower pressure. Inside duct there is high pressure fluid(because of fan) and hence will try to escape from duct according to your design. So please explain me how is it possible? Thank you buddy.

 

[Danger]

John, thank you very much for providing that new link.
Sid, I've never heard of a thrust augmenter, but it makes sense if it works in the same manner as a carburetor venturi.
The main problem with "doughnut" platforms and skirts is that they're primarily used only for small machines, such as might hold one person and be powered by something like a leaf-blower or lawnmower engine. "Steering" is achieved by the pilot shifting his weight to allow air escape from the opposite side. In this case, the machine is far too small to hold a gerbil, let alone a human. The only ways that I can see to control it, therefore, if no thrust motor is provided, are to either install rudders in place of part of the skirt or to build a weighted payload onto some sort of cross-table (such as is used in a multi-axis plotter) to move it around. Since the rudders also would require servos, either approach would add a horrendous amount of weight.

 

[sid_raptor]

I got the idea about thrust augmenters from model pulsejets.
you can find more information about how they work easily
this link has some information but i am sure you can find more detailed info aswell.
http://www.pulse-jets.com/phpbb3/viewtopic.php?f=19&t=5440"

this one is mounted on a pulsejet but the same principle works with a ducted fan aswell...

to put it simply (quote from wiki) In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy
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which means that the faster the airflow the lower its pressure (i know it does not sound logical at first but that is the way it is)

the purpose of a hover skirt is to trap air then release it under the edges creating lift. therefore the velocity of the air is not as important as its mass flow rate and that is what the augmenter increases.

 

[sid_raptor]

and yes the doughnut platform is not very good at lifting weight but i think it is a very good place to start as it teaches you the basics rather well.

 

[Danger]

Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy

That is precisely how a carb venturi works. I was just trying to use a simplified approach to the matter. There was some doubt in my mind, however, due to the vagueness of the diagram, that your version was the same thing.

 

[sid_raptor]

That is precisely how a carb venturi works. I was just trying to use a simplified approach to the matter. There was some doubt in my mind, however, due to the vagueness of the diagram, that your version was the same thing.

lol i just quickly made that in paint brush because i could not find any good diagrams of how it works...guess it caused more confusion instead..hehe

anyway yea it is very similar to a venturi but just on a larger scale and as i said before we just want more air to move into the skirt to improve the lift and this does the trick pretty well without the need for more power or fanspeed.
which in turn would let you use lighter power cells. If you really want the best performance you can order li-polymer batteries for model aircraft on many hobby sites. these are a little more expensive then your average 9v batteries but they can be incredibly lightweight.


By the way Jack can you describe what type of skirt design you are using and what kind of motor and fan you are using? If we know that then maybe we can guide you better