Hovercraft Lift: Measuring Clearance Height & Lift Force

[JBM1234]

<< Mentor Note -- thread moved from the technical forums so no Template is shown >>

Hello,
I built a hovercraft not long ago for an experiment. What I did was applying more power to the motor and looking at how high the hovercraft could go. However, the formula that I was planning to use, uses a different height, the clearance height. I would like to know if there is a way to manually measure the clearance height. ANd, if the height the I measured at the beginning, is somehow related to lift/ lift force?

Thank you very much

 

[russ_watters]

I would like to know if there is a way to manually measure the clearance height.

Welcome to PF. Do you mean that you have trouble measuring the clearance height when the hovercraft is in "flight"? I'm not being snarky when I say: a ruler. But I suspect there's another constraint (or six) here we don't know yet.

ANd, if the height the I measured at the beginning, is somehow related to lift/ lift force?

What "height" did you measure?

 

[CWatters]

and, if the height the I measured at the beginning, is somehow related to lift/ lift force?

Thank you very much

The fan increases the pressure under the hovercraft, that creates an upward force that depends on the area...

Force = pressure * area

The force lifts the hovercraft allowing air to escape around the edge. There is quite a complicated relationship between power and height. In short the fan has to be able to deliver the pressure needed for the force to equal weight at the same flow rate as the air escapes around the edge. Or rather it will rise up until that condition occurs.

 

[JBM1234]

Welcome to PF. Do you mean that you have trouble measuring the clearance height when the hovercraft is in "flight"? I'm not being snarky when I say: a ruler. But I suspect there's another constraint (or six) here we don't know yet.

What "height" did you measure?

I measured the total height, how far away it was from the floor

 

[sophiecentaur]

I measured the total height, how far away it was from the floor

One thing that would be relatively easy to measure would be the pressure under the skirt with a U tube. That should be constant (if the load isn't rising or falling). The height of the gap just affects the efficiency of the system (a hovercraft is a lot more efficient than a hovering helicopter, for instance).
As mentioned above, the actual relationship between power and height is complicated. The air escapes through the gap and it would be total area that counts, which itself would be affected by irregularities. Why did you want to measure this, aamof?
Ideally, there would be no gap at the bottom (this is pretty obvious?? ) . The only reason for having a gap is to allow for irregular ground, which will allow leakage.
Then there is the problem of efficiency of the fan; it would be near impossible to estimate the useful mechanical power going into pumping the air into the skirt. How much pumping and how much turbulence are involved? A 'good' fan design could make as much difference as anything else.

 

[JBM1234]

One thing that would be relatively easy to measure would be the pressure under the skirt with a U tube. That should be constant (if the load isn't rising or falling). The height of the gap just affects the efficiency of the system (a hovercraft is a lot more efficient than a hovering helicopter, for instance).
As mentioned above, the actual relationship between power and height is complicated. The air escapes through the gap and it would be total area that counts, which itself would be affected by irregularities. Why did you want to measure this, aamof?
Ideally, there would be no gap at the bottom (this is pretty obvious?? ) . The only reason for having a gap is to allow for irregular ground, which will allow leakage.
Then there is the problem of efficiency of the fan; it would be near impossible to estimate the useful mechanical power going into pumping the air into the skirt. How much pumping and how much turbulence are involved? A 'good' fan design could make as much difference as anything else.

I wanted to measure the height because there is an equation which related power with height which is:
Pa = hc lcu Dc ( W / Ac )^3/2 ( 2 / d ) ^1/2 http://books.google.ca/books?id=Blp...AA#v=onepage&q=theory ground vehicle&f=false" chapter 8

When I found this equation I thought it would be easy to do, I built my own hovercraft without a hole at the bottom (mistake) and I connected a propeller to a motor which was connected to a battery, and I could measure the power supplied. With a motion sensor, I measured the total height of the hovercraft when ´´´flying´´.
However, the height measured wasn't the corrected one, and now I should measure the clearance height but I am afraid the power won't affect the clearance height that much
I wanted to know if the height initially measured is somehow related to lift

 

[sophiecentaur]

I built my own hovercraft without a hole at the bottom

I am confused. Which hole and where?

 

[JBM1234]

I am confused. Which hole and where?

There is supposed to be a hole under in the skirt to let air flow. Without the hole, there is no clearance height to be measured I think.

 

[berkeman]

Welcome to the PF.

I measured the total height, how far away it was from the floor

That's not what increasing the motor power of a hovercraft is for. Motor power is increased to allow for a larger diameter hovercraft, and to increase the payload capability. Who cares how high it rides -- if there is sufficient motor power the craft will ride with the skirt just barely off the surface you are hovering over, no?

 

[JBM1234]

Welcome to the PF.

That's not what increasing the motor power of a hovercraft is for. Motor power is increased to allow for a larger diameter hovercraft, and to increase the payload capability. Who cares how high it rides -- if there is sufficient motor power the craft will ride with the skirt just barely off the surface you are hovering over, no?

Yes, but my experiment is to se the relationship between the power input against the height to see if the power supplied to the motor actually makes the hovercraft hover higher

 

[berkeman]

Yes, but my experiment is to se the relationship between the power input against the height to see if the power supplied to the motor actually makes the hovercraft hover higher

Then you should probably add an extra variable, which is the gross vehicle weight. Adjust the weight that is being lifted by adding payload weights. You will end up with a graph of hovercraft deck height versus motor power and gross vehicle weight that will help you understand when your motor is sized too small to effectively lift the deck up to the skirt height.

 

[berkeman]

We need a picture to help this discussion, IMO...

https://images.boats.com/resize/wp/2/files/2015/04/hov-pod-hovercraft.jpeg

 

[Klystron]

For examining rotation speed versus lift the OP might be interested in using a scale model of a hovercraft. Early inexpensive electric lawnmowers, while equipped with small wheels, hovered as the spinning blade produced appreciable ground effect. Replacing the cutting blade with a (dull) fan blade could provide a simple model for a full-scale vehicle. The re-purposed mower is easy to weigh and to measure height above ground compared to a vehicle. My old e-mower had a throttle to vary engine rotation but using a rheostat with vernier would allow finer control of engine RPM.

A suitable model could be equipped with flexible skirts and lift (height) measurements compared for different configurations and payload.

BTW the operator guided the e-mower somewhat like a floor buffer, titling the device slightly and applying pressure on the handle; not unlike pressing down on the handlebars to turn an adult tricycle. Static tests could be devised using a an old spring scale to indirectly measure lift.

 

[sophiecentaur]

I built my own hovercraft without a hole at the bottom (mistake)

Having been told about the hole, I think the reason could be as follows. You need a reasonable flow of air past the fan or it will not be effective - the fan will not slow down when blocked; it will speed up because it is doing no work. (Remember the blocked hairdryer effect?). This may not be the case if a single fan is used for lift and propulsion so perhaps you could make this a bit clearer??

 

[sophiecentaur]

to see if the power supplied to the motor actually makes the hovercraft hover higher

Higher than what? A diagram is urgently needed so that we are not all thinking different models in our heads.

 

[sophiecentaur]

. Without the hole, there is no clearance height to be measured I think.

Ah - you mean the gap around the bottom of the skirt. That fits what I was thinking. The system depends on a flow of air and not just static pressure as there would be if the craft rested on a balloon. The fan needs the moving air in order to be a fan.

 

[JBM1234]

Thank you all for your help. I think that, with the formula I have, if I cut a hole in the skirt, my experiment could have two paths. I could either look at the effect of an increase in the cushion area ( that could be done by cutting a bigger hole every time) to the power supplied to the motor in order to maintain the hovercraft at a clearance height of X centimeters. Or, I could set a constant cushion area, and look at the effect of an increase in weight to the power required to maintain the hovercraft at a clearance height of X centimeter.

I think both experiments have a quite straightforward answer. However, my teacher has never seen an experiment with a hovercraft done by one of his students before, and I think he would be able to figure out the effect of changing the weight. As he doesn't know the terminology, I think changing the cushion area would be a bit more creative and unexpected.

What do you think? what idea do you like the most?

 

[berkeman]

(Please note that all schoolwork assignment threads should be posted in the Homework Help forums, and follow the Template that is provided there. No big deal in the case of this thread, but I'll move your thread to the schoolwork forums now...)

 

[berkeman]

I think that, with the formula I have, if I cut a hole in the skirt, my experiment could have two paths. I could either look at the effect of an increase in the cushion area ( that could be done by cutting a bigger hole every time) to the power supplied to the motor in order to maintain the hovercraft at a clearance height of X centimeters.

Sorry, for me this makes no sense. Please post a diagram or picture of what you are asking about. The area of the bottom of the skirt equals the deck area (see my picture). What hole are you changing the size of?

I could either look at the effect of an increase in the cushion area ( that could be done by cutting a bigger hole every time)

EDIT/ADD -- The skirt of hovercraft is vertical-ish. I still don't understand what you are wanting to vary in this experiment.

 

[Klystron]

What I think is that you said hovercraft without specifying the size. Please tell how large is your model?
How much does it weigh roughly?

Also, does "hole" mean "gap between ground and bottom edge of skirt? thanks.

 

[CWatters]

Cutting holes in the skirt sounds like a mistake to me. The purpose of the skirt is to reduce losses around the edge.

 

[JBM1234]

I agree, however, in the formula
Pa = hc lcu Dc ( W / Ac ) 3/2 ( 2 / d ) 1/2

uses hc which is the clearance height, defined as the height between the floor and the gap on the skirt. In page 522 in the following link is a diagram where u will see hc shown.
https://books.google.ca/books?id=Bl...EwAA#v=onepage&q=theory ground vehicle&f=true

 

[JBM1234]

Sorry, for me this makes no sense. Please post a diagram or picture of what you are asking about. The area of the bottom of the skirt equals the deck area (see my picture). What hole are you changing the size of?
EDIT/ADD -- The skirt of hovercraft is vertical-ish. I still don't understand what you are wanting to vary in this experiment.

In the skirt, there should be a hole under it to let air flow out, the distance between the hole and the floor is callled the clearence height. Ac is called the cushion area, so, if I make the hole bigger and bigger, the cushion are will be reduced.

 

[sophiecentaur]

I could either look at the effect of an increase in the cushion area ( that could be done by cutting a bigger hole every time) to the power supplied to the motor in order to maintain the hovercraft at a clearance height of X centimeters

As far as I can see, the only point in the "hole' you are talking about is to allow the fan to have some air to work on before the craft lifts off the ground (a sort of priming effect). Once it lifts up, there will be enough air flow for it to work and the hole is actually working against you by leaking useful air. A possible experiment could be to find what minimum size hole is needed to get the fan working usefully. That would very with the design of the fan you are using and I guess you only have one fan.
As stated above, a DIAGRAM is needed.
EDIT: Is this a language thing? When you write 'hole' do you mean 'gap'? This is where the diagram would help a lot>

 

[berkeman]

I think I found a better picture of what the OP is trying to do. All is clear now...

https://cdn.trendhunterstatic.com/t...ler-m200g-hovercraft-to-begin-production.jpeg

 

[Klystron]

I think I found a better picture of what the OP is trying to do. All is clear now...

https://cdn.trendhunterstatic.com/t...ler-m200g-hovercraft-to-begin-production.jpeg

View attachment 236879

no skirt design ultra-cool pic

 

[sophiecentaur]

no skirt design ultra-cool pic

Agree but that's not what I would call a hovercraft. It's more of a ground-effect helicopter and I wonder how much fuel it would eat up, compared with a hovercraft with real skirts and what sort of load it could carry. (Hopefully, me on one occasion)