Subject:
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Re: "real" LEGO Hovercraft ? (with/without batteries/RCX "onboard")
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 29 Nov 2002 17:28:52 GMT
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Original-From:
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Jim Choate <ravage@einstein.ssz./NoSpam/com>
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Viewed:
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892 times
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On Fri, 29 Nov 2002, Steve Baker wrote:
> You know that the pressure of the air inside the skirt multiplied
> by the area of the ground enclosed by the skirt has to equal
> the weight of the hovercraft.
There's an echo in here ;)...you'll get to the answer faster if you'll
think of mass and force instead of weight.
> If the skirt made a perfect leak-proof seal against the ground,
> and there were no leaks anywhere else, you wouldn't need a fan
> at all...you could just fill the skirt with air one-time and
> it would hold the weight of the hovercraft forever.
>
> The only thing the fan has to do is to replace the air that
> leaks out from underneath the skirt.
Which is where the ratio of the input plenum to the skirt (the output
plenum) comes into play.
> That's a function of the perimeter of the skirt and the
> quality and nature of the skirt.
No, that's a function of the area of the exposed 'ring' or torus around
the bottom of the skirt which lets the air out. Think of it as a box with
two holes in it. The input hole is fixed and the output hole changes
area; the input hole moves a fixed volume of air per time and the bottom
hole has variable volume of air per time (mediated by STP) [1]. You design
the skirt to keep this output area as small as possible.
If that gap gets very big for very long you're going to dump your pressure
and the hover settles.
[1] This is a simple problem, think of a bucket with a fixed input flow
and a variable hole in it, and you're asking to find the volume of
contained water at time tau given the function describing the variance
of the output hole...for a hover sitting still this function is a
constant (at least for our purposes).
> It's going to be near impossible to figure out the rate of
> air loss under the skirt - that requires aerodynamics which
> are beyond the abilities of most (if not all) of us!
Actually it doesn't (but if it does, so what?). It does require you to
think in terms of volume/time however (hint: a given propeller will move a
certain volume of air in each rotation with various boundary conditions).
And how do you make the very bottem of the skirt flexible enough it
follows the ground, -but- not so flexible that the internal air pressure
blows out the seal...
You really don't need anything more than a basic physics book and some
algebra and geometry.
If folks will stop thinking in terms of open plenum skirts (ie they are a
simple sheet or curtain hanging from the running board to the ground) and
more into 'constant volume' skirts (eg tubular skirts) you'll make a lot
more progress a lot faster.
A compound design you might try is a tubular skirt with a very short
sheet skirt attached at the bottem of the tubular skirt. This way the
tubular skirt gets your height managed and the sheet skirt will take care
of ground coverage (which tubular skirts are bad at).
One of the primary maintenance tasks of real world hovercract is the
replacement of the ground side skirt edge, they wear a -lot-.
> However, some QUALITATIVE things are evident:
>
> * Minimising the perimeter length of the skirt is a good thing
> because it reduces the amount of air loss. The size of the
> gap between the bottom of the skirt and the ground must play
> an important factor here - and that area is the 'flying height'
> of the bottom edge of the skirt times the perimeter length.
Not sure what you mean by 'perimeter length' but I suspect you're talking
about the length of the line of ground contact with the skirt. If you
reduce this you also -increase- the necessary forces and air volume needed
to get this thing off the ground to start with. Probably -not- a good
design constraint to play with. If we assume we're going to stay with a
open plenum or curtain skirt (we can use this same assumption for tubular
skirts) then we need to look at the -area- of the ground enclosed by this
perimeter, rather than the perimeter itself. That should be taken as the
area of the running board in most designs.
An additional factor is that with smaller ground surface covered you need
increased pressure inside the skirt, which means when you do get a gap
the air moves out at a very high speed (rho=mv). You want just enough
pressure to get the skirt inflated and a reasonably steady bleed of air
around that ground contact line. This means that you want the skirt to be
only just enough above ambient pressure to do the job. That is why you
don't need a lot of power with a good design.
> The flying height will minimise itself - if the air loss is
> too great, we'll lose air pressure, the hovercraft will start
> to sink - which will reduce the amount of air loss - which will
> allow the air pressure to build up again.
No it won't, what it will do is blow the bottem of the skirt out and dump
all your pressure and settle the hover. The input plenum is fixed
volumetrically. Additionaly, you'e got momentum effects, so that when one
corner starts to dump it will lower, and this -raises- the opposite
corner thus dumping there. It's somewhat similar to a see-saw (but don't
take that too literally). And around and around we go...
You're hover will bob like a bouncing ball, that is considered a -very-
bad design. This effect is one of the primary reasons most hovers work
over water and area of smooth surfaces only. It makes the job of keeping
ground contact easy because it's smooth.
> * Decreasing the air pressure inside the skirt also reduces the
> amount of leakage around the edges.
It reduces the -rate of leakage-.
> The speed that the air
> flows out from under the skirt must be related to the pressure.
Difference in pressure.
> If you let the air out of a pressurized container, it flows out
> fast to start with when the relative pressure is high - and then
> slows down to zero as the pressure drops.
Assuming that on inflow replaces that escaping. Not true in a hover. Your
basic idea is right, you need to refine the variables you're thinking a
bit.
> The trouble is that these design aims are contradictory.
Pardoxical is a better way to look at it. Why I love engineering ;)
Look into "Para-Consistent Logic" :)
> So, a large footprint seems like a good idea. However, it
> all depends on how the speed of the air leaking out from under
> the skirt varies as a function of the air pressure inside.
Volume of air leaking out from the skirt.
> If halving the air pressure (P) halves the speed of the air that
> leaks out (S):
>
> S = P x constant
It's not linear, it's exponential.
> While we are looking at non-lego solutions, you could note that
> balsa wood construction is lighter than foam polystyrene in many
> cases because it can be much thinner and still be strong enough.
Bass Wood works great, it's nearly as light as Balsa but a lot more
strength.
> In terms of engine RPM and propellor design, we know that the
> propellor has to push enough air under the skirt to exactly
> equal the lossage from the edge of the skirt.
The propellar most move enough volume. One way to think about this volume
is to ask the question:
How far will the propellar move as it turns if it is left unconstrained?
If the propellar is constrained then the air will do the moving...
You're on the right track.
--
____________________________________________________________________
We don't see things as they are, ravage@ssz.com
we see them as we are. www.ssz.com
jchoate@open-forge.org
Anais Nin www.open-forge.org
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