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 15:56:27 GMT
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Original-From:
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Steve Baker <SJBAKER1@stopspammersAIRMAIL.NET>
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Viewed:
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3853 times
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pixel wrote:
> but wher is the fun?
Non-lego motor -- for lower weight and higher RPM.
Non-lego skirt -- because there isn't anything in Lego
that can do that.
Non-lego propellor -- because the Lego ones are crap
Non-lego decking -- for lightness and rigidity.
...hmmm it's looking a lot like a no-Lego solution!
We all know that electric hovercraft are possible - there
are a couple of R/C hovercraft at my local toy store. Building
one from scratch would be easy...however, the challenge here is
to build one out of Lego...either 'pure' Lego or 'mostly-pure'.
> you said CD is too big
> ?
> CD has 5 1/4 inch so it's not much bigger than you said - 4 inches
> but i suppose you are right!
Remember, the area goes up as the square of the radius. Going from
a 4" radius propellor to a 5.25" CD increases the AREA swept out
by the propellor by a factor of 1.72 - so your CD is not just a little
bigger - it's 72% larger!
I don't see the reason to restrict ourselves to 4" propellors
though. What's the logic behind that?
Presuming you have appropriate duct-work, a larger prop spun
at the same speed as a smaller prop will generate more air flow -
and that's what we need here.
If our motors have poor RPM but high torque (as is certainly
the case with the geared Mindstorms motor) then a larger propellor
makes a lot of sense...it's just like changing the gear ratio's
driving it - but without the frictional losses in the gears train.
Sure there are complicated problems of losses in the propellor
design - but I suspect the size of the prop is a small factor
compared to the poor aerodynamic efficiency of any 'toy'
propellor we might pick up.
> have a look here
> http://web.mit.edu/sp.742/www/motor.html
...whoever maintains that page needs to play with the newer
geared Mindstorms motor and do an update!
> i must check the weight of lego-motors to use your formulas
> but they are useful - thanx
>
> i've been trying 12V
> and read somewhere that 18V quadruple torque!
> so:
> aprox:
> 0.37W * 4 = 1.5W
> and 1hp = ca 750W
> so lego motor has 0.002 hp
> so
> we can lift ca 0.3lb
> so i think lego motor cannot lift itself
> so end of topic isn't it?
That's not true.
It takes ZERO energy to support something against gravity.
My chair is supporting my (not inconsiderable) weight using
no energy at all.
So, supporting the weight of the hovercraft on an air cushion
doesn't necessarily have to consume more energy than the lego
motor can provide.
If you sealed up the bottom of the skirt so it was like a
balloon with no holes in it, the air pressure would support
a LOT of weight with no energy expenditure at all.
The motor is *ONLY* keeping the pressure topped-up because of
the air leaking out around the bottom of the skirt.
That's the difference between a hovercraft and a helicopter.
I'm quite prepared to say that a pure lego helicopter that can
fly freely (no counter-weights!) is impossible. But I wouldn't
rule out the possibility of a pure lego hovercraft.
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://web2.airmail.net/sjbaker1
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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