The story of my model hovercraft

I would never have the patience to write down the story of my model hovercraft.  But luckily I corresponded steadily with my old friend Eric Goldstein (who now runs Goldstein Hovercrafts, selling model hovercraft kits that are a snap to put together) while I was working on it, and I have collected the relevant items here so that you can "listen in" on my end of the conversation....

Subject: Re: batteries Date: Fri, 28 Feb 1997 19:03:49 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> > Yeah, and I tried a regular 9-volt plus 8 Mikey Jordies for a
> > total of 21 volts (Never mix batteries of different types!),
> > and boy did that thing fly! Turning it on or off would move
> > my hand I was holding it with. And it sounded like it wasn't
> > under much load... a drive belt could make it go even faster...
> Did you have the prop attached????????????

Yes, that's what pushed my hand -- it pushing on the air.

I just tried it again with a regular 9-volt plus 8 fully charged 1.2 volts... It's enough of a breeze that it inspires me to build an infrastructure around it so that it can make the infrastructure hover!

I haven't lost any hands yet though. So maybe it's not all that strong...

Subject: Re: expensive computer fan does 27 cubic feet per minute...y Date: Fri, 28 Feb 1997 19:03:55 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> I'm curious about how you attached the prop to the motor.

I just did what you told me: Use heat-shrink tubing and just jam it on.

Subject: Re: gears Date: Sat, 01 Mar 1997 15:48:11 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
It's so nice out, I'm gonna sit on my porch and make it!
Subject: better Date: Sat, 01 Mar 1997 21:26:36 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
I tried it at 19.3 volts (it's the 9-18 volt motor) on the porch (I received orders against trying it further inside) using the nicad 8-pack plus a non-rechargable Osco 9-volt, and it lifted itself, with the batteries on it, enough to skim about.

I still think it needs a skirt.

Subject: Re: balance Date: Tue, 04 Mar 1997 02:54:01 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> When I got up this morning, I noticed that your craft righted itself nicely along the axis of the battery pack,
> but was more unstable along the axis that is perpendicular to the length of the battery pack.

I hate to be so annoying, but I don't believe you. I mean, you may have noticed it, but I bet it was coincidence. But I'm such a geek I'll have another look at it myself. The batteries are slowly getting weaker, but it just keeps on hovering... (isn't that some competitor to eveready though?) It hovers lower than it used to, and I think it's much less stable. The weird thing is the skirt shape. On the outside it goes straight down instead of bulging out. But then, once it gets to the floor, it goes straight back up on the inside, slowly evening out until by the time it's at the tape near the prop, it's nearly level, right next to the foam. That is, it bulges *inwards*, exactly the opposite of an inflated inner tube. So the pressure must be greater underneath the whole thing than in the skirt. And yet, in contradiction, I think some air flows from the skirt into the lower section where the skirt isn't taped down. So I don't really know what I think. But where the skirt touches the ground, it just barely touches, like a thin book stood on its side, not at all like a balloon. It's really great that you can just look at it and see exactly what shape it is.

Maybe I can make movable pieces that cover the holes that go through to the bottom, and then as the battery dies, I can cover more of the holes, and it will keep on working...

Subject: skirt Date: Tue, 04 Mar 1997 17:47:53 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
So I blocked off two of the six holes, with tape, and it hovered perfectly again without listing at all.

The batteries are running down; you can hear it getting slower.

After blocking off the holes, the skirt took on the following shape:

  ========================================= . . . . . . . . . ======
   |                   ,,--''
   |             ,,--''
   |       ,,--''
   | ,,--''
That is, it goes straight down, just touches the ground, and then goes straight in to where it's tacked to the hull on the inside.

This seems to correspond to good performance.... I'll have to work out a theory....

Subject: Speaking of which... Date: Wed, 05 Mar 1997 07:34:03 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
I blocked off two *more* holes, so now only two of the six are letting air through. Crazy, huh? Now the skirt bulges on the inside, demonstrating that there is higher pressure inside the skirt than under the craft.

Intermission: What do you call the area under the craft but not inside the skirt? I'll call it the void.

Anyway, it looks nice, it has high friction and doesn't slide, and it is not stable. That is, it sucks. I'm going to recharge the batteries before uncovering the holes, though, just to be sure. My guess is that ideally the pressure is just the same inside the skirt as in the void. This means that IT IS A MYTH that the skirt needs higher pressure than the void. Maybe it even needs slightly lower pressure!!!

-- Skeptical in Hinsdale

Subject: skirt Date: Wed, 05 Mar 1997 22:16:54 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
I tried two, four, and six holes with fresh batteries (though not with 18 volts, which would obviously make more sense than 10 volts), and the results were exactly the same. Best stability was at four holes, with the skirt going straight down to the ground, then straight in to where it was tacked up. Best stability, mind you.

At 4 or 6 holes, it glid with no problem; at 2 it preferred to stay in the same place unless I pushed it. All *exactly* the same as when the batteries were lower. And I measured the batteries a lot. When it was running, and when it wasn't. When it was running, you could watch the voltage go 10.56, 10.55, 10.54, 10.53, etc., but then it seemed to level off around nine and a half, which is what the batteries are rated at anyway (8 x 1.2).

Subject: oh, the anticipation Date: Thu, 27 Mar 1997 22:58:54 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
I have the batteries all charged up, and the ufo is sitting just over there... According to my calculations, using twice as many batteries to run the same motor will make it go twice as fast but the batteries will run down twice as fast. Oh well. Just have to tie the battery packs on, and ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ!

Clearly, you get the most out of your battery pack if you charge it up and then bask in the knowledge that you could use it *any* time you want. You can bask in this knowledge for *days* without losing any significant amount of charge from the batteries!

Subject: FULL POWER!!! Date: Fri, 28 Mar 1997 14:51:36 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
So I put everything together, with a nice way to easily put in and take out the batteries, and nailed the duct on (the tape was getting loose), and after much effort I was finally able to take it outside for its test run. "Will it work on the grass?" was the main question.

So I took it to the porch, hooked up the wires so the motor started spinning. A lot faster than before, I suppose, but nothing terribly fearsome. I started it upside down (I can reach the wires that way), and it sort of sucked itself onto the floor. Then I picked it up, and whoa, the duct came off with some excitement. I guess it was getting sucked down a little too much for those nails! I then pondered the contraption I was holding in my hands. See, I would have set it back down and fixed it, but I realized that I could no longer just set it back down, since now the bottommost part (the whole thing being upside down) was a rapidly spinning propeller, with no duct. It was blowing quite a strong breeze against my face, so I realized I could just set it down right side up and it would hover in spite of missing its duct. So I did this, and as I set it down, the skirt inflated, the pressure built up underneath the propeller, and Poweee!, the propeller flew off and hit the ceiling. After realizing what exactly had happened, I managed to disconnect the wires and bring it back inside. Then I walked over here and wrote this note. I can hardly stop laughing! I think I'll be benefitting from a high excitement, low battery drain mode for many more minutes to come!

Subject: Re: FULL POWER!!! Date: Fri, 28 Mar 1997 17:08:53 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
So I took it outside again, and the guy upstairs wanted the lady next door to see it, and she wanted her kid to see it, so I had to wait a bit before ... before what, blowing it apart again? But anticipation being the name of the game, I waited until it turned out her kid was taking a bath, so they called over some kids who were playing ball across the street, so they could see me turn this thing on.

Oh, yes, I nailed the duct back on, with the biggest nails I have! And I wedged that prop on there pretty good, too.

So I turned it on (upside down), and lifted it up, and it didn't come apart, and I was relieved. I tried to set it down, but every time I set it down it would bounce back up. It was too bouncy to set down. So I tried it on the grass, and it sort of worked but it didn't slide anywhere, so I put it back on the sidewalk, very carefully so it wouldn't bounce, and it slid here and there and went for a walk down the sidewalk. If the skirt hit the grass, the whole thing would spin some and it would come back from the grass. It went over to the driveway and found a local minimum. I went over and pushed it back and it did fine.

The upstairs neighbor said to one of the bigger kids that he should try pushing down on it (as I had shown the neighbor once long ago), to see just how strong it is. I went over to the craft and tried pushing down a little to see for myself how strong it was, and zing!, the propeller shot off. It went up, up, way up, and was around the top of the tree in front of our house before it decided to head downwards. It's a big tree. It sailed downwards, spinning the whole time, so it fell kind of slowly, like those one-bladed seeds that spin around as they fall. I had time to walk over to where it was falling (in the street) and try to catch it, but it stopped spinning or something and I missed. Anyway, I unplugged the motor, and now I have to figure out how to attach the propeller better.

Subject: Re: FULL POWER!!! Date: Fri, 28 Mar 1997 18:01:34 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
Well, I jammed the propeller on there real good, I mean real good, and I went back out to try out low-performance grass tests without the pressure of spectators for high-performance action.

The grass is kind of wet cause it drizzled all morning. In tall green grass it won't move. It hovers, but it's kind of stuck. In low yellow (dead) grass, mud, gravel, or whatever else, it works fine -- drifts around by itself in search of low ground. In a combination patch of say half green grass, it works pretty well.

If it's moving and the front side "plows in", i.e. becomes lower, then it has a hard time going further -- forward thrust pressure (supplied by my hand, of course) only worsens the problem. If, however, you press down on the back, so the back is lower, then the front lifts up and the craft will fairly easily slide forwards. This is only an issue on the wet grass, but the fact that aft weight ameliorates the problem is good to keep in mind.

I peered at the skirt to see what was happening when it bounces. What's happening confirms my opinion that what I've learned so far from this model is applicable to full size craft. What's happening is this:

Craft up in air => skirt pressure makes skirt cross-section be like:

______________________ <-- craft body
          |          |
<-- more   \        /  <-- skirt              <-- shape "A"
    craft   '-,__,-'
Craft on ground => plenum pressure similar to skirt pressure produces:
         '-,_        |
             '-,_    |                        <-- shape "B"
Shape B extends slightly lower than shape A does. As you set the craft down, as soon as the bottom of shape A touches the ground, plenum pressure starts to build up, producing shape B. Shape B is not produced in a gradual way, but similar to when you crack a whip, a gradual motion becomes very fast at the end. The skirt would snap into shape B with a noise. Since shape B is taller than shape A, this process pushes the craft up, making a noise at the point when shape B is fully attained. But the craft keeps going up due to inertia, which means that shape B is lifted off the ground by the craft body, and the skirt returns to shape A.

You have to set the craft down very slowly and firmly so that shape B is attained without accompanying vertical upward motion.

You said that full size crafts bounce, for example I think mine did once. So this supports my claim that full size craft skirts also take shape B just like my model's skirt does, not shape A, as is commonly believed.

Also, several times I noticed the propeller working its way up the shaft, so I would cut the power and jam it back down before continuing. And it didn't fly off at all! This seems sub-satisfactory, though.

Oh, yes, I tried pushing down to see how much pressure it could bear. Well, it didn't budge downwards. I pushed harder. It wouldn't sink. I pushed harder. Nothing. I was afraid I'd break it before it would give in. I finally got some gumption and slowly but firmly pressed really hard, and it slowly, slowly sank. I should figure out how to make a platform on it so I can pile things on and measure just how much it really can bear. R/C and thrust stuff will be absolutely no problem at all. I'm not sure whether it could support András, though. He's three months old today. He weighs twice what he did at birth (almost). He is huge. He smiles and grabs things.

I think higher power increases the speed with which shape A changes to shape B. So bouncing happens at high power. High power also increases the hover height between the ground and the bottom of the skirt, but this doesn't change very much.

Subject: Re: Date: Sun, 30 Mar 1997 19:15:45 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> Cause I just looked in "Light Hovercraft Design" by Chris "Neoteric"
> Fitzgerald, and in "Hovercrafting as a Hobby" by James Perozzo, and both
> have diagrams that show a cross section of an inflated skirt shaped the
> way you've seen your skirt shaped.

Well, I nobody had ever told *me* about this. So I still discovered it, independently!

> Fitzgerald points out that some racers
> pump up their skirts really hard until they become almost cylindrical.

That would seem to me, based on my experiments, to be a bad idea.

Subject: Re: Date: Sun, 30 Mar 1997 19:15:45 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> Oh, I meant here: Bob wanted to make sure people constructed skirts
> that were the right shape. But maybe he was just being lazy in his drawings
> since the details didn't matter.

Aha! But it does matter!

It matters when you cut the pattern for the end of the skirt, for it to fit together with another one at a 90 degree angle (e.g. rear corner of craft).

If it bulges, you cut:

_____________________________ . . .
     |_______________________ . . .
But if it goes straight down and then straight in, you cut:
_____________________________ . . .
       ',____________________ . . .
Subject: Re: Date: Sun, 30 Mar 1997 20:55:40 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> > I'm making a thrust duct! (For the model.)
> Wow! I can't wait till I'm out of this academic self-imposed hell and can
> come see! Are you using rubber bands for belts? (I'm so interested
> in belts now!)

I was writing a big long response but my machine crashed so I had to reboot. Well, it wasn't all that long. But still.

The answer is no belts, ugly unfinished duct, taped onto the craft, tried it once and it worked, though I think the craft needs to be balanced for good performance.

Want to try it outside but it's dark and I don't want to risk losing a propeller! ZZZZing!

Subject: Re: Yay! Date: Mon, 31 Mar 1997 01:36:08 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I fixed my thrust motor so that prop'll never come off. I filed down the metal part that sticks out and spins, and then melted some tubing into the notches I made, and then melted some more tubing onto that. Not just shrank the tubing, mind you, since that is not sufficient, but set it on fire until it bubbled and melted into just the right shape, and then got hard as it cooled. Well, we'll see how long "never" lasts this time...
Subject: Re: Date: Mon, 31 Mar 1997 15:08:35 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I'm kind of dejected because my model keeps flying apart, or else it's unbalanced, or else the batteries run down, or else the skirt pressure isn't quite right, or .... How can this be, when its performance was so amazing the other day? If God were my friend, he'd tell me.
Subject: Re: Date: Mon, 31 Mar 1997 17:44:17 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> I was thinking about your super-attached prop. If you actually succeed in
> welding it on there good, you'll probably then decide you're really
> interested in experimenting with different prop pitches!!!!! :-)

No problem, it flew off right away! I can't figure out how it managed to do so, but it did.

I am now using little "hook pins" (like what I made for your gas tube) made out of paper clip pieces, and the pins fit into the notches I filed. It seems to be working fine. And it's easy to remove if necessary.

Subject: Re: modeling idea Date: Wed, 02 Apr 1997 00:37:26 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
One problem with my model is that since I don't want to build another one right away, I'm reluctant to make permanent structural changes before I know what I'm doing. And it takes a while for me to think I know what I'm doing. And in the mean time, I make only non-permanent changes, which tend to come undone at inopportune times (sometimes leading to propellers going where they're not supposed to and inflicting permanent structural changes anyway...).
Subject: Re: modeling idea Date: Wed, 02 Apr 1997 00:56:49 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I think my craft's instability is related to my initial hole-covering experiment. I think I have too few holes covered. I removed the tape covers because now they are obstructed by the batteries, but according to the shape of the skirt, they're not obstructed enough. So maybe that's why it's unstable. I mean, it used to be unstable in that case even without having a bunch of stuff way up high on top. But I'm worried maybe this theory isn't right.
Subject: Re: modeling idea Date: Wed, 02 Apr 1997 01:08:11 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> I think my picture of what you're up to and what problems you are having
> is diverging from reality. I didn't realize stability of trim was a
> problem for you -- I thought it was just stability of structure!!!!!!!

My craft lists. It flies apart. It falls apart. It shakes itself around so that it isn't like it should be, even when it hasn't come apart. I think if I put in a couple more nails, it will mostly just list instead of having these other problems. Then maybe if I plug up a hole, things will be ok. Maybe. And recharge the batteries.

Subject: Re: modeling idea Date: Wed, 02 Apr 1997 01:15:35 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> So you have a big ol prop mounted thrust-wise, and now the craft tips
> backward? I was hoping you'd figure out a solution to the tall skirts
> are unstable problem, even if the solution only worked for electric crafts
> that were model-sized and could sling their weighty batteries in "the void".

My thrust prop+duct is in the traditional location, and I put some metal in the front of the craft to balance it, and the craft tips forwards. Sometimes backwards, but usually forwards. Never level. And it doesn't have very high overall acceleration. Maybe having twice as many batteries is a significant weight factor. But you'd think that batteries would be an overall help regarding lift!

Hmm, the more I think about it, the more optimistic I get.

Part of the problem is I was expecting it to just work, with no further effort except for just making it.

Subject: Re: modeling idea Date: Wed, 02 Apr 1997 02:00:37 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> So I guess the prop is the problem. (It was so stable when I saw it).
> Why not use that mini-prop I gave you and keep it as low as the batteries?

No! I shall not do this! Matt is inventor of new technik, ya. He use same lyftmotor and same driftmotor. He no succumb to traditional low-flying technik, nay! He use no fancy-fancy glasfiber. He try new konstruktion idea, even so if it make his mind to expire.

Matt is saving mini-prop for future idea the which is to him yet unknown.

> You could balance it with another mini-prop pointing the other way!

For total of no thrust? Ho ho! Or do you take my Mickey-Maus idea? I plan two driftmotoren just as the ears to Mickey-Maus.

Subject: Re: duct Date: Wed, 02 Apr 1997 20:18:50 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I think a big problem was low power. I'm recharging batteries now. I ran it until the motors stopped, around .6 volts (from 16 cells!!!)

I can only recharge 8 at a time. But once they're all done, I'm sure it will work! Maybe a piece of tape or two to block off some more area, or perhaps some more holes to the skirt, or perhaps taping the skirt the rest of the way down... Whatever. It's bound to work. I hope.

Subject: Re: duct Date: Thu, 03 Apr 1997 11:19:00 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
Well, after recharging the batteries, it hovers great! It doesn't list; in fact, it can bounce!

I taped down the little holes remaining at the edge of the duct where I didn't completely tape it down before. This noticeably increased the skirt pressure, so I think I can open up more of the main prop/void opening. It doesn't work on big bushy grass. Oh well. Who needs that anyway.

I definitely need remote control. It tugs pretty hard wanting to go. I made it go in circles with a string, but quickly got tired of spinning around so fast (or jumping over the string) while it was zooming around me.

Subject: Re: duct Date: Thu, 03 Apr 1997 15:25:56 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I went to the hobby store and they said 20 volt controllers only exist for boats, and they run 75-130 bucks apiece, so if I want one for each of two props, and one for the lift, well, that's a lot. Plus, I'd need a radio, and the controls are always set up so that one knob moves left/right (intended for steering), springing back to the middle, and one knob moves up/down (intended for power), staying wherever you put it instead of springing back to the middle. So, it would be hard (i.e. not the cheapest) to find a radio that's appropriate for controlling two parallel thrusts. And on top of being outrageously expensive and having awkward controls, the big boat controllers don't reverse.

So I left without buying anything. I went to Radio Shack and asked what's the cheapest remote control toy they have, and got a remote control car for $9.97 (clearance item). It has a kind of goofy control set up: There's one trigger. When you press it, the motor goes. When you press it again, the motor goes in reverse. The third time it goes forwards again, and so on. When you're not pressing it, it doesn't go. The circuitry does this reverse-every-other-time thing, as opposed to a mechanical device, so I can't change it. Not sure quite yet how I'll hook it up to the hovercraft.

Subject: Re: duct Date: Thu, 03 Apr 1997 17:13:25 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
I have 16 cells, or about 20 volts, hooked up to both motors. The motors are in parallel, so attaching the second motor doesn't slow the first one down very much; it just makes my batteries drain faster.
<missing header>
I put on a steering device. I took the motor from the $9.97 car out of its gearbox housing and just stuck a 4" prop right onto it. I then put it onto a foam pylon sticking straight up on the front of the hovercraft. The prop is aimed sideways, so as to just spin the craft. The forward-reverse nature of the control just means that pressing the trigger alternates between steering to the left and steering to the right.

I put it together, tested the remote control unit, and the steering prop worked fine. Prop spins forwards, prop spins in reverse, etc, just like it's supposed to. So I take it outside to try it out with the lift motor going. I turn on the lift motor, so the craft is hovering. I press the trigger. Nothing. I try holding the antenna this way or that, but get no response. At one point, the steering prop started going when I wasn't pressing the trigger! That was the only time it went! It just wasn't working at all.

Subject: Re: Date: Sat, 05 Apr 1997 20:45:51 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> I wasn't sure I understood what the problem was, but if I
> said "radio interferance from the 2 20 volt motors", would it
> shed any light on the issue?

Well, *I'm* not sure I understand what the problem is! I thought maybe it's a power problem, cause it surely gets different power when the lift motor is also being powered.

Whether this sheds light would probably have to do with how correct it is...

Also, the motor looked exactly like the Radio Shack 1.5-3 volt motor, so I took one of those and hooked it up in place of the motor that came with it. I press the trigger, and the prop starts to spin, then it reverses and starts to spin the other way, then it reverses again, and again, and again, and, well, that sure seemed useless no matter how you look at it. So I put the correct motor back on, which, as you say, has a couple of resistors and a capacitor on it, for reasons I couldn't figure out. You say it's to cut down on radio interference?

Subject: Re: Date: Sat, 05 Apr 1997 20:48:06 -0600 From: Matthew Cook <> Organization: Wolfram Research To: Eric Goldstein <> References: 1
> well, there are sometimes two of these thingies, but usually
> three, one between the two terminals, and one for each terminal
> and soldered to the case of the motor. They stop radio interference
> Mostly. Another thing is to route the antenna in such a way that
> it gets the least interferance. This ussually means keep it away
> from all the motors, wires, and batteries.

Hmm, I'll try rerouting the antenna, but I'm telling you, it totally didn't work, and I want it to totally work. And the antenna isn't coiled around one of the other motors or anything, so I'm suspicious about how much can be fixed by just moving the antenna from here to there.

<not an email -- conclusion summary>
I put a big fat capacitor on the back of each motor, in parallel with the motor. This made the remote control totally work! We went to try it out on water, on a tiny pond in front of an office building (after office hours). It worked at first but quickly the steering stopped working. There was a slight breeze, which pushed the craft. Mostly it pushed the duct for the thrust motor at the rear of the craft, which was sticking up like a big sail for the whole craft. This meant the slight breeze was mainly pushing just the rear of the craft, which meant that the craft got oriented to be facing directly into the slight breeze. So, the craft, unsteerable, was trying to go absolutely directly against the slight breeze, with the net result that it stayed perfectly still! Helpless, we watched as it sat there, fighting the slight breeze, until the batteries slowly died down. Finally it lost enough power that it listed onto the water, then the skirt deflated and it sat in the water (all batteries submerged!). As the thrust died down, it started being pushed by the slight breeze, and after what seemed like forever, it finally arrived, quite wet, at the side of the pond. Good thing it's made of foam and can't sink!

While we were pondering what had gone wrong with the steering, a late-working employee came out of the office building and watched our experiments (luckily we had a spare set of batteries). He recognized the model as a hovercraft, which immediately identified him as a Good Guy. We said we were having trouble, and he said he was "just an electrical engineer", so he couldn't help us. Somehow we had failed to notice previously that the office building had a sign in front saying "McDonnell Douglas" on it. What good fortune! Little did he know that our problems were in radio control, rather than aerodynamics (which is what he had been assuming, and why he said "just an electrical engineer"). When I explained the problem, he looked things over and said the transmitter surely just didn't have enough power to transmit very far. Experiments proved him correct. The radio control unit had been designed for an indoors toy, and just couldn't transmit over distances on the order of a small pond.

This little adventure is where the two model hovercraft pictures are from.

Disorganized model links

High End Electric Motors:

Alex's Models

Eric Goldstein's Models

Subject: Check it out... at the library! Date: Sun, 16 Mar 1997 16:42:36 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
REF: STB £14.95 (US$24.00) plus p&p
For the first time Kurt Schreckling Strahlturbine Book is now available in English. It explains how to build Kurt Schreckling's turbojet motor, the FD3/64 containing full construction, photographs and working drawings.

REF: MJE £14.95 (US$24.00) plus p&p
Following neatly on from kurt Schreckling's book on the FD3/64, Thomas Kamps brings the construction and running of gas turbine engines up to date. The book includes highly detailed and well illustrated building instructions which the advanced model builder can use to make and even design his own jet engine.

Subject: river landmarks Date: Tue, 18 Mar 1997 16:54:24 -0600 From: Matthew Cook <> Organization: Wolfram Research To:
next time you take a river trip, you can be prepared with a bunch of landmarks (i.e. big docks) you'll see along the way, so you'll know exactly where you are (at least within a mile)!