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...
I just did what you told me: Use heat-shrink tubing and just jam it on.
I still think it needs a skirt.
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...
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....
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
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).
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!
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!
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.
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.
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.
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:
_____________________________ . . . | | | ', ', ', ',____________________ . . .
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!
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.
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.
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.
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.
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.
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.
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.
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.
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?
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.
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.
Eric Goldstein's Models
MODEL JET ENGINES
BY THOMAS KAMPS
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.