Hovercraft Question?greenspun.com : LUSENET : Junkyard Wars : One Thread
The Hovercraft challenge was great! However, I was sure that the Long Brother's craft would be at a disadvantage for these reasons:
Being powered by only one engine, how could forward thrust be throttled down without losing the hover cushion? Did you guys keep the pedal to the metal and just steer? It also seemed that almost all the weight was on the back end and the driver did not sit at the front but in the middle. Why didn't the Craft tip backwards? (Is it because most of the downward air thrust was also at the back?)
In most cases, wouldn't air escaping from the high side of the skirt prevent tip-overs like the Scrap Daddy’s had? (Was their skirt too long?
-- El Kid (email@example.com), January 20, 2001
It doesn't take a lot of pressure to keep the hovercraft off the ground. Force = pressure * area. The area of the craft was about 8'x12'?? That would be 13824 square inches. If it weighed 300 lb, it would need pressure of f/a or 300/13824 or .02 psi.
I think the key is that you have to supply air to the skirt faster than it leaks out. Of course that is easier as the gap between the skirt and the ground will lessen when less air is supplied.
-- Michael (Canadian P. Eng.) (firstname.lastname@example.org), January 20, 2001.
anyone catch what motor the Long Bros. craft had?
also, it seems that the winner borrowed a lot from a design from universal hovercraft's UH-10. The local building supply place has most of the parts. the brings up the question of what the cheap/common motor option might be.
then there's the skirt...
-- Mighty Mik (email@example.com), January 20, 2001.
I liked the single engine design too. As to the control, if you don't like the way it is moving, just let off the throttle, and "plop", then start over. I have never driven a hovercraft, but I can see that they drive a lot like an airboat on snow. My friends and I took an airboat to Illinois one year (1976 I think) after a blizzard. We got a write up in the local paper, as people were calling in about a UFO that they had seen. We had to tie it up whenever we parked it because it would tend to blow away. The motors that I noticed on the hovercraft were one Briggs twin 16HP, and one Kohler single cyl ten HP. Waddy, "Wreckspert" for the "Rusty J's"
-- Waddy Thompson (firstname.lastname@example.org), January 21, 2001.
the single flaw that destined the scrap daddys machine to an untimely demise was that the skirt wasnt tall enough...from what i could tell, it looked like the deck was maybe 8 or 10 inches(254mm)from the ground, this didnt allow for any roll,pitch or yaw without the deck coming periously close to the ground, or contact! which we saw!....the comercial units that are quite large have skirts in the range of 6-8 ft. tall! and can go over objects almost as high with out damage! as for speed control...slow the engine, and the craft sets down on the ground causing drag! then throttle back up when the desired position or proximity is achieved!"mashing" the throttle doenst work too well as these vehicle steer almost as good as a hockey puck! it was quite a good episode!
-- tim (email@example.com), January 21, 2001.
I think the answer to your question El Kid is that the long Bros. machine would hover at just off idle. So backing off the throttle a little would only reduce their forward momentum & the extra drag from the skirt if they backed off more would slow them down nicely. I thought it was a great machine. The scrap daddies machine did not have enough lift to keep the skirt inflated enough so it would lose its hover in the turns & dig in to the sand. I think if they had just revved the motor a little more driving the hover fan it would have worked much better. They had it at a fast idle!!! Their machine look to heavy too for the fan they had though.
"Rick The Rocket" future Junkyard Wars contestant.:-)
-- Rick Lawrence (firstname.lastname@example.org), January 21, 2001.
The ScrapDaddys' were aiming for (and got) 6 inches of cushion.
I think their main problem was weight distribution; since they had a steel frame, there are plenty of weight around the periphery of the craft, so there might be a tendency to rock side-to-side (hence the digging in and ripping of the skirt). Using duct tape to secure the skirt in retrospect wasn't the best of ideas as well (too easy to tear).
The Long Brothers' weight distribution was strictly near the centerline of the hovercraft (where the driver is sitting and where the engine and fan duct was mounted) because their platform was made of styrofoam, which probably means there wouldn't be too much rocking.
I was surprised at how the flimsy construction materials of the LOng Brothers lent to such a stable-running platform compared to the steel- framed Scrapdaddy machine.
-- Thomas (email@example.com), January 21, 2001.
Since a hovercraft is actually suspended in the air while it is running, all the force to turn it in corners has to come from the motor. This effect alone would make a lighter machine more manuverable if it has the same power as a heavier one.
-- Waddy Thompson (firstname.lastname@example.org), January 21, 2001.
I have a question. Didn't the shape of the race path contribute to the demise of the ScrapDaddys? A rectangular track causes there to be 4 90 degree turns and since the craft is riding on a cusion of air, there's no friction and therefore nothing to keep it from rolling when the turn is made. It seemed to me that a circular or eliptical path would've made more sense.
Does someone with more experience know where I'm coming from?
-- Arachnerd (email@example.com), January 24, 2001.
I don't think a square course matters-- A hovercraft with a rear- driven steering fan or rudder can sideskid into a turn, and a well- designed one will not roll during the maneuver. The Long Brothers didn't have a rolling problem-- clearly the more stable (and superior) machine.
-- Thomas (firstname.lastname@example.org), January 24, 2001.
One point I'd like to clarify. The Long Brother's machine was probably LESS stable than the Scrap Daddys'. Putting all the mass on the centerline meant that it required very little force to roll the craft. Unfortunately for the Scrap Daddy craft, their perimeter weighted craft not only required more force to start it rolling, it also required more force to STOP the roll. The tippy Long Bros craft could be righted by leaning to the other side a bit.
-- John Calderwood (email@example.com), January 24, 2001.
I don't know John, that Long Brothers craft looked pretty stable to me when it was running.
The reason why I think it is less roll-prone than the Scrap Daddies machine is because of this:
1) the Long Bros machine's lightweight styrofoam platform with centerline mounted heavy machinery makes it easy for the pressurized air to equalize right under the light platform edges, thus dampening roll. The styrofoam is actually bending a bit under the centerline, so the edges of the platform would bend upwards slightly, providing a stabilizing force with a vector deviating somewhat from the vertical at the edges similar to aircraft with dihedral in its wings (the purpose of dihedral in aircraft is stability, of course). The bend also drops the CG somewhat.
2) the scrap daddies also had soft, deformable fabric covering the frame. That means there can be localized areas with slightly uneven pressure under the skirt, which I believe would contribute somewhat to instability.
-- (firstname.lastname@example.org), January 24, 2001.
-- Vassili Kustaff (email@example.com), December 15, 2003.
Can we use tracing paper as the hovercraft's skirt
-- Ryan Toh (firstname.lastname@example.org), November 09, 2004.