So You've Always Wanted a Hovercraft... (Video)

What little boy or girl never wanted a hovercraft? Something loud that could travel over water, pavement, maybe even over a plowed field or through a swamp? Ben King obviously wanted one, so after he grew up and got his PhD in physics and found a good job, he founded Lone Star Hovercraft. Timothy Lord interviewed Ben at the Austin Mini Maker Faire, and we also found some video of Ben flying (is that the right word?) one of his hovercraft on a lake that we spliced into the interview to liven it up a little. Vroom!

Comments

Been there, done that

[catchblue22]

When I was a teenager, the father of a wealthy school friend won a hovercraft in a card game. It looked quite similar to the one above. It was powered by a Bombardier snow mobile engine and was extremely loud. It would only hover when the fan was running, as the airstream for the hovering air came from a diverted stream of about 1/3rd of the prop wash air. Steering it felt a lot like trying to push one of those Ikea shopping carts that has four pivoting wheels...during a turn, you end up going sideways for a time. Going over water, it felt not unlike being on a loud boat or a seadoo. Going over land, it felt like being on a loud ground vehicle. The cool part came when we could drive it over a mud flat which alternated between sand and water. It really was an unusual sensation. The problem was that it ate fuel like crazy. It was far worse than a regular boat. The other problem was that when it came to a rest, the sand started to grind down the bottom. We did mitigate this by adding some fiberglass enforced wooden rails. Overall, it was great fun as a teenager, but even if I had the money to dump on such a toy, I doubt I would.

Re:Looks fancy and all, but...

[Solandri]

Their problem is (1) active suspension, and (2) lack of directional stability.

(1) If your car is stopped at a red light and its hybrid engine shuts off, you're burning no energy. A hovercraft stopped at a red light is still burning energy to maintain the air cushion. Same thing is true at speed - the car's suspension keeps the chassis off the ground at (close to) zero energy cost. A hovercraft is always burning energy to stay off the ground.

(2) When you drive a car on a road, the wheels are physically locked (up to the coefficient of static friction) with the road. You have to exert a significant amount of torque to the car before the wheels unlock from the road and the car starts to spin/skid. So a car is pointed in the direction it's traveling nearly all the time. This reduces directional control to a simple one degree of freedom problem - the more you turn the steering wheel, the faster you change direction.

With a hovercraft, the slightest torque on it will change its orientation. Even an airplane does better - its high forward velocity generates a stabilizing aerodynamic force on the tail to keep it pointed somewhat in the direction of travel. OTOH, a hovercraft's slower forward velocity means it needs to rely on vectored thrust for orientation stability. So now you've got a direction of desired travel which is mostly uncoupled from the direction the hovercraft is pointing (yaw). And if you do get turned away from the direction you're traveling, a righting moment to yaw it in the right direction again will also impart a small translation, thus changing your direction of travel slightly.

It's actually more akin to piloting a spacecraft in 2D than it is driving a car. You can do tricks like spinning 360 degrees without changing your direction of travel (much). Which is fun in theory, and perhaps useful if you're in combat. But it's added complexity which makes piloting one more dangerous that driving a car for the average layperson.

Also, the advantage that it can travel over water is a bit of a misnomer. At low velocities, a hovercraft on water acts pretty much the same as a displacement hull. The air cushion sinks down until it's displacing the hovercraft's weight in water. Moving forward then involves pushing the hovercraft uphill over the front lip of the depression in the water it creates, just like a displacement hull. You're only slightly better off than if you were in an amphibious car. As you pick up speed, the wave resistance begins to decrease. The tradeoff point where it becomes more efficient than a planing hull varies with size, but it's typically around 30-50 knots, which is why they haven't displaced planing boats as the recreational watercraft of choice - there's little to no advantage at these speeds. For it to be nearly as efficient as traveling on a solid surface, you have to be moving at close to 100 knots over the water.

Re:I'd love one of those

[HornWumpus]

I more or less always pack in the woods. Better make the first shot count.