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Pretty much letting my mind free-wheel.

Assume a fleet of air-supported hover-craft were to replace cars/etc on the streets. Assume also that the present traffic-signals/pedestrian rules remain unchanged.

As I understand hover-craft come to a gradual stop; similar to a train.

What would be the equivalent of disc-brakes on hover-craft? i.e. How would you improve braking capability on a hovercraft?

p.s. Friction, Aerodynamics, Inertia hence the post here in the Physics forum but please feel free to vote as OT ...

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closed as off-topic by jinawee, DavePhD, Kyle, John Rennie, Qmechanic May 29 at 17:47

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Can't give a hard Yes/No answer to this question. What I'm doing therefore is to upvote the helpful answers, and accept the most helpful one with the least number of votes... Hope it works with you guys –  Everyone Nov 24 '11 at 7:32

8 Answers 8

up vote 2 down vote accepted

To deal with lift fan failure, you'd need some landing pads anyway. If you design those properly you can use them for braking, too. The biggest problem might be how you'd lose the air cushion rapidly, in case of emergency braking.

Maglev trains use a similar solution as they've got the same problem.

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An anchor?

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But this might wear down quite quickly. So it might be more cost effective to lower some sort of tire which does not slip on the surface with ABS by means of disc-brakes or even eddy current braking. –  fibonatic Feb 4 at 14:14

Electromagnetic induction.

All roads the hovercraft drive on need to have spatially-varying permanent magnetic fields. The hovercraft has a circuit with high induction sitting on board, but the circuit is usually broken. When you want to brake, close the circuit and power drained by the induced current will slow the hovercraft.

This could potentially be used to store the energy as well. The magnetic fields would have to be pretty strong, though. Perhaps installing superconductors underneath all the roads could do it, if that ever becomes feasible.

Another option would be to simply use the hovercraft's fan to brake. Usually, the engine drives the fan, which blows air out the back. You could make it so when you pull on the brakes, it works the opposite way. The air blows the fan, which sends energy back to the motor, or gets stored in a capacitor somewhere. This stops the hovercraft. You'd never be able to stop on a windy day, though, unless you stop actively by continuing the power the fan with the motor, but changing its direction.

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This is actually like something I've been thinking of, but didn't yet think of asking it on SE. I will now! Anyway +1, I love using electromagnetism. –  mikhailcazi Feb 4 at 13:56

Do you mean air-supported or magnetically supported?

Air-supported hovercraft need to be able to generate adequate horizontal thrust, for accelerating, decelerating, turning, whatever.

Just to give you an idea of how much thrust you can get from a propeller, a Cessna 172 weighs about 2000 lb and has propeller thrust of about 400 lb at full power. That's 20% of its weight, or about .2g acceleration. That's not too bad compared to rubber tires on asphalt, that can do about .5g .

Then if they really need to, they can lower some tires :)

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Is mag. levitation also called "Hovercraft"? –  Georg Nov 21 '11 at 11:58

Well not sure why no one suggested it yet, but simple rocket science might do the trick. From the Newton's motion laws, which I think a hover craft follows pretty well, you need to put a force perpendicular to current velocity with opposite direction.

So one would just need some thrust engines with adjustable thrust vectors and nice Stability Augmentation System. Higher thrust, sharper break.

And an engineer to calculate the power and feasibility of the solution. But I think it is pretty possible to implement in the real world.

The main pro of rocket science is that your breaks are independent of the ground and external systems, which I think is the very nicest feature of a hovercarft, and giving it up just for breaking is a no-go for me.

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A motor in the front position should be adjusted to the same power of the back motor. When the two will be switched on together, the like forces will cancel each other and not allow the craft to move.

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The forces will cancel each other out, but that won't stop the craft. Considering the vehicle is already moving and then removing all forces from it will simply stop it from accelerating. It will keep moving with the same velocity it was going with. You'd either need to switch off the first motor, or have a second motor which operates at double the power. –  mikhailcazi Feb 4 at 13:54

I think according to Newton, the like forces will cancel and the hovercraft will not accelerate :) That means it will carry on at the same velocity. You would need to switch the rear-facing fan off and turn the front-facing fan on to decelerate.

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The hovercraft could contain a storage-area full of bowling balls. When you need to brake, the system ejects a bowling ball straight ahead with a velocity of:

$$v_{ball} = \frac{m_{hovercraft}+m_{ball}}{m_{ball}}\cdot v_{hovercraft}$$

This using conservation of momentum, which is calculated on the spot by the hovercraft's computer.
Ejecting a ball straight ahead with that speed will transfer all the momentum to it, stopping the hovercraft.

Of course, you could use something heavier, but that would increase the effort your system would have to make to keep the hovercraft afloat.

Oh, also, the people would need to watch out.

Hovercraft_Bowling_Ball


On a more serious note, a safer way to use this same principle could be using an air tank at the front of the hovercraft. At braking-time, a piston pushes this air ahead with a calculated speed (using the same COM formula).

  • If you need to brake slowly, the air would be pushed out comparitavely slowly, so that the your deceleration is more gradual.

  • Emergency brakes would need the piston to push the air out VERY fast.

Considering that you're talking about hovercraft's being used widely, the only thing which would cause a problem is the 'whoosh!'es heard everytime a hovercraft brakes. And that if you emergency brake and stop right in front of a pedestrian, s/he may be blown away.

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