What would happen if...

[TransformerRobot]

I was wondering about what would happen if an air cushion hovercraft had an air-intake fan that spun at 500,000 RPMs.

 

Would that cause the craft to malfunction, or would it give it more lift and faster acceleration?

[Greg H.]

Well, let's look at the math.

 

If we assume a four foot diameter fan, running at 2500 rpm (at the hub), that yields a prop tip speed of 357 mph.

The same prop spinning at your 500k rpm reaches prop tip velocities of 71,400 miles per hour, or something like three times the escape velocity of the Earth.

 

My guess is it would explode in a wonderous spray of shrapnel and kill everyone in the immediate vicinity.

 

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Calculations courtesy of http://www.hoverhawk.com/propspd.html

Edited May 8, 2014 by Greg H.

[TransformerRobot]

Well, let's look at the math.

 

If we assume a four foot diameter fan, running at 2500 rpm (at the hub), that yields a prop tip speed of 357 mph.

The same prop spinning at your 500k rpm reaches prop tip velocities of 71,400 miles per hour, or something like three times the escape velocity of the Earth.

 

My guess is it would explode in a wonderous spray of shrapnel and kill everyone in the immediate vicinity.

 

---

Calculations courtesy of http://www.hoverhawk.com/propspd.html

 

Well, since that won't end well, what if the RPM ratio was 25K?

 

Or, if trying to make the fan more powerful, would the craft get faster if a small jet engine was in place of the propulsion fan?

[Greg H.]

 

Well, since that won't end well, what if the RPM ratio was 25K?

 

Or, if trying to make the fan more powerful, would the craft get faster if a small jet engine was in place of the propulsion fan?

At 25,000 RPMKs, you end up with a speed of Mach 4 at your propeller tips, which would probably result in the catastrophic failure of the fan.

 

Attaching a jet to it would, all other things being equal, make the hovercraft go faster, assuming the jet produces more thrust than the original fan. That's a far more complex equation and depends on a variety of other factors, such as can your hovercraft even lift the jet engine plus mounting plus fuel off the ground.

Note: I'm not advocating trying this in any fashion. I have a feeling it can only end poorly and with someone being severely injured.

[TransformerRobot]

Okay, how safe would it be if the RPMs were lowered to 7500? Still too much?

 

What if I just added a second fan to the rear end, and a pair of light 12 inch wings at the sides?

[Greg H.]

What are you actually trying to do?

[TransformerRobot]

What are you actually trying to do?

 

I'm just trying to learn how a hovercraft would be made to go faster?

 

The fastest a craft has ever gone is 137.4 km/h. Do you know how boring it must be to hold races with a vehicle that slow?

[dimreepr]

 

I'm just trying to learn how a hovercraft would be made to go faster?

 

The fastest a craft has ever gone is 137.4 km/h. Do you know how boring it must be to hold races with a vehicle that slow?

 

You should try it first, don't forget there are no brakes.

[TransformerRobot]

 

You should try it first, don't forget there are no brakes.

 

Well, then how can people take hover racing as seriously as they do Formula 1 or Hydroplaning?

 

I thought people were attracted to racing competitions because of the blood-boiling speeds?

[dimreepr]

It’s the danger that’s exciting not just raw speed, taking a corner on a bike at 70 mph when the same corner at 65mph feels like the limit is far more exciting than 190mph on a straight.

[TransformerRobot]

That's what they were doing on those corners? I thought they were doing at least 130 in cornering.

[dimreepr]

I was talking about my own experience as an example of how low speeds can thrill; in terms of motogp bike racing, it depends on the corner, some are as low as 50mph or as high as 200.

 

Although when you have no brakes and tricky steering 85mph seems fast enough, especially when a tree’s looming.

[TransformerRobot]

Then what can be put on a craft to make the steering easier?

[Greg H.]

Then what can be put on a craft to make the steering easier?

Not much. Steering a hovercraft is already pretty easy - you make them spin in a circle if you really need to.

 

The problem here is you're trying to make it work in a way it was not intended to. Never forget, a hovercraft has no physical contact with the groud, the skirt not withstanding. It is essentially riding ona frictionless surface, so it depends on its thrusters for steering and braking.

[TransformerRobot]

Not much. Steering a hovercraft is already pretty easy - you make them spin in a circle if you really need to.

 

The problem here is you're trying to make it work in a way it was not intended to. Never forget, a hovercraft has no physical contact with the groud, the skirt not withstanding. It is essentially riding ona frictionless surface, so it depends on its thrusters for steering and braking.

 

So there's not any major danger with 2 hovercraft colliding provided both pilots are buckled in?

 

What about if they hit trees, rocks or guardrails? And if the craft were racing on paved areas, how much could the skirt take before there would be a tear in it?

[Greg H.]

 

So there's not any major danger with 2 hovercraft colliding provided both pilots are buckled in?

 

What about if they hit trees, rocks or guardrails? And if the craft were racing on paved areas, how much could the skirt take before there would be a tear in it?

There's as much danger as if you were in a car at the same speed. All the energy has to go somewhere. The collision might be slightly less damaging, given the hovercraft's frictionless nature compared to the ground (i.e. they may tend to bounce off one another more than say, cars), but I expect there would still be some damage.

 

As for how much wear the skirt can take, that's completely dependent on what it's made of, how durable it is, and will vary from manufacturer to manufacturer.

[TransformerRobot]

There's as much danger as if you were in a car at the same speed. All the energy has to go somewhere. The collision might be slightly less damaging, given the hovercraft's frictionless nature compared to the ground (i.e. they may tend to bounce off one another more than say, cars), but I expect there would still be some damage.

 

As for how much wear the skirt can take, that's completely dependent on what it's made of, how durable it is, and will vary from manufacturer to manufacturer.

 

What is the strongest fabric available for making the skirt?

 

Could something as powerful as Kevlar be used?

[dimreepr]

Ripstop nylon/polyester is cheap and very durable and here's how to make a skirt.

[John Cuthber]

Not much. Steering a hovercraft is already pretty easy - you make them spin in a circle if you really need to.

 

The problem here is you're trying to make it work in a way it was not intended to. Never forget, a hovercraft has no physical contact with the groud, the skirt not withstanding. It is essentially riding ona frictionless surface, so it depends on its thrusters for steering and braking.

In my very limited experience, steering a hovercraft is a pig.

Sure, you can spin it round so you are facing the other way- but you tend to keep on traveling in the same direction you were originally- with the additional problem that you can't see where you are going.