supersonic waves vs hovercraft

[crashforce]

Evening all, not sure if this is going in the right area but you may be able to help me.

For my own interest im wondering that the effects of supersonic waves would be on hovercraft travelling above the speed of sound? Putting aside other issues such as weight and skirt wear etc how would the physics under the skirt be effected?

The only problem i can see is the ridiculously high under skirt pressure needed to counteract the massive forces on the front of the skirts (unless of course it was always to run on a smooth surface in which case the skirt could be covered with an aluminium shroud to protect it and reduce drag.

hope so one can provide some information
Regards,
harry

 

[IRstuff]

You can't get air to go supersonic, except in the case of explosions.

TTFN

 

[GregLocock]

Unless you trap it inside a closed volume. grin

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[crashforce]

the air wouldnt need to be going supersonic though would it? because the fans and the air pocket are moving at that speed??

just wondering what the waves travelling under the craft would do tthe pocket of air? i.e a high pressure wave moving under would case a verticall lift on the vehicle like it was moving over a waves at sea?! or would it because the skirt deforms to allow it to ride smoothly over obstacles at sea?!?!

Id love to see a hovercraft designed for the quarter mile or a speed record. i know a few of the huge ferry hovercrafts that ran/run the channel are capable of 130kph with no passengers simply because of the near zero friction levels

 

[IRstuff]

Forget the waves. You can't get air moving outward on the leading edges, hence no lift on the leading edge.

TTFN

 

[rb1957]

it would be interesting to see a hovercraft travelling at 600mph ... that's gotta be more than twice their maximum speed today.

there are a huge number of real world issues (like stability and control travelling over an uneven surface, and all the crap you'll run into/over), but ...
wouldn't the shock wave form ahead of the vehicle (like it does on a plane) so the airflow behind the shock is subsonic. yes, the drag force would be Huge, but is it (as IRstuff posts) impossible ? wouldn't the air close to the skirt be able to flow forward (driven by a sufficiently large (sorry, Large) fan) ? heck, if you had to couldn't you install a crane (not a skyhook, but attched to the hovercraft) to lift the nose up ? yes, i know the hovercraft would have to supply more than twice the force needed (by now i think we're talking above a portable nuke power plant !) ... but is it impossible ??

 

[berkshire]

I am wondering if the rigid sidewall hovercraft design of the type built several years ago by Vosper Thornycraft would be capable of this. They allowed ram air to enter under the front when the speed was high enough.
B.E.

 

[GregLocock]

Lok at the design of a shockwave rider. It is essentailly what is being proposed.

Cheers

Greg Locock

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[crashforce]

cant find any info on the shockwave rider?

what i was thinking was an aerodynamicall outer hull with a leading edge lower than the hovercraft skirts kinda like the body sits over a spaceframed car. I like the idea of allowing high speed air under the front deliberately after all why pump it in when you can reduce drag using the stuff in front of you!

Obviously space would be the biggest problem but for aperfomance only application the quarter mile or a land speed record you could change the propulsion to rockets and then youve only got 1 engine to to fill the skirt. Or even use smaller rocket engines to do that youd just need a system with very cool exhaust gasses or a way of cooling them.

ive got no plans on building one just wondered if it could feasably be done

thanks for the info guys

 

[rb1957]

maybe you could have a dedicated fan, mounted to lift the nose, sort of lift the fwd engine in the F35 ... wouldn't need to be a full engine (with combustion and hot exhaust), could be a shaft powered fan (like the competing F35 design).

sounds like a tricky design/control problem ... i mean if you supply too much lift you'll rasie the nose (and flip the vehicle ?) ...

don't quite see how the fan would work, if it's exhausting into a skirt (back pressure?) ... maybe dump the skirt ? and suspend the vehile on a cushion of fan exhaust ??

 

[swearingen]

The key may be to design it to be a hovercraft at low speeds and reconfigure to be a ground-effect vehicle at high speeds. I know some research has been done about a very large ground-effect cargo vessel for the Atlantic. Sort of the capacity of a ship with the speed of an aircraft...

 

[GregLocock]

google ekranoplane

Cheers

Greg Locock

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[RPstress]

Re current speeds of Hovercraft, there's a rumour that they got the first stretched N4 up to 100 kts in the Solent during trials, but I never met anybody who was there.

The last trip by an N4 across the Channel recorded an average over-water speed of 64 kts. They could reasonably easily do 70 kts on a calm day.

I'm not sure what a BH7 or an unladen LCAC is capable of.

A long way to go before the sound barrier!

 

[crashforce]

very interesting reading on the ekranoplane!

obviously standard hovercraft are a longway off of these speeds in the same way that standard passenger cars are.

one interesting point i hadnt considered before is that y placing the ekranolanes wings behind the engines they get all that extra air flowing around the wing and more lift. could this not be used in current plane/hovercraft manufacturing to increase lift and efficiency?
how would it effect the rest of the bodys aerodynamic qualities?

 

[RPstress]

Propellor aircraft with wing-mounted engines generate a significant extra lift when the throttles are opened. Pushers like the Beech Starship lack this effect.

C17 has a blown flap with the engines tucked up under the wing more than usual (though not as extreme as the YC-15 it was developed from).

I think it's a tradeoff between less efficiency in cruise vs more lift for takeoff, but I'd have to ask an actual aerodynamicist to be sure.