Reduced volume flow in a hovercraft

[hover guy]

I was wondering what will happen to a hovercraft which is designed for a given air film thickness, is. slightly lifted above its air film, will the hovercraft start behaving like an helicopter, which uses the action- reation phenomenon to keep itself in the air or it will still continue to slip on the air film.

 

[flyboy_1234]

most modern 'hover crafts' are miss-named and should be called 'skimmers' as they sit on a cushion of air formed by the skirt that lightly scuffs along the ground. I suspect what your referring to would be a true hovercraft which utilities an air curtain to form the edge of the air pocket.

In both cases the craft is lifted by the higher pressure under the craft much like a lid on a pot of boiling water "hovers" above the pot and can be easily spun. To take the analogy further, if you held the lid 1 inch above the pot it would take a LOT of boiled water to hold the weight of the lid... similarly the skimmer/hover craft would take a large jump in power to float above the ground effect... may as well just jump in a helicopter. My old skimmer uses a 5 h.p. engine for lift, and that's probably overkill, while a helicopter uses... you get the picture.

 

[hover guy]

thanx for the reply flyboy, but I wanted to know wether a skimmer if lifted with some external force, such that partly the weight is divided in between air film and a force such that both of them are in perfect equilibrium, then ,will the hovercraft still be able to produce a lift even if the force has lifted to a position above its air film...what will be the value of height of skirt to completely shift its weight to the balancing force.

 

[CWatters]

The main lifting force is not "helicopter" action of the fan blades but the pressure increase below the platform. You don't have to increase the pressure much because it's multiplied by the area. In a TV demo a large car transporting hovercraft was driven over raw eggs. The pressure was low enough that they didn't break.

If the skirt formed a perfect seal with the ground the lifting fans could be shut off once the required pressure had been achieved. However in the real world the skirt leaks and the fans have to provide the required pressure AND flow rate. If an existing craft were to be raised up an additional 2 meters the skirt leakage would increase dramatically and the lifting fans couldn't deliver the required higher flow rate. The pressure would fall and the craft would sink back down until the skirt leakage reduced. I suspect this is self regulating...the craft lifts up until the skirt leakage matches the flow rate provided by the fans but no more.

 

[hover guy]

thanx, for the reply, I guess pressure is the key, once it is disturbed by an external force , more volume of air will try to compensate its lift,creating a suction of air,and letting it escape faster, hence there will be less convergence of velocity into dynamic pressure thus the hovercraft will come down to stabilize itself.