MadSci Network: Physics |
First of all I am not a hovercraft specialist, however... This is actually a fairly complex question. Basically hovercraft tend to (or try to) hover at a fairly constant height. This height is determined by the lift from the air cushion and the weight. Lift from the air cushion is primarily a function of pressure in the air cushion. A hover craft sits on a nearly static cushion of air (as opposed to wing in ground effect vehicles). This air cushion is almost always contained using a "flexible bag" of some sort (the skirt). Within this "bag" there will be a certain pressure of air, and it is this pressure which determines how much weight the hovercraft can lift. At very light weights the bag will be less deformed and there may even be a area between the bag and the ground. There is often an "air curtain" just below the skirt that also helps contain the air cushion, so the skirt may be hovering some height above the ground but the air cushion is mainly contained within the bag itself. (Very early hovercraft models had no skirt and could only hover very close to the ground, also some basically skirtless hovercrafts or skates are used for shifting loads in factories where clearance is not an issue). So as the hovercraft is loaded any hover height below the skirt will be lost (as it is hard to maintain air pressure here), and the bag may deform slightly as the load increases. However this change is generally small compared to the total height of the skirt. As the hovercraft is loaded the air pressure in the cushion is usually increased. So basically if you have a light load and max power going on your engines, producing max pressure you will hover at your skirt height plus a bit more underneath. The reason you are not blasting skywards even though the pressure in the skirt is much greater than that required for the load is because that pressure is lost rapidly below the skirt. Load up your hover craft and the hover height below the skirt goes first (hardest place to maintain pressure) until your engines are producing just enough pressure to lift your weight plus the loss of air around the bottom of the skirt. The max weight that a hovercraft can take is a function of this max pressure of the air that can be maintained in the skirt (the air cushion). Load beyond this limit and height starts diminishing rapidly. http:// www.hovercraft.org.uk/download/download.htm http:// collections.ic.gc.ca/science/english/eng/projects/air.html for wig http://home.mira.net/ ~radacorp/ground_effect.html http://www.ae.metu.edu.tr/~gulkiz/ wig.html
Try the links in the MadSci Library for more information on Physics.