Quote:
Originally Posted by freebeard
Always climbing out of a hole in the water.
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Nope, the difference in speed over differing surfaces isn't "
getting over hump speed - aka the hole in the water" it's surface drag and some parasitic drag.
Ice is really flat and nice and hard, and even when the skirt accidentally touches it the coefficient of drag is fairly low.
Water can actually pull down on the skirt as if it's alive, this can cause a "plow-in" when the skirt is pulled under. Plus water unless it's a still pond is uneven and the air bearing seal (ideally a constant level) takes more energy to maintain.
The various bulbous bow skirts with encapsulated air-spring reserve and perforations to allow air to escape near the surface and help prevent
parasitic drag were all developed the hard way - experience.
My hovercraft is rated as max-speed of 45 mph. In practice over water it is 30-35 mph and 50-55 mph on ice. I have no air speed indicator but rode a motorcycle for years - my estimate is that I once went 65 mph over ice on a large inland lake and strong tail wind - it was very scary as I was in fear of going head over tail should cushion pressure falter and plow-in ensue.
These things called hovercraft are pushed by a wind (thrust), and wind is quite the foe if it's going against you. Once I was trying to get on cushion in a head wind to get back home across the lake, and a storm front was moving in. I had to turn around and go on a beech, then while on land the cushion was stable and headed out over the choppy lake into the headwind with all it had and never let up. I estimate I was going less than 15 mph ground speed and barely made it home - almost ran out of gas.
Same craft that unexpectedly was doing 65 mph on ice, could barley do 15 mph, and all dependent on the wind and choppy waves.
Here is a hovercraft that has some unusual features:
https://www.neoterichovercraft.com/technology.php/
Video Link:
https://www.neoterichovercraft.com/buck-bg.webm
Quote:
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Neoteric’s reverse thrust system surpasses jet aircraft in efficiency: while such aircraft deliver only 18% thrust in reverse, our hovercraft deliver 60%. Neoteric hovercraft are so efficient that they can achieve a speed of greater than 25 mph (40 km/h) in reverse and are the only hovercraft in the world that can accelerate over hump in reverse.
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Good descriptions on this site.
DiscoverHover CURRICULUM GUIDE #19
DRAG
http://www.discoverhover.org/infoins...de19-drag.html
Quote:
A hovercraft, such as the DiscoverHover One, is able to glide or slide easily because there is so little contact friction with the surface over which it is hovering. Still, there are forms of friction which come into play, and these frictional forces are usually known as drag. Drag occurs in several forms, the most familiar being wind resistance, or form drag, which is created by the hovercraft having to push aside air as it moves forward. This effect increases more and more as the hovercraft’s speed increases. Streamlining the shape of the hovercraft decreases the wind resistance, resulting in higher top speeds. While wind resistance is always present, it becomes a much greater problem at speeds of 50 km/hr [31 mph] and above. Since DiscoverHover hovercraft usually don't exceed 50 km/hr [31 mph], wind resistance is not as noticeable as it would be on other light hovercraft.
A hovercraft operating over water is subject to three other forms of drag not experienced on solid surfaces: wave drag, skirt drag, and impact drag. Wave drag (called hump drag at low speeds) occurs when lift air under the hovercraft pushes down on the surface of the water. Some of the water is displaced from under the hovercraft, creating a depression in the water. The total weight of the water displaced is equal to the weight of the hovercraft and pilot, according to Archimedes’ Principle. As the hovercraft starts moving forward, the depression moves with it and forms a small wave in front of the bow. This causes the bow (front of hovercraft) to rise and the stern (back of hovercraft) to sink slightly. The hovercraft is, in effect, trying to fly uphill. As the hovercraft increases speed, the bow wave increases in size. Eventually, the hovercraft will reach a speed where it's moving faster than the wave and will “climb” over it. Technically known as planing speed, this is commonly referred to as “getting over the hump”. At this point the hovercraft will accelerate rapidly. The moment before planing speed is reached, wave drag is at its greatest. When traveling above planing speed, the lift air under the hovercraft doesn't have enough time to depress the surface of the water and the wave drag decreases dramatically.
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