How much air does a hovercraft need? The amount of air a hovercraft needs obviously depends on its weight, its size, the speed of its fans, and the quality of the skirt around it.
Hovercraft work by producing pressurized air underneath them, which lifts the hovercraft off the ground. A second fan at the back of the hovercraft provides thrust, moving the hovercraft forwards. It is complicated to produce the cushion of air underneath the hovercraft, but a hovercraft can move a lot more efficiently than a boat or other vehicle because they are subject to less friction. Friction on a boat or a vehicle comes from either air pressure or contact with the ground. The majority of it comes from the sea or the ground. When you look at a boat moving through the water, you can see the wake left behind it. This wake is made by energy lost from the boat to the sea. The more energy you have to use to overcome friction, the less efficient your vehicle is. If you can raise a vehicle over the ground on a cushion of air, then there is less contact with the ground, and friction is very greatly reduced. The main types of friction that a hovercraft is subject to are air resistance, drag from the leaking air, and spray off the water, which is a lot less than that of a boat.
Hovercraft differ in design, but generally they have a large air intake fan at the back that forces air out through a vent at the bottom. They always have a large skirt around their base, which is designed to keep the air in. These skirts were not invented until hovercraft had been around for a while, and they drastically improved the hovercraft because they could get more lift with less air. The skirt is made of flexible rubber so that it can balloon with the air and move over the contours of the ground. It also needs to collapse when the hovercraft is switched off.
To understand how a hovercraft works, we need to remember that air behaves like a liquid. It flows just like a liquid and is governed by the same laws that govern liquids. These are called Bernoulli’s principle and were discovered by Daniel Bernoulli in 1738. One of his principles is that fluids flow from an area of high pressure to an area of low pressure. This is the principle behind why a plane can fly. The wing of an airplane is curved so that the air takes longer to go over it than under it. This creates an area of lower pressure on top of the wing. The higher pressure air under the wing wants to move into the area of low pressure and pushes upwards, giving the plane lift. A hovercraft gets lift from the same idea, by using a pressure difference, but in a slightly different way. To give it lift, there needs to be greater pressure under the hovercraft than above it. When you increase the pressure of the air, you are squeezing more air into the same volume so that the number of molecules increases. As the pressure under the hovercraft increases, the air molecules want to move into the lower pressure air above the hovercraft. They cannot escape or go around the hovercraft because of the skirt, so they lift the hovercraft up.
To work out how much air you need to lift the hovercraft, you need to know its weight and the area of its surface. It is surprising how little air pressure is needed to give a hovercraft lift. You also need to know the speed at which air can be pushed under the hovercraft, and you need to know the speed at which the air leaks out from beneath the skirt. The basic formula to work out the pressure is to divide the weight of the hovercraft by the area of its base, which gives an answer in pascals. For example, a small one-person hovercraft might have a mass of 150 kg, with the rider. If it has a radius of 1 m, then that works out to be a pressure of 468.5 Pa. Atmospheric pressure is 101,325 Pa, which means you only need to increase the pressure of the air under the hovercraft by 0.46% to create lift. You need more pressure to lift a larger hovercraft, but even then, it is not a lot. The SR.N4 Mk III is a passenger and vehicle carrying hovercraft that is 320 tons when fully loaded. The same calculation gives a requirement of 2,450 Pa, which is only 2.5% of atmospheric pressure.
Two other things that need to be taken into consideration are how quickly the air can be pumped under the hovercraft and how quickly it leaks out from under the skirt. The air cushion under the hovercraft is not air-tight because if it were, the hovercraft would be floating on a balloon, which would create friction. That means, air can leak out. To ensure that the air pressure under the hovercraft stays the same, you need to make sure that air leaking out equals air going in. If you can do this, then the hovercraft will float. And this is what I have learned today.
Sources
https://www.discoverhover.org/infoinstructors/guide8.htm
https://www.amphibiousmarine.com/how-does-a-hovercraft-work
https://www.checkmateflex.com/resources/insights/how-does-a-hovercraft-work
https://en.wikipedia.org/wiki/Hovercraft
https://en.wikipedia.org/wiki/SR.N4
https://en.wikipedia.org/wiki/Bernoulli%27s_principle
Photo by Hien Nguyen huu duy: https://www.pexels.com/photo/hovercraft-in-arctic-20863923/