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 Air pressure
Surface area to weight ratio
Thrust
Friction
Pros and cons
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| Compressed air contained by the skirt creates lift |
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Air pressure
A hovercraft hovers by creating a cushion of air with sufficient pressure to support the weight of the craft and passenger(s). A fan blows air underneath the hovercraft and the air is trapped between the craft and the ground by a piece of material called the 'skirt'. As the fan blows more and more air molecules under the craft, air pressure is increased.
Molecules are the minute particles of which all materials – solids, liquids and gases – are composed. Each material has its own kind of molecule, consisting of a particular combination of atoms. Air, for example contains molecules of nitrogen, oxygen, argon, water vapour and carbon dioxide.
Molecules dislike being crowded together. When compressed, they will try to spread out, or, if they are trapped, like those in a hovercraft's skirt, they exert a force on the surfaces containing them.
In order for a craft to hover, air pressure inside the skirt must be raised until the air molecules inside are exerting a force greater than the force acting down on the craft – in this case the combined weight of craft and rider.
Because a hovercraft cushion isn't sealed, as a car tyre is, more air must be continuously blown in to maintain pressure.
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Surface area to weight ratio
A successful hovercraft will have a high ratio of ground surface area (that is, the area of ground encompassed by its skirt) to weight. This is important because the larger the area a weight is spread over, the smaller the force (per square metre) needed to lift it will be.
The air pressure needed to lift a craft can be determined by dividing the total weight of craft plus passenger(s) – the laden weight – by the ground surface area of the craft.
Reducing the amount of air pressure needed by increasing ground surface area means a craft requires less power, so it is more efficient. However, a larger ground surface area also results in more leakage and drag. Drag is the force with which air or water resists the motion of an object. It is sometimes called 'air resistance' or 'water resistance'. See also Friction below.
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| Demonstrating Newton's third law: to every action there is an equal and opposite reaction |
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Thrust
A hovercraft needs power to generate thrust as well as lift. Newton's third law states that to every action there is an equal and opposite reaction. Hovercraft engines force air out at the back of the craft, and the craft reacts by moving forwards.
Some hovercrafts have two engines: one to generate thrust, and one to generate lift. An 'integrated' craft, on the other hand, has only one engine; the airflow is divided between lift and thrust.
It is vital that the pitch or angle of the fan blades providing thrust are correct. If the blades are pitched too flat, they will just cut through the air rather than displace it. If they are pitched too steeply, they will just churn the air into vortices.
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| Hovercraft generate little friction because they float on a cushion of air |
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| The less contact the skirt has with the ground, the less friction is generated. Skirt contact increases over rough surfaces. |
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Friction
Hovercrafts have several advantages over other modes of transport. Firstly they can move freely over water, land or ice surfaces, providing those surfaces are not too rough or porous. They also generate very low levels of friction – provided skirt contact with the ground is kept to a minimum (which is of course best achieved over smooth surfaces).
Friction is a force that appears whenever a solid object rubs against another. It also appears when a solid object moves through a gas or liquid. Friction always opposes movement and it disappears when an object becomes stationary.
However smooth they seem, surfaces are always irregular at the microscopic level. Also, there are forces of attraction between the molecules in the contact regions of surfaces that grip each other or 'weld' together. The harder the surfaces press together and the closer the molecules in each surface get, the stronger the grip. Friction results from the breaking of this grip and from the irregularities of the surfaces ploughing against each other.
The molecules of air are few and far between compared with those of a solid object. This means that air can pass over solid objects without generating much friction. As a hovercraft sits on a cushion of air, it too generates only small amounts of friction (from air resistance and from the contact of the skirt with the ground).
Newton's first law states that every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. Friction and drag are two such external forces, which act to slow a moving body down. So, as hovercraft generate only a very small amount of friction, they are very efficient, requiring less power to keep moving (and to get underway in the first place).
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| The Krazy Karters' polystyrene craft is very buoyant. They'll float even if they lose power |
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| The Karters' segmented skirt means a tear won't be disastrous |
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Pros and cons
Martial Mechanics
- The Martial Mechanics' engine, from a microlight aircraft, is ideal: it gives them the power they need to build an integrated craft, and it's very light too.
- The team effectively build a boat hull and attach a skirt underneath. The streamlined bow makes it much easier to move through the water and to lift up again should they lose power at any point.
On the other hand ...
- The Martial Mechanics opt for a single-skin 'bag skirt'. Though simpler to make than a segmented skirt (which is what the Karters opt for), a single tear will mean a serious loss of air pressure. Also, a 'bag' skirt creates more friction than a segmented skirt, because it curves under itself, so more material is in contact with the ground. This will make the Karters' machine less efficient and slow it down.
Krazy Karters
- The polystyrene used as a base for the Krazy Karters' craft is very light and ensures their craft is buoyant. This turns out to be quite important: when they lose power over the water, their hovercraft effectively becomes a boat!
- As well as creating less friction, the Karters' segmented skirt means that if they get a tear, it won't spell curtains, because the surrounding segments will inflate a bit more to fill the gap. Also, the segments can move independently over objects, so less air pressure is lost because a better seal with the ground is maintained.
- The double-engine design means there is no complicated ducting to worry about.
On the other hand ...
- Having two engines makes for twice as much work on the build day. There is also twice as much that can go wrong. Even more importantly, two engines mean increased weight.
- The Krazy Karters' front engine, providing the lift, is found to be lacking in power.
- The polystyrene blocks form a very blunt front end. This is fine on land but not so good over water. When the skirt deflates, the blocks create a bow wave, making it more difficult for the craft to lift back out of the water. In the event, these disadvantages cost the Karters victory.
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