Background Essay: Air Power: Making a Hovercraft

Friction is defined as the resistance to sliding motion between two surfaces in contact -- a solid object rubbing against another solid, or a solid object moving through a gas or liquid. In some situations friction can be useful, for example, a brake pad rubbing against a wheel rim slows a moving bicycle. However, in many other situations friction causes problems and needs to be reduced. Solid, plastic, or liquid materials called lubricants are used to reduce the effects of friction. Petroleum jelly, motor oil, and the ball bearings contained between a wheel and its axle are examples of common lubricants.

Hovercrafts work on two main principles: lift and propulsion. A fan blows air straight down underneath the vehicle. A skirt -- a piece of material fastened around the bottom edge of the base of the vehicle -- traps the air between the vehicle and the ground or water. As the fan continues to blow, the air pressure inside the enclosed space increases. Under pressure, molecules try to spread out, and when prevented from doing so, they exert a force on the surfaces that contain them. If the air pressure inside the skirt rises so much that the air molecules exert a force greater than the downward force produced by the weight of hovercraft, it lifts the vehicle. The cushion of air between the ground or water and the vehicle acts as a lubricant. In simple configurations where a great deal of force is not required, adequate pressure can be attained without a skirt.

Some hovercrafts have two engines that drive separate fans: one to generate lift, the other to generate thrust, which propels the vessel forward. Integrated lift-propulsion systems use only one engine to perform both functions.