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Sometimes called an air-cushion vehicle or ACV, a hovercraft rides on a cushion
of air instead of wheels to go over many surfaces. A multi-blade fan forces
air under the hull of the hovercraft, creating a high-pressure region called
the lift air cushion and making the craft float. The hovercraft lifts off the
surface it's resting on when the lift air pressure is greater than the total
weight of the hovercraft divided by the area of the lift air cushion. It then
hovers just above the surface. Because most surfaces are uneven, additional
height is needed so the craft can travel without getting the bottom of the hull
caught on anything.
To increase the clearance between the bottom of the hovercraft and any uneven surfaces, a flexible fabric skirt is attached to the bottom outside edge of the hull. This creates a wall that traps the lift air, forcing the hovercraft to rise higher above the surface, 6-9 inches [15.24 – 22.86 cm] in most cases, giving the hovercraft a smooth ride and allowing it to clear obstacles.
Because the hovercraft only puts a very small pressure on the surface it’s riding over, it can easily be flown over mud, short grass, sand, water, ice, snow, or pavement. Hovercraft also use air to move forward. Many hovercraft use an engine with an airplane-type propeller or multi-blade axial fan to push air behind the hovercraft, creating forward thrust. Often, a circular enclosure called a thrust duct is built around the propeller. By using a thrust duct built so the tips of the propeller travel within 1/8 inch [0.3175 cm] of the inside face of the duct, the thrust output of the propeller can be increased by 10-15%. (Fowler. Circa: 1993) The thrust determines how fast the hovercraft can go and how steep a grade (like a boat ramp) it can climb.
Steering a hovercraft is accomplished using rudders, similar to those on an airplane. The rudders are mounted behind the thrust duct to put them directly in the flow of the thrust air. Their position also gives them a mechanical advantage in turning the hovercraft, similar to the effect that moving a long lever is easier than moving a short lever. As the rudders turn, the thrust air is deflected left or right, forcing the hovercraft to change direction. Rudders are made with a symmetrical airfoil profile to minimize air drag and increase their efficiency. Most hovercraft use at least 2 rudders, some as many as 5. In the following handouts we will investigate the science behind what makes a hovercraft work.
