WHY A HOVERCRAFT HOVERS: PRESSURE AND LIFT
Lift air, like other gasses, is considered to be a fluid because it takes the shape of the container surrounding it. In the case of a hovercraft, the air takes the shape of the bottom of the hovercraft, the inside edges of the skirt, and the surface it's hovering above. The fan that blows air under the bottom of the hovercraft keeps pushing more and more air below the hovercraft, thus increasing the pressure in the air cushion. The pressurized air cushion exerts a force on its container (the bottom of the hovercraft, the skirt, and the surface the hovercraft is resting on). When the force this pressurized air exerts on the surface grows to equal the weight of the hovercraft, it becomes buoyant (like a boat in water) and begins to float on air.
When a hovercraft hovers, it will lift as high as the skirt’s designed shape will permit. Lift air begins escaping through the gap between the bottom of the skirt and the surface it's over. The size of this gap will be large enough so that the same amount of air escapes through the gap as is pushed in by the fan, keeping the pressure inside the air cushion constant. Usually, this air gap will be 0 to ˝ inches [12.7 mm] between the skirt bottom and the surface and is called daylight clearance.
Sketch by J. Benini
Pressure is defined as the force exerted on a surface per unit area of the surface.
Pressure = Force ÷ Area
P = F ÷ A
In order to calculate the lift force of a hovercraft, we solve this equation for the force.
F = P · A
The lift force is therefore the air pressure inside the
air cushion multiplied by the area enclosed by the hovercraft skirts.
Now we can find the lift force by multiplying the pressure times the area.
This hovercraft produces 350 lb [1508 N] of lift force and will therefore
be able to support up to 350 lb [1508 N] of total weight and still hover.
This means if the actual hovercraft weighed 100 lb [444.8 N], then it
could carry 250 lb [1112 N] of people and cargo.
In the case of hovercraft, there are two forms of pressure that can be measured: static pressure and dynamic pressure. Static pressure is the pressure of a stationary region of air, while dynamic pressure is the pressure of air that is in motion. Static pressure is what lifts the hovercraft. If you measure the pressure of the lift air cushion by placing a manometer (a device that measures pressure) just under the skirt, you will obtain a different value than if you were to measure the pressure further inside the cushion. This is because the air is moving rapidly out of the bottom of the skirt, so you could be measuring dynamic and static pressure at the same time. At the cushion center, the lift air is more static.
The Discover Hover One hovercraft is an integrated hovercraft. Only one propeller is used to provide both lift and thrust air. Other hovercraft designs have separate lift and thrust systems. The sole purpose of the fan is to maintain the pressure inside the air cushion under the hovercraft. A multi-bladed fan is used for lift because it's better (more efficient) at pumping pressure than a propeller with just two blades. A separate propeller mounted on the back of the hovercraft is responsible for driving the hovercraft forward.
Sketch by J. Benini
In the integrated hovercraft, the lift air is divided by a splitter usually located at the bottom of the thrust duct, as shown above. By placing this splitter just after the propeller, a portion (usually 1/3 of the total air supply) is forced by the propeller and directed down into the air cushion by the splitter in order to maintain the pressure inside the cushion. The rest of the air is forced behind the hovercraft, propelling the hovercraft forward. A diagram of the various paths the intake air travels in an integrated type of hovercraft is shown below.
Sketch by J. Benini
Continue to Experiment 4.1