Hovercraft: Lift Using Only Air - Article

Hovercraft: Lift Using Only Air

The idea of lifting an object using only the air is not a new one. However, the hovercraft is among the newest ways to achieve lift using air flow. Other vehicles which generate a lifting effect using air are helicopters (rotary wing) and airplanes (fixed wing). A fundamental comparison will be provided here.

Hovercraft Lift

Lift air flow for a hovercraft is contained in a skirt, which is a flexible material enclosure surrounding the exterior edges of the craft. Hovercraft achieve lift in part by spreading out the weight to be lifted over a large enough skirt area to allow air pressure to overcome the pressure from the weight. For example, if the total weight to be lifted is 500 pounds and the area of the craft is 100 square feet, the overall pressure of the weight would be 5 pounds per square foot (500 pounds divided by 100 square feet).

Since there are 144 square inches in a square foot, there would need to be approximately 0.035 pounds per square inch air pressure to lift the total weight of 500 pounds (5 pounds per square foot divided by 144 square inches). Because the air pressure needed is so low, the hovercraft remains one of the more efficient ways to lift weight using air.

Airplane Lift

Airplanes use wings as a method to generate lift. The difference in air pressure necessary is produced from a variety of factors which influence the air on the upper and lower surfaces of the wing. The Coanda Effect indicates that a fluid follows along a curved surface instead of flowing straight off of the surface. Bernoulli's Principle states that a flowing fluid speeds up, and reduces pressure, along a curved surface.

It is this difference in pressure that results in lift. The pressure above the curved upper surface will be lower than the pressure below the flat lower surface. To generate this pressure differential, the air must be moving over the surfaces of the wing, in other words the airplane must be moving forward. Conversely, hovercraft need not be moving forward to achieve lift.

Helicopter Lift

Helicopters are also capable of generating lift without forward motion. Lift for helicopters is also the result of a type of wing, however, the wing in this case is not stationary. Instead of moving the air over the wings, the wings are moved through the air on a rotating shaft.

The rotating wings must be able to support the entire weight of the helicopter, and its occupants, at the single point of the central shaft. This places the shaft under much more pressure than many of the assemblies on a hovercraft. Additionally, helicopters require more power per unit lift than a hovercraft of the same weight.

Conclusion

The differences between operating characteristics of conventional aircraft and hovercraft are derived from the differences in their methods of achieving, and sustaining, lift using only air.

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