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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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