Kite.
Courtesy of https://airandspace.si.edu/stories/editorial/how-kites-fly
The first known man made flight is in the form of kites, which dates back as far as 5th century BC. What is important about kites which relates them to wings is that they are heavier than air. This means that they are not just floating due to buoyancy, they are somehow overcoming the gravitational pull downward. These early designs were based off insects and birds, with thin large sheets designed to 'catch' the wind. Although very primitive this can be considered the first type of foil. By utilizing a force from wind in one direction people were able, by offering resistance, to produce a force in another direction. This lead to message carrying and even kites capable of lifting humans off the ground.
As illustrated on the right, how a basic kite works is quite similar to a modern wing. The most important element of any wing is that there must be flow of the fluid, be it air or water, along the surface. Unlike a modern wing a basic kite is almost in a perpetual state of stall as there is very little pressure difference between the leading and trailing side of the kite. A stable flying basic kite will not travel in the vertical of the horizontal direction because the lift, drag and gravity are net neutral. Yet when a gust of wind hits the lift will outcompete the drag and gravity forces causing the kite to move.
As illustrated on the right, how a basic kite works is quite similar to a modern wing. The most important element of any wing is that there must be flow of the fluid, be it air or water, along the surface. Unlike a modern wing a basic kite is almost in a perpetual state of stall as there is very little pressure difference between the leading and trailing side of the kite. A stable flying basic kite will not travel in the vertical of the horizontal direction because the lift, drag and gravity are net neutral. Yet when a gust of wind hits the lift will outcompete the drag and gravity forces causing the kite to move.
Air foil. |
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https://nptel.ac.in/courses/101106035/026_Chapter%205%20_L19_(03-10-2013).pdf
The lift a foil generates is due to its shape. Specific foil shapes can be seen in nature in different birds flight capable creatures. Some foils will generate lift at higher velocities while others are designed for lower velocities. For example a soaring glider operates at low velocities and its wings are designed longer and have more relief than say a fighter jet which has shorter and flatter wings. When designing a wing this becomes very important, for if the structure cannot withstand the drag forces or lift forces which it undergoes it risks catastrophic failure. Wings are designed to reduce drag as much as possible while still generating lift in order to be most efficient. These principle are used for in applications such as planes in order to generate upwards lift, or in instances such as cars to provide downforce. Most foils are supported through an internal structure which gives them rigidity to hold up against the
Courtesy of http://www.mackiteboarding.com/closed-vs-open-cell-foil-kites-for-kiteboarding-and-snow-kiting.htm
Some foils use their own airspeed to give them structure. As the air chambers in this foil fill with air the structure becomes rigid. Foils such as these are used for large container ships, kite surfing, paragliding, skydiving and more. These are very effective because they are very light thus the downward force of gravity is relatively easy to overcome. Thus in comparison to aircraft wings these types of foils can operate under much lower air speeds.
When a foil does not have enough lift to combat gravity and drag, usually due to a lack of air speed, the wing will stall. This is a type of cavitation where the foil cannot support itself and once this critical point is reached loses all lift until the fluid flow reaches a velocity around the foil which will 'reactivate' the lift.
When a foil does not have enough lift to combat gravity and drag, usually due to a lack of air speed, the wing will stall. This is a type of cavitation where the foil cannot support itself and once this critical point is reached loses all lift until the fluid flow reaches a velocity around the foil which will 'reactivate' the lift.