The Ancient Chinese discovered that kites with curved surfaces flew better than kites with flat surfaces. Lilienthal and Cayley, in the 1800s, demonstrated that a curved surface produces more lift than a flat surface. This led to the conclusion that a wing needs to have camber. That is, the top needs to be slightly curved, like a hump. The bottom is left flat or straight. An object with this shape is called an airfoil. Often, the words "wing" and "airfoil" are used interchangeably, but they shouldn't be. Airfoil shapes are designed to generate as much lift as possible while incurring as little drag as possible.
| 1. Set up leading edge and trailing edge and construct chord line between them. | |
| 2. Add curvature with camber line. | |
| 3. Wrap thickness about camberline to form upper surface. | |
| 4. Wrap the same thickness under the camber line to form lower surface. | |
| 5. Final airfoil shape. | |
Camber causes the air that flows over the top of the airfoil to move faster than the air that flows beneath it. In the 1700s, Daniel Bernoulli showed that a fluid that flows faster over a surface will create less pressure on the surface than fluid that flows more slowly. This concept later became known as Bernoulli's Principle. Further, since air is a fluid, air follows Bernoulli's Principle. Thus, we have a situation where there is less air pressure on the top of an airfoil than underneath. This difference in pressure will cause the wing to move. That is, the difference in pressure will generate a force. The force that is generated is called "lift". Bernoulli's Principle applies only to subsonic flight.
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By virtue of its shape alone, an airfoil will generate lift as air flows over it. However, even more lift can be generated by the airfoil if it is tilted with respect to the airflow. This tilt is called an airfoil's angle of attack. As the wing is tilted, the air flowing over the top of the wing flows even faster than the air flowing underneath. As the difference in the speed of the two airflows increases, the difference in pressure increases also. Remember that it is this difference in pressure that generates the lift force. So, as its angle of attack increases, the wing generates more lift. Think of an airplane taking off - remember, airplanes always take off heading into the wind. As the airplane speeds along the runway, it is already feeling the effect of the lift generated by the shape of the airfoil. Farther along the runway, the pilot pulls the nose up. This increases the angle of attack of the wings which causes more lift to be generated. |
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The wings also provide lift through Newton's Third Law of Motion which states that for every action there
is an equal and opposite reaction. As the wing moves though the air, the lower surface
of the wing deflects some of the air downward. As Newton's Third Law of Motion explains,
an additional force is generated. The deflected airflow underneath the wing is the action.
The reaction is that the wing moves in the opposite direction (in this case, upwards).
This means that the development of low pressure above the wing (Bernoulli's Principle) and
the wing's reaction to the deflected air underneath it (Newton's third Law) both contribute
to the total lift force generated.
However, there can be too much of a good thing! The airfoil's ability to create lift is dependent on the airflow remaining smooth. Think of a stream flowing gently around a rock. The water's flow changes direction to go around/over the rock, but it remains smooth - it doesn't get jumbled or choppy - and it hugs the rock as it flows around it. Now, if that rock were a larger rock, the water would hit it, get all jumbled up and then eventually move on. The flow around that rock would not be smooth. The same thing happens with a wing. Up to a certain angle of attack the air will flow smoothly along the surface. The wing acts like a small rock. If the angle of attack becomes too great, an effect similar to throwing a big rock in a stream is created. The air will get all jumbled up and not flow smoothly around the airfoil. If this happens, lift will not be generated. We say the wing "loses its lift" or "stalls".
Stalls can be caused by real-life flying situations. If the engines quit or a sudden gust of wind hits, the airplane's forward speed decreases. The airflow over the wing decreases and the amount of lift drops. The weight force then takes over and a potentially hazardous situation results. Fortunately, pilots spend many hours learning how to recover from a stall. Flight simulators are used extensively to train pilots in how to recognize an oncoming stall and prevent it. If a stall should occur, pilots learn through simulation how to maneuver the airplane so the generation of lift is restored.
Another cause of an airplane stalling is wing icing. When ice builds up on a wing, it changes the shape of the airfoil. The new shape stalls more easily. What's worse, the ice also adds weight, so this can be a dangerous situation. Obviously the solution to this problem is getting rid of the ice. Airlines spend many hours during the winter months de-icing wings before takeoff.
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