The Work of Wings
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.
|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.
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.
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 totallift 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".
Rectangular Straight Wing
Tapered Straight Wing
Rounded or Elliptical Straight Wing
Slight Sweepback Wing
Moderate Sweepback Wing
Great Sweepback Wing
Forward Sweep Wing
The Swing-wing Design
High Lift Devices
Slats are located on the leading edge of the wings.
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.
The shape of a wing greatly influences the performance of an airplane. The speed of an airplane, its maneuverability and its handling qualities are all very dependent on the shape of the wings. There are four basic wing shapes that are used on modern airplanes: straight, sweep (forward and back), delta and swing-wing.
The straight wing is found mostly on small, low-speed airplanes. General Aviation airplanes often have straight wings. These wings provide good lift at low speeds, but are not suited to high speeds. Since the wing is perpendicular to the airflow it has a tendency to create appreciable drag. However, the straight wing provides good, stable flight. It is cheaper and can be made lighter, too.
The sweepback wing is the wing of choice for most high-speed airplanes made today. Sweep wings create less drag, but are somewhat more unstable at low speeds. The high-sweep wing delays the formation of shock waves on the airplane as it nears the speed of sound. The amount of sweep of the wing depends on the purpose of the airplane. A commercial airliner has a moderate sweep. This results in less drag while maintaining stability at lower speeds. High speed airplanes (like fighters) have greater sweep. These airplanes are not very stable at low speeds. They take off and descend for landing at a high rate of speed.
The forward-sweep wing is a wing design that has yet to make it into mass production. An airplane (like the X-29) is highly maneuverable, but it is also highly unstable. A computer-based control system must be used in the X-29 to help the pilot fly.
Simple Delta Wing (top) and Complex Delta Wing (bottom)
Delta Wing" width="267" height="351" />
The swing-wing design attempts to exploit the high lift characteristics of a primarily straight wing with the ability of the sweepback wing to enable high speeds. During landing and takeoff, the wing swings into an almost straight position. During cruise, the wing swings into a sweepback position. There is a price to pay with this design, however, and that is weight. The hinges that enable the wings to swing are very heavy.
High Lift Devices
The trailing edge of the wing is equipped with flaps which move backward and downward. These are not to be confused with ailerons, which are also located on the trailing edge of the wing, but have an entirely different purpose. The flaps increase
the area of the wing, and the camber of the airfoil. With this increase in area, the airflow has farther to travel which spreads the pressure difference between the top and bottom of the wing over
a larger area. An equation for the lift force is
lift = pressure x area
Given this equation, if the area increases the lift increases also. Conversely, if the area decreases, so will the lift.
Slats are located on the leading edge of the wings. They slide forward
and also have the effect of increasing the area of the wing, and camber of
Flaps and slats are used during takeoff and landing. They enable the airplane to get off the ground more quickly and to land more slowly. Some airplanes have such large flaps and
slats that the wing looks
like it's coming apart when they are fully extended!
The trailing edge of the wing is
equipped with flaps which move backward and downward. These are not
to be confused with ailerons, which are also located on the trailing
edge of the wing, used to make the aircraft roll (each side is raised/lowered in the opposite direction, the raised side is the roll side).
The flaps increase the area of the wing, and the camber of the airfoil.
With this increase in area, the airflow has farther to travel which
spreads the pressure difference between the top and bottom of the wing over
a larger area. An equation for the lift force is:
Given this equation, if the area increases the lift increases also. Conversely, if the area decreases, so will the lift.Slats are located on the leading edge of the wings. They slide forward and also have the effect of increasing the area of the wing, and camber of the airfoil.
Flaps and slats are used during takeoff and landing. They enable
the airplane to get off the ground more quickly and to land more
slowly. Some airplanes have such large flaps and slats that the wing looks
like it's coming apart when they are fully extended!
Spoilers are devices that are located on top of the wings. Spoilers have the opposite effect from flaps and slats. They reduce lift by disrupting the airflow over the top of the wing. Spoilers are deployed after the airplane has landed and lift is no longer needed. They also substantially increase the drag which helps the airplane to slow down sooner.