Aeronautics

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.

1. Set up leading edge and trailing edge and construct chord line between them.

Chord
2. Add curvature with camber line. Camberline

3. Wrap thickness about camberline to form upper surface.

Upper Surface
4. Wrap the same thickness under the camber line to form lower surface Wrap Thickness 2
5. Final airfoil shape. 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.

 

FanBy 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.

 

AirflowThe 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".

 

Stall Formation

 

Rectangular Straight Wing

.Straight Wing

Tapered Straight Wing Tapered Straight wing

Rounded or Elliptical Straight Wing

Rounded Straight Wing

 

Slight Sweepback Wing

Slight Sweepback Wing

Moderate Sweepback Wing

Moderate Sweepback Wing

Great Sweepback Wing

Great Sweepback Wing

Forward Sweep Wing

Forward Sweep Wing

 

The Swing-wing Design

Swing Wing Design

 

High Lift Devices

High Lift Devices

Slats are located on the leading edge of the wings.

Spoilers

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.

Wing Design
The amount of lift produced by an airfoil depends upon many factors:

  • angle of attack
  • the lift devices used (like flaps)
  • the density of the air
  • the area of the wing
  • the shape of the wing
  • the speed at which the wing is traveling

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)
A delta wing looks like a large triangle from above. Because of the high sweep, airplanes with this wing can reach high speeds - many supersonic airplanes have delta wings. Because of the high sweep, the landing speeds of airplanes with delta wings are very fast. This wing shape is found on the supersonic transport Concorde.

Simple and Complex <a href=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
When an airplane lands it is desirable to fly as slowly as possible. Ideally for landing, an airplane would have a large wing with a very cambered airfoil. However, airfoils designed to perform well at slow speeds are not good for flying at faster speeds, and vice versa. Airplane designers have developed a set of features that allow the pilot to increase the wing area and change the airfoil shape to compensate for this.

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 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.

Trailng EdgeThe 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:

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 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.