Why do Airliners have Swept Back Wings?

Why do Airliners have Swept Back Wings?
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Race Against Air Friction and Turbulence

History of Swept back wings

In swept wing technology the airplane wings are swept back at an approximate angle of 35 degrees. This angle varies as a result of an increase in wing length. This idea came about as a result of the technology battle between Germany and Europe to make an effective weapon during W.W.II. This technology was first introduce by a German aeronautical engineer in 1940. The concept was successfully tested on the Messerschmitt Me 262, the first operation jet fighter craft. This technology was later adopted by US engineers for their fighter planes such as the North American F-86.

Airliner stability at high speed

Airliners, like all airplanes, fly because of the greater air pressure on the bottom side of the wings than on the top side. Air moves more smoothly on the top side of the wings than on the bottom. But when airliners move faster and faster, the air speed that moves over the top side of the can wings exceed the speed of sound. This results in the vibration in plane body because of the resulting shock waves. To try to understand this concept by analogy, when you move your hand through water slowly, the water passes over it softly. But when you pull your hand faster, the result is turbulence and disturbances. The same is the case when an airliner moves from slow speed to high speed. The figure below shows an airliner with swept back wings. (Click the image to enlarge, and then click Back in your browser to return to this page.)

How this problem was solved by swept back wings?

When speed increases, so do turbulence and drag, as a result of air friction on the wings. Swept back wings technology was introduced to solve this instability and vibration in supersonic jets at high speed. Wings are angled towards the back end, which creates an imaginary increase in wing length. This creates an “illusion” for the air that the plane is moving slower than actually it is.

Airliners can achieve high speed, almost twice the speed of sound in the case of the Concord. Air travels further on the top side, thus safeguarding the airliner from shock waves. The idea of swept back wings was tested on high speed supersonic planes.

The results were worse for airliners moving relatively slower than the speed of sound. Wings are also made thinner to reduce the air friction to achieve high speed. The results were opposite when the airliner moves relatively slowly (slower than the speed of sound).

Another technique called “swept forward wings “is used to make the flight of slow moving aircraft more stable. Straight wings are also adopted for small planes with short takeoff and landing (STOL), low speed, and for less fuel consumption. The fighter jet F-111 has variable sweep wings. They can move forward and backward at 16 degree and 70 degree respectively. Here the combination was achieved to allow the fighter to fly more stably from low speed to supersonic speed.

Mathematical explanation

We can make an illusion that airliner is moving slower than actually it is by bending its wing backward by an angle say “r”.The plane is moving with speed of velocity V. Velocity have two components.Vsinr that is along the wing and other is VCosr which is at 90 degree to wing and is flow velocity. This is clearly shown in the third figure below. It is obvious from fig (c) that VSinr which is along the wing has no effect on the plane and can be ignored. The horizontal component of velocity VCosr has the effects. But one should note that both the velocity components are smaller than V.

Summary and conclusion

Airliners with swept back wings have the following advantages.

  1. More lateral stability.
  2. Less turbulence when speed abruptly changes.
  3. Less air friction, as wings are designed thin and fine.
  4. Air velocity is split into two individual components. Velocity component that is along the wings has no effect on airliner. The 2nd velocity component that is perpendicular to wing has the effect, but it is less than actual speed of airliner i.e. V. So airliner can fly at much higher speed.