By Don Lincoln Ph.D., University of Notre Dame
The most tangible example to explain how plane wings work is when one extends an arm outside a speeding car’s window. Holding the arm straight, one can feel how the movement force changes as the direction of the arm does. Tilting the hand upward will lift the whole arm. Plane wings work the same way, but what exactly is the reason they make the plane fly?
The most common way to explain how plane wings work is to use Bernoulli’s equation, which focuses on fluid pressure and speed. However, it is a wrong assumption. Mathematically, the lifting force generated by the difference of pressure above and below the wing is significantly less than the opposite force from the plane’s weight. So how do plane wings actually work?
Air Circulation
Unlike what was commonly believed, the air passing the top of the wing does not meet the air from the bottom at the wing’s end. The movements create small vortexes of air that can even be seen by window-seat passengers if there is enough humidity in the air. The air flows towards the tail of the plane on the top of the wing and towards the nose of the plane on the bottom of the wing, hence, constantly circulating around the plane.
The lower pressure above the wing causes this circulation, and it will be counter-clockwise if the wing moves towards the left and air flows over the wing towards the right. To balance the rotation, a counter-rotation of the airflow is set up over the wing, where the airflow circulates in a clockwise manner. This creates pressure differences from the top and the bottom of the wing. This pressure is affected by other factors, as well.
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Factors Affecting Pressure
The shape of the wing, the angle at which the wing is tilted from horizontal, the speed of the air flowing over the wing, and the viscosity of the air are factors that can lower the pressure around wings. The same thing will happen if the movement happens in any other fluid, gas, or liquid. Thus, the viscosity should not be taken for granted, especially that it changes with altitude. Putting all these together will show how plane wings work and make them fly through one of the basic laws of physics.
This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on Wondrium.
Newton’s Third Law of Motion
The plane flies because the wings push the air down, and the air, according to Newton’s third law of motion, pushes it back in the opposite direction. As mentioned in the introduction of the article, the most familiar example is how the arm stretched out of the car’s window moves. If the fingers are held straight next to each other, tilting the hand will change the whole angle of the movement and lift of push down the arm.
In the case of the plane, the air pushes down the plane, and the plane pushes it back up with the same force. The same thing happens at the bottom of the wing: as it pushes the air down, the air pushes it in the opposite direction, i.e., up. But this is not all.
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What Else Creates the Lift?
With an angle of 10o for a plane wing, the bottom of the wing only generates about a quarter or so of the force needed to make the plane fly. The lower pressure above the wing goes beyond what Bernoulli explains: air deflection by the wing. The difference of pressure combined with air circulation is beyond what Bernoulli predicts and can keep the wing up in the air. Why does it fail sometimes?
How Does a Plane Stall?
A plane stalls when it loses lift and begins to fall. It happens when airspeed is too low or if the wing has too big of an angle measured from the horizontal. In this case, the airflow over the top of the wing stops and the pressure below the wing turns into the normal air pressure, killing the force to hold the plane up in the air. To save the plane, the nose should be pointed downwards to create enough speed and re-establish the low pressure on the top.
In conclusion, wings work with the speed, pressure, and the generated air circulation based on the Bernoulli principle, but not as Bernoulli expects.
Learn more about From the Sky Down: How Falling Works.
Common Questions about How Plane Wings Work
How planes fly should be explained through how plane wings work. They are the leading element in flying, used for lifting, turning, landing, and controlling the airplane. The pressure, speed, and air circulation formed around them combine to make flying a huge metal object possible.
Not all the wings are curved, but the common belief is that they are curved to create pressure difference and, consequently, the lift force. But the reality is that the curve does not create enough lift force against the plane’s weight. However, it creates enough pressure difference to create air circulation and other elements that combine and make plane wings work.
Only if the wing is stretched from one side of the plane to the other, in a way that the plane is located in the middle, it can fly with one wing. This means, in the current technology and how plane wings work, it is impossible to fly with only one wing at one side. Losing one wing while flying, is a disaster.
Many low-speed airplanes have straight wings, which extend from the body of the plane at right angles. These wings are efficient and can provide good lift at low speeds, but not at high speeds. Even though how plane wings work decides how the plane flies, many small jet-like planes use their engine power to do what wings alone cannot offer.
