For a plane or bird to fly, its wings must produce enough lift to equal its weight. Most wings used in flight are a special shape – called aerofoils (or airfoils). This shape is needed to help generate lift.
The explanation for lift has been traditionally attributed to a Swiss mathematician named Daniel Bernoulli (pronounced Ber-noo-lee). However, recently, many scientists have debated whether the use of the Bernoulli principle to explain how wings work is, in fact, correct.
Many feel that using the Bernoulli principle, commonly taught in schools, is either incorrect or should not be used as a single explanation for lift. This is an interesting example of how science ideas are constantly being challenged. Many people now argue that angle of attack, based on Newton’s third law of motion, is a more effective explanation for lift.
It appears there are actually a number of explanations for lift that include the angle of attack and the Bernoulli principle and that these explanations work together to explain how lift is produced.
The angle of attack – Newton’s third law
Newton’s third law of motion states that, for every action, there is an equal and opposite reaction. Based on this law, wings are forced upwards because they are tilted, pushing air downwards so the wings get pushed upwards. This is the angle of attack or the angle at which the wing meets the airflow.
As air flows over the surface of a wing, it sticks slightly to the surface it is flowing past and follows the shape. If the wing is angled correctly, the air is deflected downwards.
The action of the wing on the air is to force the air downwards while the reaction is the air pushing the wing upwards. A wing’s trailing edge must be sharp, and it must be aimed diagonally downwards to create lift. Both the upper and lower surfaces of the wing act to deflect the air.
The amount of lift depends on the speed of the air around the wing and the density of the air. To produce more lift, the object must speed up and/or increase the angle of attack of the wing (by pushing the aircraft’s tail downwards).
Speeding up means the wings force more air downwards so lift is increased. Increasing the angle of attack means the air flowing over the top is turned downwards even more and the air meeting the lower surface is also deflected downwards more, increasing lift.
There is a limit to how large the angle of attack may be. If it is too great, the flow of air over the top of the wing will no longer be smooth and the lift suddenly decreases.
Birds and planes change their angle of attack as they slow to land. Their angle of attack is increased to ensure their lift continues to support their weight as they slow down. Wings and tails need to be movable so that their shapes can be changed to control their flight.
The Bernoulli principle
To understand this principle, we need to understand air pressure. Air is composed of several invisible gases that have mass. This mass is made up of molecules, moving in rapid random motion, and exerts a force called air pressure. We are unaware of this pressure because it is evenly pressing all around us. If the air pressure is not even, the greater pressure pushes an object in the direction of the weaker (or lower) pressure.
In 1738, Bernoulli found that, when a gas (like air) moves, it exerts less pressure. According to Bernoulli’s principle, the faster air moves, the less air pressure it exerts (this is not the same as the force exerted by a wind), because the molecules in the air become more spread out.
Normally, air moves along smoothly in streams, but airflow is disturbed when a wing moves through it, and the air divides and flows around the wing. The top surface of the wing is curved (aerofoil shape). The air moving across the top of the wing goes faster than the air travelling under the bottom. Because it’s moving faster, the air on top of the wing has less air pressure on the wing than the air below the wing. In other words, air below the wing pushes on the wing more than air above the wing.
This difference in pressure combines with the lift from the angle of attack to give even more lift.
It used to be claimed that the air travelling over the top of the wing took the same time to reach the back of the wing as the air travelling along the bottom. This has been shown to be incorrect, but it has been shown that the speed of the air over the top is faster than the speed of the air under the bottom.
The shape of the aerofoil is different for different aircraft. It is designed to give the best trade-off between lift and drag for each aircraft. On many aeroplanes, the bottom of the wing will curve downwards slightly instead of being flat. On other aircraft, such as gliders, it will curve upwards. On a stunt plane, which is just as likely to fly upside down as it is to fly the right way up, the curve on the bottom of the wing will be the same as it is on the top.
Nature of Science
Science ideas and concepts are constantly being challenged. Scientists are currently considering the Bernoulli principle concerning its explanation for flight. Many scientists now believe it is incorrect to use this principle as it has been used. However, part of it is compatible with Newton’s third law of motion as an explanation.