What is the name of the principle that explains why airplanes can fly?

What is Lift?

Lift is the force that holds an aircraft in the air. How is it generated?

It has been an extremely active debate among those who love flying and are involved in the field regarding which is best for describing how aircraft get the needed lift to fly? Two theories (Bernoulli’s equation or Newton’s laws and conservation of momentum) are the subject of an intense debate between scientists.

Who is correct?

The truth is that:

No One Can Explain Why Planes Stay in the Air

Let’s first learn a little about both theories and do some simple experiments to illustrate them.

These experiments will help you understand better these two theories. Each experiment is easy to be replicated at home and it is designed to use everyday items that can usually be found in the home.

Even if all these experiments are very simple and funny they will introduce many advanced concepts and notions from physics like fluids, pressure, atmosphere and energy conservation. Also, they will help you understand some “tricks” used in sports (baseball, ping-pong) as well as some of the boomerangs and kites “secrets”.

When a moving flow of gas is turned by a solid object, lift occurs. According to Newton’s Third Law of action and reaction, the flow is turned in one direction, and lift is generated in the opposite direction. Because air is a gas with free-moving molecules, any solid surface can deflect a flow. Both the top and bottom surfaces of an aircraft wing play a part to flow turning. Neglecting the role of the upper surface in turning the flow results in an incorrect lift theory.

Computer drawing of an airliner showing the lift vector.
aerodynamic lift theories; image via NASA

Aerodynamic lift is a very complex phenomenon but by the end of this section you will have a pretty good image about it.

Lift: Newton principle for kids

Newton Lift Theory: Airplanes fly because wings deflect air downward so that in reaction the plane is forced upward.

Sir Isaac Newton(1643-1727)

Sir Isaac Newton is better known for his discovery of the law of universal gravity (every object in this universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centers).Another law of motion discovered by Newton states that for every action, there is an equal and opposite reaction(Newton’s Third Law). As Newton’s laws suggests, the wing must change something of the air to get lift. To generate lift a wing must divert air down; lots of air. His theory is pretty and can be demonstrated easy. The kites are maybe the best example to demonstrate his theory in flight.

Newtons Third Law of Motion is pretty intuitive and easy to understand. Watch this short video produced by NASA:

Lift: Bernoulli principle for kids

Bernoulli Lift Theory: Airplanes fly because the pressure above the wing is smaller than the pressure below the wing.”

Daniel Bernoulli(1700-1782)

Daniel Bernoulli, an eighteenth-century Swiss scientist, discovered that as the velocity of a fluid increases, its pressure decreases. Bernoulli’s principle applies to any fluid, and since air is a fluid, it applies to air.

To understand how and why Bernoulli’s Principle works, we can consider the following experiment (M. Mitchel).

“Take a room full of children, and ask each child to start running at top speed. Children will start bouncing off each other, and the walls, with impressive collisions (ouch!). Now take those same children out of the room, and ask them to run down the hall at top speed. Now they are all running together, and all collisions between children are much more gentle than before since they are all running in the same direction.

The children in both cases represent the atoms in the fluid, and the force of the collisions represents the pressure between those atoms. In the first case, when the speed of the group as a whole was zero, the jostling (or pressure) was high.

In the second case, when the speed of the group as a whole was large, the jostling (or pressure) was low.

There are two types of pressure involved in these experiment: static pressure and ram pressure.

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The static pressure would correspond to the jostling felt by a teacher running along in the center of the children. That pressure would be slight. Ram pressure would correspond to the body blows experienced by an unsuspecting teacher standing in the path of the oncoming crowd (big ouch!).”

In the following video you will see a nice explanation of the Bernoulli principle:

The Bernoulli explanation of flight is not so intuitive as the Newton theory and one of the most difficult element to understand is how does the air on top of the wing know it has a “date” with the air under the wing?

It doesn’t “know,” but keep in mind that there is very fast moving wind behind it, urging it to keep moving! If the air on top of the wing moved slowly, there would be a huge buildup of air in front of the wing. If you put a rock in the bottom of a river, the river still flows at the same speed! But the water immediately above the rock has to speed up, lest it be “crushed” by the water behind it. This is why a great big wide river moves rather slowly, but as it passes through a narrow canyon it speeds up (rapids!).

Another common question regarding the lift explained by the Bernoulli theory is why does air follow the curve of the wing? This phenomenon was explained by a Romanian scientist, Henri M. Coanda. It essentially states that “a fluid stream which comes in contact with a gently curved solid surface will tend to follow that surface”. This can be easily observed by placing a spoon (or other curved surface) under a gently flowing faucet, and noticing how the water bends around the spoon.

Lift Experiments

Here are some simple experiments to demonstrate Newton and Bernoulli Lift:

Newton Lift Experiments

Next time when you will ride a car on a highway pull your hand out of the window (watch for the traffic!). You will notice that if you will tilt your hand upward, your hand will get pushed up. That is Reaction lift, and it is exactly the same as this second source of aerodynamic lift for aircraft. For this effect to exist, the wing or your hand must be tilted upward, at an angle that is called the “angle of attack”.

What is LiftBecause of the high speed of the car or the airplane, a lot of air constantly hits the bottom surface of the wing or the palm (bottom) of your hand. After that air has hit that angled surface, it gets deflected downward, pretty much at the angle of the angle of attack. Therefore, the air now has a new VERTICAL movement downward due to the collision, which occurs due to a downward FORCE being applied to it. Newton said that there must therefore be an equal and opposite UPWARD force on the wing / hand. The same forces occur when you fly a kite.

When the kite’s angle of attack is zero, it will fall. Like an airplane, a kite is heavier than air and relies on the motion of the wind past the kite to generate the aerodynamic lift necessary to overcome the weight of the kite. The movement of the air past the kite also generates aerodynamic drag that is overcome by constraining the kite with a control line. The interaction of these forces determines the overall performance that varies with the design of the kite.

Bernoulli Lift Experiments

Experiment 1:

Take two pieces of paper, one in each hand; hold them close to your face and blow between them. You will notice that the pieces of paper will get closer one to each other, contrary to your expectations. Next, take the end of one of the pieces of paper in your hand so that the length of the paper falls over your hand. Now blow over the sheet of paper. It will rise! Can you explain why?

Experiment 2:

I this experiment we will use water instead of air. Both of them can be treated as fluids.

A mass of water flowing uniformly from a faucet shows neckdown; it is wide and thick at the top, and it tapers to become narrow and thin at the bottom. As the water falls from the faucet it accelerates, and as the velocity of the fluid flow increases the area of the fluid flow decreases; the stronger atmospheric pressure overwhelms the weaker static pressure in the quickly flowing water and compresses the water stream.

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Note: Think about other experiments that can use water instead of air to demonstrate the lift force.

Experiment 3.

When a boomerang is thrown, it is held nearly vertically. The cross-sectional shape is asymmetric, that of an airfoil. As it is thrown, it spins and creates a “Bernoulli Lift” which acted toward the left and makes the boomerang to fly in a circle, back to you.

Note: The boomerang is not actually held exactly vertical when throwing, but slightly tilted to the right. The rotational spin therefore creates the Bernoulli force vector that is slightly upward of being straight horizontal to the left. This small vertical component of the force vector overcomes the vertical weight vector of the boomerang, which keeps it from crashing down. Eventually, as aerodynamic drag slows down the boomerang’s spin, the Bernoulli force vector also reduces. Once the vertical component of it drops to less than the weight of the boomerang, it falls and crashes.

From these experiments we can see that both “Bernoulli” and “Newton” are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object.

Newton and Bernoulli do not contradict each other. Explanations which are based on Newton’s and on Bernoulli’s principles are completely compatible. Air-deflection and Newton’s Laws explain 100% of the lifting force. Air velocity and Bernoulli’s equation also explains 100% of the lift

You will understand this better if you will consider the spectacular “Up Side Down” flight of an airplane.

People who understand the logic behind Bernoulli lift immediately realize that an upside down wing cannot really produce any Bernoulli lift. They are correct! Watch carefully the next time you see such an upside down aircraft flying. They must depend entirely on Reaction Lift, and therefore they must keep the nose of the airplane noticeably higher than usual, to get the greater angle-of-attack they need. Their situation is actually rather dangerous, because of the natural instability of relying entirely on Reaction Lift.

This sort of demonstration confirms everything we have described here. If ONLY Bernoulli Lift existed, no upside down flight would be possible. If ONLY Reaction Lift existed, then an aircraft could use the same angle-of-attack either shiny side up or upside down. The fact that maybe 1/3 greater angle-of-attack is necessary suggests that around 1/3 of the normal lift is probably provided by Bernoulli Lift (for that speed and altitude) while the other 2/3 is normally provided by Reaction Lift.

Now you can better understand why the takeoff and landing are the most potentially dangerous parts of a flight. The relatively slow speeds involved in both takeoffs and landings mean that very little Bernoulli Lift then occurs, and therefore you have situations that are nearly completely Reaction Lift.

The pure Reaction Lift flight is extremely unstable (and therefore potentially dangerous). The fact that Reaction Lift is also far less efficient regarding usage of power (due to truly massive turbulence created), the engines must be operated at very great power, but of course part of that is necessary anyway to accelerate such a large and heavy object up to flying speed! Once an airliner gets up near cruising speed, there is enough Bernoulli lift to provide a good deal of stability, as well as then using up far less fuel with the engines running far easier.

Because modern airliners are operated by companies that intend to make money, they try to have the heaviest payload that is safe. This is why purely Bernoulli Lift aircraft are commercially impractical. It has been found by practice that a combination of Bernoulli Lift and Reaction Lift, where the Reaction Lift predominates, especially at low speeds, represents the most cost-effective and safe choice.

In a sense, Bernoulli Lift might be thought of as representing stability and consistency, while Reaction Lift might be thought of as more brute force lift that is less easily managed.

New Theories

Computational fluid dynamics (CFD) simulations and the so-called Navier-Stokes equations, which take into account the actual viscosity of real air, are the domain of modern scientific approaches to aircraft design.

The solutions to those equations, as well as the output of the CFD simulations, produce pressure-distribution predictions, airflow patterns, and quantitative results that serve as the foundation for today’s cutting-edge aircraft designs. Nonetheless, they do not provide a physical, qualitative explanation of lift on their own.

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