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 How Planes Don't Fly Debunking a standard explanation of lift

You might think that physicists and engineers understand the basic principles of flight extremely well. However, debate still rages among many scientists over the fundamental concept of what gives lift to a wing.

How could there possibly be confusion over such a fundamental issue? Some physics educators think that the problem is essentially that although physics courses are quite effective at teaching students to use a mathematical representation of the world, conceptual learning is often neglected.

This seems to be the case for the problem of flight. A lot of people reading this are probably already thinking that the problem is trivial - Bernoulli's Law, known for almost 300 years, answers the question of what makes a wing feel lift. However, the answer is not quite that simple, or, looking at it another way, the answer is even more simple.

The standard (wrong) explanation

The following paragraph gives the standard, but wrong, explanation of lift. See if you can detect the problem with it.

An aerofoil tends to be reasonably flat underneath and curved on top. Air flowing over the top of the wing therefore has to travel a greater distance than air going under the wing. This means the air flowing over the top of the wing is moving faster. By Bernoulli's Law, this means the pressure above the wing is lower than the pressure below the wing. The pressure difference results in an upward force, providing the lift required.

Have you noticed the built-in assumption in that explanation? It assumes that the air flowing above and below the wing should arrive at the back of the wing at the same time. This is often called the "principle of equal transit times". However, it is wrong. Air travelling over the top of the wing actually arrives at the back of the wing sooner than air passing below.

This is not to say that Bernoulli's Law is wrong. It can still be used to explain flight, however, it is often misused. Before getting back to that point, let's see how lift can be explained without resorting to Bernoulli's Law.

Lift without Bernoulli

One of the simplest (but often misused) laws of mechanics is Newton's third law - that every action has an equal and opposite reaction. This is enough to give us most of the explanation we need for lift.

Lift can be considered simply as a reaction force. A reaction to what? A reaction to the force exerted by a wing on air, forcing it downwards. A wing will fly with some "angle of attack". That is, the wing is tilted to the oncoming air stream. The result is that air is deflected downward.

It is the downward deflection of air that creates the reaction force of the air back on the wing, and pushing upward. Pilots all know that more lift can be created (up to a point) by increasing the angle of attack on a wing. It is fundamentally why pointing a plane upward will tend to move it upward.

If you don't care about the exact details of pressure and flow lines of air over a wing, our explanation is done. A wing forces air down so the air forces the wing up. We have lift!

The Coanda effect

If we probe more deeply into this question, we find that the Newton's third law explanation has some built in assumptions that need justifying. One of these is that the air actually is deflected downward. Experimentally, this has been shown to be the case and there is a good theoretical explanation. It goes by the name "The Coanda effect" but is not as mysterious as it sounds.

The Coanda effect is simply the tendency for a fluid to stick to a surface over which it is flowing. This includes when the surface bends. You have seen this before when you have put the spoon under a running tap. The water follows the back of the spoon and can be deflected quite a lot.

One way to explain the Coanda effect is to realise that there is a skin friction effect between the surface of an object and the fluid moving past it. This tends to slow the surface layer down. The effect of this slowing is like brakes being applied to the wheels on just one side of a car, or those wheels driving in mud. The greater resistance tends to pull the car toward the side of the resistance. In a fluid, this pulling to a side keeps the fluid flowing along the surface of an object, even when it bends.

Seeing as wings fly at some angle of attack, the air follows the path of the wing and is deflected downward. At this point, Newton's third law can take over. Or, if you still want to know what Bernoulli says about all this, you can think in those terms.

Back to Bernoulli

The basic principle behind using Bernoulli's Law in this problem (both in right and wrong explanations) is that there should be a lower pressure created above the wing. This does indeed happen but we need to be careful in understanding why that is the case.

Seeing as the air well above and below a wing is essentially unaffected by the wing, it flows in a straight horizontal path, from the wing's point of view. All of the action happens in a channel centred on the wing.

One way to think about the flow is to consider the wing to be like a blockage in the channel of flowing air. The blockage, and especially, the upper, highly curved surface at the front of the wing, constricts the air, forcing it to flow faster and, hence, at a lower pressure.

Alternatively, you can think of the air being forced past the front of the wing having to expand to fill the "shadow" region behind the tilted wing. This expansion of the air flow results in a lower pressure.

Either way, the lower pressure above the wing causes a net upward lift force.

The more detail in which you wish to examine this situation, the more complicated it gets. There are even a few more levels of detail that we could go into to describe what is happening. However, we'll leave the discussion here for now but if you want to read more about the topic in greater detail, have a look at the following web pages.

Other resources

A Physical Description of Flight
An excellent article that dispels many of the myths and misunderstandings associated with applying Bernoulli's law to plane wings.

Misinterpretations of Bernoulli's Law
The way Bernoulli's Law is often taught and explained in textbooks is misleading and sometimes incorrect. This article goes into detail about the problem and explains some alternative ways of thinking about Bernoulli's Law.

Encyclopaedia Britannica on Bernoulli's Law
A discussion of Bernoulli's Law including derivations and a good way to think about the Law.

How Airplanes Really Fly
An essay that debunks the myth that lift on an airplane wing is due to a longer travel path over the wing than under the wing.

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