How Planes Don't Fly
a standard explanation of lift
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.
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.
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.
standard (wrong) explanation
following paragraph gives the standard, but wrong, explanation of lift. See if
you can detect the problem with it.
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.
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.
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.
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.
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.
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.
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!
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.
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.
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.
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
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.
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.
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.
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.
way, the lower pressure above the wing causes a net upward lift force.
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.
Physical Description of Flight
An excellent article that dispels many of the myths and misunderstandings
associated with applying Bernoulli's law to plane wings.
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.
Britannica on Bernoulli's Law
A discussion of Bernoulli's Law including derivations and a good way to think
about the Law.
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.