There are many theories of how lift is generated.
Unfortunately, many of the theories found in encyclopedias, on web
sites, and even in some textbooks are incorrect, causing unnecessary
confusion for students.
The theory described on this slide is often seen on web sites and
in popular literature. The theory is based on the idea that lift is
the reaction force to air molecules
striking the bottom of the airfoil as it moves through the air.
Because this is similar to the way in which a flat rock thrown at
a shallow angle skips across a body of water,
it is called the "Skipping Stone" theory of lift.
It is sometimes called a Newtonian theory of lift, since it involves
Newton's third law, but to avoid confusion with the correct Newtonian
theory
of flow turning, we shall call it the "Skipping Stone" theory.
Before considering what is wrong with this theory, let's investigate
the actual flow around an airfoil by doing an experiment
using a Java simulator which is solving the correct
flow equations. Below the simulator
is a text box with instructions. Be sure that the slider on the right
of the text box is pulled to the top to begin the experiments
Let's use the information we've just learned to evaluate the
"Skipping Stone" Theory.
This theory is concerned with only the interaction of the
lower surface of the moving object and the air. It assumes that all of the
flow turning (and therefore all the lift) is produced by the lower surface.
But as we have seen in our experiment,
the upper surface also turns the flow.
In fact, when one considers the
downwash
produced by a lifting airfoil,
the upper surface contributes more flow turning than the lower surface.
This theory does not predict or explain this effect.
Because this theory neglects the action <--> reaction of
molecules striking the upper surface, it does not predict the negative
lift present in our experiment when the angle of attack is negative.
On the top of the airfoil, no vacuum exists.
Molecules are still in constant random motion on
the upper surface (as well as the lower surface), and these
molecules strike the surface and impart momentum to the airfoil as
well.
The upper airfoil surface doesn't enter into the
theory at all. So using this theory, we would expect two airfoils
with the same lower surface but very different upper surfaces to
give the same lift. We know this doesn't occur in reality. In
fact, there are devices on many airliners called spoilers
which are small plates on the upper surface, between the leading
and trailing edges. They are used to change the lift of the wing
to maneuver the aircraft by disrupting the flow over the upper
surface. This theory does not predict or explain this effect.
If we make lift predictions based on this theory, using a knowledge
of air density and the number of molecules in a given volume of
air, the predictions are totally inaccurate when compared to
actual measurements. The chief problem with the theory is that it
neglects the physical properties of the fluid. Lift is created by
turning a moving fluid, and all parts of
the solid object can deflect the fluid.
You can download your own copy of the program to run off-line by clicking on this button:
BUT..... this theory is not totally inaccurate. In certain flight
regimes, where the
velocity
is very high and the
density
is very low,
few molecules can strike the upper airfoil surface and the Newtonian
theory gives very accurate predictions. These are the conditions which
occur on the Space Shuttle during the early phases of its re-entry
into the Earth's
atmosphere
at altitudes above about 50 miles and at
velocities above 10,000 mph
(hypersonic conditions).
For these flight
conditions, the theory gives a good prediction. However, for most normal
flight conditions, like those on an airliner (35,000 feet, 500 mph), this
theory does not give the right answer.
