NASA's "avkids" aerodynamics textbook online

[William Beaty <billb@ESKIMO.COM>]

I heard about a new NASA site, an online aerodynamics text aimed at K-8
grades, so I immediately took a look at the section on "lifting force."

Their explanation is better than many, but still has problems.  I sent the
following comments to buzz@cislunar.com. This is about the instructor's
chapter found at:

  http://wings.avkids.com/Book/Flight/instructor/forces-01.html
  http://wings.avkids.com/

The chapter has a long list of references.  What happens when many of the
references contain typical misunderstandings of airfoil function?  Meme
contagion!   Errors leap from old books to new.

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William J. Beaty                            SCIENCE HOBBYIST website
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> Diagram at http://wings.avkids.com/Book/Flight/Images/airfoil.gif

The diagram actually depicts a zero-lift condition; it DOES NOT show how a
wing creates lift.  (Or perhaps it depicts a wing flying in GEM or Ground
Effect Mode near the floor of a wind tunnel?)  Cambered wings cause the
upper air stream to flow faster, but cambered wings also deflect the
oncoming air downwards.  If the air remains flowing horizontally as shown
in this diagram, then the lifting force is zero and the airplane will
fall. See the first diagram on http://amasci.com/wing/airfoil.html


> If two particles were released from the same point at the same time, one
> on each streamline, they would start out moving together. As they
> approached the front of the airfoil, however, their velocity will start
> to change. Initially, each will start to move faster; their velocities
> will increase. As they turn back downward along the back half of the
> airfoil, they will slow back down to their initial freestream velocity.
> Due to the shape of the airfoil, the air moves faster over the top of
> the airfoil than it does on the lower surface. The faster air leads to a
> lower pressure (from Bernoulli's Law) on the upper surface and hence a
> net force is produced.

The above explanation is simply the classic "hurry over the hump"
misconception...  except the final part has been removed.  The missing
part always discusses the need for the two particles to rejoin at the
trailing edge of the wing.  Simply removing that incorrect phrase does not
remove the error from the rest of the explanation.  An entirely new
explanation is required.


Here's the best way to explain lift that I've yet encountered.  First take
a look at a flat (uncambered) wing which has positive angle of attack.
When air flows over this wing, the air above the wing flows faster, AND
THE AIR BELOW THE WING FLOWS CORRESPONDINGLY SLOWER.  For every gain in
extra MPH above the wing, there is some missing air-speed below the wing.
The difference in the two velocities is proportional to the angle of the
wing's trailing edge. Because the trailing edge deflects air downwards
MORE than the leading edge deflects air upwards, air below the wing gets
backed up and it flows slower, while the air above the wing must race to
fill the vacuum which otherwise would be formed. (This is the essence of
the Circulation Theory of lift explained in advanced aero textbooks.)


Now look at a **cambered** thin wing which has zero angle of attack. When
air flows over this wing, the air above the wing flows faster, just as in
the example of the uncambered wing above.  The air below the wing flows
slower.  And as before, the difference in the two velocities is
proportional to the angle, but this time it is proportional to the angle
that the trailing edge makes with the oncoming air.  We know that the
difference in air velocity occurs because of "Kutta's Condition," because
the rear half of a cambered wing acts as if it has a positive angle of
attack, and because the air leaves the wing at a downward angle from the
wing's trailing edge.  This causes the air below the wing to flow slow,
while the air above the wing flows fast (see discussions of Circulation
Theory in aero texts.)

In other words, both cambered and uncambered wings can only create a
pressure difference BECAUSE the trailing edge of the wing acts to deflect
the oncoming air downwards.  The shape of the wing does create the lifting
force... but only because the wing is shaped so the trailing edge forces
air to flow downwards.  Everything hinges on the angle of the trailing
edge of the wing.


A third situation exists (and one that creates lots of misunderstandings):

What happens with NON-THIN wings?  If a wing is thick, or in other words
if it has a bluff streamlined shape, then the air that's both above and
below the wing will speed up, and this speeding up of the air is added to
the velocities discussed in the two examples above, and it covers up
certain important features.  If the thick wing has some camber, then
something confusing occurs:  the air below the wing DOESN'T flow slower
(with a slowness corresponding to the speeding up of air above the wing.)
Instead the thick wing speeds up the air while the camber-effect slows it
down, and as a result the air below the wing barely changes speed at all.
And the air ABOVE the wing flows even faster than it did with a thin wing.
Extra speed has been added to the flows both above and below the wing.
The result: if you base explanations on thick wings, you will mask the
slowing of air below the wing, you will remove the whole reason that the
air above the wing flows faster, and you will spread confusion.  Instead,
base your explanations on thin wings (cambered thin wings or tilted flat
thin wings.)


> Most airfoils today have camber, or curved upper surfaces and flatter
> lower surfaces. These airfoils generate lift even when the flow is
> horizontal (flat).

Not true.  If the air which flows off the trailing edge of a cambered
airfoil is flowing horizontally, then that airfoil has zero "circulation",
and the air flowing above and below the airfoil has the same speed, and
the airfoil generates no lift. The lifting force is always proportional to
the angle the trailing edge of the wing makes with the oncoming air far
ahead of the wing.  That's Kutta's Condition, that's the central concept
in understanding lift.

You could instead say it this way:  "Since the back half of a cambered
airfoil is tilted downwards, cambered airfoils are still able to create
some lift even when the airfoil as a whole is not tilted at all."


> The Wright brothers used symmetric airfoils in their airplane design.

Wrong.  The Wright brothers used asymmetric (cambered) airfoils.  The
point about the Wright's wings is that their wings were ***THIN WINGS***,
and therefore their upper and lower surfaces had identical path lengths.
But why is this important?  Well, it shows that the curved shape of the
wing is critical, and shows that the difference in path length is not
important. (And why is the cambered shape important?  It's important
because cambered wings create upper/lower velocity difference via Kutta
condition, where Kutta condition refers to the angle of the trailing
edge.)


> In order to generate lift with a symmetric airfoil, the airfoil must be
> turned (tilted) with respect to the flow, so that the upper surface is
> "lengthened" and the lower surface is "shortened".

Not correct.  The above sentence still has some of the "path length"
misconception where the two particles of air supposedly race to meet each
other at the trailing edge.  Since the "path length" explanation is wrong,
there's no reason that "lengthened" or "shortened" surfaces should ever be
mentioned at all.  They don't explain anything except to people who are
still trapped within the "path difference" misconception.  Lifting force
is not created by lengthened or shortened surfaces, it is created by the
angle of the air flowing off the trailing edge.


> Airflow deflection is another way to explain.

No.  Airflow deflection is a critical part of any explanation since it
explains WHY the air above the wing flows faster.  Airflow deflection
explains why the shape of a wing is so important.  I think you meant to
say "Newton's Laws give us another way to explain."


> To understand the deflection of air by an airfoil let's apply Newton's
> Third Law of Motion. The airfoil deflects the air going over the upper
> surface downward as it leaves the trailing edge of the wing.

Yes, but the airfoil also equally deflects downwards any air going over
the lower surface. Note that this process can be almost completely hidden
if you use a thick wing as an example.  When using a thick wing, the upper
and lower airflows approach the trailing edge at two very different
angles, and both change their angle as they flow off the trailing edge.
This is confusing.  Better to use a thin wing where the angle of the
upper/lower airflows is the same as these flows move past the trailing
edge.



> According to Newton's Third Law, for every action there is an equal, but
> opposite reaction. Therefore, if the airfoil deflects the air down, the
> resulting opposite reaction is an upward push. Deflection is an
> important source of lift.

Deflection IS NOT an independent source of lift.  In fact, deflection
creates 100% of the lifting force.  Pressure difference also creates 100%
of the lifting force.  They are two different ways to explain lift; they
are not two different sources of lift.


> Planes with flat wings, rather than cambered,
> or curved wings must tilt their wings to get deflection.

...and planes with cambered wings have the deflection built right in!



> Another way to increase the lift on a wing is to extend the flaps. This
> again lengthens the upper surface and shortens the lower surface to
> generate more lift.

Not true.  The pathlengths of the wing surfaces do not figure into the
lifting force.  (Discussing pathlengths is an echo of the "hurry-over-the-
hump" misconception.  The pathlength explanation is wrong, and path
lengths should not be mentioned at all.)

Actually, by extending the flaps we increase the angle that the trailing
edge of the cambered airfoil makes with the oncoming air.  This causes the
airfoil to fling air downwards at an even greater angle, which increases
the circulation and increases the difference in upper/lower pressure, and
essentially makes a cambered wing act even more "tilted" even though the
wing as a whole might have zero angle of attack.  The increased angle of
trailing edge causes increased "Kutta" effect, which speeds up the upper
air and slows down the lower air, which creates a larger pressure
difference, which creates increased lifting force.