Incorrect diagram in grade K-6 textbooks. In the lefthand diagram above, the air approaches the wing horizontally and also leaves the wing horizontally. This is incorrect; it violates Newton's laws, since by F=ma there cannot be a lifting force unless air is accelerated downwards. The wing must deflect the horizontally-moving air downwards, as shown in the right-hand diagram. (Note that the right-hand diagram is actually a 2D slice of a 3D flow, not a purely 2D depiction.)
Actual windtunnel photographs show that wing surface-length is irrelevant.
Pulsed smoke streams illustrate that the air flowing above the wing is
greatly outracing the air flowing below. Parcels of
air which are
divided by the leading edge DO NOT recombine at the trailing edge.
Therefore the "equal transit time" explanation ("wing shape" explanation)
force falls apart.
(The above image was made by late Aerodynamicist Alexander Lippish, The
German Me-163 creator, at Collins Radio windtunnels, USA, in 1953. It was
published in his book. Alexander Lippish was an expert on smoke in
windtunnels and his smoketrails were world-famous.)
VIDEO: Windtunnel Smoke Pulses U. Cambridge Dept. of Eng. 2003
ANIMATION: Potential flow, NACA 12, NACA 4412
The above flim clip of a windtunnel experiment depicts a single "plane"
air as it approaches a thin airfoil and is sliced into upper and lower
portions. Note that this airfoil is NOT "curved above and flat below."
Instead the upper and lower surfaces are approximately equal in length.
Note that the air flowing above the wing quickly outraces the air flowing
below. The air flowing above and below the wing never rejoin again. The
real reason for the rapid flow of air above the wing is never explained in
textbooks using the "wing shape" explanation of lift.
(This image is made by Aerodynamist Martin-Ingelman Sundberg at KTH windtunnels in 1992. Sundberg, who first saw syncronized smokepulses when visiting a windtunnel maker in USA 1962, made this smokepulse video to show how ICAO pilot education was wrong in explaining winglift with "airflow longer path over wing".)
The confusing aspects of "airfoil shape" shown above
can totally obscure the true nature of aerodynamic lift.
Many authors point out that asymmetrical airfoils give positive lift even
if the angle of attack is zero. They offer this in order to prove that
"wing shape", and not "attack angle" should be the explanation of choice.
But there is a problem here. To determine if an airfoil is tilted, we
cannot rely on construction of the geometrical attack angle. Geometrical
attack angle is very sensitive to tiny bumps on the wing's leading edge,
since tiny bumps can change where we draw the main 'chord.' Yet tiny bumps
on the leading edge can have little effect on deflection of air, while the
tilting of the airfoil shown in the fourth section can have an enormous
effect upon the deflection of air and upon lifting force. "Kutta
Condition" shows that the angle of the trailing edge is critical to
production of lifting force. SMALL FEATURES ON THE LEADING EDGE CAN CAUSE
US TO TILT THE ENTIRE WING, WHILE WE DENY THAT WE HAVE DONE SO.
To determine the effective attack angle, we cannot trust the simple
geometrical rules. To determine whether an asymmetrical wing is REALLY
set to zero angle of attack, we instead must take seriously the concept of
"Kutta condition," and inspect the trailing edge of
the airfoil to see if it directs the air downwards. Or put simply: the
angle of the trailing edge IS the angle of attack, and
the angle made by the main chord of the airfoil has little effect on
the lifting force.
The above fluid simulation from Saab Aircraft shows
phase lag between upper and lower air parcels after an airfoil has passed.
Air travels much faster over the top of the airfoil, and then it never
rejoins the air which has travelled below. Note that the airfoil has
deflected the air downwards.
(This image is a Navier-Stokes 2D airflow calculation around a SAAB
340 wing made by Aerodynamist Krister Karling,SAAB Aerospace)
Note that the asymmetrical (cambered) wing at the top of the diagram has
been adjusted to produce zero lifting force. There is no "slip" or "phase
delay" between upper and lower airflows. In the middle and bottom
diagrams, the angle of attack is progressively increased, which also
creates an increasing lifting force. The increasing angle of attack also
increases the phase delay between upper and lower air flows.
So not only is the common "wing shape" or "path length" explanation wrong, but it even conceals some of the most interesting phenomena in airfoil physics: the fact that the time delay between upper and lower airflows is proportional to the attack angle and the lifting force!
Prof Klaus Weltner notes that in the famous publication in NACA report 116, Prandtl may have become the origin of the "Transit-Time Fallacy." Prandtl depicts a lack of circulation in a flow diagram producing significant lift. The roughly vertical lines in this diagram are incorrect, and only show the flow for a zero-lift condition. Yet the rest of the diagram depicts "Kutta Condition" and strong lift. Those vertical lines instead should be drawn to show an enormous offset or "phase delay:" where the upper parcels vastly outrace the lower parcels, and never meet up after passing the trailing edge of the airfoil.