You probably think that a wing lifts an airplane because the airflow moving over the top has a longer distance to travel and “needs to go faster to have the same transit time as the air travelling along the lower, flat surface.” Well, you are wrong.
Explains University of Cambridge’s Professor Holger Babinsky:
I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.
He created the short video below to hopefully put this misconception to bed.
Professor Babinsky explains:
What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes—lower pressure on the upper surface and higher pressure on the lower surface. This is why a flat surface like a sail is able to cause lift—here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.
In fluid dynamics, air is actually considered to be, and typically modeled as, a fluid. When modeling fluid movement, a scientist named Bernoulli discovered that a faster moving fluid has exerts less pressure than a slower moving fluid. Everyone who has ever played with a garden hose has experienced this principle. If you put your thumb over the top of the hose, to get the same amount of water through the hose, the water has to travel much faster. Once you remove the obstruction, the water returns to its normal flow rate.
This is the principle acting on an airplane wing. The air (fluid) moving of the top of the wing encounters an obstacle that it must go around and therefore its speed increases and its pressure drops. The difference in pressure between the bottom and top of the wing results in more pressure at the bottom, thus pushing the wing upward into the sky. This is lift.