So we all know how planes fly, right? The top of the wing is rounded and the bottom of the wing is more straight. Air takes longer to travel over the top of the wing than the bottom, which results in more pressure on the bottom, hence the lift. Right?
As it turns out, no.
This is what I was taught, and it’s what I’ve always believed (it’s even in most lower-level text books), but it’s simply not true. This concept, called the Equal Transit Time Theory does not generate nearly enough lift to keep a plane in the air.
The main reason planes fly is far simpler: wings force air downward, which in turn force the wing (and therefore the plane) upward.
The primary actor at play here is called the Angle of Attack (AoA), and it’s easy to conceptualize. We’ve all put our flattened hand outside a car window while the car was in motion. You noticed that if you angled it straight on you could hold it steady, but if you angled the front edge upward you created massive lift that forced your arm up in the air.
The same concept works for kites, helicopter blades, sailboats, ceiling fans, and planes. In fact, winglike surfaces can generate lift almost regardless of the shape of their top and bottom surfaces. Notice that ceiling fan and helicopter blades are basically symmetrical, yet they create a downward columns of air just fine. You can actually use a barn door to generate lift…if you were so inclined.
In other words, lift simplifies nicely to Newton’s third law of equal and opposite reactions: air goes down, wing goes up.
Finally, if you’re still harboring any fond feelings for the equal transit time theory, ask yourself a simple question:
If the top vs. bottom wing shape is so important, how can planes fly upside down?
Alas, there are actually multiple ways of describing, and calculating with varying degrees of precision, the way in which lift is generated. There is the mathematical/engineering approach, and there is the physical approach, which is what I’ve described above. John D. Anderson said it best:
It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.
The one thing the experts agree on, however, is that the way most have been taught about how planes fly (the Equal Transit Theory) is absolutely incorrect. ::
[ There are, of course, other factors at play other than angle of attack, e.g. the Coandă Effect. The point of this piece is to illustrate that the primary factor with flight is the forcing of air downward (to which Coandă contributes) rather than equal-transit. ]
References and Notes
Keep in mind that the fact that Equal Transit Time theory is incorrect does not mean that the Bernoulli Principle is flawed. The Equal Transit Time Fallacy is based on a misunderstanding of the Bernoulli Principle, and does not diminish the principle itself in any way.
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