Aerobatic pilot flies upside-down. Notice the symmetrical airfoil as well! Credit: USAF.

How Planes Really Fly


Recently, Miles reported on the story of it being too hot for some planes to fly in Phoenix, where the daily highs were pushing 120°F.

The issue there was that the smaller regional jets were not rated to fly in such extremely high temperatures. It isn’t just a matter of quality control–it’s a matter of physics.

Hot air rises (think hot air balloon), and it rises because it hot air expands, making it less dense. The higher the temperature of the air, the “thinner” the air gets–compared to a 60°F day, the density of the air is almost 10% lower during a 120°F day.

The manufacturers hadn’t provided data on how these planes would fare in these higher temperatures. This doesn’t mean they wouldn’t be able to fly, but ground control decided not to take the risk. A totally understandable case of “better safe than sorry”.

But why would less dense air be an issue for planes? Why were there worries that the planes wouldn’t be able to fly?

That’s because an airplane’s wings need to interact with air to create lift, the force which propels a plane upward. The thinner the air, the harder it is for the wings to produce lift.

How does a wing generate lift? Well, you might remember this explanation from your high school physics class: the air is split by the wing and, because the wing is curved to be longer on top, the air above has to flow quicker around the top of the wing than the bottom to meet up with the air stream and keep going past the wing. The faster-flowing air on top produces a lower air pressure, which pulls the plane up and generates lift.

Incorrect theory of lift, debunked by NASA.
Incorrect theory of lift, debunked by NASA.

Too bad this explanation is bogus.

Almost everyone–physics teachers, fluid scientists, and even many pilots–took this explanation for granted up until recently. There should have been indications that this was incorrect, if we had all thought to ask some simple questions, questions such as: Why do planes with symmetrical wings fly? Why can planes fly upside down?

Aerobatic pilot flies upside-down. Notice the symmetrical airfoil as well! Credit: USAF.
Aerobatic pilot flies upside-down. Notice the symmetrical airfoil as well! Credit: USAF.

These questions were sorted out five years ago. University of Cambridge Professor Holger Babinsky decided to image how smoke moves around an airfoil in a wind tunnel to figure out exactly what’s happening.

He found that the air does split up as it hits the wing and does travel faster along the top of the wing, but it doesn’t meet up with its bottom half. And why would it? This was a weird assumption everyone was making that just had absolutely no basis.

Instead of meeting up after the wing and continuing straight past it, the split currents of air are both directed downward. This “turning” of the air is what provides lift for the aircraft.

This explains why planes can fly with symmetrical airfoils or upside down: it’s the angle of attack bending the air around the wing that produces the lift, not so much the lower pressure of the air above the wing.

It’s amazing how often the wrong “equal transit time” theory is still cited. For example, listen to what a University of Minnesota webpage says on the matter: “The airfoil shape produces unequal lengths across the top and bottom of the wing. Air splitting at the front of the wing must rejoin at the back of the wing so as not to create a vacuum.”

Again, there is absolutely no evidence for the split column of air having to meet up. If you’re looking for more on this, NASA has a good, thorough breakdown of the situation.

The correct explanation is still dependent on air density to get a plane flying, though, so it’s a good thing that the excessive heat in Phoenix grounded flights. And, as far as a science misconception goes, it’s better than, say, not believing in climate change.

Whether you choose to believe it or not, science is science, and rising temperatures will continue to ground planes–at which point we’ll have much bigger issues to worry about than how planes fly.

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