Wait, You Mean Bernoulli Had It All Wrong?
Instead of Christmas cards this year, I’m sending out a class-action apology to all those primary students I led so seriously astray with my misguided whiteboard talks. If you’re an instructor, you’ve surely done the same during the standard foreplay before committing lift.
You’ve sketched the wing section, drawn in the airflow above and below the cambered surface, name dropped Danny Bernoulli and, after batting away questions that might reveal your shallow grasp of L/D, you moved straight to what’s important: tower light gun signals.
But the lift explanation always includes—at least mine did—what Bernoulli actually discovered, which is that when a fluid—liquid or gas—accelerates, its pressure drops. This has remained a bullet point of accepted physics for almost 300 years now and since we’re living in the same universe, it shouldn’t have changed.
Well, not according to Professor Holger Babinsky at the august Cambridge University, who posted a YouTube video asserting that aeronautical engineers, aerodynamicists, designers and instructors have been getting it wrong—or at least explaining it wrong—for years. Actually, this isn’t a new presentation nor are the assertions found within exactly groundbreakers, either. But the video recently resurfaced on social media and I’ve been getting pummeled by it.
Says the accompanying article, even Albert Einstein reputedly screwed this up in believing that the air on top of the wing accelerates because it has farther to go than the air on the bottom of the wing. See the analysis in the video. The debunking part is that Babinsky claims we’ve been saying for years that the air accelerates because the top camber forces it to travel farther. We have? If I ever used this explanation, I don’t recall it and a cursory look through some of my aviation reference materials doesn’t flog this explanation, either. In his classic Stick and Rudder, Wolfgang Langewiesche boldly said forget Bernoulli entirely and that the wing keeps the airplane up by pushing the air down. That’s actually a Newtonian explanation and knocks a leg out from the straw man erected in the video because it doesn’t mention sentient molecules having a meeting at the leading edge and agreeing to reconvene at a specified time at the trailing edge. (But they gotta rush off; might hit traffic around those protruding rivets at two-thirds span.)
Since my Ph.D. is in light signals not aerodynamics, I referred the video to my friend David F. Rogers, who is an actual aerodynamicist and has taught same to would-be naval aviators at the Naval Academy for many years. Any validity to the video’s claims?
“This is basically nonsense,” he says of the video. “I’ll assume that you know what a stagnation point is and that the flow decelerates to zero at a stagnation point. Knowing that, look at the video again. Notice that there is a stagnation point on the bottom surface. So, the flow in the smoke line that passes around the bottom surface has to decelerate to zero and then accelerate again to some velocity. That takes time. Is there a stagnation point on the upper surface? No.,” he explains.
Look closely at the smoke lines and you’ll notice that they’re more tightly spaced on top of the wing than on the bottom, meaning they’re moving faster. We haven’t translated the worm hole yet, so as Bernoulli postulated, the pressure is lower on top of the wing and higher on the bottom. This has been proven empirically and why it is so is buttressed theoretically with something called the Kutta condition. It explains how the air cleaves at the leading edge, accelerating across the top surface. This is neither exotic nor difficult to understand, but read further here.
The straw man part of the video’s argument is the claim that so many assert that parcels of air on the upper and lower airflow must reach the trailing edge of the wing at the same time, having been cleaved at the leading edge. Supporting straws are that this entirely explains lift. But of course, it’s more complicated than that and if you’re a wiser man than me, you’ll say as much and move on to clicking on how shocked you’ll be at how Loni Anderson looks today.
But … no.
At this juncture, the discussion devolves to tastes better/less filling. “This is like the argument between the physicist and the aerodynamicist/engineer. The physicist approaches the problem from a fundamental change in momentum viewpoint,” says Rogers. That’s the Langewiesche argument that there’s a downward deflection of the airstream that creates a reaction force in the vertical direction that translates to lift and drag.
“The physicist has applied Newtonian physics in its purist form. S/he then claims that the force is notáa result of a pressure change, Ó la Bernoulli, and concludes that the aerodynamicist does not know what s/he is talking about. Of course, this is also nonsense. They are both coming to the same conclusion but using different approaches. Both are using the same Newtonian physics,” Rogers says.
So why get sucked into this click bait in the first place? Because in Don Quixote’s day, he had to ride three days just to find a lousy windmill to tilt. But on the internet, gimbal-mounted windmills come at you like fifth-grade dodgeball and after one has bounced off my forehead five times, it’s time to act. If I hear the music, I’m gonna dance.
I’m waiting on tenterhooks for Professor Babinsky to tackle ADS-B. Then we’ll really be having fun.