Getting Lift Straight

As a pilot, an aerospace engineer, and an aerospace educator, I’ve been wrestling for at least the last year with how lift and aerodynamics were being taught. I’ve written a few blogs about it and protested to AOPA, the FAA, and the Civil Air Patrol in an effort to broach the problem. I have also dived back into my college aerodynamics books and other aerodynamics texts I own in an effort to re-examine what I knew and to see where I thought the holes were in my own knowledge. The timing was good. As I was doing this, I stumbled on Doug McLean’s excellent book: “Understanding Aerodynamics: Arguing for the Real Physics”. It has filled in a lot of the holes my college education didn’t. I’m still digesting the book; you’ve got to be interested in the subject to hang with it and is best digested if you have some technical background. That said, I STRONGLY encourage the above named organizations to go through section 7.3.3, “A Basic Explanation of Lift on an Airfoil, Accessible to a Non Technical Audience”. In particular, AOPA needs to bounce it off their “Essential Aerodynamics” course and the FAA needs to do so against their “Pilots Handbook of Aerodynamic Knowledge”. (I intend to take care of my CAP cadets, since I’m in the organization.) They are not the only ones by any means; many instructor written textbooks need to do the same.

Doug talks about the wing creating a downturn in the flow and that the wing’s reaction is lift (Newton’s Third Law). Most sources are interpreting this as the wing creating a downward “jet” of air (in the same way that a rocket engine reacts to a gas jet) and then confuse downwash over a wing as the downward jet the wing is reacting to. But his next statement points to the real meaning: “The aspects of the interaction that need to be explained further have to do with how the moving fluid actually pushes back.” He goes into that in the next section ( entitled “Lift is Felt as a Pressure Difference on the Airfoil Surfaces”. In other words, while the downward turning air is extremely important to the overall physics of the flow and lift generation, the lift is felt as a pressure difference that acts on the wing. (This is what I’ve been trying to say when I’ve protested against folks saying the wing pushes down on the air; it actually does…but…it doesn’t somehow shoot a big jet of air toward the ground that pushes the airplane into the sky. There are lots of momentum changes going on in the flow and engineers can calculate lift by analyzing that, but the answer is the same.)

Thankfully, even before I got ahold of Doug’s book, my own revisiting of the subject had convinced me that part of the issue was that the subject needed to be presented as a wing/air system instead of as individual pieces with more emphasis on the critical factors (like angle of attack) and less emphasis on poking the mechanical bear. I do agree that the cause and effect relationships of the flow need to be touched on; this was something that is lacking from every text I have (as Doug mentioned, college level aero texts tend to emphasize the math and skip the physicality, which drove me bonkers but I shrugged my shoulders and went on.) Like Doug, I am a big believer that technical subjects need to be taught correctly, that dumbing them down too much is a mistake, and there is usually a way to get to the right explanation if you try. Many people don’t, assuming that it’s okay to take any liberty; but the problem is often that doing so is like having an accident: everything appears to be okay until somebody gets hurt. As I said earlier, if you’re looking for a good explanation of how lift works, grab section 7.3 of his book and be sure to read and understand it.

Thankfully, the explanation of “Lift” on Wikipedia has already been updated while leaning heavily on Doug’s work, so the next generation of learners will be getting the right stuff. I’ll be putting together my own little pitch on this I can use with my own students, even while I go work on my Advanced Ground Instructor rating knowing I have to buy the FAA’s wrong answers on the aerodynamics portion of the test. Hopefully, this mess will get straightened out in the next year or so. We’ll hope truth can overcome the Internet and the current trend to believe in what you want to, even if it’s wrong. Otherwise, we risk running afoul of the words of Richard Feynman: “For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.”