Shooting an Aerodynamic Elephant

Since the reason I spend time blogging is an effort to “give back” by passing on my experiences and knowledge, I’ve focused some articles on the current turmoil in aerospace education concerning the generation of lift. The misinformation and misconceptions are quite widespread; they are now affecting most information sources pilots trust to be correct and that we are tested on. (Yes, I am now speaking to you, FAA, which all the pilot training organizations like AOPA, Gleim, and instructor published pilot books march in locked step with.) Worse, this misinformation is being passed on as fact by educational organizations entrusted with training youth. The bottom line is that we will likely suffer for a decade or more with people not really understanding how a wing works and having completely wrong ideas about it; hopefully, it will result in only having folks busting test scores and looking ignorant and not in an actual accident somewhere. The problem is that is not guaranteed and people usually find a way to make the most improbable things happen. Which is what proper education is supposed to guard against.

Wanting to help tackle the problem, I knew I needed to make sure that what I was writing, thinking, and teaching was correct; so, I began going back to my various aerodynamic texts and refreshing myself on the generation of lift. Some of those are texts I used to get my pilots’ ratings, some of those are texts still tagging along with me after my aerospace engineering degree (“Foundations of Aerodynamics”, Kuethe and Schetzer; “Airplane Aerodynamics”, Dommasch, Sherby, and Connolly; “Theory of Wing Sections”, Abbott and Doenhoff), and others were texts I had come to respect as good references (i.e., “Aerodynamics for Naval Aviators”). As I looked at what the controversies were and how they were spreading, I started re-examining what I knew and looking for the truth. As I integrated all I knew and went through the various arguments, I began realizing there was more to the picture than what my engineering education had taught me, though none of it was wrong. (I couldn’t say the same for the information I was seeing primarily on the Internet and that was creeping into pilot’s educational books, probably because of both an incomplete understanding by their authors and their reliance on Internet sources, which are sometimes difficult if not impossible to independently verify…not that anyone appeared to be going to the trouble.) I was specifically focusing on the shortcomings of the Bernoulli explanation (though I knew its basic heart of using pressure distributions to explain lift was correct) and how Newton’s Third Law was being misapplied..and, later, how the Coanda effect was being incorrectly drawn into the explanation of basic lift. (We NEVER discussed Coanda during any engineering class I took and my literature search only turned it up in a NASA paper on high-lift devices…for very good reasons!) I became convinced that a lot of the problem was centered around not considering the wing and air as a “system” and that the application of Newton’s Third Law really had more to do with interactions in the pressure field than it did with any kind of direct mechanism where “the wing pushes down on the air” (a.k.a AOPA).

Then, along came Doug McLean tackling the problem with his book: “Understanding Aerodynamics: Arguing from the Real Physics” (Wiley).

I can’t say enough good things about this book. It has become my main resource for enhancing my own understanding of this subject AND tackling the issue on the aerospace education front. It is a very technical read; but for most folks interested in understanding what’s really going on, if you do nothing but read Chapter 7 (“Lift and Airfoils in 2D Subsonic Speeds”), then you can come to understand where the “truth” lies.

I expect to be working on an explanation of the generation of lift that relies heavily on what he had to say; but all I want to address today is this. I strongly disagree with him on one thing: he believes it’s easier for most folks to understand lift using Newton’s Third Law. Most folks experience Newton’s Third Law lies only with some kind of reaction engine (i.e., jet and rocket engines), and so they erroneously think that the wing lift occurs in exactly the same way (leading to little jets of air pushing out of the wing). But this is NOT where Newton’s Third Law comes into play with a fixed wing. This sentence from McLean’s book captures it well: quote: “The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences”. This pressure difference is how the fluid pushes back on the wing; in other words, lift is a reaction to the downward turning of the flow that with other mechanisms generate the pressure forces that lift the wing. Lift is NOT just a reaction to downwash or any direct jet of air shot off the wing by air deflection.

BANG! I just shot one “aerodynamic elephant”. It’s not the only one alive, but it’s the only one I’m going to spend a bullet on right now. I just don’t want to hunt all afternoon. (You can wait for more from me but, frankly, buy McLean’s book and you can read along as he beats me to it and shoots them all.)