Stick and Rudder is a book describing the art of flying, published in 1944. I have no intention of learning to fly, but was once again drawn to the book because of it’s interesting explanations of what keeps an airplane in the sky. The author’s opinion is that “angle of attack” is the key element in producing lift on a wing. Basically this means that air hitting the bottom surface of a wing “pushes” it up. This is in direct contradiction to the aeronautical engineers story that high wind speed above the wing creates a vacuum that “pulls” the wing up. This is supposedly caused by Bernoulli’s principle which states that as flow velocity increases in a fluid, the pressure drops. This applies on a wing where the air flow over the top is faster than on the bottom, resulting in a lower pressure on top, “pulling” the wing up.

The problem is that there is nothing to pull – things to not move toward regions of lower pressure because they are “pulled”, they move because they “pushed” by the region of higher pressure. The universe does no “suck” but it does “push.” The logic behind this is related to the fact that particles in a fluid generally do not have any connection between themselves, therefore they can’t pull on each other. The don’t act like a chain where all of the links are attached to each other, allowing the chain to be pulled. It is more like a line of individual blocks where if you pull the first one you get the first one, not all of them. However, if you push on the end one, the entire line of blocks moves.

It turns out that the forces created by the angle of attack are exactly the same magnitude and direction as calculated by using Burnoulli’s principle, so in fact the aeronautical engineers are not “wrong” but perhaps they aren’t “right” either. The difference is not about how you design a wing or airplane, but it does make a difference in how you fly it. Not an actual difference, but a vastly different “mental model” of what is going on. It is my contention that an important element in operating high speed machinery safely is having a correct mental model of what is happening so that when things are not happening as expected there is an enhanced opportunity to quickly, and accurately, take unplanned steps to get out of trouble (or better yet, avoid getting into trouble). Perhaps there is something about these two models that make one superior to the other in the mind of the pilot. A recent problem with the Boeing 737 Max crashing during takeoff was in a large measure due to unexpected behavior of the flight control system, and the pilot’s mental model not matching what was actually happening. The crashes almost certainly could have been avoided in the pilots had a better/clearer understanding of what was happening to the lift on the wings given the flight conditions.

Besides the idea of lift being created by increased pressure created by the angle of attack of the wings, I came upon a couple of other interesting ideas that were new to me until I thought them through. One idea that the author presented is that the thing that causes an airplane to go up or down is not the “elevators”, but rather the throttle. If you speed up, the plane will go up, if you slow down, it will go down. If you stay the same, it will stay the same. Somehow I had the idea that pulling the “stick” back, thus pointing the nose higher into the sky, caused the plane to go up. Actually, it causes the lift to decrease because the increased angle of attack slows the plane down, so the lift is less and the plane goes down (depending upon various characteristics of the plane). The thing that adjusts the speed of the plane is the stick, but adjusting the angle of attack and hence the “friction” on the wings. Pull back on the stick and you slow down, push forward and you go faster. Increase throttle and you go up, decrease it and you go down. The ailerons can, and usually do, similarly unexpected things. Tip the plane to the left as if you want to turn to the left and it is likely to turn to the right (while tipped to the left). Spins, stalls, and all sorts of other highly undesirable outcomes can easily result from the disconnect between the pilot’s expectations based upon their mental model of what is happening, and what is actually happening.

In any case, I found the book to be an interesting study of what I thought I knew about flying and airplanes and what is actually going on with them. It got me almost, but not quite, interested enough to want to go flying in a small plane to see how it works “in the air.” All of this is very interesting, and important, from the perspective of my profession as a System Safety Engineer. It is often critically important that there is an alignment (consilience?) between what is happening within the mental model developed in the mind of a machine operator and what is actually happening with the machine. Even small differences can quickly lead to disaster.