Bill Crawford’s Flightlab Blog

One of the peculiarities of flight training is that it’s mostly procedural. You learn to do lists of things. That’s a simplification, of course, but not so far off. One obvious fact about procedures is that they’re experienced in sequence. The stages of flight are managed a task at a time. An instructor knows a student is catching on when procedures happen without prompting, and begin to be accomplished with smoothness and authority—as if the tasks weren’t separate at all, but part of a confident flow.

But a procedural approach is never completely revelatory, because it teaches mostly pilot behavior, while de-emphasizing any observation of aircraft behavior. The trouble is that knowing how to control stuff doesn’t automatically translate into understanding why stuff happens. (This was impressed upon me earlier this year, when I asked a prospective unusual-attitude student to explain stalls and he began telling me about clearing turns.) Aircraft behave the way they do because of any number of things going on concurrently, often without the pilot being explicitly aware. Despite the emphasis on procedures—on feeding the aircraft one proper input after another—aircraft aren’t mere procedural creatures. They’re also creatures of simultaneity and compromise; with various opposing forces and moments all happening at once. The aircraft is always busy arbitrating, balancing things out.

Unfortunately, even a basic understanding of aircraft behavior faces a road block right from the start, courtesy of the bungled theory of lift that clueless instructors usually slap you with first. The theory of lift is just a horror show. I’ve been reading aerodynamics for years, and I still can’t connect the dots on that one. But over those years I’ve realized that you don’t need to command a complete theory of lift to understand why things happen in aircraft—or to anticipate what’s about to happen if you don’t get your act together. For that, it seems to me, you basically need two areas of knowledge. Both subjects can get complicated when addressed in detail, but they’re within any pilot’s ability to understand.

First, you have to understand the nature of pressure patterns over the surface of a wing: specifically, how those patterns change and migrate in response to angle of attack and in response to the wing’s own motions. Fundamentally, the distribution of pressure determines aircraft control. It’s differences in pressure that make control possible.

Second, you have to understand how an aircraft responds to displacement of its velocity vector. The velocity vector is simply a conceptual arrow sticking out of the aircraft’s center of gravity and pointing in the direction in which the aircraft is actually moving (but not necessary aimed). Usually, the velocity vector lies on what’s called the aircraft plane of symmetry. That’s a vertical plane intersecting the aircraft’s longitudinal (roll) axis and its vertical (yaw) axis. Typically, aircraft are symmetrical on either side of the plane. I’ve started referring to the plane of symmetry as the plane of coordination, because in coordinated flight the velocity vector lies on it. In uncoordinated flight, the velocity vector points to one side or the other—for example, toward the center of a turn if the aircraft is slipping, and away from the turn if the aircraft is skidding. There are a couple of instances in which it behaves differently, but in general the ball in the turn coordinator points in the lateral direction of the velocity vector.

The important feature of the velocity vector is that it describes both an aircraft’s angle of attack (the vector’s vertical angle) and its sideslip angle (the vector’s horizontal angle). This feature is easier to understand from drawings. I would have include some with this column if I didn’t want you to visit my website, www.flightlab.net. Notice you’re being cleverly manipulated and are losing control: Go to website, cupcakes, go to website, ice cream, go to website, donuts….go to www.flightlab.net, click on Course Notes, click on Axes and Derivatives.

The following are why the velocity vector is a central character in the description (or prediction) of aircraft behavior. Notice the repetition of the word “stable.” Unstable aircraft don’t always behave as described—that’s why we call them unstable. (By the way, just so you’re in an appreciative state of mind, the following paragraph is the most complete description of aircraft behavior, in the least number of words, ever written!)

(1) A directionally stable aircraft yaws the plane of coordination into alignment with the velocity vector if the two become displaced. (2) A laterally stable aircraft rolls away from the velocity vector if the vector becomes displaced from the plane of coordination. (3) A longitudinally stable aircraft maintains the velocity vector at a constant, trimmed angle of attack unless the pilot commands otherwise.

Really understanding the above takes time—there’s a lot of unpacking to do. To my mind it’s time well spent. Pressure patterns and the velocity vector are the keys to the kingdom. They’re especially central to the academic side of unusual attitude training, since it’s usually easier to save yourself from something weird if you have the frames of reference needed to understand what’s going on. Understanding reinforces procedure. Without the former, the latter will sooner fade.