Even in 1800, it was understood—by at least one person, a certain George Cayley—that two forces, weight and drag, were antagonistic to flight and that there must be two corresponding forces to overcome them. Today, we speak of thrust, drag, lift and weight, but—with the sole exception of weight—these forces did not then have agreed-upon names. By the time of the Wright brothers, what we know now as “drag” was being called “drift,” a term of such obvious ambiguity that the Wrights themselves dumped it in favor of drag.
Words have many facets. They possess both meanings—often more than one—and connotations, and they often suggest more or less than what they strictly mean. Those connotations can affect the way we think about flight.
The word “lift,” for instance, implies a vertical force aligned with, and opposite in direction to, the gravity vector. We lift things upward because gravity is pulling them downward. The words used by the principal skyfaring nations of the early 20th century all reflect the same notion of raising or supporting a weight. The French word portance, cognate with the English “porter” and “portable”; the German Auftrieb, meaning literally “up-drive”—or Lilienthal’s expression, hebende Kraft (“lifting force,” with hebende akin to “heave” and “heft”)—and the Russian pod’yomnaya sila, meaning “lifting force,” all embody the idea of a force in an upward direction, opposing gravity.
The connotations of “drag” are less unanimous. In Wilbur Wright’s mind, the English word reflects an association, quite possibly unconscious, with something pulling backward on an object or restraining it from behind. The French trainée, somewhat similarly, suggests a track or trail left behind something—in any case, something like a wake that is behind the moving object. In contrast, the German Widerstand and the Russian soprotivleniya (both meaning “opposition” and “resistance”) suggest something pressing on it from the front. If you think drag is a pull from behind, your mental conception of it, and the ideas you might conceive of how to reduce or eliminate it, will be different from those of a person who thinks of it as a pressure on the front. If you slice a teardrop-shaped body in two crosswise, which piece has less drag—the one rounded in front and flat in back or the one with a flat front and a tapered aft end?
Because drag is produced by motion through the air, we intuitively understand that the drag force is aligned with the relative wind and, therefore, with the path of the airplane. That is correct. But lift is different; because of the choice of a word, we unconsciously think of lift as acting upward, the opposite of weight. That is a mistake.
The direction of the lift force is actually determined, just as that of the drag force is, by the path of the moving body, but whereas drag is aligned with the path, lift acts at a right angle to it. It is not really a “lift” but rather a cross-path force. Such a force can act in any direction—up, down, sideways, all around the clock. Indeed, although we speak of a “side force” on the vertical tail and of a stabilizer “pushing downward,” we logically ought to apply the term “lift” indiscriminately to any object in a fluid flow.
But what to call it? Strangely, no term comes to mind. “Drag” is both colloquial and appropriate, and while it imposes a supposition of acting from behind (as opposed to the synonymous “air resistance,” which implies acting from the front), in every other way it conveys an accurate impression of what is meant. “Lift” does not. If we could force ourselves to call drag the “axial force,” then we could call lift the “radial force.” But forget that—even I won’t do it.
Read More from Peter Garrison: Technicalities
There is a thing in aerodynamics called “normal force.” Normal, in this context, does not mean what it usually does; it means “perpendicular” to some assumed axis. Aware of the prejudicial implications of the term “lift,” aerodynamicists call the total cross-path force on an airplane at a right angle to the plane of the wings the “normal force.” But even though “path-normal force” would be a technically correct substitute for “lift,” it is not likely to catch on.
Because we call it “lift,” almost everyone fumbles the question, “How much lift does a climbing airplane have?”
In steady, horizontal flight, an airplane’s thrust is equal to its drag, and its normal force is equal to its weight. Now, suppose we push the throttle forward in order to climb. What happens?
First, thrust increases. Next, the airplane begins to accelerate. Because it is longitudinally stable, however, it does not want to accelerate; it wants to maintain its trimmed speed—that is, a certain angle of attack governed by the position of the elevator. (For the sake of what tattered shreds of clarity may remain, I am ignoring the possible effect of increased slipstream energy on the horizontal tail.) In order to maintain the trimmed speed, the airplane pitches nose-upward, returning to its trimmed angle of attack.
Let’s say, for argument’s sake, that the airplane settles in an attitude that is 5.74 degrees more nose-up than before. This particular angle, in addition to being realistic, has the convenient property that its sine is one-tenth. To put that in more-familiar language, an airplane with a 10-to-1 glide ratio would descend at an angle of 5.74 degrees to the horizontal.
Now let’s look at our four forces again. Weight is unchanged. Drag is the same in magnitude—we’re traveling at the same indicated airspeed as before—but is now tilted, along with the flight path. One-tenth of the drag now acts downward, increasing the effective weight.
Thrust, on the other hand, is increased because we pushed the throttle forward, and one-tenth of the thrust force is now acting upward, against weight. Because thrust is now greater than drag, the net effect is to reduce the effective weight of the airplane.
Speed and angle of attack are unchanged, so lift (the path-normal force), like drag, is unchanged as well. But what we call lift is a devious thing. The “lift vector” is now tilted backward because the flight path is tilted upward, and so the lift produces a drag component that is, again, one-tenth of itself. But that drag is also tilted and produces a weight increment that is one-tenth of itself, or one-hundredth of the lift. Lift is a double agent; under the appearance of being the friendly antagonist to weight that its name suggests, lift is actually working for drag as well as weight.
Lift lifts, but it also drags and weighs—and, in a glide, thrusts. In a vertical dive, lift is horizontal; it does nothing at all. Does such a force deserve to be called “lift”? How can we trust it to hold us aloft? Does it even deserve a security clearance?
So, it’s time for “lift” to get a new name. Suggestions are welcome. I’ll start.
How about—drum roll—“magic”?
This story appeared in the March 2021 issue of Flying Magazine