A reader inquired about the purpose of small vanes, resembling the wings of a Concorde, that he noticed in a picture of a Cessna 303 in the December 1996 issue of Flying. The fins, six of them in all, are located near the leading edges of the intersections of the wing with the fuselage and the engine nacelles. In each case the little fin, which has a narrow-delta planform no more than two or three inches wide at its trailing edge, is set at a positive angle with respect to the wing surface, forming a converging channel between itself and the wing.
Lest I be guilty of over-reliance upon the paperback writer's cheap expedient of suspense to keep you reading, I will reveal right now that they are vortex generators. Their purpose is to flush low-energy air out of the intersections. They do this by generating a tip vortex, just as any wing at a non-zero angle of attack would. The vortex, if you could see it, would look like a slender rope trailing from the vortex generator and following the contour of the intersection. In fact, when atmospheric conditions are right you can sometimes see just such vortices, made visible by condensation, trailing from vortex generators on the nacelles of some airline jets such as big-engine 737s. They appear at the same time solid and ghostly, and are capable of the most delicate, graceful and agile responses to changes in attitude or G-loading. They are, in fact, quite beautiful. In aerodynamics, aesthetic enjoyment is a frequent, albeit unintended, side effect.
Usually, we think of vortices as drag producers. Wingtip vortices, which are evidence of the effort needed to levitate thousands of pounds of stuff upon thin air, do not speed up an airplane. But under some circumstances small vortices can be used to reduce drag. To understand this hair-of-the-dog effect, it's helpful to think a little about what drag is. It is not merely a fancy term for colliding head-on with air.
When we talk about aerodynamic phenomena, we tend to treat the airplane as a stationary object with air flowing past it, as though it were mounted in a wind tunnel. While this view, which has the advantage of keeping the airplane before us for more than a second, is physically equivalent to the more realistic one of the stationary air and a moving airplane, it hampers an intuitive grasp of what's going on. Instead, for the moment let's imagine ourselves standing still as an airplane flies by. If we could see the air, it would be at rest as the airplane approached and would be pushed aside as the airplane passed. In the airplane's wake, some air would be moving in the direction the airplane moved, as if trying to follow it. This entrained air would swirl around in random ways for a few minutes before again becoming motionless. The effects would be similar to those we observe when we stand at the side of a road and a truck roars by.
The air entrained by a perfectly streamlined body consists of only a thin film adjacent to its surface. Within this film, called the boundary layer, there is a progressive change in speed: The closer to the surface it is, the closer the speed of the entrained air is to that of the moving body, until the molecules in immediate contact with the surface-which, at their scale, is of a mountainous roughness-are carried along at the speed of the airplane. The boundary layer, which is very thin at the nose but grows thicker and thicker toward the tail, takes its momentum from the airplane. After the airplane has passed, the air in the boundary layer continues to be carried forward by that momentum. It mingles with nearby stationary air, creating swirls and eddies which viscosity eventually damps out.
Nearly all of the drag of a well-streamlined object manifests itself as the momentum of the boundary-layer air that it leaves in its wake. Several factors influence boundary-layer momentum, but the dominant one, and the one most intuitively obvious, is the thickness of the boundary layer. The thicker the boundary layer, other things being equal, the more air there is in it and the greater the drag it represents.
Now air, like water, does not like to flow uphill. "Uphill," in the case of air, means from low pressure to high. Just as momentum can carry moving water up a slope for some distance, it can also propel air against rising pressure. But not for long. In the rear portion of a streamlined body, where its surfaces are converging, pressure rises. As the air closest to the surface, which is being carried along at nearly the speed of the airplane, meets this rising pressure, its momentum is insufficient to drive it up the pressure hill, and instead separation occurs.
Separation means that the boundary layer detaches from the surface and rides up over a layer of invading air that is flowing in, either from behind or from either side. This is what happens when a wing stalls. High pressure boundary-layer air from below the wing curls around the trailing edge and advances upstream, prying the boundary layer away from the surface. As the boundary layer leaves the wing surface, the curvature of its path around the wing flattens, and lift, which is the result of bending the path of passing air, is reduced.
