A debate simmered in certain obscure quarters a couple of years ago over the relative merits of the Newton and Bernoulli explanations of lift, even though they're just two sides of the same coin. An airplane produces many kinds of disturbances in the air as it passes by, and you can argue all day about causes and effects; but lift and drag ultimately boil down to what the airplane feels-that is, to the forces applied directly to its surface.
Everything would become clear if we could just be airplanes for a little while, and feel on our skin the push here and the tug there whose end result is the miraculous levitation of thousands of pounds of deadweight.
We can't. But computers can make those feelings visible, and perhaps in that way make clearer just how it is that airplanes stay up. One type of representation has become a staple of sales brochures because it is graphically arresting and at the same time implies technical sophistication: It is the "spectrum plot," in which some item of interest, like surface pressure or friction, is represented as a rainbow of colors. Here are two such plots, representing a 172RG in flight seen from above and below. In this case, the quantity being displayed is pressure or, more exactly, the pressure coefficient, or Cp. A pressure coefficient of zero represents ambient static pressure; a coefficient of 1.0 is the dynamic pressure of moving air at the flight speed, as the pitot tube would feel it.
The first choice you have to make when creating an analysis like this-this was done, by the way, with a program called CMARC, which is sold by a business in which I participate-is how you are going to map color to pressure. The full range of pressures on the airplane in the pictures is from 1.0 to maybe -1.5, with the lower figure representing the "suction" near the leading edge of the wing. (At large angles of attack, leading edge suction can get down to the neighborhood of -5.) The spectrum plot provides more useful detail, however, if you confine the available colors to a narrower band of numbers. In this case it runs from -0.4 to +0.4. In other words, any pressure lower than -0.4 reads as magenta, and any pressure higher than 0.4 reads as dark blue. Ambient pressure is a medium green.
You'll notice that the spectrum used is not the true spectrum of visible light, which goes from red at one end to violet at the other. Violet looks reddish, confusing one end of the spectrum with the other, and so the color range used is from magenta (that is, fuchsia or deep pink), representing low pressure, to blue, representing high.
A recurrent pattern is visible in several parts of the airplane. Going from front to back, the color starts as blue, changes to pink, and then travels back through green to blue. The leading edges of the wings and the fuselage, for example, are dark blue, as is the base of the windshield; that is where the "total pressure" of the oncoming air is felt (and explains why cars often have their ventilation air intakes at the base of the windshield). Perhaps surprisingly, however, the same dark blue exists behind the rear window. This is the area of "pressure recovery," and, although many people would suppose that pressure behind an airplane would be low, it is, unexpectedly, higher than ambient. This is due to a kind of bounce effect, whereby the energy stored as air speeds up to squeeze around an object is released as it converges behind. It's never possible to recover all of that energy-if it were, things would have no drag - but part of the trick of designing low-drag bodies and airfoils is to get the most out of the pressure recovery.
Pressure is inversely proportional to speed - this is the principle famously associated with the 18th-century Dutch mathematician Daniel Bernoulli - and as air speeds up when it passes around obstacles, pressure drops. The drop is most conspicuous on the wings, where the large magenta area on the upper surface represents a significant part of the lift. The underside of the wing is also reddish, indicating lower-than-ambient pressure, but the effect there is much weaker. On the horizontal stabilizer the same phenomenon appears inverted, because in this case the horizontal tail is "lifting" downward to balance the airplane.
The magenta zone on the top of the wing grows less extensive toward the tip because the wing is twisted in order to ensure that the root works harder than the tip. The stall, when it occurs, will begin at the root and spread outward. Very near the tip, twist has gone so far that there is even a small patch of quite low pressure on the underside of the leading edge. Because of wing twist, the ailerons will be effective even when the inboard portion of the wing has stopped flying.
Looking up at the airplane from below, you can see a small magenta patch where the wing and fuselage meet. The fuselage is still bulging slightly here, and the combination of its curvature and the wing's gives an extra kick to the air. This is an example of what causes interference drag, which occurs when flow speeds up because of the combined effects of two surfaces, or when one surface is simply immersed in the pressure field of another. The extra speed causes extra friction, and in the absence of a fillet or fairing may lead to flow separation and extra drag farther downstream. A similar pressure "shadow" can be seen on the side of the fuselage beneath the stabilizer.
One of the most powerful effects of the airplane's passage is the pair of tip vortices that it leaves in its wake. But these are already behind the airplane, and have no effect on it. The only telltale sign of the tip vortex is the little patch of low pressure near the trailing edge of the wingtip; this is the core of the vortex, an area of intense low pressure produced as flow spills, like a breaking wave, around the wingtip.
Spectrum plots like these can be used to investigate skin friction, boundary layer thickness and shape, laminar flow and turbulent separation. They have become a standard part of every aerodynamicist's toolbox, but they can convey a great deal of knowledge and insight to nonspecialists as well.
