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.
