BLOCK THAT COEFFICIENT!

I have to admit that when I encounter equations in a book, I usually just skip over them. Often some phrase like, “It may be readily seen that...” precedes them, and so I figure that since plenty of other people must be readily seeing whatever there is to see, there’s no reason that I need to as well.

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NEVERTHELESS, I think of equations as something like fibre in food: nutritionally null, but indispensable to mental digestion. So...

It may be readily seen that L = .00119V²CS.

That thing that just shot past was the classical equation for the lift force. What it's saying is so beautifully simple that it's worth a close look even from hardened equation-evaders.

L stands for Lift. V stands for speed ("velocity") in feet per second, and S for area ("surface") in square feet. CL is the lift coefficient - I'll get back to that later. The curious little number .00119 stands for what air is like at sea level, basically, how heavy and thick it is. The equation says that sea level air moving one foot per second exerts a pressure of .00119 pound, or about 1/50th of an ounce, on an area of one square foot. This force is called the "dynamic pressure" of the moving air. 

We can conveniently measure the dynamic pressure with a pitot tube and a manometer, a fluid-filled transparent U-shaped tube, or with an airspeed indicator, which amounts to the same thing. If we take several measurements at different speeds, we discover that the pressure is proportional to the square of the speed; double the speed and the pressure quadruples. If you go 100 times the speed – 100 feet per second or about 68 mph – the pressure is 10,000 times greater than at one foot per second: 11.9 pounds per square foot. Now we are in the range of pressures that can support an aeroplane, provided that our wing has a lift coefficient of 1.0.