WHY CAN AEROPLANES FLY UPSIDE-DOWN¿

In 1945, the National Advisory Committee for Aeronautics, precursor of NASA and source of much of our knowledge about practical aerodynamics, published a hefty compendium of aerofoil data, NACA Technical Report 824, which later appeared in book form under the title Theory of Wing Sections.

- PETER GARRISON

THE NAMES OF THE AUTHORS, Abbott and von Doenhoff, have become household words wherever aerofoils are regularly discussed at the dinner table.

TR 824 presented, in the form of graphs, the characteristics of a great many aerofoils as measured in wind tunnel tests. Aeroplane designers and – perhaps even more numerous – would-be designers would pore over these graphs, alert to subtle differences that might make one aerofoil more suitable for a particular purpose than another.

To these aficionados of things round in front and pointed in back, aerofoils had distinct personalities: svelte and seductive 64-210, matronly 4418, dutiful and workmanlike 23012, exotic and mysterious 747A315.

But if one could free oneself from infatuation with one profile or family of profiles – “profile” and “aerofoil” are synonymous, by the way – and look with a cold eye on all these characters as a group, what was striking was that, as with people, their similarities were far greater than their differences.

If you superimposed all the graphs of lift coefficient against angle of attack upon one another, shifting them right and left as necessary to cancel the effects of camber, you found that they all formed a single slanting line that fanned out at its top and bottom ends into a spray resembling a spent firework. Remarkably, the line was practically straight; an aerofoil gained the same amount of lift when its angle of attack increased from five to six degrees as it did when it increased from one to two.