Bumbling Bees and Fast Flies

I was a very small child when, while ducking a little yellow and black buzzbomb, I heard an adult say, "You know, some scientist once figured out that bumblebees can't fly." Back then I hadn't the least understanding of aerodynamics, but I knew hokum when I heard it. That bee flew perfectly well.

Certainly a lot of northern children hear the same story every summer, as they marvel at (or flee from) Alaska's fuzzy, hardworking native bees. The aerodynamically impossible bumblebee makes a wonderful story at the expense of someone who thought mathematics was more true than observable reality.

That particular know-it-all is nearly as fictitious as the flightless bee. John McMasters, principal engineer on the aerodynamics staff of Boeing Commercial Airplanes, reports in a recent issue of American Scientist magazine that he spent some time trying to track down the villain who gave his profession such a black eye.

He found the tale was circulating in German technical universities back in the early 1930s, in the infancy of true aerodynamic science. But the closest he came to its real source was an anecdote about a party attended by both a biologist and a Swiss aerodynamicist (whom McMasters kindly leaves unnamed). During the dinner conversation, the biologist asked for enlightenment about the aerodynamic capabilities of bees and wasps. The aerodynamicist made a few assumptions, did some quick calculations, and concluded that bees shouldn't be able to fly.

The error was in the assumptions, not the calculations. The Swiss scientist had assumed that bee wings are relatively smooth flat plates; that, plus the bees' small size and slow flight, led him to assume that airflow over the wings would cause them to stall easily. Eventually the aerodynamicist checked an insect wing with a microscope and saw his error, but it was too late: the story had developed wings of its own.

A magnified cross section of an insect wing shows many remarkable zigs and tags. The extraordinarily thin membranes between the tubular, vein-like struts arch up, curve down, angle and bend in ways that would give a jet pilot nightmares. Yet it is possible to draw a smooth contour enclosing all the highs and lows of the wing, and that contour will look like an elegant airfoil-like a cross section through the wing of an airplane.

As far as the passing air is concerned, the moving insect wing is a symphony of different airfoils, all playing together to provide perfectly adequate characteristics of lift and drag. The insect can take off, circle, or fly swiftly in---well, a beeline---as well as it needs to.

Lest people assume that the simple moral of the story is observe first, calculate second, McMaster provides another insect-based anecdote in which a scientist erred by speaking out about what he thought he'd seen, uncorrected by calculations.

During the 1920s, an entomologist hiking through New Mexico's high country reported seeing deer botflies buzzing past at terrific speeds. They looked like brown blurs, and he estimated that they were traveling at about 400 yards a second. The story was widely reprinted from the technical joumal in which it originally appeared, and the speedy botflies made several record books.

They also greatly annoyed another scientist, Nobel winner Irving Langmuir. Langmuir was a chemist, but he really did know hokum when he heard it. He did the calculations. A speed of 400 yards a second at the altitude where the observations took place---near 12,000 feet---is faster than the speed of sound. The New Mexico slopes would be echoing with botfly sonic booms, and any hiker unfortunate enough to be struck by a fly going that fast would get a wound equivalent to one from a large-caliber pistol bullet. Of course, the fly could only attain that velocity once in its life, since its head would be crushed by the dynamic pressure of its speed.

Langmuir then observed: he attached a botfly-sized weight to a thread and whirled it to find the upper and lower bounds of speed at which it looked like the described blur. The mean value of the measured speed turned out to be 10 yards a second.

The real moral may be: trust both observation and calculation, but only so far. You don't need to wear bullet-proof vests for protection from hurtling botflies, but don't bug a bumblebee. They fly well enough to catch up with you easily.