Water flowing over a pot in the sink
illustrates the transition from laminar to
turbulent flow. The same thing happens on a
wing, but invisibly.
Since the speed differences between adjacent layers are small and air doesn’t have a lot of internal friction, as long as the air remains smoothly layered the total resistance is low. But after the laminar boundary layer has traveled a certain distance along the surface — about half the chord length or a little more on a good laminar profile — the layers break up. This is the transition point that is visible in the sink. Beyond this point, air particles no longer move parallel to the surface but bounce back and forth across the boundary layer.
Now the movement of air in the boundary layer is retarded not just by friction between adjacent layers, but also by collisions among packets moving at different speeds as they eddy toward and away from the surface. This is a much more efficient way of transferring momentum from the air to the airplane, and it doubles or triples the drag.
Experimentalist
I met Paul Lipps about eight years ago. He was a retired electronics engineer — he spent his career working for the General Electric Co. on Atlas and Minuteman missile radar-tracking systems — who had built a Lancair 235 that he liked improving. He put a lot of work into induction, cooling and wing optimization and had the 135 hp two-seater cruising at 175 knots or so. He also designed an electronic ignition system that is now produced and marketed by his friend Klaus Savier under the brand name Light Speed.
Paul and I met in person only two or three times, but my computer still contains an exchange of several hundred e-mails. He liked words, and he would sometimes send me lists of weird neologisms or elaborate puns. I’m not generally crazy about Internet-borne entertainment, but Paul’s e-mails were pretty good. He also liked to fulminate against the EAA, which, he said, had forgotten the meaning of its E.
Paul hadn’t. He had measured just about everything about his airplane, even the stagnation temperature rise. Self-taught, he had absorbed the principles of aerodynamics to a point that, while short of the professorial, stood well up in the gifted-amateur range. His odd-looking, pointy-tipped Elippse propellers, based on a propeller theory of his own invention, achieved some remarkable speed gains in Reno races. He would cut and try, often recording speed changes down to half a knot, a precision which, as I told him, I found frankly implausible. He didn’t mind. He reported gains, losses and lack of any change at all with equal alacrity: He was, to my mind, the quintessential salt-of-the-earth homebuilder, equally adept with brain and hands.
Paul was sloppy. He didn’t mind hastily knocking together a new cooling baffle out of some random scrap from the floor of his hangar, sealing it with casually applied high-temperature silicone and then, if it worked as hoped, leaving it as is. He knew what mattered to him and what didn’t, and looks didn’t. But aerodynamic carelessness bugged him: Once, as I was about to depart from a visit with him, he cast an indignant glance at the bug-splattered leading edges of my wings, got a wet rag and wiped them clean. Laminar flow mattered — didn’t I know that? The airplane did seem a little faster on the return trip.
