Pilotage

Columnist and pilot Mark R. Twombly is a partner in the ownership of a Piper Twin Comanche.

The Matterhorn-White-painted aluminum skin that forms the top surface of the wing of our airplane is nice and smooth — to a point. It would be smoother were it not for a malicious thunderstorm that passed directly overhead, took aim, and let loose a torrent of flinty little nuggets of ice.

This provocation of nature occurred before we owned the airplane. The hail damage didn't stop my partner from buying it from the previous owner, but I have to say the dings weren't as obvious before we had the airplane repainted a couple of years ago. The old paint had lost its gleam, which is why we had it repainted, but as we later learned, the tired, chalky beige color served to soften the shadows that now call attention to every pockmark.

I've taken to putting some spin on the situation by saying that the airplane isn't hail damaged, it's dimpled. Like a golf ball. If you believe golfing's spinmeisters, the 300 to 500 little dimples on the average ball reduce its drag in flight, which enables it to travel more than twice as far down the fairways as would a smooth ball.

Seems to me that if dimples work for a simple little ball that doesn't really fly but is launched into the air, then dimples should work on something as elegant as an airplane wing, which truly does fly. (See " Test Pilot," June Pilot.)

The fact that I can walk any airport ramp and not find an intentionally dimpled airplane wing has me a little worried about the veracity of my argument, but then again I could be way ahead of the game. If nothing else, seeing them as dimples rather than dents puts our blemishes in a more positive light.

A bit of research, however, uncovered some discouraging theories. To sum up the science, dimples reduce drag on a golf ball because it is a sphere, and they have little such effect on an airplane wing because of its airfoil shape. When launched, a golf ball creates a drag-producing wake of turbulent low-pressure air. Dimples generate a thin layer of turbulence in the laminar airflow around the front half of the sphere. This helps the air stay attached to more of the trailing half of the ball, thus reducing the amount of drag-producing, low-pressure wake turbulence.

The characteristic shape of an airfoil with its tapered trailing edge is designed to preserve laminar flow around the entire airfoil. In other words, there isn't any significant trailing-edge turbulence to mitigate. Dimples have no role to play on an airfoil. The low-pressure area is on the cambered side — the top surface of the wing — and that is what creates the lift we so depend on.

For all of its importance to the basic function of an airplane, the wing doesn't seem to get the attention that it deserves. One explanation may be that the relatively simple wing of a light general aviation airplane doesn't reveal much of itself in terms of the work it is doing. It doesn't go up or down or back or forth; it doesn't spin; it doesn't make any noise. It offers no visible proof that Daniel Bernoulli was correct when he postulated that the pressure in a stream of fluid is reduced when the speed of the stream increases, as the slipstream does when it travels over the curved top surface of an airplane wing compared with the bottom surface. We must take him at his educated word.

The one common opportunity we have to view a wing in all of its functioning glory is when we are sitting with our noses pressed to the inside windowpane next to a coach-class, over-wing seat on a commercial airliner. We can see that powerful lift is at work when the tip of the wing flexes upward at the moment of rotation on takeoff. On touchdown, that same wing tip droops when the burden of lift is removed.

One other piece of evidence occurs only under the right ambient conditions on takeoff and approach. Vaporous trails snake from the drooping leading-edge slats to the trailing edge of the extended, massive flaps hanging off the back of the wing. We may not be able to see the airflow on the underside of the wing, but at least it's half of a wind-tunnel demonstration of the Bernoulli principle in action.

But back to the dimples. I hear what the aerodynamicists are saying about them, but I cling to the notion that we may be netting some small gain from the hail pattern on our wing. On the other hand, we've installed just about every official drag-reduction device available, and we don't seem to be going appreciably faster.

This seems to support the view of the golf-ball manufacturing executive who was dismissive of wondrous claims of performance gains made possible by technological advances in equipment. "In spite of space-age balls and clubs," he is quoted as saying, "the average score on the PGA Tour has improved but one stroke over the past 17 years."