Laminar Flow in the Kitchen Sink

Pinpointing the elusive nature of laminar flow and what it means for pilots.

Laminar Flow

Laminar Flow

** 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.**

(March 2012) It is understood among pilots that laminar flow is something good. But exactly what laminar flow looks and feels like eludes us, because air is invisible and so we never see the difference or the transition between laminar and turbulent.

One common example of laminar flow in everyday experience is smoke rising from a just-extinguished candle. (Candles surpassed cigarettes in popularity as journalistic laminar-flow analogues late in 2008.) If the air is still, the smoke rises for a few inches in a perfectly smooth column. This is the laminar portion: The paths of all of the tiny “packets” of smoke — packets being imaginary volumes of very small size, but significantly larger than individual molecules — are parallel.

Then, suddenly, the flow breaks apart and the column becomes unsteady and several times wider than before. This is the turbulent state: The paths of packets are no longer parallel, but eddy and swirl from side to side within the rising column.

The problem with the rising-smoke example is that it does not involve a surface, and so it is difficult to intuitively relate it to flow over the wing or fuselage of an airplane. Nor does it tell us anything about why turbulent flow is draggier than laminar.

The other day I noticed an instance of laminar transition that bears a bit more resemblance to flow on an airplane. I was rinsing the bottom of a pot, which, because the pot was quite tall, was within half an inch of the faucet, so that water flowed smoothly onto it without splashing and spread out radially in all directions. A few inches from the faucet was a plainly visible line beyond which the water’s appearance changed. Inside that arc, water spread smoothly outward, like combed hair; beyond it, it appeared bumpy, tremulous and, in short, turbulent.

You might expect the transition line to move in or out as you open or close the faucet. It doesn’t, or at any rate it moves very little, until you reduce the flow almost to a trickle. At that point, the transition line disappears and the flow becomes entirely laminar.

The reason the drag is greater in the turbulent region has to do with the nature of drag itself. (It’s convenient to think of the airplane as standing still, like the pot in the sink, and of the air as blowing past it.) The airplane’s surface slows down air close to it, forming a thin layer, called the boundary layer, at the bottom of which is an infinitesimally thin layer that adheres to the surface and does not move at all.

The word laminar means layered, because a laminar fluid resembles a stack of thin lubricated films. At greater and greater distances from the skin, the layers slide faster and faster, until you arrive at the “free stream” outside the boundary layer. The laminar boundary layer is paper-thin at the leading edge of the wing, and grows to perhaps an eighth of an inch in thickness on, say, a smooth sailplane wing.

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.

Paul died of lung cancer last October at the age of 77. A few months earlier, he had built a flasher for my anti-collision lights, using a clever circuit he had designed. It is a tiny, rather disorderly looking circuit board about the size of two sugar cubes.

It works perfectly.

They Weren't Under Oath
An airliner, its nosewheel turned sideways and jammed, is approaching to land. Passengers crowd the terminal windows. As the jet is about to touch down, a pickup speeds out onto the runway and places itself under the nose. The nosewheel descends into the bed of the pickup, and airplane and pickup roll safely to a halt together.

It’s a piece of viral advertising, of course, and pretty transparent. The none-too-artfully computer-generated jet is more or less a 727; when did you last see one of those in airline service? The pickup accelerates to the jet’s approach speed — probably on the order of 120 to 130 knots — quite effortlessly — and carries the load with equal aplomb.

The 727’s nose gear probably carries around 7 percent of the airplane weight on the ground. So that pickup — a Nissan Frontier, to give the devil his due — not only had exceptional acceleration and top speed but also remarkable load-carrying capacity: somewhere around 7,000 to 10,000 pounds in the bed.

A similar video, called 405, made to showcase the desktop computer-generated imagery abilities of its creators, involves a dude who is cruising along the notoriously busy 405 freeway in West Los Angeles in a Jeep Grand Cherokee. He looks a bit uneasy, because his side of the freeway is, but for him, completely empty. We know the reason: The police have closed the freeway to allow a DC-10 with unspecified mechanical trouble to land on it. Our hero does a horrified double take when he sees the looming jet in his rear-view mirror, like the “closer than they appear” tyrannosaur in Jurassic Park. The jet’s nose gear hits him from behind, the car lurches, the nose strut crumples, and the heavy’s nose settles onto the roof of the car.

The CGI here is quite a bit better than that in the Nissan ad, but the physics isn’t. I hate to split hairs, but the nose load would be somewhere around 15,000 to 20,000 pounds — hard to take, even for a Jeep.

But at least this one isn’t trying to sell you a Jeep.