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Lift

On a bright, crisp autumn morning, a quick call to the local airport confirms that a plane is available. I'm off. The routine is familiar. Inside the plane, switch on the Master, There's power and fuel. The flaps work. I walk around the plane to inspect control surfaces, fuel, oil, prop... Confirming with a checklist, the plane appears to be in good working order.

Takeoff is effortless. Given full throttle and a little steering, the air is so smooth that plane seeps into the sky . The wheels as they spin down and the roar of the engine grab the senses and push the mystery of flight out of mind. Meanwhile the wings look just as they do when the plane sits awkwardly tied to the ground. They have the characteristic dull leading edge and bulging upper surface. A flat underside meets the upper surface as it tapers into a narrow trailing edge.

The treasures of flight unfold in the climb. The countryside is laid before me; an expression as varied as the human existence. Fields appear as patchwork, the snap-green of alfalfa pieced with the chocolate of freshly-plowed loam and the manila of drying corn. A quarry, swarming with yellow trucks, bores into the earth for rock. Slicing through the rolling hills, ribbons of tar carry the morning commuters. Trimmed and heading south the plane climbs out of the airport pattern. There is relativity little to do. A time to think on lift.

The air continuously flows by the wing. I imagine tracing the flow of air -- drawing streamlines. Ahead of the plane, the streamlines are as parallel as railroad tracks. The wing acts as a blunt chisel, splitting these streamlines apart. Split by the wing, streamlines drape over the upper surface and hug the underside. To maintain the continuous flow of air, one bit of air above the wing must travel faster to meet its neighbor traveling under the wing.

One short section of these streamlines is a parcel of air. The energy of this parcel may be split in two. One part is the kinetic energy of uniform motion - the energy anything acquires just by moving relative to something else. The other part is the energy of interaction between the air parcel and its neighbors, the pressure of the parcel; the greater the pressure the harder the parcel pushes on its neighbors. The two parts add togther to make the total amount of energy of the parcel.

Before my wing comes along, every parcel of air moves relative to the wing at the same speed; the air flows by continuously. When my wing slices through the air and a parcel travels over the wing, the parcel must move faster over the wing. The greater the speed, the greater the kinetic energy. Even so, the total energy remains constant. Since kinetic energy has increased then the other part - air pressure - must decrease. Thus, the parcel of air exerts less pressure on its neighbors and the wing!. Adding up all the contributions of every parcel of air, the flow over the wing creates a slight vacuum, sucking the wing upwards and lifting the airplane. You can see why the wing is more or less level on the bottom. This ensures that the parcels of air below the wing move along almost undisturbed, keeping the pressure just as it was before the airplane came along.

We can see this effect with just two sheets of paper. Try blowing them apart! To try this, hold one sheet in each hand so that they hang vertically. You will be looking at the edges of the paper. Now, try to blow them apart. As you blow harder and harder between the sheets they come closer together! In the middle the air moves quickly, reducing the pressure. The streamlines are sucked together and the paper is drawn into the flow.

The valley and its town have vanished behind the tail. Below me, the long legged ridges swing from the northeast to the southwest in wide arcs, dividing up the land into narrow valleys. A ridge line rises up, richly carpeted in autumn's colorful pointillism. I finish the climb, trim the plane for straight and level flight, and rest in the balance between the wings' lift and the persistent tug of gravity.

© S. Major 1993-2004 Last modified 11 April 2004

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