In this age of 747s and Concordes and supersonic jet fighters, it’s sometimes hard to realize that airplanes have existed for less than a century.

Even manned gliders, which came before powered airplanes, have only been around for slightly over a hundred years. In fact, 1991 was the 100th anniversary of the first glider flight by aviation pioneer Otto Lilienthal.

Lilienthal believed that the best way for humans to figure out how to fly heavier-than-air craft was to first figure out how birds fly–after all, birds are heavier than air, too. He began systematic aerodynamic studies of bird flight, and in 1889 published his results in a book, Bird Flight as the Basis of Aviation (well, since he was German, it was actually called Der Vogelflug als Grundlage der Fliegerkunst). In 1891 he took the really brave step of putting his theory into practice in a glider of his own design.

Over the next five years he made more than 2,000 flights in a series of 18 gliders, which kept him in the air longer than all previous aviators combined who had flown in heavier-than-air craft. On some of his flights he soared as high as 250 metres. Unfortunately, he made one too many flights; he crashed on August 9, 1896, and died the next day. (The Wright Brothers later made a gift of $1,000 to his widow, Agnes, in gratitude for Otto’s contribution to aviation.)

One of the things that Otto discovered in his studies of bird flight was the importance of a cambered (arched) wing. To understand why it’s important (vital, in fact), you have to back up a little bit.

OK, actually you have to back up quite a lot, more than a century, to a fellow named Daniel Bernoulli, a Dutch-born Swiss mathematician. In 1738 he published a book called Hydrodynamica, in which he formulated what is now called Bernoulli’s Law. It states that as the speed of a fluid increases, the pressure inside the fluid, or exerted by it, decreases.

In scientific terms, gases and liquids are both fluids, so Bernoulli’s Law means that fast-moving air has a lower pressure than slow-moving air. You might not think so if it’s blowing in your face, but that’s just because you’re in its way; even in that situation, the swift moving air that flows around your head after your face blocks the flow is at a lower pressure than the stationary air behind your body where the wind can’t get at it.

What does all this have to do with Lilienthal and heavier-than-air flight? Well, on an arched wing, the air has to go further to get over the arch than it does to simply flow underneath it. That makes it flow faster on top of the wing than on the bottom, which, by Bernoulli’s Law, means that there is lower air pressure on top of the wing than on the bottom. The relatively high-pressure air underneath pushes the wing up into the low-pressure area. When the force of that push is greater than the total weight of the wing and the aircraft it’s attached to, the aircraft flies. This is how every airplane from an ultralight to a 747 gets off the ground.

Of course, airplanes have more to be concerned about than just lift. Three other forces are also involved: gravity, thrust, and drag. Gravity is what lift has to overcome; thrust and drag are also opposites.

Thrust is the forward motion of the aircraft through the air. It may come from a propeller, which you could think of as a rotating wing that produces lift at right angles to the airplane’s main wing, or from a jet, which sucks air in and forces it out behind the plane at high speed. Thrust has to overcome drag, which is the natural reluctance of the air to get out of the way of large objects blundering through it.

In addition to balancing these forces, aircraft designers have to be concerned about control. It doesn’t do you much good to get the airplane into the air if it immediately careens out of control. (Passengers complain about that even more than lost luggage.)

The three elements of aircraft control are pitch, roll and yaw. Pitch is up and down, roll is round the horizontal axis from nose to tail, and yaw is round the vertical axis. Bernoulli’s Law comes into play here, too. In a conventional aircraft, pitch is controlled by elevators in the tail. If they’re tilted up, the air striking them will force the nose up, and will also increase the camber of the wings, making the air flow even faster over the top and slower on the bottom, thus increasing lift and making the plane rise. If the elevators are tilted down, the opposite happens.

Roll is controlled by ailerons on the outermost trailing edges of the wings. These are deflected in opposite directions–one up, one down–to make one wing rise (the one where the aileron goes down) and the opposite one to go down.

Yaw is generally kept to a minimum. The upright fin is itself an airfoil that provides directional stability, again by Bernoulli’s Law: if the plane yaws, the airflow is no longer even on both sides of the fin, and the higher pressure on one side will force the rudder back to a point where the pressures on both sides are equal. The rudder, a flap in the tail fin, is used sparingly as necessary to achieve the same effect.

Otto Lilienthal never lived to see powered flight; he’d been dead for seven years when the Wright brothers achieved immortality. But were he miraculously returned to life today, he could take one look at the wing of the largest passenger jet and say, in chorus with Daniel Bernoulli, “Ja, I knew it would have to look like that.”

Permanent link to this article: https://edwardwillett.com/1992/04/flight/

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