Today, let's think about stability. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Try a little experiment with me. First balance a long pencil on end on your fingertip. If you can manage that -- and I gravely doubt that you can -- then try to lift your finger upward, keeping the pencil balanced as you do.

Launching a rocket is that kind of balancing act. The main nozzle pushes the rocket upward. Smaller nozzles direct jets off to the sides. When the rocket starts to tip over, one of these fires to realign it. That's the sort of thing you try to do by moving your finger to realign the pencil.

The rising rocket is a completely unstable system. Left to its own devices, it'll always tip over. When I was a kid building model airplanes, I knew that some designs were stable and others weren't. A WW-I biplane looked nice on the shelf. But, without a pilot at the controls to make corrections, it'd stall, or tailspin, or find some other way to crash. But models of gliders or Piper Cubs flew wonderfully well when I set them loose in the air.

Walter Vincenti at Stanford University tells us why this was so. He looks back over the history of flight and finds that airplane designers had very different views of stability. Most 19th-century builders were trying to invent stable flying machines. But the Wright Brothers' airplane was unstable. They knew the trick wasn't to make the machine stable -- it was to make it controllable.

But the Wright brothers were bicycle-makers, and all of you know that a moving bike is unstable -- when you quit steering it, it falls over. At the same time, it maneuvers perfectly well when it's under control.

Instability suited WW-I biplanes nicely. Their very instability made them highly maneuverable. Stable airplanes couldn't be made to maneuver like that.

It wasn't until flying went commercial in the 1930s that designers took much interest in stable airplanes. Commercial flight could get very dangerous if it had to have constant close control. Commercial airplane designs remained stable until fairly recently. But today's airplanes are equipped with sophisticated automatic controls that give the pilot an illusion of stability. Today, we've gone back to unstable designs that need lots of control. But human beings have to do only a small part of the work of controlling them.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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From The Art of Flying, 1911