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.
(Theme music)
