Today, we worry about airplane stability in 1916. 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.
By 1916, aeroplanes had barely been
invented. Designers at every level of sophistication were
furiously sorting possibilities and configurations. The
new science of aerodynamics was revealing unexpected
traps -- things beyond the reach of intuition.
Great Britain was bending to the task of fighting WW-I
when, in 1916, a small book on aeroplanes appeared. It
had two sections bound under one cover. One told about
aeroplane design; the other about stability. War meant
secrecy, so this necessarily represented thinking from
before the war, when aeroplanes were little more than a
The section on design tells how forces interact. How far
off the engine's thrust axis should the wing (or wings)
be tilted? The forces exerted by each of a biplane's
wings might act differently. The weight of a human being
was a big fraction of the weight of a delicate old flying
machine. A pilot could upset flight by shifting his
weight with respect to the aeroplane's center of gravity.
Designers had to be concerned with the pilot's strength.
In a tight turn, he had not only to turn the rudder
against the wind, he had also to turn it against
centrifugal forces. Charts, graphs, and equations treat
all these issues in fine terse detail.
But then, like the fourth tempter, arriving quite out of
the blue in T. S. Eliot's play Murder in the
Cathedral, part II of the book rears its head. This
section on inherent
stability precedes the expectations of readers.
For the issue of stability had only recently emerged like
a toad from under a rock. Should aeroplanes be stable in
flight? Every fiber of our intuition told us they should.
Disturb the level flight of an inherently stable
aeroplane, and it'll return to steady motion. A truly
stable airplane, placed in the air at any speed or angle,
would right itself and glide safely to Earth. But that
misses a serious subtlety.
Few, if any, airplanes are that stable. Many modern jets
would simply sink like a stone. The designs of the
then-newly evolving combat biplanes would grow
increasingly unstable. They could easily go into a
tailspin from which only a skilled pilot could recover.
And so the author struggled to resolve a debate that was
only just taking form. Was stability good or bad? After
forty pages on the mechanics of stability, he cast his
lot with stable airplanes.
But it was too soon; flight was too young. He had too few
of the new flying machines to guide his thinking.
Aeroplanes were now going to war, and they'd all have fly
like stunt planes.
A century ago this designer gave us a fine double legacy.
Any engineering student could learn so much from his
discussions of stability. But that same student could
learn just as much from the author's acceptance of the
commonsense, but misleading, idea that stability should
be an end in itself.
I'm John Lienhard, at the University of Houston, where
we're interested in the way inventive minds work.