Today, the Wright Brothers' propellers. 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.
This Wednesday, December
seventeenth, 2003, marks one century since the
Wright Brothers' famous flight. I thought there'd
be nothing more to say about it when I found a
recent article by three engineers who've studied
the propellers of the first Wright Flyer.
Two eight-foot diameter propellers pushed (rather
than pulled) the airplane forward. They were made
of laminated spruce and a sprocket-driven bicycle
chain drove them. Power was supplied by a
twelve-horsepower, four-cylinder, homemade engine -- the first
internal combustion engine to be made of cast
aluminum.
We look at those propellers and hardly see them.
Indeed, if we do see them, they strike us
as uninteresting. They look like narrow sticks,
widening slightly toward their flat tips. And yet,
like the rest of the airplane, they represent a
huge departure from anything that'd been made
before them.
Propellers had only recently been put to use
driving ships. But the dynamics of a propeller in
water are much different than in air. Water has a
thousand times the density of air. In water, one
must avoid arranging the flow to give a low
pressure anywhere on the blade. That leads to
cavitation, which attacks the surface of a ship's
propeller.
The Wrights knew they had to get maximum thrust
from a very small engine. They settled on a
propeller design with an airfoil cross-section.
They twisted its blade so the pitch was high near
the hub where it moved slowly, and lower out at the
tip.
Then they tested a reduced-scale model, measuring
the propeller's rpm and thrust. When they were
done, they had performance curves. They knew each
propeller would provide 67 pounds of thrust at
thirty miles an hour -- enough to get the job done.
But they'd also learned enough so they could change
the propeller speed to meet different flying
conditions. Their primitive engine had no throttle;
it operated only at one speed. However, they could
change the propeller speed by switching out the
sprockets between flights.
Now the authors of this new article reconstruct the
Wright propellers from old records, and from
fragments of the original propellers. They do wind
tunnel tests. The Wrights weren't able to test
propellers in their small tunnel. They could only
predict how the propellers would act in forward
motion.
In the end, computer modeling shows that the
Wright's propeller design was almost optimal for
their low airspeeds. Old photos of propellers from
the not-quite-successful airplanes of people like
Samuel P. Langley,
Gustave Whitehead, and Hiram
Maxim all show the same primitive flaring
triangular shape. Those airplane-makers were
nowhere close to their contemporaries, the Wright
Brothers.
And so, while people try to challenge the Wright
Brothers' priority, the vast base of solid
engineering continues to tilt in their favor --
detail by detail. Did they invent the airplane? Oh
yes indeed. I think we can safely say they really
did.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
R. A. Ash, C. P. Britcher, and K. W. Hyde,
Prop-Wrights: How Two Brothers from Dayton Added a
New Twist to Airplane Propulsion. Mechanical
Engineering (100 Years of Flight Supplement),
Dec. 2003, pp. 11-15, 39.
From Kaempffert, The New Art of Flying,
1910.
Detail from a 1904 photo, showing original form of
Wright propellers (Library of Congress)
A later form of Wright propellers (Kaempffert,
1910)
The Engines of Our Ingenuity is
Copyright © 1988-2003 by John H.
Lienhard.
