Today, aerodynamics and automobiles. 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.
The first cars were made
before the Wright brothers flew and before they'd
even started doing wind tunnel tests. Wind tunnels
were used in airplane design from the start. Car
designers were far slower to see that aerodynamics
also affected their work.
At the turn of the twentieth century, Germany set
up a number of high-level technical universities,
which put them way ahead in aerodynamic research.
In 1921, it was the Zeppelin Airship Works that
first studied automobile streamlining in wind
tunnels.
Automobile drag can be a serious gas-eater. When I
was a kid, I entertained myself on long auto trips
by putting my hand out the window and turning it at
various angles to the wind. The forces, even on a
child's small hand, were quite strong. And small
changes in the shape and orientation of my hand
made huge differences.
The usual measure of aerodynamic efficiency is the
drag coefficient, CD. It
compares the drag force, at any speed, with the
force it'd take to stop all the air in front of the
car. Drag coefficients for the first boxy autos
were up over 0.7. Instead of letting the air slip
past, they brought most of it to a halt.
In most of today's cars that figure is down to a
scant 0.3. Of course there's more to it than just
lowering the drag. It's easy enough to reduce drag
if we let ourselves create other aerodynamic
problems when we do. A car has to be designed for
negative lift. The wind should press it solidly
down against the road. And cars must not be
vulnerable to crosswinds.
As early as 1907, a streamlined racing car called
the Rocket reached 132 miles per hour before
it became airborne. It had a low drag coefficient,
but it was still a bad aerodynamic design.
Streamlining was the new
design icon in the 1930s, yet that was more an
illusion of speed than real drag reduction.
Only a few cars had been wind-tunnel tested. The
famous Chrysler
Airflow was the exception, with far less
drag than most of the cars following it.
It took time for engineers to see that they had to
smooth the bottom of an automobile as much as the
top. It took time to see that sharp corners on the
front of a car were terrible drag-inducers. Only in
the last generation did 18-wheelers sprout those
strange-but-effective, drag-reducing cowls over
their cabs. And only since WW-II has wind tunnel
testing been a regular part of car design. Only
recently has accurate computer simulation let
engineers use rapid trial and error to improve
aerodynamic designs.
That's why today's cars offer so little drag. And
of course they're looking more and more alike. As
designers work with increasing knowledge of design
limitations, they close in on optimal designs that
cannot vary much from one car to the next. It's
just because our cars are such fine machines that
we no longer look at the wild sunspray of
possibilities -- which got us to where we are.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
