Suppose you've just designed a gigantic new passenger
now you want to test it out for real. You could spend millions of
dollars building it out of shiny titanium
metal and race it down the runway to see if it actually flies—but what
if you got your calculations wrong? What if your airplane takes off for
twenty seconds, then suddenly drops like a stone and lands on a city
packed with 5 million people? It's not the best way for testing
something so dangerous. That's why airplane designers try things
out on the ground first using scale models in wind tunnels. Let's take
a closer look at how they work!
Photo: The fan blades inside
one of the giant wind tunnels at NASA Langley Research Center. Note the man inside!
Photo by courtesy of
NASA on the Commons.
Designing planes that will fly quickly, efficiently, and
economically is all about making air flow smoothly over their wings and
past their tube-like bodies. This is called the science of
aerodynamics. Once a plane's up in the air, there's no easy way to see
how air is moving past it (though an experienced test pilot will have a
good idea what might be causing problems). If there's a major design
defect, the plane won't get into the air at all. That's why every
modern spacecraft and airplane is
tested on the ground first in a wind tunnel: a pipe-like building
through which air blasts at very high speed.
Photo: The basic idea: fix the plane on the ground and
blow air past it.
Photo of an F-86 aircraft, mounted in the 40 x 80 foot full-scale wind tunnel at the NACA Ames Aeronautical Laboratory, Moffett Field California, taken in 1954. Note the engineer standing underneath the plane.
By courtesy of NASA on the Commons.
The basic idea of a wind tunnel is simple: if you can't move the
plane through the air, why not move the air past the plane instead?
From a scientific point of view, it's exactly the same. If a plane
drags (causes air resistance) when it soars through the sky, air will
drag in exactly the same way when you fire it past a stationary model
of the plane on the ground.
There's nothing to stop you building a super-giant wind tunnel and
testing a life-sized model of your plane—and, indeed, the American
space agency NASA does have wind tunnels like this. But most of the
time it's much cheaper to use a small, scale model of the plane in a
much smaller wind tunnel.
Photo: Testing a full-size replica of the 1903 Wright Flyer air plane in the NASA
Ames Full-Scale wind tunnel. By courtesy of NASA on the Commons.
How does a wind tunnel work?
A wind tunnel is a bit like a huge pipe that wraps around on itself in a circle with a fan in
the middle. Switch on the fan and air blows round and round the pipe.
Add a little door so you can get in and a test room in the middle and, hey
presto, you have a wind tunnel. In practice, it's a bit more
sophisticated than that. Instead of being uniformly shaped all the way
round, the pipe is wider in some places and much narrower in others.
Where the pipe is narrow, the air has to speed up to get through. The
narrower the pipe, the faster it has to go. It works just like a bicycle pump, where the air speeds up when
you force it out through the narrow nozzle, and like a windy valley
where the wind blows much harder, focused by the hills on either side.
Photo: A wind tunnel is like a giant pipe.
Note the wide outer sections and the much narrower inner section where the tunnel produces
high-speed air in the central test laboratory.
Photo of the 16-foot high-speed wind tunnel at the NASA Ames Aeronautical Laboratory, Moffett Field, California, taken in 1948.
By courtesy of NASA on the Commons.
Having a wind tunnel with narrow sections is an easy way to build up
more speed—and speed is something we need lots of. To test a supersonic
airplane, you need wind speeds about five times faster than a
hurricane. And for testing something like the Space Shuttle, you need to blow your wind
round ten times faster still. Some wind!
Key parts of a typical wind tunnel
Artwork: A plan view of a typical wind tunnel.
Poke your head inside a wind tunnel and—providing your ears don't get blown off—you'll find something like this:
Compressor: The fan (or fans) that produce the high-speed wind.
Supersonic, high-speed test section: The model airplane is placed in here.
Vanes: These are airfoils positioned in the corners to turn the air through 90 degrees without losing energy.
Acoustic muffler: Wind tunnels are noisy places! Mufflers help reduce the noise and more accurately simulate a
realistic air flow.
Subsonic, low-speed test section: There's a smaller test chamber round the other side where the air moves a bit slower.
Access doors: The scientists have to get in somehow!
Air dryer: This section removes moisture from the air flow.
Here's one of NASA's own wind tunnel cut-away drawings, showing similar features (and a few more I left off):
Artwork: Key features of the Ames 14ft trans-sonic wind tunnel.
By courtesy of NASA on the Commons.
Photo: Want to do a bit of wind-tunnel testing but can't afford the millions
you'll need to spend on all that fancy equipment? No problem: there's an app for it! Search for "wind tunnel" in your
favorite app store and you'll find quite a few simulations you can play with on your smartphone or tablet.
This is a screen capture I made with a free app called Wind Tunnel Lite, from Algorizk, which lets you test
a few basic shapes (like cars and airfoils) at different wind speeds. There's also a pro version that lets you control far more things (propeller thrust, fluid viscosity, friction, and wind speed). Well worth a look for teachers!
Air is invisible, so how do you see whether a plane is performing
well or badly inside the tunnel? There are three main ways. You can use
a smoke gun to color the airstream white, then watch how the smoke
shifts and swirls as it passes the plane. You can take what's called a
Schlieren photograph, which makes variations in the air speed and
pressure show up so you can see them. Or you can use
anemometers (air-speed measuring instruments) to measure how fast the wind is going at
different points around the plane. Armed with your measurements and
lots of complex aerodynamic formulas, you can figure out how good or
bad your plane is and whether it would really stay up in the sky.
Once you're happy, you can build yourself a prototype (a test model)
and try it out for real—or persuade someone else to try it out for you.
Test pilots earn amazing amounts of money because of the risks they
take. But they're an awful lot happier buckling themselves into their
seats knowing everything they're about to try has already been tested
in a wind tunnel!
Although wind tunnels are best known for testing out new planes and space rockets—vehicles that speed
through a (theoretically) static airstream—they can also be used in the opposite way: to simulate how fast-moving winds
affect static structures, such as high-rise buildings and bridges. Architects and structural engineers need to consider
not just the loads that high winds impose on their designs (literally, whether buildings could blow down), but how things like skyscrapers catch the wind and bounce it down to ground level, creating "down-draughts" and potentially dangerous vortices that can blow people off their feet. Problems like this are easy to study—and correct—using realistic models in wind tunnels.
Most people would agree that the Wright brothers pulled off a neat trick when they made
the first powered flight in December 1903. Some trick! They'd spent years studying aerodynamics and
perfecting the design of their wings, which they called "aeroplanes."
While the Wrights made most of their tests outdoors, modern planes are more likely to be tested indoors—thanks to
the insight of self-taught British aero engineer Frank Wenham (1824–1908), who invented the wind tunnel in 1871. Unlike huge modern tunnels, Wenham's original had (as he put it himself) "a trunk 12 feet [3.7m] long and 18 inches [46cm] square, to direct the current horizontally, and in parallel course" and the air that whistled around it traveled no faster than 64km/h (40mph).
Compare that with the world's largest modern wind tunnel at NASA Ames Research Center, which is over 100 times longer (430m or 1400ft long), has a test section with an overall area measuring 24m × 37m (80ft × 120 ft), and produces winds of up to 185km/h (115mph). Modern wind tunnels like this owe a huge debt to forgotten pioneers such as Wenham, whose insights helped to usher in the modern science of aerodynamics—allowing millions of us to take to the sky every single day!
Wind Tunnels Utilized in New Ways by Walter Tomaszewski. The New York Times, August 30, 1970. An article from the Times archive explains how wind tunnels were used for things like skyscraper design in the late 1960s. One notable pioneer of this work was Jack Cermak of Colorado State University.
The 1901 Wright Wind Tunnel: Wright-Brothers.org, undated. A fascinating photographic look at the tunnel the Wrights used for their experiments (the second one in the United States).
Low-Speed Wind Tunnel Testing by Jewel B. Barlow, William H. Rae, and Alan Pope. Wiley, 2015. One of the classic textbooks, particularly concerned with automobiles, bridges, and other low-speed applications.
For deeper technical detail, it's well worth taking a look at patents—and here are a few examples I've pulled out for you. There are dozens more on file, some covering the design of the tunnel and others focusing on how models can be supported or moved to simulate realistic airplane movements. You can find many more by searching the USPTO database (or an alternative such as Google Patents):
US Patent 1,635,038: Wind tunnel for flight of models by Elisha Fales, July 5, 1927. Fales worked for the US Army Air Service and made important contributions to the science of aerodynamics. In 1918, working with Frank Caldwell, he built the first high-speed (though still subsonic) wind tunnel in the United States to test propeller designs.
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