Wind tunnels

Last updated: April 29, 2008.

Suppose you've just designed a gigantic new passenger airplane and 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 so, in practice, 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 Great Pictures in NASA (GRIN).

Grounded in science

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 tube-like building through which air blasts at very high speed.

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.

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 Great Pictures in NASA (GRIN).

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.

How does a wind tunnel work?

The basic idea of a wind tunnel is crude and simple. It's like a giant drainpipe 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 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 wide 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 drainpipe. 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 NACA Ames Aeronautical Laboratory, Moffett Field, California, taken in 1948. By courtesy of Great Pictures in NASA (GRIN).

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 aeroplane, 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.

Measuring airflow

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 colour 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 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.

Photo: Testing a new jet engine in a wind tunnel. By courtesy of NASA Glenn Research Center (NASA-GRC).