European transonic wind tunnel offers unique ability to test flight safety | Science| In-depth reporting on science and technology | DW | 05.07.2013
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European transonic wind tunnel offers unique ability to test flight safety

Before new aircrafts go into production, they have to undergo intensive testing. And part of that takes place at the European transonic wind tunnel in Cologne, Germany - one of two such facilities in the world.

Even Dieter Schimanski has to sign in. He takes the telephone receiver in one hand and dials a few of numbers. His colleague picks up at the other end. "I'm coming in now," he says as he waits for his colleague's response. "Yes, okay, good," comes the reply. He waves his ID at the scanner on the security door and it opens. This is the door to the wind tunnel.

For well over 20 years, the engineer has been working with aircraft, aerodynamics and flight safety. But experience doesn't necessarily pay in some cases - right now, it's all about safety, because work in the big hall involves high pressure and nitrogen. There's a submarine-like structure with massive walls in the hall, with walkways around it. Metal scaffolding allows you to see underneath. It's loud in there.

The submarine-like building looks as if it is floating in mid-air. Dieter Schimanski, the researcher in charge of flight tests, points to its massive outer wall. "That is the cover of the wind tunnel and that is the pressure tank, which at the moment is filled with [...] nitrogen gas," Schimanski explains. The tunnel is at room temperature, but can be cooled to 160 degrees below zero Celsius (256 degrees below zero Fahrenheit).

pressure tank

Twelve meters in diameter - the massive pressure tank

One of two wind tunnels worldwide

In order to test the model under real conditions, Schimanski and his colleagues must be able to simulate the low temperatures aircraft experience when flying at an altitude of 10,000 meters (32,800 feet). The European transonic wind tunnel (ETW) in Cologne and NASA's wind tunnel are the only two facilities in the whole world which allow researchers to simulate such conditions. Plane manufacturers have to build their model aircraft, on a 1-to-30 scale, before sending them to Cologne for testing.

model aircraft

Schimanski with an aircraft model built at a scale of 1 to 30

The models look like large toy planes. But the dimensions are surprising - the aircraft look tiny in the huge wind tunnel. The high airflow speeds come about, explained Schimanski, as turbulent air is collected in the absorption chamber and blown out through a nozzle. The simulation reaches wind speeds of up to 900 kilometers (560 miles) per hour.

Despite the elaborate test process, it's cheaper for manufacturers to simulate a real flight at the ETW than to put new planes in the air.

Security checks and final tests

aluminum pipes

Many pipes are needed to ensure the air remains free of moisture

Dieter Schimanski holds his identification card to the card-scanner, and the door opens. He leaves the hall and shows off the extensive grounds where the center is located. The sun shines outside. The test site is situated in a rather open area - due primarily to security concerns. The area could be easily evacuated in the event of an accident or nitrogen leak. Dozens of aluminium pipes are set up here, with some leading back to the hall. An air-drying unit ensures that no moist air could condense the wings of the model in the tunnel.

Schimanski casts a final glance over the area. Then he hears a noise: air flowing into the dry-air unit. "All systems are checked a final time. All the bellows are blowing now," he said, nodding. Everything is in order. The test preparations are complete. The engineer enters the control room. The aircraft model is now in the wind tunnel.

The test starts. The model simulates the climb after take-off. Just like a real airplane, the model can turn, thus realistically simulating what happens in the air. The aircraft is monochrome, except that on the wings - where it comes into contact with the wind force – there are various colors.

"We use temperature-sensitive colors, so the flow of wind is visible," said Schimanski. This allows the client to see exactly how the wind affects the wings, he added. There are many small dots with fine fibers glued to the wings that move in the wind. This test is particularly important for takeoffs and landings.

aircraft in wind tunnel

There are many possibilities in the wind tunnel

A great deal of emphasis is also placed on having a smooth surface for the aircraft, as this is vitally important for the plane's aerodynamics. "Our models are highly polished so that the surface also has a ratio of 1 to 30. Our models are so reflective, you can use them as a mirror when shaving," Schimanski said, smiling.

Year-long preparations

But attention is not only paid to the surface of the aircraft. There are also special requirements for the models' material. Sometimes it takes up to a year to finish one model. "We use a special sort of material because it has to be able to withstand extremely strong forces. If we test it at 160 degrees below zero Celsius, the material cannot afford to be brittle," Schimanski pointed out.


Fine fibers imbedded in dots on the wings indicate wind direction

The test day has gradually come to an end. Testing can last anywhere from a few days to several weeks. The aircraft model alone costs around half a million euros. But the effort is worth it; even minor improvements with the test model can lead to large energy savings with bigger planes later on. In this case, it's certainly the small details that make millions.

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