When aircraft fly, they generate turbulent airflow behind them known as 'wake turbulence', which can affect following air traffic. The German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) has developed a method to measure wake turbulence in flight. This method was put to the test during flights with DLR's Falcon 20E research aircraft.

When assessing wake turbulence during cruising flight, it is particularly important to measure the strength and intensity of the vortices and the behaviour of an aircraft flying into wake turbulence. As cruising flight takes place at altitudes of over 10,000 metres, it is impossible to carry out such analyses from the ground.

Measurement campaign behind large aircraft
For the test flights, DLR researchers flew through the airspace above Mecklenburg-Western Pomerania and Brandenburg as often as possible and through the centre of the wake turbulence caused by typical airliners.

"This was a very complex undertaking. It placed high physical requirements on the test team and the test aircraft," said Dietrich Fischenberg, a researcher in the DLR Institute of Flight Systems in Braunschweig.

"An essential element was good collaboration between all participants: the DLR Institutes of Flight Systems and Atmospheric Physics, the DLR Flight Experiments department, German air traffic control (Deutsche Flugsicherung; DFS) and, last but not least, the pilots who permitted us to fly through and measure the wake turbulence," Fischenberg continues.

A DLR staff member working with DFS during the tests put together a real-time selection of potential aircraft in the test zone, and air traffic controllers informed the relevant pilots of the tests taking place. They then asked for important flight data needed for the research and brought the Falcon closer to the airliner. Two hundred tests were carried out in wake turbulence at ranges from five up to a maximum of 25 nautical miles behind the airliners.

Information on wake turbulence for new types of aircraft as well
The data collected is now being assessed using software developed by DLR. Researchers are now gaining insights into the strength of the wake turbulence at varying distances behind an aircraft, even at great altitudes.

Furthermore, they can determine what happens when the turbulence behind the aircraft sinks and at what point it is safe for other aircraft to fly through the wake turbulence. This new method is also being used to gather information on the flight dynamics of and loads generated on the approaching aircraft.

The strength of the turbulence after it has sunk is currently of particular interest. It allows assessment of the effect that sinking wake turbulence has on aircraft flying at narrow vertical separations in advance.

There is something else that this new method can help with; when integrated into the certification process for new, very large aircraft, it might be possible to accurately determine when and where it is safe to fly through the as-yet undetermined wake turbulence. "That is still a long way off, but it would greatly simplify the certification process for new aircraft," says Fischenberg.

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