Spraying water into the rear-engine exhaust plume of a Lockheed Martin F-16 significantly reduces ground level noise, the Dutch National Aerospace Laboratory's researchers have discovered.

Planning is now under way for more tests to optimise the injection using less water and altering the shape of the injector's nozzles. At the Netherlands' Leeuwarden air base, 12 injectors sprayed, at an angle, 1,000 litres (265USgal) of water per minute at a pressure of 56bar (811lb/in2) and at a temperature of 16°C (62°F) into the aircraft's rear exhaust plume as it tested its engine.

NLR says: "Water alters the airflow, which reduces the shock waves and turbulence. What we don't yet know, however, is exactly how the water alters the airflow. We hope the camera recordings will help us see that."

A high-speed camera positioned to the side of the aircraft filmed the variations in the plume, which allowed researchers to observe in real time the way in which the noise was reduced. To record the noise levels, 36 microphones were directed at the F-16's exhaust to make recordings both with and without water injection. In studying the noise reduction the researchers are taking into account the F-16 engine's power output, the water pressure and the distance between the microphones and the aircraft.

Source: Flight International