| If an engine nozzle is noisy when
it flies, no one will care how quiet it was on the ground.
The Nozzle Acoustic Test Rig (NATR) is one of three test
stands located inside the anechoic dome of the Aeroacoustic
Propulsion Laboratory. It is unique because it can simulate
a hot engine nozzle in flight.
Tests in 2001 are just one example of how useful data
from the NATR can be. Researchers wanted to prove whether
or not serrations in the engine nozzle (often called
chevrons) could reduce noise. Half-scale models of
a set of nozzles with and without chevrons were tested
in the NATR, and measurements were compared against
calculated sound pressure levels.
Careful analysis of the data indicated that a nozzle
could be optimized to achieve a 2 EPNdB noise reduction
in flight. The optimized design was built and installed
on the NASA-owned Lear 25 and flight tests in Arizona
proved the researchers to be right. The NATR results
were closely matched by the flight test measurements
for overall noise reduction and directivity.
If researchers had to rely on flight test data alone
without the high-quality data from the NATR, optimizing
the chevron nozzle would have been much more expensive.
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