Whatever other qualities they may possess, jet fighters are not known for their quiet engines. They could soon become considerably less noisy without any loss in performance, though, thanks to a relatively simple alteration.
In tests performed within an anechoic chamber at the University of Cincinnati, aerospace engineers started by firing up a 1/28th-scale model of the F404 jet engine used in the F-18 fighter aircraft. Once they had established a baseline for the amount of noise created by its exhaust plumes, they tried swapping in different exhaust nozzles.
After some experimentation, it was found that a nozzle with a ring of small triangular fins on its inner surface altered the exhaust flow in such a manner that the perceived engine noise was reduced by five to eight decibels. That might not sound like much, but as lead scientist Prof. Ephraim Gutmark points out, "Typically, engine companies are happy even getting a half-decibel improvement because decibels represent a logarithmic scale."
Additionally, it was determined that use of the nozzle would not negatively affect the performance of the engine. In fact, the noise reduction could allow jets to last longer – in a phenomenon known as acoustic loading, planes can be literally shaken apart over time by the excessive vibrations caused by their engine noise.
Of course, quieter planes should also be easier on pilots and flight crews. According to the US Department of Veterans Affairs, hearing loss and tinnitus are the leading causes of military disability claims.
The US Naval Air Systems Command (NAVAIR) plans to test full-scale versions of the finned nozzles on F-18 Super Hornets, later this fall (Northern Hemisphere).
Source: University of Cincinnati
Are they implying that the internal fins would not reduce the noise of a higher bypass turbofan?