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Back in the Vietnam War era, special ops needed a quiet means of inserting soldiers behind enemy lines to install eavesdropping equipment. They modified a Hughes 500 to do the job. One of the things done was the addition of a muffler to the jet exhaust. How did it work? It looks very different from the lobes and serrations used to quiet the cores and fan exhaust in turbofan engines.

"I'd stand on the [landing pad] and try to figure out the first time I could hear it and which direction it was coming from. I couldn't place it until it was one or two hundred yards away."

With all recent the interest in municipal vertical flight this might be useful in modern helicopters as they are much more intrusive than this.

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  • $\begingroup$ Interesting. Never heard of such a thing! $\endgroup$
    – Romeo_4808N
    Commented Mar 7, 2020 at 4:17
  • $\begingroup$ Basically a large automotive-type muffler. They don't need thrust from it and probably don't mind a small drop in efficiency due to the priorities of the mission. A commercial aircraft is more concerned with performance and fuel efficiency. $\endgroup$
    – Max Power
    Commented Dec 6, 2022 at 4:10
  • $\begingroup$ @MaxPower Do you have knowledge of its construction or is this opinion? $\endgroup$
    – Pilothead
    Commented Dec 6, 2022 at 6:36

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