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Saw an interesting aircraft at the Miramar airshow
today. The Lockheed YO-3A Quiet Star was designed during the Vietnam war as a
low altitude night recon aircraft which relied on near silent propulsion
for stealth. It is now owned and flown by NASA-Ames as an acoustical test
platform to gather inflight acoustic data on helicopters - its so quiet it
doesnt corrupt the data. Here's a website on the airplane:
It started life as a Schweizer 2-32 sailplane,
greatly modified. It used a Continental IO-360 geared 3:1 to turn a very wide
blade prop at very slow RPM.
Most pertinent to me was the engine muffler. Hard
to describe so take a look at the picture.
The exhaust enters this faired section on the side
of the fuse. There's a steel chamber at the front that simply provides some
volume for the exhaust to expand into. Behind that chamber is about 8 feet of
oval section pipe filled with holes and a fishmouth at the end. This entire
faired area and the fuselage side under the fairing is lined with fiberglass to
absorb the sound. So the fairing is the muffler.
I suspect this design would actually work for
the rotary because the perforated section is so large that it would dissipate
the shockwave and the cool air flowing through the whole thing as well as the
gap between the perf tube and the fiberglass would prevent the fiberglass from
getting beat up. Mounted on the bottom of the fuse I think it could be pretty
unobtrusive compared to a large muffler slung underneath the airplane. And it
looks like it should be pretty good aerodynamically as well.
Mike Wills
RV-4 N144MW
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