Maybe, but all your bases are belong to us.
Oregon too.
******************
I do have more than just gibberish . . . as we know (I think we know) the
largest percentage of aircraft engine noise is from the prop, especially
the tips, beating their way through the ambient air at just under the
speed of sound (and sometimes over, for the really noisy ones like a
T-6). Hartzell came up with what they called their Q-tip prop some years
back in order to provide a reduction in noise signature for their
high-dollar customers who have noise concerns. Prince did the same thing
although in his case his claim was that creating the prop equivalent of a
wing winglet interrupts the tip vortex thus reducing drag . . . just like
it does with a fixed wing. In theory one can imagine this might work -
it was one of the factors I considered when I bought a Prince when I was
shopping for a new prop to replace the Sterba 52 x 52 that the plane came
with. However, as Paul Lipps has documented in Contact Magazine,
whatever drag reduction that might result from interrupting the prop tip
vortex is more than made up for by the drag of added weight and volume
and curved structure hung out at the end of the blades where speed and
drag are where one least wants them. Paul has shown that eliminating
mass from the tip section of the prop reduces drag so significantly that
putting pointy little tips out there improves prop efficiency by such an
enormous amount that "Phantom" have been taking first place at Reno in
the Bi-Plane category since 2003. The tips on Phantom's prop are so
sharp you can easily stab yourself if not taking care when around them.
Phantom has 12 Biplane Gold Championships in 15 years as of 2018.
Something remarkable about Paul Lipps' props is not just their speed but
their silence. Phantom's O-360 may be turning 4500 RPM or more but when
they go overhead it is with a quiet whoosh.
It is said by some that Q-tip Hartzell props and P-tip Prince props are
quieter. It would take a noise meter like Oscar mentions to detect and
measure any sound reduction. When I had my Prince I never noticed that
it was any quieter, but I wasn't paying attention to that metric. See
what Jeff has to say about his Prince. He may have some data to share.
Since most aircraft engine noise comes from the prop, a possible solution
to your noise restriction is to buy a Prince P-tip if they indeed are
significantly quieter. They're pretty pricey so one would want some
measurable data.
A better suggestion would be to consult with Craig Catto
https://www.cattoprops.com/contact/
who worked extensively with Paul Lipps and well understands the intricate
relationship between inlet air position, cowling and prop dynamics. A
quiet prop is not only well . . . quiet . . . but it's lack of racket
indicates a level of efficiency. Paul used to say, "Anytime you hear a
prop making noise what you're hearing is just pure wasted energy." For
anyone interested, Paul has left us a great treasure from his engineer's
mind. This material can be found in back issues of Contact Magazine and
a surprising amount is easily available on the VAF forum. His handle was
"Elippse" (no quote marks) and although he's been gone since 2011, his
discussions all seem to be still available . . . based upon a cursory
search I did recently. For anyone interested in props, Paul is the
person to read.
Anyway . . . the point of this Stef is that you may achieve your noise
goals through giving attention to the propeller rather than by focussing
on the engine. Of course you won't want a prop as strikingly extreme as
Phantom's, but taking those fundamentals and applying them to your
particular situation could be very, very productive.
Mike
KSEE
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