Hi Neil and other Rotorheads;
Sorry for the delayed reply, had a busy week with
my kid in a hockey tournaments, as well as my own beer
league. I love this season.....
I was using an AeroTurbine 2525 several feet
downstream of my modified stock turbo until recently.
It's a long story why and how, but I had this muffler
inside a housing to keep it out of the air stream for
aerodynamics and to keep it out of the cowl. Hard to
describe and it was a half-baked idea, that I would
not do again, but it was successful in that I had no
visible muffler hanging below that aircraft and it was
very quiet. I had strived to have a quiet aircraft and
was quite satisfied with this aspect and many other
pilots on the airport grounds had made very positive
comments about how quiet it was when I would taxi past
and also on take-off and climb out. I only ever had
one cranky old guy say that it was loud, but it was
clearly a case of it being different and not like a
conventional aircraft noise, so to him it was
obviously not good and must be loud. I think we can
dismiss this one complaint.
Unfortunately I've not got any proper decibel
measurements from a proper meter, but I did install a
decibel meter app on my phone and took some
measurements from inside the cockpit during a full
power run while tied down just before I removed the
muffler, for some before and after comparisons.
Why did I remove it? Well again it's back to
that darn half-baked muffler housing idea. The radiant
heat from the muffler would radiate into the cockpit
alongside my leg and despite having the muffler
ceramic coated as well as firewall insulation on the
housing it would get a little warm on hot summer days.
That and I decided that it was time for my aircraft to
go on a diet and start shedding some pounds, so I
decided that it was time to see how bad it was without
a muffler as I had never run it before without a
muffler. Before the AeroTurbine I had a home made
version of a SpinTech.
Anyways the not-so-hard data that you're looking
for.
With the muffler in place I saw a max of 83db,
without it I saw a max of 89db. According to the app
this is equivalent to an alarm clock?
Keep in mind that this from a cell phone app that I
had just downloaded moments before the test and had
made no attempt to calibrate it so really can only be
used for before and after comparison.
So it was only a 6db increase in noise. It has
always been possible to talk easily with the pax
without the use of headsets during cruise flight. So
with such a small increase in noise, I modified the
exhaust pipe to come straight out of the rear of the
cowl and went for a flight. One of my old cowboy
buddies has a small ranch right under the departure
end of one of the runways, so I notified him I was
going for a flight and to go outside to take a listen
and tell me how it sounds. I didn't tell him why, but
he knows that I've always wanted to make sure I was
quiet. So afterwards I stopped by for a beer and first
thing he says is that it sounds quieter now. WTF?
Since he's not very technical, I'm thinking again that
it's just a different tone and he has interpreted it
as being quieter.
So I have a co-worker who lives under the
downwind leg of the circuit and he is much more
technical minded (he converted his own truck to
electric about 10 years ago) so his opinion is
probably more relevant. However since he's below the
downwind leg, it's at a point where I have reduced
power, so is already much quieter that when climbing
over my other friends place. Anyways this guy has
previously reported to me that he feels my plane is a
little quieter than most, but mostly it is a different
tone that is easy to tune out as he finds it less
obnoxious than many of the GA planes that fly over. So
I asked him a few weeks after I made the change if
he'd seen me flying over recently and how did it
sound? He said he hadn't noticed any difference, but
wasn't paying attention to the sound anymore. So while
that's not very scientific, it tells me that any
increase in sound or change in tone hasn't been enough
to make anyone take notice. So for all that reason, I
think I'm going to make this a permanent weight loss.
So clearly the stock turbo does provide some
muffling. Another thing to note is that the thin wall
321SS pipe is about 30" long and has 5 elbows in it as
it weaves it's way out. I'm not sure that would have
much effect, but I imagine it's better than a straight
pipe.
I see Dave has also responded and while I believe
he has a stock turbine housing, the turbine and
compressor wheel are not stock at all, so I don't know
how much, but I suspect this would have some effect on
sound.
As far as the longevity of the AeroTurbine, It
showed no signs of damage at after about 25 hours of
flight and probably another 15 hours of ground
running. That was before I sent it out to have it
ceramic coated and I then put about another 10 hours
on it (ground and flight) before I removed it. It
shows no sign of damage or flaking of the ceramic
coating. I'll probably use it on an old Jeep that my
kid and I are rebuilding for him when he turns 16.
Based on my experience, I would clearly chose to
pack the useful weight of a turbo, than that of a
muffler. Although that comes with it's own challenges
as well.