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.