| Well, I just made a guess and measured it at 100 ft. I'll measure cockpit noise later, when I've got some flight time in. Here is my story:
I have a Glasair Super II RG with a Mistral 2 rotor engine. I've had it in the air once, and landed immediately as there was an oil temp problem that turned out to be a measurement problem. However, what I did find that was particularly annoying was the noise. My LightSpeed Zulu handled the noise fine until I keyed the mic. Man, what a blast of noise! I could barely hear the tower.
The muffler that I was using was a 2 1/2 inlet/outlet and 3 1/2" diameter Burns Stainless device that I purchased many moons ago. It was made up with stainless scrubble innards. So, I decided to replace it with another Burns Stainless muffler, but a 2 1/2" inlet/outlet, 6 1/4" diameter, dual stage muffler. See:
Although this one has a 3" inlet/outlet, they made one for me with the 2 1/2". What a difference!!!! Here are the measurements I took from 100 feet, off the wing tip and about 30º aft. They also put in a stainless cloth material instead of the stainless scrubble. They claim that it works great for the rotary.
idle: 57-60 DB 1500 RPM: 78 DB 2000 RPM: 87DB
By RPM, I mean prop at 2.85-1 reduction ratio. I was fearful that going any higher would put my prop too close to the ground as the nose gear compressed down with power. I suppose you math-types can extrapolate different distances.
I'll be hitting the air tomorrow. While I had one short flight already, this will be much longer, hopefully and I can see what the plane will do.
The only real disadvantage of the muffler is that it hangs underneath the belly and looks ugly. But it, so far, works terrific. I hope the drag penalty is not too great.
BTW, the rest of my exhaust consists of 2" headers into a collector with a 2 1/2" outlet made with .065 321 SS.
Best Regards,
Steve Thomas _______________________________________________________
Steve,
Typically the measurement for cars and motorcycles is at 50 feet away at 90 degrees to the directin of the exhaust flow. Any in-cabin measurements wouldn't need orientation, just put the pick up about where your head is.
Bill Jepson
-----Original Message-----
From: Steve Thomas < glasair2@me.com>
To: Rotary motors in aircraft < flyrotary@lancaironline.net>
Sent: Thu, Jul 29, 2010 9:31 am
Subject: [FlyRotary] Noise Measurements
I want to take some noise measurements today with a new muffler I just
installed. What is a reasonable distance from the aircraft to be taking these
measurements? Right next to the airplane seems to be somewhat unreasonable, and
distance makes a big difference.
Best Regards,
Steve Thomas
_______________________________________________________
--
Homepage: http://www.flyrotary.com/
Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html
|