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Date: Tue, 13 Mar 2007 16:18:58 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Engine noise and changes in timeing and mixture.
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wrjjrs@aol.com wrote:

> Enerest, Please note I mis-typed the line and I believe the noise will 
> increase again as the tuning improves. There are many factors here, 
> timing, mixture and exhaust tuning. If all the other factors remain 
> the same I have found that ported engines are usually quieter when too 
> rich. This is anecdotal, I haven't recorded it with a DB meter. My 
> personal explaination is that the cylinder pressures are higher when 
> better tuned and the sudden port opening in a rotary or 2-cycle causes 
> a louder "bark" The rotary is particularly bad in this respect.
> Bill Jepson
>  
>  

Agreed.  But consider that you're at the peak of the tune curve, but 
that curve is very well defined because the fuel varies slightly with 
each injection.  Think of it as drawing the curve with a crayon vs an 
engineering pencil.  You can't really tune it perfectly.  It's like 
driving a car with loose steering or flat tires.  On one injection, you 
have the perfect amount of fuel, the next just a little lean, and the 
next just a little rich.  The rotor next to it is doing who-knows-what.  
How would you get the max HP out of that?  You couldn't, any more than 
you could win with flat tires in an Indy race.  How would that mistune 
express itself?  Well, you would have BIG BARK/little bark/BIG 
BARK/little bark (repeat at 3 to 6K/sec, ad infinitum).  This would set 
up a resonance signal in addition to, over and above the primary exhaust 
signature.  It would take some sophisticated sound sprectrum analysis to 
prove/refute this hypothesis, but it does explain a decrease in noise 
combined with an increase in HP.

-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |