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----- Original Message -----
From: "Bill Dube" <bdube@al.noaa.gov>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, December 01, 2004 11:19 AM
Subject: [FlyRotary] Re: Muffler design
>
> >
> >
> >I believe (have not done the calculations) that if you put both exhaust
into
> >the same tube that you may find 24" is sufficient to get some good noise
> >reduction. However, in order to handle double the exhaust gas flow that
> >putting both exhaust ports into one tube, you might find that a 2"DIA
tube
> >might be on the small size. I would probably think about at least a 2
1/2"
> >perhaps 3" dia tube. Unfortunately, I had little success in finding 3"
dia
> >SS discs at least .062 thick. So I did not go down the 3" path.
>
> A cross-over pipe is significantly different than combining the
> two tubes with a wye. Its length and positioning provide a set of
> resonances that can help damp the sound. The length of the cross-over
> itself combined with the length of pipe between the engine and the
> cross-over tee, and the length of pipe from the tee to the first muffler
> baffle all play a role. Not a simple system by any means.
>
> Perhaps one could nickel plate a section of 4130 airfoil shaped
> tubing for the cross-over.
>
I have seen "H" as well as "X" cross over assemblies offered, supposedly
the X is somewhat better due to the lack of 90 deg turns for the exhaust
energy. But, all are somewhat expensive and don't seem to be made for less
than 2 1/2" tubes. I agree it would be important where you position the
cross over and again is likely to be a compromise. For the moment, I think
I will leave the cross-over approach to someone else interested in delving
into new areas of exhaust suppression. Good idea about using the 4130
lifting strut shape for the cross over. Would probably have to plate it
inside and out to get decent life out of it.
I still have a bit more testing with my current approach and will try to fly
it tomorrow and see how it affects engine performance at the top end.
There are other interesting developments such as exhaust throttle bodies
which keep a valve nearly fully closed at idle and low rpm operating to mute
the exhaust and then use manifold pressure to fully open at WOT for minimum
restriction such as on take off roll.
Some of the newer autos in Europe are appear with some rendition of this
approach.
Just a whole world of exhaust alternatives out there - but, who has the time
and money to try them all?
Ed
Ed
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