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Date: Sat, 21 Aug 2004 19:48:22 -0400
From: Mike Robert <pmrobert@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EM-2 MAP readings
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Russell Duffy wrote:

> Hmm, Weber was pretty much upfront with the fact that their TB ports
> were only for balancing idle and shouldn't be relied upon for accurate
> readings at any sort of power setting. Perhaps the TWM port  is placed
> differently than the Weber TB.
>
> I'm still skeptical, but I can't think of any way to prove whether 
> they're correct or not. 
>
>  IMHO, I think your 42s should do just
> fine at the revs you're turning. In fact, I think the intake charge
> velocity created by the 42mm tract may help you. I have info here (not
> in the attic!) reference intake VNE as well as a link to a calculator
> for gas velocity in a pipe. The link is
> http://not2fast.com/gasflow/velocity.shtml - you'll have to do the
> metric/English conversion thing but the math is sound. I'll post the gas
> velocity  thing later when I can get to the info. I want to say 600 f/s
> but will confirm.
>  
> Neat calculator, but let's see if I'm using this correctly. 
>  
> I'm looking at one rotor, since each TB bore and intake runner feeds 
> only one rotor.  The displacement of a 13B is 1.3 liters, so one rotor 
> is .65 liters.  My handy converter turns this into .023 cubic feet per 
> rotor, per revolution.  Multiply this by 7000 rpm, and I get 161 cubic 
> feet per minute.  I enter this into the gas flow program, leave the 
> temp at 100F, and put in the runner ID of 1.625".   That gives me a 
> mean velocity of 186 ft/sec.    
>  
> From the notes at the bottom of the program screen, and your 
> recollection of 600 fps, my 186 fps figure doesn't appear to be 
> stressing my intake size at all.  The real question is- did I screw 
> something up in the way I figured this? 
>  
> Feeling better about my intake all the time.  Pity the area is covered 
> with thunderstorms today.  I really don't want to test my new XM 
> weather that bad :-)
>  
> Thanks,
> Rusty (replacing home security system destroyed by lightning)
>  
>  

Rusty, I don't see anything wrong with your assumptions or math. One 
fact that throws things off is the very strong pulsatile nature of the 
rotary intake. The formulas work well with steady state, steady flow 
situations. You start throwing in intake resonances, the rotary's 
inherent strong intake pulses generating same, etc., all bets are off. 
But - by personal experience, the wraparound manifold you have should 
work well with the <=7K steady state area you plan to operate in.

     -Mike