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Date: Thu, 03 May 2007 08:51:43 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] How does the air know?????????
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tonyslongez@cox.net wrote:
> This may be a silly question but, (here goes) How does the air know?
> how does the air know the length of the intake( runners) tubes versus the lenth of say a plenum?  What if I have a really long plenum that acts like a long tube. how does the air know when it has reached an intake tube or port.
>
> Do you understand what I'm trying to ask? air is air ! 
>
> Is it all just a preasure game with these intakes?
> How many different types of preasure are there?
>
> I get the length part of it. I guess where I'm confused is the difference between a plenum and an intake runner, can a plenum act as an intake runner?
>
>   
Air has mass.  When you start sucking it down a tube, you get momentum.  
People usually don't think of momentum in tubes, but it can be 
significant.  I was working as a security guard when one of the 
supervisors got overly busy and shut off valve to the main water supply 
pipe to quickly.  It ruptured the pipe and create all sorts of commotion.

So, you've open the intake to your first rotor and started sucking air 
down it at a rate of 200mph or thereabouts.  The rotor goes around a bit 
more and shuts the intake off.  Where does that momentum go?  Why, it 
bounces off of that cast iron rotors side, and back up the intake.  The 
pressure will go right back out the intake and into the free airflow if 
there is nothing else to catch it, but if you play the game right you 
can have that pressure wave traveling down the intake of the other rotor 
just as it is opening up.  This results in a little extra pressure to 
ram more air into that cylinder right before it closes.  When it does 
close, there's a resulting pressure wave bouncing back.  If the runner 
lengths are identical, the first rotor will catch the second's bounce.

On the exhaust, you want the opposite to happen.  You want a negative 
pressure wave.  It turns out, that when the positive pressure wave 
(cause by the exhaust opening up) hits a plenum, it reflects a negative 
wave back down the exhaust pipe.  The maximum power situation is when 
that negative wave gets to the exhaust port just as it is opening for 
the next rotor face.

That's the FL30 view.  Runner diameter, runner length, temperature, air 
density,  the moon's phase and a thousand other parameters all seem have 
some effect on how those waves behave.  That is where all the fun begins.