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Date: Wed, 26 Sep 2007 14:35:30 -0500
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Crabs
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George Lendich wrote:
> Ed and Bill,
> 
>         Just some ponderings as I progress along my way of understanding
>         different carb developments.
>          
>         I notice with a slide carb ( round on round opening) you get 
>         more of a straight line function of air to fuel ratio as it
>         progressively opens.
>         With a flat slide on round opening you get 'diminishing returns'
>         ( my term) as you get past half way.
>          
>         I notice that some carbs have eggs shaped openings, I guess
>         that's to even out the percentage of fuel/air mix to percentage
>         of opening and to compensate for the enrichening at higher RPM
>         i.e. more air to fuel at higher rpm at the fatter end (top).
>          
>         I was wondering what was wrong with a square carb - not
>         completely square mind you, but with some roundness to corners
>         say 1/2 diameter.
>         Wouldn't this provide for a more linier opening to fuel/ air
>         ratio mix OR does this pose other problems such as air flow and
>         drag/ turbulence? 
>          
>         George (down under)
> 
>     Typing your subject line a bit fast there George?
>     Actually Ford tried a completely square opening just prior to going
>     to FI. The attempt was to make the carb throat into a supersonic
>     nozzle. It had a square, or rather a rectangular opening with a
>     specially shaped ramp on one side that moved. It was electronically
>     controlled. That was suposed to be it's weak point the electronics
>     would faill and the nozzle wouldn't work. Then it was just a crappy
>     wierd looking carb! Hard to beat the two circles system. Many have
>     tried and many have failed. If your making a new carb, just go round
>     and save yourself some trouble. You need the linear change in
>     opening to keep everything working properly. You can do it other
>     ways, (the Predator carb uses two barn door type flaps that open
>     into the throat, leaving a square opening), but EXPECT to have a
>     BUNCH of development time in it to get it to work.
>     Bill Jepson
> 
>      
>     Bill,
>     Thanks Bud - I'm hearing you. I won't attempt the degree of
>     complexity you are mentioning - but it's all very interesting!
>      
>     I thought about an electrically controlled needle that moved up and
>     down that was governed by an O2 sensor reading, but it would require
>     very fine adjustments, probably a small servo motor - beyond my
>     capacity. Until I win Lotto that is!
>      
>     Marvelous how the mind probes the different concepts. As soon as I
>     sent the last e-mail I realised a completely square carb would have
>     too much opening at the lower end - hence the egg shape that has
>     been used.
>      
>     I'm now thing half round at the bottom ( to centre) and then square
>     sides at the top half.
>     It's worth looking at - but blending the shapes i.e. carb to round
>     inlet tube might be a neat trick.
>     George ( down under)
I wouldn't pretend to be a fluid dynamicist (see, I can't even spell it) 
but there is a good reason to use round. Round has the least 
surface-to-area (volume) ratio. Lower drag on the air.

Charlie