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Date: Tue, 09 Sep 2014 07:23:45 -0700
From: Brent Regan <brent@regandesigns.com>
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To: Lancair Mailing List <lml@lancaironline.net>
Subject: Re: Bellmouth intake design
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Adam asks "can someone point me to any drawings or papers which spell 
out the optimum shape for a bellmouth? Just has we have specifications 
for NACA scoops and airfoils I imagine someone did the research for air 
intakes."

There is an optimal design for ducted fan inlets (turbofan engines) 
because the duct inlet contributes about a quarter of the total engine 
thrust. The engine is accelerating air over a curved shape, just like 
what happens with the wing.

Induction air inlets are not as sensitive as all you are trying to do is 
maximize the diameter of the vena contracta and to do that all you need 
is a nice fat radius, say about a quarter the inlet tube radius. So for 
a 3" diameter, use a 3/8" radius minimum.

Brent Regan