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Date: Sat, 17 Jun 2006 22:18:12 -0500
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Exit area smaller than intake was External Diffusion
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Ed Anderson wrote:

> Thanks Charlie, I'll read the Low Flow Articles.
>
> Its no secret that you reduce exit airflow drag by accelerating it to 
> the outside airstream velocity.  We also know that if we make an area 
> smaller that the air velocity will speed up.   I will be very 
> interested in how they do it without incurring drag.  It would take a 
> higher pressure area in front of the narrow exit in order to force the 
> hot air through a smaller area. Since mass flow will remain constant, 
> you can increase the velocity and get the same amount of air through a 
> smaller exit area or increase the area and get a larger volume of air 
> through at a slower velocity.
>
> But when you decrease the exit  area you generally will experience 
> pressure increase in front of the narrow area (which in this case is 
> inside the cowl).  That pressure increase in back of a core or fins 
> would tend to oppose the air flow coming in to the determent of good 
> cooling.  If they had exhaust augmentation then I could see how that 
> would work.
>
> But, I should stop speculating and read the articles and see if that 
> improves my understanding of how a smaller exit improves cooling.
>
> Ed 


I wouldn't pretend to understand it; I just remember reading that it can 
be done. BTW, Tracy S. did mention accidentally getting some extractor 
effect that improved his oil temps while cutting his exhaust pipes in an 
attempt to tune them.
C.