Message-ID: <405A54D3.3080000@netdoor.com>
Date: Thu, 18 Mar 2004 20:02:59 -0600
From: Charlie & Tupper England <cengland@netdoor.com>
Reply-To: cengland@netdoor.com
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Rev-2 Runs!
References: <list-3100428@logan.com>
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Russell Duffy wrote:

> > FWIW, the CAFE Foundation's research has shown that using a cooling
> > exit shape very similar to Finn's cheek exits might be the most
> > efficient way to get air out of the cowl.
> >
> > Charlie
>
> Interesting, but what about cooling drag? Right now I have way more
> cooling than I need. I was hoping to reduce cooling drag by closing off
> those exits - potentially with the few inches of the cone removable for
> hot summer days.
>
> Finn
>
> This is all very interesting.  On the rev-1 version, I was planning to 
> try to use the cheek extension area as the only air exit, because it 
> looked to me to be the lowest pressure area.  Traditionally, the 
> bottom of the cowl is used, but I can't believe that's a low pressure 
> area.  The top of the fuselage would get into high pressure due to the 
> front of the canopy, not to mention the ramifications of an oil or 
> water leak.  To me, the sides of the fuselage seem to be the natural 
> choice for air exit. 
>
> This did work, but only to a point.  I found that I had too much 
> cooling with the bottom, and cheeks open, so I blocked off the bottom 
> opening.  That would have been much better in cruise, but was too 
> little for climb.   The new cheek exits are noticeably smaller than 
> the old ones, so I'm sure it wouldn't work as the only exit.   
> Of course I also have a muffler coming out the bottom now, so I don't 
> have the option to close that off anyway.   It would have been easy 
> to block off the cheeks on the old cowl, since I already had those 
> bulkheads, but for some reason, I never tried it.   
>
> I'm also interested in the drag penalty for leaving the cowl cheeks 
> open.  I don't mind the look of the open cheeks (hold 
> the proctologist jokes)  and they do allow for easy inspection of most 
> of the engine.  For now, I'll be happy to leave them open for the 
> extra cooling, and inspections ability, but once I start cleaning up 
> the airframe, they'll have to get closed and faired if they're causing 
> me any significant drag.  
>
> Do we have any aerodynamics folks who could comment on this (in plain 
> English <g>) ?
>
> Thanks,
> Rusty

Here's the link to the CAFE stuff. Open 'local flow II & scroll down to 
the photos of the 'bluff' exit.

http://www.cafefoundation.org/research.htm

Not a lot of detail, but what I've read indicates that the idea is to do 
'velocity recovery', or the inverse of the pressure recovery done to 
feed the heat exchanger. They talk about the bump accelerating the 
outside air & creating a low pressure area that's filled by the 
accelerated exit air through the exit duct from the heat exchanger. The 
inside is as important as the bump on the outside.

If you think about it, it's not that different from the bottom cowl 
opening on most tractor style planes except that it has nice round 
surfaces instead of corners that allow the air to stagnate in the duct.

As evidence that the idea has some merit, I recently saw pics of some of 
the really fast Reno racers & they had air exit bumps like this.

Charlie