Received-SPF: pass
 receiver=logan.com; client-ip=204.127.217.102; envelope-from=ceengland@bellsouth.net
Message-ID: <4700070B.20304@bellsouth.net>
Date: Sun, 30 Sep 2007 15:28:59 -0500
From: Charlie England <ceengland@bellsouth.net>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2) Gecko/20070222 SeaMonkey/1.1.1
MIME-Version: 1.0
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Another cooling question
References: <list-2360892@logan.com>
In-Reply-To: <list-2360892@logan.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Bobby J. Hughes wrote:
> Analysis only valid if cores are identical in construction (resistance
> to air flow will likely be different with the different cores).
> 
> Charlie,
> 
> Good point. We do have a known relationship of how the cores behave
> using the temps before and after the exit area was enlarged. 
> 
> My notes show oil at 220-225 and water steady at 190 before enlarging.
> Mark reported oil at 215 climb and 190 cruise and water at 160 (cruise?)
> after enlarging. 
> 
> Exit area improvements. 95-95 deg OAT if my notes are correct. 
> 
> Oil 225 -215= 10 climb
> Oil 220-190 = 30 cruise
> 
> Water 190 - ? = ? Climb (no data)
> Water 190-160= 30 cruise.
> 
> It is clear that enlarging the exit area is made improvements to both
> oil and water cooling. 
> 
> Bobby (hope I never need to understand the other 80% of the equation)
> 

I'm not a fluid dynamics person, but I can read & see what seems to work 
for others, & the stuff that really works has just as much attention 
paid to exit as entrance.

Tracy pointed out earlier that smoothing the airflow *out* makes a big 
difference.


Enlarging the exit works, but it seems to be a brute force method. Some 
of the newer accounts I've read indicate that the exit can actually be 
smaller than the inlet if everything is done right.

If there's no shaping of the exit air flow, it will be totally chaotic 
in the cowl until it tries to re-accelerate from ~40mph back to 180 mph 
to squirt through the exit area. Slow & chaotic seems to me to equate 
with high pressure & high drag, just what you don't want behind the heat 
exchangers. Letting the air leave the heat exchangers & go where it 
wants seems to me to cause the same problems, on a slightly smaller 
scale, that separation causes in the inlet duct.

The P-51 has an exit duct. The fastest guys flying conventional air 
cooled engines in homebuilts also have exit ducts. They attempt to 
confine & shape the area behind the heat exchangers (cylinders/cylinder 
heads) so the air doesn't have room to continue to slow down & 'recover' 
pressure before it needs to speed up again at the exit point. They also 
put big round (as in 3" radius) transitions at the bottom of the 
firewall, so the air doesn't separate when it tries to turn the corner 
from straight down to straight aft.

Does this stuff pass a sanity check with those that know more about 
aerodynamics? If so, am I explaining it where it makes sense?

Charlie