Subject: Stock Oil Cooler
From: Paul
Date: 9/9/2000, 7:15 PM

Russell Kent wrote:


Ken Powell wrote:


Paul,
The stock oil cooler is 19.625 * 4.5 * 2 for an area of
176.625 sq. inches.  It appears that this cools fine up to 185
HP (Tracy), but the duct needs to be correct.  Note that some
parts of the country have 100 degree days in the summer and
Arkansas is one of those so I am anticipating cooling problems
just like everyone else.

If the oil provides 1/3 of the cooling in a rotary, and we
are saying that you need 500 to 1000 sq in for water cooling, it
is no wonder that we are having lots of oil cooling problems due
to the small size of the stock cooler.  Why does the stock oil
cooler seem work so well despite it's small size????  It is
built very rugged, but does not seem special from a cooling
perspective.  Most importantly, do you believe that the stock
oil cooler will be enough to handle from 200 to 240 HP?  I'm
worried about not being able to cool our PP engine.  I realize
that Fred Morino's work shows us that the larger the radiator
the lower the drag, all other variables being equal.


As this chart that Paul sent out on Sat 01-Sept-2000 shows, the oil
absorbs about 6.5% of the total heat while the water absorbs about 17%.
That is just shy of a 1:3 ratio (actually 1:2.6).  So if a water radiator
is 500 to 1000 square inches, the matching oil cooler would need to be
190 to 380 square inches.  But this computation ignores the fact that the
oil can (and should) be much hotter than the water.  A hotter working
fluid will shed heat more rapidly (more efficiently) to the passing air,
so the oil cooler can be slightly smaller.  As a WAG, let's use 10%.  So
that gets us to 170 to 340 square inches.  I think this explains why the
stock 176 square inch oil cooler works barely adequately in some cases,
and is insufficient in many other cases.

As a rule-of-thumb, using a 1:2.5 ratio between the oil and water
radiators' sizes sounds pretty good.

Russell Kent


 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org