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Been out of the loop for about a month - computer crash.
New computer setup now and trying to catch up on 600+ emails. My take
on cooling for what its worth:
1) I like (and used) the idea of separating the cooling
circuits as much as possible. Air to oil for the oil cooler and separate
inlets/ducts for the oil cooler and radiator. That way when one or the
other doesn’t work they are as isolated as possible making it easier to
diagnose and fix.
2) Don’t bother with trying to take advantage of any
sort of thrust recovery. As far as I can tell, thrust recovery in the P-51 is
legend. Never found anything in the way of proof that it really occurred. It's
hard enough just designing and implementing something functional.
3) Go ahead and use the theories available on proper
inlet/diffuser design, but don’t get too wrapped around the axle if you cant
design/fabricate/fit the theoretical optimum. Don’t fall into the Paul L trap of
designing for the rest of your life at the expense of finishing and flying. I
fell victim to this and spent a couple of years building inlets and
diffusers.
4) Oversize your inlets/exits a little beyond what you
think you'll need. Wouldn’t you rather overcool on initial flights and have to
figure out how to reduce your cooling than to see oil and coolant temps climbing
through redlines at 500 AGL on your first flight?
5) If you have more room for a bigger radiator, use
it. You'll pay a weight penalty, but more core surface area has the potential to
reduce cooling drag if the diffuser/inlet is right.
6) On a tractor airplane, coolers in the front. Everyone
wants to copy the P-51 belly scoop for aesthetics, theoretical advantages
(ref #2 above), etc... Coolers in the cowl take advantage of prop blast for
better ground cooling, weigh less by eliminating long plumbing runs, and are
less subject to boundary layer effects and other air inlet obstruction
issues.
7) Copy something that works. When I started my cooling
system design there werent a lot of options, but now there are quite a few guys
flying. I like what I have (really wanted to get rid of the cowl cheeks) but
could have flown at least 3 years sooner if I'd just copied Tracy. If building a
canard I'd copy Al G's setup in a heartbeat.
Just my take on it. Note that my rules arent about
designing the best system, but about trying to get something that will
work first time out. There's enough to worry about on the first flight and
its nice to have a cooling system that works from day 1.
Mike Wills
RV-4 N144MW
Sent: Sunday, December 20, 2009 1:58 PM
Subject: [FlyRotary] Re: Oil Cooling
I've been reading everything I could on cooling after my first attempt
flopped. Today I got my new cooling set up complete enough to ground test. I
copied (my version) from several of you, notably Dave Leonard's radiator and
Tracy's RV-4 oil cooler. Hat's off to Ed Anderson for all the under cowl duct
work on his website too.
I did achieve the stable temps at low power that Dave L told me are
critical. I just now got done running my RV-8 13B turbo for over 20 minutes and
got stabilized temps of 140F water and 170F oil. Once the oil cooler thermostat
opened, the temps dropped a few degrees.
Outside air temp 53F, 81% RH. Perhaps I should have run it longer, but
worries about leaks, fires etc. always make inspections prudent.
I suppose I have overcooled my plane, and added a big drag bucket on the
chin, but hopefully it will be safe to fly, and I can work back from 'too cool'
for more speed later.
After reading Tracy's post about adding the oil cooler exit duct, and
different flow resistance of the two coolers, I am glad I devided the
two.
What I think lead to any success I might be having, is that I devided the
air suppies and outlets. The oil cooler gets 100% of the right cowl cheek air,
and the heated air exits the right side of the cowl. The radiator gets air from
the highest pressure point on the airframe: under the prop spinner. I left out
the upper lip because I believe this area has attached laminar flow. The
radiator exit air passes out the cowl bottom in the usual way.
The left cowl cheek is for Engine intake, plus surface cooling air on the
engine, turbo, & fuel system. It shares the bottom outlet with the
radiator air. I'm still working out the this duct.
.
-----Original
Message----- From: Tracy Crook <tracy@rotaryaviation.com> To: Rotary
motors in aircraft <flyrotary@lancaironline.net> Sent: Sat, Dec 19,
2009 5:46 pm Subject: [FlyRotary] Re: Oil Cooling
In retrospect it does
seem obvious but I had never thought about matching the airflow characteristics
of the water and oil heat exchangers before. I should know by tomorrow if
this is just wishful thinking. Those heat exchanger volume figures you
mentioned are at best just a rule of thumb but still useful. Other details
can make a world of difference. For example, my RV-4 has had the same
cooling components since day one but cooling on that first flight looked
hopeless. Now it is fine on even the hottest days. Heat exchanger
size isn't everything. My 20B has significantly less than 1.5 times the
core volume of my 13B installation. I'm hoping that better diffusers and
other details will make up for the relatively smaller heat exchangers. I
can't think of any reason why the 20B would need more than 1.5 times the 13Bs
cooling requirement if all other factors are the same. That's a big if of
course. Tracy
On Sat, Dec 19, 2009 at 7:15 PM, George Lendich <lendich@aanet.com.au>
wrote:
Tracy,
All that makes perfect sense and leads me to a
question which has more curiosity value than anything else. What actual size
did you settle on for the 20B. I'm curious to know if the 20B requires more
cooling than 1.5 times a 13B.
Calculating the ( rule of thumb) radiator size of
approx 600 cu" for 200hp, giving 3 cu" per
HP, the size of the Mazda oil cooler then gives a .8 per cu" per hp.
I wondering if this holds true for the 20B and indeed the single
rotor.
George ( down under)
Just an update on my RV-8 / 20B
oil cooling experiments.
On the theory that airflow patterns
inside the cowl were blocking airflow through oil cooler, I installed a
partial exit duct behind the radiator directing the airflow downward toward
the cowl outlet. It looked very restrictive but flight tests showed
almost no affect on water cooling (which is OK) but a significant
improvement in oil cooling. I further restricted the airflow
through the rad by putting some roof ridge vent material inside the inlet
diffuser. This gave a tiny increase in water temp but a further
improvement in oil cooling. Long story short, after several
more tests it became apparent that back pressure under the cowl was having a
major effect on the oil cooling. I have no idea why my instrument
did not read the pressure correctly. It works fine on the bench and is
properly referenced to the static system in the plane. The
temptation is to keep changing the cooling outlet scheme until the internal
cowl back pressure is low enough to get the cooling good enough. My
belief is that this would lead to a very high drag solution. You may
remember the experiment I did by flying with the cowl removed. The
cooling was never a problem then (except perhaps too much cooling) but the
drag was enormous. The fuel burn was 60% higher at the test airspeed of
130 mph.
The conclusion I eventually came to was that the rad (because
of it's relatively low air flow resistance) is hogging the airflow capability
of the cowl cooling outlet. (cowl flap did not have enough effect to fix
the problem). Keep in mind that the oil cooler is a thick AC evaporator
core that is very restrictive. The current experiment is to
replace it with a much less restrictive (to airflow) oil cooler. I found
the largest cooler that would fit in the same location as the AC core and I'm
using the same diffuser as before (slightly modified to fit the larger face of
the new cooler). This cooler is only 2" thick and core volume is 30%
less than the AC core. It is slightly larger in volume than an RX-7
cooler. Without any back pressure (flying with cowl off), the AC core
had way more than enough cooling capacity (146 F oil temp on a 93 degree day)
so I'm hoping that this smaller cooler will be enough. Should be ready
to flight test it this week.
I should point out another symptom. Power
setting (and therefore airspeed) had very little effect on the cooling
(i.e., it didn't get much hotter at high power as long as airspeed went up as
well. Things got hot fast in climb however. This also
indicated to me that cooling was limited by airflow through the system rather
than by the oil cooler's ability to transfer the heat to the air. If the
cooler is simply too small, more airflow will not help much.
Tracy
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