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Date: Sat, 19 Dec 2009 10:21:41 -0500
Message-ID: <1b4b137c0912190721v3b264735q81eaecf245c68c5@mail.gmail.com>
Subject: Re: [FlyRotary] Re: Oil Cooling
From: Tracy Crook <tracy@rotaryaviation.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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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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<br>Just an update on my RV-8 / 20B=A0 oil cooling experiments.=A0 <br><br>=
On the theory that airflow patterns inside the cowl were blocking airflow t=
hrough oil cooler, I installed a partial exit duct behind the radiator dire=
cting the airflow downward toward the cowl outlet.=A0 It looked very restri=
ctive but flight tests showed almost no affect on water cooling (which is O=
K)=A0 but a significant improvement in oil cooling.=A0=A0 I further restric=
ted the airflow through the rad by putting some roof ridge vent material in=
side the inlet diffuser.=A0 This gave a tiny increase in water temp but a f=
urther improvement in oil cooling.=A0=A0 Long story short,=A0 after several=
 more tests it became apparent that back pressure under the cowl was having=
 a major effect on the oil cooling.=A0=A0 I have no idea why my instrument =
did not read the pressure correctly.=A0 It works fine on the bench and is p=
roperly referenced to the static system in the plane.=A0=A0 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.=A0 My belief is that=
 this would lead to a very high drag solution.=A0 You may remember the expe=
riment I did by flying with the cowl removed.=A0 The cooling was never a pr=
oblem then (except perhaps too much cooling) but the drag was enormous.=A0 =
The fuel burn was 60% higher at the test airspeed of 130 mph.<br>
<br>The conclusion I eventually came to was that the rad (because of it&#39=
;s relatively low air flow resistance) is hogging the airflow capability of=
 the cowl cooling outlet.=A0 (cowl flap did not have enough effect to fix t=
he problem). =A0 Keep in mind that the oil cooler is a thick AC evaporator =
core that is very restrictive.=A0=A0 The current experiment is to replace i=
t with a much less restrictive (to airflow) oil cooler.=A0 I found the larg=
est cooler that would fit in the same location as the AC core and I&#39;m u=
sing the same diffuser as before (slightly modified to fit the larger face =
of the new cooler).=A0 This cooler is only 2&quot; thick and core volume is=
 30% less than the AC core.=A0 It is slightly larger in volume than an RX-7=
 cooler.=A0 Without any back pressure (flying with cowl off), the AC core h=
ad way more than enough cooling capacity (146 F oil temp on a 93 degree day=
) so I&#39;m hoping that this smaller cooler will be enough.=A0 Should be r=
eady to flight test it this week.<br>
<br>I should point out another symptom. Power setting (and therefore airspe=
ed) had very little effect on the cooling=A0 (i.e., it didn&#39;t get much =
hotter at high power as long as airspeed went up as well.=A0=A0 Things got =
hot fast in climb however.=A0 This also indicated to me that cooling was li=
mited by airflow through the system rather than by the oil cooler&#39;s abi=
lity to transfer the heat to the air.=A0 If the cooler is simply too small,=
 more airflow will not help much.=A0 <br>
<br>Tracy<br>

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