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