On 06/06/2010 07:07 PM, Erich Trombley wrote:
> I have a one off custom FWF that uses the stock engine mount, water
> radiator and oil cooler (located one behind the other which in many
> aspects is close to what you have done). I had a very similar
> experience to yours in that I conducted my first flight in January
> when temps were mild and as summer approached I ran into major
> cooling issues just as you described.
It looks indeed like your setup is quite similar to mine.
Just like you, I can't drop the oil cooler without major modifications.
> This is what I did to solve my cooling problem which was pretty
> straight forward: I enlarged the 2" diameter holes to create a
> horizontal teardrop. The extra air flowing over the top of the
> engine was sufficient to keep it cool both on the ground and during
> climb.
I have two naca ducts in the top cowling with a diffuser angled down so
it dumps the air on top of the cylinders. After some testing, I believe
that in my situation the answer should not be found in increasing the
cowling airflow. Reasons are:
1) There are no signs of elevated temperatures inside my cowling. No
color changes, no melted plastic parts, etc.
2) I had the large 3" stock cowling inlets plugged, but openen them
again in the hope that this would enhance cooling. Well, it didn't.
There was a very slight improvement in ground cooling, but not in flight.
3) As the Rotax engine is mainly water cooled, it is more efficient to
cool it by water than by air. All air taken away from the front of the
airplane reduces pressure recovery and results in more drag. So we
should try to use the cooling air as efficiently as possible. True, if I
remove the cowling completely the engine runs much cooler. But how much
drag are we creating with that?
The more I learn about this subject, the more clear it becomes to me
that two radiators in series is a bad design. The front radiator has no
free exit, the air flow is impeded by the second radiator. This severely
reduces air flow through the radiator. Less airflow is less cooling. The
second radiator receives hot air from the first radiator. Less
temperature difference is less cooling. Also this radiator suffers from
reduced airflow because of the drag of the first radiator.
So, as it already turned out that two radiators in series doesn't work
well in my case (and in the case of the majority of the forum members)
I'm going to change it.
(In your situation it works, but even with your enlarged cowling
openings, you are still balancing on the red line and have to limit
climbing to less than Vy. It is better than what I have now, but now I
have to work on it anyway, my goal is to solve the issue completely.)
The idea is now to take the oil radiator completely away, so the water
radiator can vent the air freely. For the oil radiator I will take a
smaller one (everyone reports too much oil cooling once the oil cooler
is lowered and it is no longer fed with hot air), fit it somewhere else
in the cowling, and connect it with a scat tube to a dedicated air inlet.
Additional advantage is that I can put a butterfly valve in the scat
tube and so reduce the cooling air for the oil cooler in winter (or
during warm up), which I think is better than a thermostat (less oil
line connections, and less drag: we are not going to spend energy to
drag air through a radiator while we actually don't want to cool the
stuff inside at all). For the moment I will dump the exit air from the
oil cooler inside the cowling. The 914 equipped Dimona does it that way,
and it tows all day long gliders with full power at no more than 50
knots, in hot weather, without any cooling issues. If really needed I
can give it a dedicated output, but for the moment I will try it this way.
The Rotax manual says that the water radiator releases 30 kW and the oil
cooler 7 kW. So, the oil cooler accounts only for about 20% of the total
heat release. I think it is a good idea to separate it totally from the
water radiator.
Thanks for sharing your experiences!
Frans
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