Gentlemen,
Although my initial cooling issues have been improved upon, I'm not
satisfied at all with the cooling of my 914.
Now, with increasing ambient temperatures, I have to severely limit my
climb outs. Cruising is ok. During climb, both the water temperature and
oil temperature approach the red line, more or less at the same time.
Water temperature recovers faster than the oil temperature after I power
down. At this time I can barely make it to circuit height and then have
to level off, build more speed, and then limit my climb with 200 ft/min
until I'm in cooler air. The engine has now 32 hours on it, so it might
still improve somewhat further, but I don't expect miracles from that.
I had to do something.
First I made an additional exit in the cowling, under the turbo, hoping
to provide more airflow. Whether the airflow in the cowling improved I
don't know, but it had zero effect on the water and oil temperatures.
Temperatures inside the cowling seem to be ok anyway, no discoloured
spark plug indicator labels (still bright yellow), no melted tie-wraps, etc.
My conclusion was that the problem is not in cooling of the cowling, but
somewhere in the efficiency of the radiators.
So then I built a new radiator inlet, somewhat larger than previous one,
with a lip to avoid digesting the boundary layer and making sure the
inlet receives ram air, angled straight into the airflow, and with a
diffuser for which I copied a profile out of a famous book.
It was no improvement over my "free style" sleeker inlet and diffuser.
None. Very frustrating, as I ruined my nicely shaped and painted
original inlet for this.
I need to do something, but to avoid ruining again something in order to
solve this problem, I need to hear some experiences:
1) A shroud over the cylinders. I don't have it, but instead I have two
naca ducts in the upper cowling, curved down to release air aimed at the
cylinders.
Has anyone ever tried whether the Rotax shroud (or home made shroud)
makes any difference? Sure, it will cool the cylinders, but does it have
any effect over the water temperature and/or oil temperature? Or is this
again going to be a loss in time and effort?
2) In my setup the oil cooler receives exit air from the water cooler. I
know there is a "trick" mentioned in the build manual for hot climates
to lower the oil radiator 2 inches to receive cold air instead of warm
air, but I didn't opt for this as I don't consider our climate as
extremely warm, and in winter the oild could actually get too cold.
Has anyone tried both options, and did it make much difference? Also,
did it have any influence over the water temperature? Relocating the oil
radiator will be a large project, and I fear that after that I will
still have to limit my climb outs to avoid the water from boiling.
Yesterday on the airfield I took a closer look at the Rotax 914 equipped
Dimona, which is used by the glider club to tow all day long gliders,
with full power at 50 knots, in all ambient temperatures. They never
have cooling problems.
What I saw was shocking. The water radiator is fully exposed to the
airstream, no diffuser whatsoever, it is just part of the cowling, as if
someone took out some skin of the cowling and fitted the radiator in the
resulting hole. The oil radiator is mounted in an angle behind a naca
duct (I was taught that a naca duct does not provide ram air and
shouldn't be used for cooling a radiator?). Both radiators just dump the
warm air inside the cowling! It can't be easier than that.
Then, at the bottom, the Dimona has a cowl flap which just opens the
cowling to dump *all* air overboard. If it is closed, all radiators, as
well as all cowling ventilation, is blocked.
How can this system, which looks less sophisticated than our system,
while violating all rules and knowledge, work so well?
Anyway. At the moment I'm grounded, can't fly like this, and after
spending a week trying to improve things with zero effect, I'm out of
options and I'm inclined to leave the airplane in the hangar and find
another hobby. :-(
Seriously, what should I do?
Frans
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