Constant Speed Propeller

Sam Purpura wrote:

I have the same engine/propeller on my Europa. It will constantly run at 5750 inTO, 5450 in climb and 5050 in cruise. I’m in the California desert and the airflow (single opening) to the oil cooler is almost closed off and directed to the radiator and it works best with about a 90kts or more of airflow. Think you might be over pitch with the low rpm and low airspeed and probably improve the airflow through your cooling system. It was a bit of experimenting to get it sorted out, but worth it.

Hi Sam,

With the exception of Cruise, your rpms are the same as mine. My Cruise is usually 5200-5350 rpm.

Air speed, in cruise, is around 135 knots indicated (145-147 TAS)

I too aim for 90 knot climb out.

Coolant temps are good in the air (low 90C's), not so wonderful on the ground (up to115C) - I can live with/manage this.

Oil temps are good (high 90C's), until I go to Cruise where I see a gradual rise to a sustained 110-115C.

At this stage, in my flight testing, I have not gone over 6500ft (AMSL) and am only now going into our winter, so have little data on cooler OAT performance. No matter the OAT to date, my Sonex will have to accommodate ground temperatures up to 45C next summer (not that I usually fly when its this hot). For me, this means I must get my cruise oil temp down below 100C.

Note:  RPM's quoted are CS Automatic mode - I can vary this at will, by switching to Manual mode.

I intend to bring my Sonex home to my workshop, for a major cooling system rethink.

First job will be to determine if the oil cooler has an internal restriction:

A "loose" contamination (eg paint flakes/swarf/etc) should be fairly easy - I can remove the Rotax  cooler from the engine,  flush it out with a solvent (petrol?), blast it with compressed air and examine the "stuff" that comes out. If a significant something is ejected, this will be a welcome relief, - that the cooler can be reinstalled and better oil cooling will ensue.

If nothing is ejected - how can I test the cooler for effective flow? (suggestions please).

If oil cooler flow is judged to be satisfactory, I feel that the only remedy, for the high oil temperatures, is to increase cooling capacity - add a second cooler in series, install a cooler with greater cooling capacity. Your thoughts & suggestions, rational for/against:

A second cooler? 

B. A single higher capacity cooler?

Perhaps improved airflow would help. Have you tried creating a low pressure area where the air exits the cowling. That has helped me getting the temperature down with a simple flap lowered into the airstream from the bottom of the cowl.

Hi Sam,

I will try to list the cooling  modifications from the start;

Cowling constructed to facilitate aft cooling of coolant (radiator). The idea was that all cooling air would exit through the radiator. Steps taken in design & installation, to make cowling as air tight as possible. Inlet nostrils opening, approximate same as exit air (exit air restricted by radiator)

Static, ground, test runs - no modifications to cowling - both coolant & oil temps unacceptably high

Cut small hole, in cowling, in front of oil cooler location - some improvement too oil temp. Small hole, progressively enlarged to about the same area as oil cooler "face".

Oil cooler fitted with rubber baffles, to ensure all incoming air routed through cooler - some improvement

Starting with small hole and progressive enlarging a "belly" hole cut in bottom of cowling - some benefit on ground.

Installation of a pilot activated cowl flap, directly behind exit air - some improvement.

Combination of above mods. brought ground testing temperatures, just within Rotax limits.

Flight testing starts - first flight abandoned due to high oil temperatures (120+C). Cylinder head temps 95C in air, 110 -120C on ground.

"Boxed in" oil cooler. Fitted flexi air duct, from box to dedicated exit air hole, in the side of cowling. Oil cooler air circuity completely separate to cowling air - Big improvement, with  constant vigilance, oil temps (in cruise) can now be kept at 110C 

Large belly hole filled in - no change to flight temps but ground opps. see head temp. rise..

Oil hoses, "reinforced" with springs to prevent collapse - no change to temperatures

Sensor in oil cooler exit air will see 45C (oil cooler working)

If flight conducted with CS in "Climb" mode, max oil temps 104C. "Cruise" mode will see temps slowly rise to 110C. If CS Manual selected and prop moved slightly to "Fine" (100rpm rise for same throttle setting) oil temp. will drop up to 4C.

Note: Head temps. in flight remain, remain around 95C 

For a while now it has seemed to me that its the load on the engine that is resulting in oil temp rise above that which the cooler can accommodate. To me this suggests:

# Oil cooler has an oil flow restriction 

# Oil cooler not "large" enough for heat load in cruise

Hi Sean,

The 912/914 Illustrated Parts Catalog has oil radiator options as large as 234 CM^2.  The 912iS has similar options.  However, the 915 IPC offers a larger oil radiator of 300 CM^2, which is Rotax Part Number 886107.  I’ve seen some SLSA manufactures offering this larger 915 radiator on the 912iS engines.  If you don’t already have this, it’s another option for you to consider.