Quickly heating oil for motorglider in-air restart operations

This is a very interesting topic!

We like your idea about installing an oil heater. Check out the Tanis Pre-Heater system https://www.rotax-owner.com/en/videos-topmenu/product-reviews/149-tanis that can be purchased from many locations (LEAF, Spruce, etc.)

It is safe to progressively push past idle until the engine reaches operating temp (within reason). Since every aircraft is different, you will have to determine and test what RPM you require to keep the aircraft in the air following the restart. Obviously the lower the RPM, the better when trying to warm up the engine. 

You may want to think about installing an oil and coolant thermostat into your system so the fluids can bypass the coolers. This will help with getting the fluids up to operating temperatures more quickly. 

What type of motor glider do you have? If it's a Stemme or a Diamond, you will run into certification issues when installing "aftermarket" non-cert components onto your aircraft, so keep that in mind and contact the aircraft manufacturer to see what options they have/suggest.

I deal with the same problem in my Ximango motor glider, plus living rather far north. Following the ground warming procedures in the air may never get the oil up to 50 deg C / 120 deg F. Here's the key, and the trick:

The temperature requirement is really an oil flow requirement. Oil flow goes up with increasing rpm to match engine requirements UNLESS, as is the case with cold oil, the oil pressure gets high enough to fully open the bypass at ~ 75 psi. Then flow doesn't increase with rpm. So, you're basically good at any rpm that doesn't peg the OP gauge.

So:

1) Start engine.

2) Observe oil pressure. Increase rpm until the pressure stops rising ~75 psi. Nudge the throttle back just a little.

3) Wait ... the OP will drop. When it drops noticeably to like 60-65 psi ... nudge the rpm higher until the OP again hits the high limit.

This will get you into "zero sink" at ~3,300 rpm fairly quickly. Especially if one does some blocking of airflow through the radiator and cooler before taking off. The Ximango is very over-cooled for most local conditions and I'll have some cardboard/tape covering on even up to 70 deg F.

Heating the oil tank while in the air will not be effective, especially since oil drains from the tank into the case when the engine is off. "Ground warming" while in the air barely works, if at all, because the cooling is way out of balance with the heat being internally generated.

@Jeffry Stetson, that's fantastic information! Esp. because I'm in a northern clime as well. Do you have a sense of oil pressure vs. temperature vs. oil weight? For example, if you start the engine while it's cold soaked at (say -20C), is it a struggle just to idle the engine without tripping the overpressure relief valve?

@Rotax-owner, thanks for the tip. I will be checking the coolant and oil for thermostatic valves, although, as you suggest, with a certified airplane options are somewhat limited.

I'd be reluctant to do a -20C start without preheat. IIRC, the review of the Tanis heater on this site did tests about the cranking resistance at very cold temperatures. Because of differential shrinking of metal components and case, bearing clearances approach or reach zero. Too, the air-cooling would be so effective that any sort of warm up at reduced power might be impossible. Kinematic oil viscosity grows very quickly with reduced temperature, regardless of oil "weight". https://wiki.anton-paar.com/en/engine-oil/ for example.

I don't live/fly in a cold climate however this conversation has peaked my curiosity - could you use a fully synthetic (as long as no AvGas in tank) motorcycle oil ? - I have seen motorcycle oils as low a 5W, 10 &15W are common. Car oils can go as low as 0W