Thread: AM I hurting the Engine?

We already have a good amount of tape covering the cooler intake. I noticed my last flight while doing a touch and go, when it was time to power up my oil temp was below 124deg and my RPM arc was RED. The engine still produced power and I lowered the nose and eased back some on the throttle until it was back in the green during climb out. I would have aborted the take off if I had noticed it before I had already commited to take off. This kind of thing makes me nervous because of my lack of experience. Has anyone else had this happen and how do you deal with it? Is it better after landing to do a taxi back and let it get hot again or should I put more tape over the cooler? I think it was about 40-50deg F. I noticed keeping some power in helps it stay warm but is this something that needs to be paid close attention to and am I hurting the engine?

Read the Rotax Operator's Manual. Nothing you did would require a notation in the engine log and continuing the takeoff was the smart thing to do. I (and Rotax) think it's important to get the oil up to temp at least once during a flight to boil off condensed water. OTOH, I'm less sure of the importance of maintaining a higher oil temp for the duration of a flight. I run a high quality full-synthetic oil (Mobil 1 MT Racing) and I find it hard to believe that it would not be effective at lower temperatures. I make sure I'm above the red mark on the oil temp gauge before I launch and then not worry about it for the duration of the flight. As to the RPM, I had my idle speed set to 1500 and I try to keep it in the green as much as possible while taxiing. Brake pads are way cheaper than a gearbox overhaul. All of this is my untrained opinion. Take it for what it's worth.

Hi James,

No issue with the engine just keep flying. You evidently have a Dynon which sometimes gives pilots a little overload on info and then it might be a bit confusing trying to put this info in the proper perspective. Dynon has in its program a setup for the Rotax perameters and it wants the oil temps over 120F so when you glide or idle for a long period the oil temps drop back to 120F or lower and the rpm gets flagged on the Dynon.

Hi Jim,

If your idle is set to 1500 rpm then you are beating your gearbox and dogs to death with the 10.5:1 compression. You also stand a much bigger chance of losing an engine on approach at those low idle rpms. It should be in the neighborhood of at least 1750 -1850. When you guys warm up at first run of the day or just when it's a little cool out it is far better to idle until warm at 2200-2400 rpm to reduce the vibration. 1400-1500 rpm for a regular idle on a 100 hp is a gearbox killer. The 80 hp can get away with that because of the lower compression, but not the 912ULS.

Gee Roger, I had it set to 1500 because it was closer to 1300 when I got the plane. The Rotax manual says 1400. If I set it to 1800, I don't think it would ever land (:

What would be the failure mode of losing an engine on approach? I know that it windmills a lot faster than 1500 with the throttle pulled to idle.

I'll talk it over with Jerome M. when he does the annual in a couple of weeks.

Hi Jim,

Your right Jim, the Rotax manual does say 1400, but it is not for your engine. It is the rpm for the 912Ul not the 912ULS.
All the engines are like that from the Rotax factory and should be set up by the plane Mfg. or the owner since they are sometimes purchased separately. It is evident that yours and unfortunately others weren't done. I have had to set many CT's off that very low rpm. The proper rpm for the 912ul 80 HP is down around 1400 because of its lower compression, but it is an absolutely bad idea to do that with the 912uls 100 hp at 10.5:1 compression. 1750-1800 rpm idle rpm is much better. I guarantee you that you are hammering your gearbox at those low rpms. This is one of the important points covered in the Rotax classes. That's why it is so important and a Rotax requirement to attend one of their classes. You can call any of the Rotax Service centers like CPS, Lockwood or Leading Edge and ask them about the 912ULS recommended idle rpm.
Controlling your air speed is done with the stick and not the throttle. When our friends from the UK said they needed to be down at low rpm to land in 300m I took that challenge and I can do it at 2800 rpm like I land all the time. If you pull back the stick the plane will slow down below flight speed. For me it's all about hitting my target on the ground. The runway is not my target, but only a spot on it.

The failure mode is the engine compression is too great and stops the engine. It has happened and many times. Not to mention the damage you cause to the engine/gearbox.

My Pilot Operation Handbook (Flight Design Pilot Operating Handbook, CT2K and CTSW LSA Sport Models, rev 2) says to "make certain the oil temperature is above 124 F" before takeoff (paragraph 8.3).
I believe the reason for this restriction is that at any lower oil temperature, because of higher oil viscosity, at the full oil pump flow rate (max rpm) the pressure differential across the oil filter element may exceed the filter bypass valve pressure setting. In other words, it takes so much pressure to push the cold oil through the filter; some of it may pass through without being filtered. At cold temperatures, we keep the engine rpm down (below 2000 rpm for the first 2 minutes then below 2400 until the oil reaches 124 F) to ensure that all of the oil is being filtered.
So, while not a real serious issue if it happens once in a while, I don't think you should make a practice of opening the throttle if the engine is below 124 F. And sense you never know when you might need to add power, I think you should avoid letting the oil temperature drop below 124 F in flight… no doubt easier to do here in Southern California than in Minnesota.
Mike Koerner

Hi Mike,

The first 2+ minutes are important. Rotax wants you to idle at around 2200 right from the start. It really isn't an oil flow issue at that point, but a bad vibration one below 2000 rpm and allowing the metals in the engine especially the cylinder areas to heat and expand to there proper tolerances. vibration being a big one below the 2200 mark for the gearbox. Let it idle at 2200-2400 right up to 120F's. It runs much smoother at these rpms. You will not have any oil flow issues here. This comes right out of one Rotax school after another and has never changed.

yep.. paragrah 3.6)

http://legacy.rotax-owner.com/SI_TB_...-014&S_TYPE=MS

I have to point out that with all my avionics on (AP, xponder, Dynons, etc.), I have to idle over 2200 RPM to keep from draining the battery.

Hi Scott,

You shouldn't have any issue with everything on except the landing light. I have the same instruments, except the AP and so do many other. Do you idle with all the lights on?