Rotax 912ULS minimum oil temp 124°

[jnmeade]

Does anyone know the reason for the minimum oil temperature reading of 124° before take-off? Not just because it's in the POH, the Rotax engineers thinking behind setting the parameter? How much slack is there in this number? Is there a safety factor built in?
I'll ask the Rotax people at OSH, but curious if anyone already knows.

[roger lee]

Usually the minimum temp is 120F (50C) and it is set there to make sure the different metals have expanded and warmed before they are stressed at our high close tolerance rpms.

[jnmeade]

Here's the case in point that illustrates my query.
I said the oil temp was 124° before take off. That number is taken directly from the CTSW POH 8.3.11 Oil Temperature. It's the pre-take-off check list.

Roger cites figures of 120/50, which can be found in the CTSW POH at 3.4, where it says the min/max oil temperature measured at the oil inlet of the engine is 50/120 (BTW, 50C converts to 122F).

The Rotax manual says in 10.1.2.4 that oil minimum operating temperature is 50C and repeats the math error by using 120F.

We see that FD and Rotax are close but not exact in their temp spec callout and calculation.

Roger explains that the reason for a minimum oil temperature is to make sure the different metals have expanded before they are stressed by high rpm. I'm glad to know that. Does CHT or EGT offer any information we can use about expansion of metal? I'd think CHT might, at least. Does oil temp do a better job of telling us that information?

The question remains - what is the allowable deviation from the minimum oil operating temperature? 0°? +- 10°? That is my real question. I'm not looking for an excuse or permission to operate outside the specs, I'm only asking how tight the specs are?

[roger lee]

The metal temps heat up faster than oil and you want to also assure yourself that the oil is at an operational temp for a good flow through the system and all the engine metal mass is as close to equal or operational temps as possible. The Rotax Operators manual uses the 120F/50C. Actually 50C is 122F. Why did Rotax pick 120F/50C I don't know why that number over let's say 125F or 115F? It really doesn't make any difference because once you are up there all is good. Most of us are up even higher than the 120F since we don't hit the throttle that very second any way to take off. We usually do a mag check and by the time you taxi into place to take off the temps are higher any way. If you lived some place really cold and could not get the temp over 115F could you take off, sure and with out any issues. Rotax had to draw the line in the sand some place and the 120F/50C is where it's at.

To do a quick calculation of centigrade to Fahrenheit in your head do it this way and it's right all the time.

(Cx2)-10% + 32=F


50C x 2 = 100C -10% = 90C + 32F = 122F

20C x 2 = 40 - 10% = 36 + 32F = 68F

5c x 2 = 10 - 10% = 9 + 32 = 41F

Piece of cake.

[drseti]

Usually the minimum temp is 120F (50C)

My Rotax manual says 50C. The German version doesn't translate that into colonial units, so I'm not sure where the 120F comes from. When I do the conversion, I get 122F, which is what my Dynon is programmed for.

[roger lee]

The temps are always in the Rotax Operators manual.
Version April 1/2010, pages 2.2 (912UL) and 2.5 (912ULS).

The CHT minimum temps are also 120F/50C.

There are several misprints in the Rotax manuals. Some of this is just from the translation.

50C is really 122F, but they have it listed at 120F.

Many probably don't know that the compression ratio for the 912ULS that has been listed for years as 10.5:1 is really 11.0:1. Just another misprint.

[Jack Tyler]

Just a related thought: When I was helping a friend balance his carbs on his 912 ULS yesterday, I noticed he'd installed a CHT gauge along with a switch so he could sample one of several temp sensors he mounted on the engine. That strikes me as a much better indication of operating temps onboard the engine (tho' oil temp has its place there, too). I would be clueless about what I am asking my Lycoming to be doing if I didn't have a 4-cylinder EGT/CHT gauge monitoring engine op temps.

[designrs]

Oil temp > 120 degrees F for operation, primarily a concern for proper warm up in colder weather.

CHT < 230 degrees F, primarily a concern on hot days and when climbing.

Can't imagine not having a CHT guage!

[jnmeade]

I asked a guy at the Rotax booth at OSH Monday. He didn't know. It's an engineer question, obviously.

[jnmeade]

There are several misprints in the Rotax manuals. Some of this is just from the translation.

It's kind of disconcerting when we have to hear from fellow owners what the accurate rendering of an approved flight manual is (or is thought to be). These manuals should be accurate and of course that means consistent.

My German is fair, but I wouldn't attempt to give the definitive English translation of a German technical manual, so I'm not sure which errors you refer to. If the example is rounding 50° C to 120° F when it is really 122° and saying that doesn't matter when in other cases we're arguing about a 1° difference, at least if show inconsistency and sloppiness and in fact reinforces the OP. That is, what numbers are really important? (Keep in mind Rotax will always say the original German documents are authoritative.)

I still don't know and likely won't know.

BTW, this is another reason why we should (have) convert(ed) to the metric system now, since we didn't 50 years ago.

[roger lee]

The difference of a couple of degrees is a mute point. Many gauges especially analog are not accurate enough to make a difference. An example is if an EGT is within 15F of the other they are consider equal when looking at temps for setup and trouble shooting. CHT's can be off too a few degrees because of the restive nature of the probe and wiring to the gauge and inaccuracies in gauges. Don't get hung up on a few degrees it just isn't necessary.
As far as the manuals go this and many other examples are taught and corrected in Rotax classes. The manuals and SB's don't always keep up with change as fast as it happens, especially the manuals or you wouldn't see Service Alerts, SB's and SI's. Use temps and pressures as guides and trends and how one affects the other as your guide to understand what is going on with your engine.

Go to the classes.

[drseti]

Keep in mind Rotax will always say the original German documents are authoritative.

Indeed, it says as much in the front of every English language Rotax manual.

this is another reason why we should (have) convert(ed) to the metric system now, since we didn't 50 years ago.

,

Ah, but we are converting over to metric, slowly, inch by inch.

[drseti]

The FAA is just as sloppy in unit conversion as Rotax is, BTW. The LSA max gross weight is 600 kg. FAA multiplies that by 2.2 #/kg, to arrive at 1320 pounds. But, of course, one kg actually weighs 2.20462 pounds, so the equivalent weight is really 1322.77. (But just try loading up an extra 2.77 pounds into your LSA, and see how quickly the FAA cites you in the event of an accident!)

[drseti]

The difference of a couple of degrees is a mute point.

I guess if you really are mute, all you can do is point.

[drseti]

Just a related thought: When I was helping a friend balance his carbs on his 912 ULS yesterday, I noticed he'd installed a CHT gauge along with a switch so he could sample one of several temp sensors he mounted on the engine.

Not a bad move, if the 912 is in an experimental. (If it's in an S-LSA, he'll need an LOA from the manufacturer, of course). Every Rotax 912 I've ever seen has two head temperature censors installed on the engine, one on the left cylinder bank and the other on the right. Useful, to be sure. But even more useful to me is having an EGT probe in each of the four exhaust stacks. My Dynon engine monitoring system displays the two CHTs, as well as all four EGTs, simultaneously as horizontal bar graphs. There are digital temp readouts as well, but I find I never really look at the digits. A glance at the bars tells me that the head temps on both sides are about the same, and around mid-range, as are all four exhaust temperatures. A deviation becomes instantly evident with even the briefest scan of the EMS.

This is especially useful if you have carb issues. Say one of your two carbs is running rich, for whatever reason. All of a sudden, two of your cylinders (#1 and 3, or #2 and 4) start showing temps dropping well below mid-range, while the other two remain normal. Instantly, you know which carb needs attention.