Oil cooler placement and ducting advice - Rotax 912ULS, CH701

[Marty_d]

Hi all,

 

After some advice about the placement of the oil cooler and whether it needs its own air supply (NACA duct + 3" SCAT hose to a moulded shroud)?

 

There's a couple of different positions from Zenith.  

From the 2002 FWF instructions they show it bolted below the radiator, like so (I possibly could get away with this given the holes in the front of my cowl)

 

However later installations show it bolted to a welded bracket on the engine mount arm, angled and with a fitted shroud which receives air feed from a NACA duct on the side of the cowling.

I'm guessing they did it that way (in later versions) because they've mounted the water radiator vertically in order to have a narrow vertical opening in the newer cowl, leaving no room for the oil cooler under the radiator.

 

My cowl has not only a large intake for the water radiator but a smaller chin intake (see yellow arrow), so I'm tempted to go with the older method.  Before I commit though - are there any advantages going with the newer method?

 

Thanks!

Cheers, Marty

[danny_galaga]

Dunno if it helps, but at least some perspective from a different system - on the Bushcat, there is one small oil cooler above the radiator. Radiator is in much the same position as the CH701 as far as I can tell. Then another small oil cooler on top of the engine. The cowl ducts air straight to the cooler ( no hoses) and then straight out again, going over the windscreen.

Edited March 29, 2023 by danny_galaga

[RFguy]

Hi Marty. Have you had time to read through the 912 and 914 (has more useful info) rotax installation manuals ? there is lots of meat in them,

 

Now, what i would be looking for is  : can you satisfy / provide the required pressure differential across the cooler?

 

That's the $64 question. The thin coolers like std rotax - well there is very much ambient temp and heat input variables, but they can work for typical 912 usage  with as little as 2.5" of water , but the deep coolers like #800075 , needs about 5 inches of water for typical loads. 

 

Slowish  aircraft like the Archer, (100 kias cruise)  they have an # 800075 aeroclassics cooler ( a deep cooler that needs lots of differential pressure to work) - it is mounted on some 4" scat from the nose of the plane and tghe cooler, with the duct mounted at the firewall. that cooler is same as factory Jabiru used for 3300.

 

so really, it comes down to if you can make pressure across the cooler. poke a tube  with water meas into the shroud at the cooler to find out ...

 

Dont forget you need to duct air over the cylinders (per Rotax) where possible. otherwise a ton of extra heat load goes into the water system usually from the rear cylinder not getting the blast.  

 

[IBob]

Hi Marty, here is the arrangement on the Savannah, with the oil cooler upper and the radiator lower. All temps remain well within specs here in NZ, in fact I have about 1/3 of the oil cooler blanked off. Rotax used to reccomend raising the oil temp each flight to 90'C, I believe they recently raised that to 100'C, I don't see that unless I make a sustained hard climb. I would say you have plenty of air intake on your cowl as it is.
I would go with that, and in the unlikely event that you need more, you could consider fitting a scoop at the air exit point underneath to lower the air pressure there and induce greater flow.
 

[Neil_S]

2 hours ago, IBob said:

Hi Marty, here is the arrangement on the Savannah, with the oil cooler upper and the radiator lower. All temps remain well within specs here in NZ, in fact I have about 1/3 of the oil cooler blanked off. Rotax used to reccomend raising the oil temp each flight to 90'C, I believe they recently raised that to 100'C, I don't see that unless I make a sustained hard climb. I would say you have plenty of air intake on your cowl as it is.
I would go with that, and in the unlikely event that you need more, you could consider fitting a scoop at the air exit point underneath to lower the air pressure there and induce greater flow.
 

Hi IBob,

 

I find the same on my Savvy here in Melbourne. The Rotax manual does indeed say you should get the oil temp up to 100 at least once a day to evaporate any condensation.

I have blanked off part of the oil radiator in order to get the temps up to 100, and even then I only really get that on a sustained climb. I even found that in summer (although we didn't get any really high temperatures during summer here this year, and I don't fly if it's really hot anyway)

It soon returns to 90 in cruise and sits there.

 

Cheers,

Neil

[trike1]

On early model Savannah’s the oil cooler is mounted behind the radiator.On hot Queensland days(say over 35degrees) the oil on climb out will get to 120 degrees.No mods that I have tried have helped in this regard.Does anyone have any ideas on what might work to bring the temp down

[facthunter]

That's above what they like even for Synthetic base oils.. Funny how some Lycomings and Small Continentals don't need oil coolers Sav's are not a plane you can increase the climb speed much and still climb. Behind the radiator or near the exhaust are places I wouldn't locate an oil cooler.  Nev

[onetrack]

At 120°C, oil is starting to lose its lubrication ability, and some of the additives are starting to break down. This affects cylinder lubrication first, and bearing surfaces second. High oil temperature cooks seals.

Extended periods of operating with an oil temperatures above 120°C creates a greater risk of piston grabbing and bearing seizure. I'd be looking hard at ways of getting that oil temperature back to around 100°C.

Lubricating oil starts to break down at 135°C, and bearing failure and piston seizure in the bore is very close at this temperature.

[danny_galaga]

41 minutes ago, trike1 said:

On early model Savannah’s the oil cooler is mounted behind the radiator.On hot Queensland days(say over 35degrees) the oil on climb out will get to 120 degrees.No mods that I have tried have helped in this regard.Does anyone have any ideas on what might work to bring the temp down

Simplest thing I can think of without mucking around with aerodynamic mods is to add an extra oil cooler inline with the existing one. Almost anywhere you mount it will add some extra cooling, even if on the firewall, unless of course it's right next to the exhaust 😄

 

[IBob]

3 hours ago, trike1 said:

On early model Savannah’s the oil cooler is mounted behind the radiator.On hot Queensland days(say over 35degrees) the oil on climb out will get to 120 degrees.No mods that I have tried have helped in this regard.Does anyone have any ideas on what might work to bring the temp down

A pic would help to visualise what you've got, Trike.

 

[Blueadventures]

Try more lip on the lower cowl, can be achieved by bolting on with say 3mm metal threads and made of 0.5mm alloy as a trial.  Gets the hot air out better.  May save fitting an extra oil cooler.  Cheers.

[facthunter]

Even more drag. Nev

[trike1]

If you Google Rotax 912 Max oil temps some publications say 285F which is 140C but no one wants to see temps that high 

[IBob]

17 minutes ago, trike1 said:

If you Google Rotax 912 Max oil temps some publications say 285F which is 140C but no one wants to see temps that high 

That's the 80HP 912UL.

[facthunter]

Also matters where you measure it. Individual bearings and piston temps will be much higher.  Gudgeon pins will often show BLUE and check temper chart to see what temp that is. Parts of the bores on Jabiru's likewise. It's hard to maintain an oil film at those temps. ( especially if it's LEAN) where you have an oxidising situation.  Nev

[RFguy]

a few pictures would be useful for us.  Rotax might say 140 is max but reality the mfrs will tell you 110C would be maximum continuous temperature . Engines oils get thinner with higher temps and this also affects the "oil bearing"  that can be provided. between bearings and journals. Not to mention cylinders and pistons....

 

If using a radiator of area of say, 160cm2 is used, (rotax minimum suggestion)  in some service note, that's for a min 75 kts aircraft. you might need bigger for slower aircraft. 

If you have a good size radiator already, that would imply something bad going on (assuming the temperature measurement is sound) , airflow throu the core is not as you expect- is the core blocked possibly ??? Is the hose between the oil pump inlet and the radiator sucking in ? there is high vaccuum there. Is the oil cooler too high pressure drop ?

****IE is the oil flow slow? BECAUSE slow oil flow will yield higher temperatures on exit from the engine.  slow flow means high temperature differences. slow flow means cooler exit from the radiator  and hotter exit from the engine . Fast flow reduces temperature difference (rise) . 

 

Edited March 31, 2023 by RFguy

[Blueadventures]

1 hour ago, facthunter said:

Even more drag. Nev

You will pick up speed if temps cool enough for more rpm.  The extra profile drag won’t matter in this case.

[Marty_d]

12 hours ago, IBob said:

Hi Marty, here is the arrangement on the Savannah, with the oil cooler upper and the radiator lower. All temps remain well within specs here in NZ, in fact I have about 1/3 of the oil cooler blanked off. Rotax used to reccomend raising the oil temp each flight to 90'C, I believe they recently raised that to 100'C, I don't see that unless I make a sustained hard climb. I would say you have plenty of air intake on your cowl as it is.
I would go with that, and in the unlikely event that you need more, you could consider fitting a scoop at the air exit point underneath to lower the air pressure there and induce greater flow.
 

Thanks all. Bob, are you saying there's no ducting or shrouds on the Sav from the intake to the radiators?

[skippydiesel]

I think you will find that Rotax(912ULS) manuals:

• Recommend oil temps should get above 100C for a sustained period at least once  per operational day. This is usually archived during sustained climb out  that is it may not occur in a climb to circuit altitude. This is to drive off water vapour and volatile contaminants from the combustion process.
• Accepts oil temperatures to 120C with a Max of 130C (above this is considered to be overheating and Rotax advise  certain action be taken).
• Continues oil  temperatures to 110 C are acceptable (not by me).
• My target cruise temperatures is 95C (+/- 3C) for both oil & coolant  

Odly, oil coolers do not usually  require a particularly strong flow of air to be effective - check out the location of many aftermarket transmission coolers . My last aircraft had a small oil cooler, without ducting of any sort, located low on the firewall, near the exit air vent - never had an oil overheating problem and I blocked much of the vent for winter (NSW) operations.

[IBob]

No ducting. But the radiators do sit close to the air scoops, though this pic is a bit deceptive, makes them look closer than they actually are (I have an LED light bar landing light bolted inside the lower lip of that upper scoop and it's well inside the scoop without being close to the oil cooler).

[trike1]

IBob. Not able to provide a photo of my Savannah install at the moment———I’ll try drawing the layout and post that (later).      The later XL and S models had improvements the oil cooler location being just one.  Interestingly there is 912 powered aircraft in SE Qld that has both its radiator and oil cooler mounted horizontally (works just fine apparently)

[IBob]

Trike, there is a factory VG on our field, I cannot remember the cooler/radiator layout, but he has no problems, has to blank off part of the oil cooler in winter to get the temps up.
Yes, best you provide pics or a sketch, otherwise it is very difficult to comment.

[RFguy]

check whole oil system checks per rotax manual  :

 

HAS A OIL SYSTEM VACCUUM CHECK BEEN DONE  PER THE ROTAX INSTALL MANUAL ?? YES / NO ???? (1.4)

HAS OIL SYSTEM AIR PURGING BEEN DONE PER ROTAX MANUAL ? (1.5) 

HAS THE CRANKCASE PRESSURE BEEN CHECKED PER ROTAX MANUAL ? (1.3) 

ARE LENGTHS AND MIN DIAMETERS AND BENDING RADII BEEN OBSERVED PER ROTAX MANUAL ?
What were the values written down in ther maintenance book ? What was the vaccum reading at the T ? 

Section : INSTALL MANUAL. Lubrication system, 1.1, 1.2, 1.3. 1.4 1.5. If this has not been done either re-educate or sack your mechanic. 

 

Edited March 31, 2023 by RFguy

[trike1]

Marty.   Didn’t mean to hijack your original post———-Hope you are getting some good info from the contributors to this forum.

[Marty_d]

All good mate, I think my problem is solved so good luck with your mate's!