Oil temperature for take-off

[Bruce Tuncks]

I can see the argument in favor of hand propping, but could you actually do enough to make a difference?  It seems to take a long time using the starter before oil pressure comes up.

This is the technique I use after a layup... crank with the mags off until oil pressure comes up. I would do this every time but I don't want to wear out the starter system. It is a quite an  indirect way to just operate the oil pump, but it sure adds no complexity. It is turning the engine over dry, but there are no combustion forces.

I would prefer a small and light add-on electrical oil pump but his is probably not necessary.

Edited January 16, 2021 by Bruce Tuncks

[skippydiesel]

1 hour ago, Bruce Tuncks said:

I can see the argument in favor of hand propping, but could you actually do enough to make a difference?  It seems to take a long time using the starter before oil pressure comes up.

This is the technique I use after a layup... crank with the mags off until oil pressure comes up. I would do this every time but I don't want to wear out the starter system. It is a quite an  indirect way to just operate the oil pump, but it sure adds no complexity. It is turning the engine over dry, but there are no combustion forces.

I would prefer a small and light add-on electrical oil pump but his is probably not necessary.

When I hand prop my aircraft I get a small rise in indicated oil pressure - so oil is being pressurised and I would hope distributed.

 

With reference to a pre start oil pump:

 

The main benefit of such a system is on little used engines or after a long lay up. Of course it may have a small benefit for every engine, even those used daily(hard to gauge). The cost:benefit, additional weight and complexity are all of concern.

 

It would seem to me that you could remove most of the complexity & weight of a permanently  fitting  pump, by coming up with a system that can just be attached temporally (plugged in) to the oil reticulation  - just connect up, turn on using an external or "ship" power source, pump for a few minutes to get good distribution (should show on aircraft gauge), disconnect and start up.

 

A stand alone pump system could be used over a number of aircraft. Each aircraft fitted with a quick connect/disconnect coupling. The coupling itself is a potential point of failure (unlikely). The weight increase negligible. The cost , spread over several aircraft and time would also be negligible.

 

What think you?

[jackc]

Between a rock and a hard place by introducing complexity.  The benefits of prelubing are known, however another potential benefit is IF an oil pressure failure occurs in flight.  The pump can externally boost oil pressure for a short time?
This may enable you to quickly land under power IF you have pressure failure......provided the motor still has sufficient oil in it.

 

http://infinityaerospace.com/wp-content/uploads/2020/07/Preoiler_schematic.pdf

 

[kgwilson]

2 hours ago, Bruce Tuncks said:

I can see the argument in favor of hand propping, but could you actually do enough to make a difference?  It seems to take a long time using the starter before oil pressure comes up.

This is the technique I use after a layup... crank with the mags off until oil pressure comes up. I would do this every time but I don't want to wear out the starter system. It is a quite an  indirect way to just operate the oil pump, but it sure adds no complexity. It is turning the engine over dry, but there are no combustion forces.

I would prefer a small and light add-on electrical oil pump but his is probably not necessary.

I get oil pressure almost immediately after cranking the engine with the mags off. I usually only do this after an oil change to fill the filter and then re-check the oil level. When starting if you do not have oil pressure within 10 seconds there is a problem somewhere, most likely at the oil pump. As soon as I look at the pressure gauge after startup I have pressure. When cold it is around 200 kPa & once the engine has reached operating temperature it sits around 350kPa.

[onetrack]

Jack, possibly the biggest single cause of loss of oil pressure in flight is failure of an external hose, piping, or fitting - or catastrophic failure punching a hole in the block or sump.

A failed bearing or some other internal pressure loss, via mechanical failure, is a less likely scenario. If the oil supply is being depleted by the former, a prelube pump is only going to pump the little remaining oil overboard.

I like engines (any engines) with little or no external plumbing for oil, they suffer from engine failure on a lower percentage basis than engines with external oil plumbing.

[skippydiesel]

3 hours ago, onetrack said:

Jack, possibly the biggest single cause of loss of oil pressure in flight is failure of an external hose, piping, or fitting - or catastrophic failure punching a hole in the block or sump.

A failed bearing or some other internal pressure loss, via mechanical failure, is a less likely scenario. If the oil supply is being depleted by the former, a prelube pump is only going to pump the little remaining oil overboard.

I like engines (any engines) with little or no external plumbing for oil, they suffer from engine failure on a lower percentage basis than engines with external oil plumbing.

Your observations/comments make a lot of sense but in reality a large number of aircraft engines run oil coolers (might this be an Au climatic thing?)

[onetrack]

No, I think oil coolers would be pretty common on the majority of aircraft engines. And yes, split oil coolers are not unknown - although they rarely split catastrophically.

 

I just believe that minimisation of external oil plumbing is the best design idea - and besides, it reduces complexity.

[Yenn]

Oil coolers are used world wide. They allow a smaller quantity of oil to remove a greater amount of heat. The usual oil quantity is about a litre to 1000cc of engine capacity. Without the oil cooler you would need a bigger sump.

[F10]

Some warbirds have an electric pre- lube system. Probably installed after receiving the first Merlin engine overhaul invoice…..I have been told, air cooled aero engines tend to have tapered piston bores, to allow for heat expansion, the cylinder heads being hotter. Winding on power before warm up, makes me wince….scoured bores and broken piston ring time. I’m a fan of semi synthetic oil, from what I’ve experienced with cars and motorcycles….can you use semi in a Rotax 912? I know full synthetic can cause rubber oil seal issues, hence the semi synthetic. I warm up till oil temp needle moves. I think considering the relatively high RPM of Rotax engines, a good thing? Certainly the compression check on my engine shows almost new engine values, which makes me stop my nervous pacing during the test! 

[facthunter]

Just about all Rotax 912's i've been behind have used synthetic oil. It  permits a higher max oil temp to be used. It's further complicated by the need for the oil to work well with the re drive clutch and gears.

  Merlins do not have roller cam followers and have been know to "scuff"  on start up, when they are not used frequently. Prelube of the upper parts (valve gear) would be a good idea.

 Aircooled cylinders are often ground with a taper so they become parallel when delivering power. It was knownas "Choke".

 It gives better ring sealing. Harley Davidson did it on the production  twin  Models AND reconditioned cylinders till about 1928. You can't bore a tapered bore easily It has to be ground and that's expensive.. The taper was only a few thou. THAT translates to around 6 thou ring end gap change when cold. Smaller at the top. Taper wear limit is usually about .005"  the other way. (BIGGER at the top).  The lower parts of most cylinders don't wear measurably on four strokes. Nev.

Edited October 2, 2021 by facthunter

[F10]

Will heck my Shell oil bottles, but I think it was mineral? They are far away in Yarram sadly! Interesting about the bores, Yes tapering a cylinder bore would be complex, easier with tapering the piston. However, the piston would get very hot very fast. I was told that on a Piaggio Lycoming GSO 430 I spent some hours behind, the barrels were tapered, maybe inserts? We were threatened with death if we didn’t warm up properly….literally with death….engine failure after takeoff type death! Nice engine, but noisy as hell!

[aro]

Rotax 912 oil is typically semi-synthetic. Fully synthetic is OK as long as you don't use avgas, but fully synthetic oil does not handle the lead as well as mineral oil. So if you use avgas, oil with some mineral content should be used.

[facthunter]

Pistons are  always tapered even in two strokes and those engines ran about .015 thou clearance cold. The crown temps are way above the lower part of the piston. The PIG was a pusher and did have cooling issues. Those engines cold would have a lot of piston noise and the pistons rock and barrel face the rings. That's why they liked you to keep power on during descents and close the cowl gills. Nev

[facthunter]

Aro it would be an additive that "copes" with the lead in Aviation oils and additives get used up and you change the oil more frequently in those situations. Synthetics usually take higher temps and are structurally not much different from mineral oils They are synthesised from LNG and can be mixed if you have to. Some older synthetics were ester based and all sorts of things and didn't inhibit rust well and didn't mix.. Nev

[aro]

20 minutes ago, facthunter said:

Aro it would be an additive that "copes" with the lead in Aviation oils and additives get used up and you change the oil more frequently in those situations.

I don't know the chemistry, but I have seen many different sources saying that synthetic oil doesn't hold the lead in suspension the same way that mineral oil does. (One source says mineral oil molecules have more branches which mean they bind better to deposits.) Synthetic oil results in more lead sludge being deposited in the engine rather than removed when the oil is changed.

 

Looking at old versions of Rotax SI-912-016 where they recommended motorcycle oils rather than only the Aeroshell oil, it specifically listed synthetic oil for use where only unleaded fuel is used.

E.g.

1 Fully synthetic motorcycle oil with gear additives. Highly recommended for high oil temperature operation (higher than 120 C / 250 F) using only unleaded fuels

...

3 Semi synthetic motorcycle oil with gear additives. Highly recommended for normal (lower than 120 C / 250 F) and high oil temperature (higher than 120 C / 250 F) operation using leaded or unleaded fuels

 

Fully synthetic oils were specifically listed for only unleaded fuels.

[facthunter]

You are going back a while and I've explained that. Mineral oils vary and synthetic ones are more quality controlled and are generally recommended where high temps are involved  are more stable and a longer life expected. Note ED (extended Drain) oils have more additives, not better oil  The additives are not oil and are used up doing the job they are there for.. Lead is a contaminant and has to be managed like sulphur etc in LPG gas.

  The friction additives in EP gear oils are the same sort of thing. Without them a hypoid wouldn't go10 Kms before the gears were destroyed. Mineral or synthetic wouldn't matter.

[aro]

I doubt that semi synthetic motorcycle oil has any additives that fully synthetic oil didn't have that made it more suitable for leaded fuel. Or that any effort has been put into synthetic oils to make them more suitable for leaded fuel.

 

The Shell website description of the Sport Plus 4 oil says it "blends high-quality hydrocarbon base stocks and incorporates synthetic technology" and "combines synthetic and high-quality mineral oil". So it is not fully synthetic. The bottle says "Synthetic Technology".

[Bruce Tuncks]

My SK jabiru was the first kit to come with an oil-cooler. It's absence earlier was the result of a simple mistake...  the sender on the prototype engine was too short, and it reported the oil temp near the sump wall instead of the real oil temp. That's all a long time ago now.