Hot or Not?

[FastEddieB]

Rotax chose to make their aircraft engines part water and part air cooled.

Well, that's two out of three.

 

Can you name the third?

[Tom Baker]

Hi Tom,

 

Just got back home from 7 weeks away.

 

"I don't completely agree with Roger that the oil lines are causing an issue. "  

 I don't believe they do always cause the problem, but I have found at least 75%  of them to slow oil flow which causes heat retention. It won't cause an ouil shortage or even show up in the psi or pressure gauge just slows the flow. FD knew about this many years ago with the hose off the bottom of the engine and returns back to the tank that was in a too tight an "S" curve.

 

"and if there were a problem the engine would be starved for oil and pressure would be low."

 This isn't necessarily true unless there is a blockage. The oil lines just have a reduced radius so it only slows, not starves. If you had an oil stoppage you have more serious issues to worry about.

 

 

Roger,

 

The oil pump can only pump so much oil. The pump determines the flow rate of the oil through the lines. The lines leading up to the pump can not slow the flow of oil without starving the pump. If they did it would mean more time in the oil cooler and lower temps.

The oil coming off the bottom of the sump needs to return at a rate faster than what the pump is using. I don't think it can sit there longer without starting to back up in the sump. Isn't that what would happen if the line was restricting the flow of oil out of the crankcase? I know if you put more water into a sink than what the drain can handle it will eventually overflow.

[FastEddieB]

Eddie, if you want to know how the CT with Rotax engine and radiator works you should ask Roger or read the Flight Design POH.

 

Cecil, if you want to know the third way the ROTAX is cooled, you might begin by asking yourself why it has an oil cooler.

[Roger Lee]

Hi Tom,

"The oil pump can only pump so much oil."  4 gpm

 

It doesn't stop oil or keep it from the tank it only slows the flow. It isn't enough to starve the tank, but is enough to allow another 10-20F in temp.The other offending oil line that is in the SW and not the LS is the one off the top right of the cooler that does a 180 degree back into the oil pump housing inlet. I put springs in these and have had good luck with bring down temps by making sure the hose ID is as wide open as it can get and not slow the flow. I have removed and re-routed  7-8 oil return lines off the bottom to make it a straight shot out the left side of the engine. I rotate the tank slightly and take the straight fitting off and put on a 90 degree. 

 

A 10-20F increase in oil temps isn't much compared to the overall temp and it's easily affected. It may be one thing or an accumulation of a couple of small things.

[FlyingMonkey]

Roger, I agree and always knew that heat was rpm dependent. I was just surprised at the heat difference visible on the gauge between 5200+ and 5000 and below.

[Tom Baker]

Roger, how does a oil cooler cool the oil?

[Roger Lee]

By transference or release of BTU's to the metal surface which should be cooler than the medium flowing through it. The cooler outside air keeps the cooler cooler than the medium flowing through so the medium gives up some heat to the cooler metal surface. The hotter the outside air the less heat transfer because the metal cooler isn't as cool. Slow the medium flow down or speed it up too far (there is a range) and you lose your BTU transfer.

Convected and radiated heat transfer

[Tom Baker]

Do you really thing that the oil is being slowed enough to degrade the heat transfer in the cooler?

[Tom Baker]

BTW, I think I was one of the first to reroute that line off the bottom of the crankcase. Without digging I'm not sure how far back the first one I did was, but it was quite some time ago. I just did another this past week.

[Ed Cesnalis]

I'm with Tom,  the pump, pumps at a fixed rate or in a fixed range and the lines in both directions have to flow faster than the pump pumps else the pump would go dry.

[Roger Lee]

"Do you really thing that the oil is being slowed enough to degrade the heat transfer in the cooler?"

 

Absolutely. It's been proven hundreds of times on many planes.

The big but is it isn't always the same thing for each plane. It can be several things and unless you have a good understanding of the system and even more importantly good diagnostic skills you may hunt that white whale for months and throw lots of dollars at it.

 

​You guys keep thinking the pump is having huge volume issues. That isn't so. It only takes a little slowing through the system to change a measly 10-20F. Here is a perfect example of what a extremely small temp change can do. The Mfg. spec has a lower and upper limit. Many times Mfg's with pumps don't do flow test, but calculated flows. My dive compressors from Thomas was exactly that.  These oil pumps have an upper and lower limit for flow and pressure and the pump works directly off the engine. It's a type of rotary pump. A very tiny flow change that is unnoticeable without instruments can easily cause a 10F change.

 

"pumps at a fixed rate or in a fixed range"

It isn't fixed it's a range.

 

Why do you think pressure changes on your gauges. It isn't fixed. Does the pump speed up and slow down? It isn't fixed. Other than some sending units being off why are there so many different pressure readings? It isn't fixed, it's a range. This isn't a exact science. It's a calculated flow and pressure range. Why do you think Rotax says 22-73 psi. None of you here knew what the oil flow was until I posted so how would you know if it was fixed?

[FlyingMonkey]

I could see a slower oil flow rate as a minus AND a plus.  Say what?!?

 

Oil flowing more slowly through the circuit is going to spend more time in the engine portion of the circuit, picking up more heat.  Big minus.  But the oil in the oil cooler portion of the circuit also spends more time in the cooler, getting more time to cool down before being returned to the engine.  Big plus!

 

So the question then becomes:  Does the extra heat pumped into the oil on the engine side exceed the cooler's ability to cool it back down on the cooler side, even while spending more time being cooled.  Roger says yes, and I have no reason to disbelieve it.  Certainly some of the airplanes he's worked with on this issue have been helped by his alterations.

 

This may not be the culprit in all engines, but it certainly could be a strong candidate to start with.  Obviously CT's issues go beyond these kinds of tweaks.

[FlyingMonkey]

It also occurs to me that a way to mitigate this issue in general would be for Rotax to offer a "high capacity" oil system with a larger return tank, and say 1-2 liters more oil in the system.  That would allow oil more time to cool as it made its way around the circuit and dissipating heat over a larger oil volume.

[gbigs]

Cecil, if you want to know the third way the ROTAX is cooled, you might begin by asking yourself why it has an oil cooler.

 

Eddie, I was directing my rhetorical question at Morden (who apparently doesn't know the answer).  So I will direct it toward you.  Why does Rotax use a radiator to aid in cooling the engine but Continental and Lycoming do not.   I think it wrong to comment on aircraft I don't own or fly but I assume your Rotax driven plane also doesn't have a radiator.

[Roger Lee]

You can't assume every engine has a heat issue with the same cooler size and you can't assume the ones that do have the same cause, just the same result. There are several reasons that temps may be higher in one and not the other.  A good diagnostician will consider them all and usually start with the most common and cheapest and work his way back. Rarely is a Rotax issue complex. Most have reasonably easy answers and resolutions. Ed's issue may be totally different than someone else's that has the same result as high oil temp. Our discussion here has focused and damned the oil cooler. May or may not be the problem. The cooler we have can easily handle to quantity per minute oil flow from our system. So max flow through it isn't an issue so that leaves a reduction in flow. The cooler can dump heat, but if oil is slowed (or air flow reduced) the engine heats it hotter and the cooler can't dump it fast enough. In this case a larger cooler would help cool the oil, but not fix the cause. 

Don't attack the result, attack the cause.

 

With so many CT's out there with the same cooler and not having the same high temps it's hard to blame the size of the cooler and not possibly a reduction in flow through that cooler or another cause. My last Ford F350 had a history of the oil cooler reducing flow through debris. Not just my truck, but that fleet group. It was suggested at 100K miles by numerous parties including Ford to change it out and not try and clean it. 

[Craig]

Eddie, I was directing my rhetorical question at Morden (who apparently doesn't know the answer).

 

See post #95.

 

I hate participating, but sometimes it gets so frustrating, you just have to give in and feed the troll or implode.

[Ed Cesnalis]

With so many CT's out there with the same cooler and not having the same high temps it's hard to blame the size of the cooler and not possibly a reduction in flow through that cooler or another cause. 

 

You keep saying this.  My response is:  I can fly with other CTs no problem.  My CT isn't very different from the others I encounter.  I here reports of others beginning to see hot oil when ambient temps approach 90*.

 

The major difference between me and Andy for instance isn't the way our CTs respond to hot conditions but instead the difference is GA vs CA.

 

Here's a vertical profile of one of my most common flights, returning from Van Nuys, Los Angeles County.  

 

 

I'm initially restricted to clear Bob Hope's Class Charlie then I need to climb to 10,000' in a short distance and of course temps at Van Nuys are likely 95* much of the year.

 

On such a day with 8,000' of initial climb needed I say the average CT will be looking at hot oil.

 

With few exceptions the other  CTs in this country take off, climb to pattern and then cruise climb another couple/few thousand feet.

 

Average CT cruises at less than 5,000' and CA CT's have to climb to 10,000' (unless they never leave the valley they took off from) and often do it from a hot desert.

[207WF]

On the oil quantity hypothesis, we once tried overfilling the oil to use the full capacity of the tank. Did not seem to make any difference. WF

[FlyingMonkey]

CT, there is probably some truth to what you are saying.  If I had to take off at 2000ft with OAT 90F+ and climb 8000ft, I'm not sure I could do it all in one climb.  If I did, I'm sure my oil temp would be pushing 250°F for at least the first half of the climb.  At least, how it was a few days ago.  Still need more testing on the minor tweaks I did regarding the radiator/oil cooler assembly, to know how it's doing now.  

 

Tomorrow is supposed to be 93°F here, I will see if I can fly a bit after work and do some testing.

[Roger Lee]

Hi wayne,

"On the oil quantity hypothesis, we once tried overfilling the oil to use the full capacity of the tank. Did not seem to make any difference."

On a dry sump it wouldn't make any difference so long as the level never got so low as to suck air. Too high it just flows out the vent tube all over the ground.

 

 

 

ED,

 

"The major difference between me and Andy for instance isn't the way our CTs respond to hot conditions but instead the difference is GA vs CA."

 

This suggest you don't have a engine temp issue. You have a flight performance issue where you demand more from your engine longer than the normal pilot?

You push the engine longer at max performance so you may see things like temps closer to max.

 

If this is the case then going after all the other engine heat causing issues won't help and all that can be done is alter your demands on the engine by possibly changing your flight performance demands or in this case the larger oil cooler would help, but may not be a cure all because it would only help the oil. Even if you can't see things tangible like oi temp that means the engine has to be under more stress too under longer flight requirements.

 

Then the bigger cooler may help if it fits in our space?

[FlyingMonkey]

I like my increased oil capacity idea.  Rotax, can you hear me?  

[Roger Lee]

Hi Andy,

 

Wrong Mfg. Rotax never visits us and they don't spec out oil system setups. They leave that to the MFG or builder.

[Tom Baker]

 These oil pumps have an upper and lower limit for floessure and the pump works directly off the engine. It's a type of rotary vane pump. A very tiny flow change that is unnoticeable without instruments can easily cause a 10F change.

 

 

 

Roger,

 

Rotax uses an internal gear type pump, not a rotary vane pump. It is a positive displacement pump. This means that for every rotation it moves a specific amount of oil. In normal operation it will move more oil than the engine needs. The pump has an internal bypass which takes the oil beyond what the engine needs and feeds it back to the pump. If you slow or restrict the oil into the pump below what it needs the pump will cavitate. If this happens you would see large oil pressure fluctuations. That is why I say that the pump is controlling the flow of oil. If the line were restricting it beyond what the pump can move there would be bigger problems that increased oil temperature.

 

You also contend that the return line to the oil tank is keeping oil in the crankcase longer and it is absorbing more heat. If the oil is staying in the crankcase longer than it should it would have to mean that the rate of oil being expelled from the crankcase is less than the engine is using. If that were the case all the oil would eventually wind up in the crankcase and not enough oil in the tank for the engine.

[Tom Baker]

Another possibility on the cooler is that the fins due to vibration have lost contact with the oil passages. If this happens you don't get the desired heat transfer.

I'm not sure how the fins are attached on our coolers, but on many they are just a press fit. All the other fixes mentioned would do nothing to fix this. There is no way to know for sure without tearing up the cooler, and the only fix is to replace the cooler.

[Ed Cesnalis]

Roger,

 

Rotax uses an internal gear type pump, not a rotary vane pump. It is a positive displacement pump. This means that for every rotation it moves a specific amount of oil. In normal operation it will move more oil than the engine needs. The pump has an internal bypass which takes the oil beyond what the engine needs and feeds it back to the pump. If you slow or restrict the oil into the pump below what it needs the pump will cavitate. If this happens you would see large oil pressure fluctuations. That is why I say that the pump is controlling the flow of oil. If the line were restricting it beyond what the pump can move there would be bigger problems that increased oil temperature.

 

You also contend that the return line to the oil tank is keeping oil in the crankcase longer and it is absorbing more heat. If the oil is staying in the crankcase longer than it should it would have to mean that the rate of oil being expelled from the crankcase is less than the engine is using. If that were the case all the oil would eventually wind up in the crankcase and not enough oil in the tank for the engine.

   this ^