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Posted

How is it possible that the turbo bearings can "heat up" with a five minute idle?  That defies logic...just sayin'

My interpretation is that the ADA guys are seeing cooler oil temps coming out of the turbo after flying because the oil itself is going through an oil cooler.  The temperature of that oil rises after landing as there is less cooling airflow (duh, the plane is sitting on the ground) and that the very hot turbo bearings are heating up the oil now which is not getting cooling air past it (other than prop wash).  So rising oil temps do make sense and for good reason...the oil is sucking the heat out of the bearings which is what it supposed to do and why we have turbo cool down times recommended to us.  That's my version of their data...

 

Posted

It's a discussion of related rates...

It compares the rate of heating to the rate of cooling that are all changing from descent through landing, to taxi and sitting idle.

Some of the things covered in the APS presentation are...

1) the heating comes from the TIT which is still incredibly hot at idle compared to what the oil can handle.  TITs are probably still over 1000°F

2) the cooling is mostly by the large flow of oil through the Turbo which is driven by the engine's oil pump. If the Turbo springs an oil leak, it won't last long.

3) the oil pump is rpm dependent, so at idle, the oil flow is at its lowest.

4) The oil temp is cooled by air going across the oil cooler. At idle the airflow is incredibly low compared to final approach speed.

5) The presentation and photos show where the cooling oil is. Where the seals are, and the bearings locations.

6) essentially they show the turbine blades sitting in hot exhaust and not enough cooling occurring after landing.

7) the only way to demonstrate this is with actual data collected from a duplicate of your engine.  

Quick question...

What TITs do you see after landing?  They would have to drop really low to not continue to heat the turbine...

Best regards,

-a-

Posted

Of course the turbo housing remains hot...it still has exhaust going through that side of turbo housing but that is irrelevant relative to how hot that housing actually gets in flight (so hot that it glows orange).  So whether the turbine blades are hot or not doesn't matter since they are not cooled by oil.  What matters only is the temp of the bearings themselves...and those bearings are wholly dependent on oil flow to cool.

Posted

From the Approved Turbo Components web site:

On Garrett systems that have a gravity drain you should idle at least 4-5 minutes. Systems that are scavenged require 2-3 minutes and Rajay systems don't require any at all.

So our 231s don't require any cool down. And our big continentals should be good after taxiing in.

http://www.approvedturbo.com/knowledge-center/faq/

Posted (edited)

CQ,

The shaft that is floating in the bearings is heated by the exhaust running past the turbine.  The glowing red case is still cooler than the turbine inside.

There are two parts to the bearings

- the Turbo case side. (Lots of oil flow and surface area for heat transfer)

- the turbine shaft side. (Not as much surface area)

The TIT is the driving force for heat to keep transferring towards the bearing surfaces...

The amount of exhaust (related to FF) is always delivering a fair amount of heat.

The concern is to cool the case and the turbine the Best we can before turning off the oil flow.  

Since it is a large balance of related rates of heating and cooling... the debate comes down to who has the better data and writing skills to generate a procedure from specifically for our applications.

TCM (or Lycoming) procedures are not written for the Mooney specifically.  Mooney doesn't have the instrumented engine or the desire to generate the data themselves.  APS seems to be the source that has the skills, tools and desire to demonstrate best practices for the Mooney community.

Their time/temp graphs are pretty interesting.  (They are specific to the engines and turbos that they have)

Last thought....  with an NA engine we typically taxi around in a very lean setting to keep the plugs clean.  Would it be better to use a higher FF to keep the EGT as low as possible with ROP cooling in mind...?

I have written this weeks after I have last seen the APS presentation.  I may have forgotten a few details.  pp thoughts only, not a mechanic or CFI.

Best regards,

-a-

Edited by carusoam
Posted
12 hours ago, thinwing said:

Well than GSX...I guess the boys at ADA will be overhauling your next turbo ,correct?Good luck with that...I think they will say...uh uh..better have main do it...they really know turbos!

My turbo won't need overhauling as I'm not cooking it on the ramp after every flight like you do.

Posted
11 hours ago, carqwik said:

How is it possible that the turbo bearings can "heat up" with a five minute idle?  That defies logic...just sayin'

My interpretation is that the ADA guys are seeing cooler oil temps coming out of the turbo after flying because the oil itself is going through an oil cooler.  The temperature of that oil rises after landing as there is less cooling airflow (duh, the plane is sitting on the ground) and that the very hot turbo bearings are heating up the oil now which is not getting cooling air past it (other than prop wash).  So rising oil temps do make sense and for good reason...the oil is sucking the heat out of the bearings which is what it supposed to do and why we have turbo cool down times recommended to us.  That's my version of their data...

 

At least you've got plenty of company. Lot's of people don't or can't understand science and how the scientific process works.

  • Like 1
Posted

So what does one do who has to taxi after they land?  I get the shut down at the hanger, but I have to taxi to the hanger, I also have to taxi up hill a bit if landing in one direction.  If it takes me 2-3 minutes to taxi and that includes some increased power should I take off again, fly the pattern and request a shorter taxi route to protect my turbo?

 

  I get the science, and I trust it, but I have yet to see anyone flying a turbo equiped piston actually shut down on the runway because thats where there turbo is the coolest.  That being said, if you have to taxi, and you increase power to do so, do you think APS will be okay if I idle at reduced power for a moment or two before I shut down?

 

 

 

 

Posted
5 minutes ago, Marcopolo said:

So what does one do who has to taxi after they land?  I get the shut down at the hanger, but I have to taxi to the hanger, I also have to taxi up hill a bit if landing in one direction.  If it takes me 2-3 minutes to taxi and that includes some increased power should I take off again, fly the pattern and request a shorter taxi route to protect my turbo?

  I get the science, and I trust it, but I have yet to see anyone flying a turbo equiped piston actually shut down on the runway because thats where there turbo is the coolest.  That being said, if you have to taxi, and you increase power to do so, do you think APS will be okay if I idle at reduced power for a moment or two before I shut down?

What APS is saying is that sitting and idling to "cool" the turbo, doesn't actually cool it. No one shuts down on the runway, but the point is not to make it worse by sitting and idling once getting to your hangar or parking spot.

My recommendation for anyone who owns a turbo or any piston driven airplane for that matter, is to invest a weekend and attend the APS course. The TSIO360MB that I own is expensive enough that spending the time and money to attend the APS course is cheap insurance. And will ensure I know how to take better care of my engine.

A second option is to email the guys who teach the APS course. They are pretty responsive. They're also very active over on Beechtalk. Ask them the question. I'm sure they'll have a good answer for you backed up by actual data.

Posted
29 minutes ago, gsxrpilot said:

My turbo won't need overhauling as I'm not cooking it on the ramp after every flight like you do.

Well that's my problem I guess..thing is I can't find any evidence that I am cooking my turbo..no oil coking,heat signature on the stack body,impeller turns smoothly ,turbo body looks good no oil leaks on shaft  ( not any like I've seen on turbo 206/421 s ,p barons or dukes.1200 hrs on this one and after last weeks annual inspection both me ,my AI and Main turbo think it will easily go to full tbo of 2000 hrs.So after reading ADA report and accepting their limitations on instrumentation (they admit they can't monitor bearing temp itself)I have to conclude in real world operations I am doing the best I can by allowing TIT to fall 200 degrees after taxi back to hanger.This doesn't take anywhere near 5 minutes.A min and a half..I'm down to 750 TIT.Oh and no discernible oil temp change at all..I think that's the vernatherm do it's thing.Ok I've enjoyed this discussion gsx...by the way ..how many hours on your turbo?

Posted
Just now, thinwing said:

Ok I've enjoyed this discussion gsx...by the way ..how many hours on your turbo?

Likewise...  I'll have to check the hours. I'm guessing it's about 600. Of course I haven't flown anything but a couple of short ferry flights since Dec. But hopefully another couple of weeks and I'll be flying again with a corrosion free wing and a new panel. The truth is I haven't had time to fly my airplane anyway. Business has had me logging 115,000 airline miles so far this year.

Posted

All the turbo repairs I've had to do were caused by things that happened at full power, not at taxi or idle power. My opinion is that what you do on the ground has very little to do with turbo life.

  • Like 3
Posted
6 hours ago, N201MKTurbo said:

All the turbo repairs I've had to do were caused by things that happened at full power, not at taxi or idle power. My opinion is that what you do on the ground has very little to do with turbo life.

The problems may have been discovered while at full power, but that does not mean that they weren't caused by conditions created on the ground.

  • Like 2
Posted (edited)

I am not going to argue, but I will provide some data.  I bought my plane in 2009 and it needed and got a rebuilt turbo.  SInce then I have put over 1,000 hours on it and have made 658 shutdowns.  Some were after 4 hour trips, some after 15 minute trips.  I tried the 5 minute cooldown before the turbo was rebuilt and it just seemed silly to me, after descents that were sometimes 45 minutes long and at reduced power, landings at even less power, and then a taxi in, it seemed to me that the engine and turbo were about as cool as they were going to get.  I stopped doing "turbo cool downs" years before I went to the APS seminar.  Their data just supported what I already knew.  At any rate, none of those 658 shutdowns involved any turbo cool down.  The plane is just now coming out of its 8th annual since that rebuilt turbo in 2009.  No problems with the turbo.  Do with that what you want.

Edited by jlunseth
  • Like 7

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