Bolter Posted April 6, 2020 Report Share Posted April 6, 2020 1 minute ago, kortopates said: Great explanation on the mechanism behind lubrication and how important turbo speed is to effective lubrication. WRT to TIT, they cool down as soon as reduce power on approach, either from pattern altitude or from the FAF. A leaner mixture than rich helps to keep them a bit higher as well as the CHTs for a possible go around, but the reduced power cools them down on the final approach rapidly to below 1200F slowly decreasing to near 1000F. By the time we're landing they are at their minimum which is below 800F, and as we taxi to the hangar they'll actually go back up closer to 900F. Idling at this point is going to get them much cooler and not as cool as it was at landing. This is the basis of the rational for why a further cool down isn't needed despite the POH warnings calling out for a 3 min cool down. In your explanation of bearing wearing primarily "start and stop events when the speed is less than lift-off speed," is the "lift-off" speed a turbo RPM that bearing and rotor shaft separate? and if so any idea if we might approach that at a typical idle RPM? Or is repeated idling in the run-up area waiting several minutes for a IFR release and similarly what Paul's @gsxrpilotcomment above about the cool down actually does more harm than good by contributing to significant bearing wear? I am glad you asked, as it helps to frame the operating regime. Liftoff speed is relatively low, compared to total rotor speed. To get a sense of scale, maybe 10% of full speed, or possibly less. It varies with conditions, slightly, but you are over and through the liftoff very quickly. Idle of the turbo should be well above liftoff and enough into a robust oil film pressure that you can beat up on the machine without concern. Therefore only total engine start and shutdown will cause wear of the bearing surface. Nothing during operation such as the long IFR release example. You would do worse to shutdown and then restart, as far as the turbo bearings are concerned. These are similar to the main crankshaft bearings, in terms of operating mechanism. Each start and stop wears out the babbit inserts. However, crank bearings get reciprocating loads from engine firing, so they may be optimized differently. From the operating profile and TIT you describe from actual practice, I would think that cooldown would not be necessary to prevent coking for a typical landing to taxi to parking situation. That is some WAG in there, of course, but your experience suggests it is OK, as well. 2 Quote Link to comment Share on other sites More sharing options...
jlunseth Posted April 6, 2020 Report Share Posted April 6, 2020 With respect to the “3 minutes at idle” recommendation, there is another issue for the 231. The design of the alternator is such that it does not charge well at idle. To sit at idle for 3 minutes requires either running at a minimum of 1100, or sitting there running off the battery with a “Buss voltage low” message. 1100 is really not idle. But I just have not found it to be a problem not to do a “cool down.” 1 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted April 7, 2020 Report Share Posted April 7, 2020 When it comes to lubrication... 1) a round shaft in a round hole... is very similar to two infinitely long parallel plates, one passing over the other.... 2) when the shaft stays centered... no pressure is built between the ‘plates’. 3) If gravity were to try and pull the shaft off center... as if the turbo has stopped... that would be similar to the plates being on an angle to each other... 4) An interesting thing occurs... upon start-up, oil is dragged by one surface, towards a narrowing gap, by the first rotation of the shaft.... as this occurs... the local oil pressure begins to rise.... 5) as the gap has closed towards zero... this local pressure screams toward infinity... (ideally) 6) as the oil pressure in the gap rises... it lifts the shaft off the opposing surface... the faster it spins... the more centered the forces are, and gravity and g-forces disappear into the footnotes of the equation... 7) For the ideal things to occur... the surfaces have to be mirror smooth, the oil film has to be in place... uniform temperature is important... 8) Oil viscosity also plays a part... the more viscous the oil is... the more pressure can build up...more centering force is available.. 9) any dirt in the system or craters in the mirrored surfaces will cause challenges... 10) air bubbles or exhaust bubbles probably won’t be noticed until they become significant... 11) It would really be cool if a turbo body temperature could be measured... It would be a variation of knowing how much heat is left in the giant steal housing... 12) Expect under ordinary X-country travel... the long descents really cool things off as best as possible... (as discussed in this thread) 13) Multiple trips around the pattern, without much of a low powered cool down period... may leave additional heat in the turbo’s body... 14) What might be causing coking... oil flow is no longer following the usual path... as soon as an eddy current occurs... the residence time becomes large within the eddy... carbon may start to deposit in areas where flow has stopped... as carbon gets deposited, the eddy gets worse... The oil pressure discussion covering the pressure between the near parallel plates... is called the ‘lubrication approximation’. It shows up in a narrow section of transport phenomena... Bolter, is this something you are familiar with? (This is a bit of a stretch for me) PP thoughts only, not a rheologist... Best regards, -a- 1 Quote Link to comment Share on other sites More sharing options...
Bolter Posted April 7, 2020 Report Share Posted April 7, 2020 @carusoam The flat plates metaphor is a good one. Communicates the converging wedge idea well. The dominant equation is "Reynolds Equation" which is a simplified version of Navier Stokes for fluid flow. Over 100 years and the math still works :-) My work is actually with bearings that work with air (we design and make air foil bearings), but I started with gas turbines using conventional oil lubricated bearings. Like any topic, the more you dig, the more there is to learn. 1 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted April 7, 2020 Report Share Posted April 7, 2020 Ahhhh... (way cool!) Mr.(s) Navier and Stokes have visited here before... https://mooneyspace.com/search/?q=Navier&quick=1 We have a few people around MS that are familiar with both the fluid flow and the math... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Will.iam Posted December 23, 2022 Report Share Posted December 23, 2022 @Mikosch i saw your 252AY at aerocountry yesterday. Ironically you have the exact same paint scheme as mine 252 so they are twins. i find it strange that yours does not have the anti-static wicks that mine do which begs the question what function do they serve besides costing me money when someone bumps into one and i have to replace it. Anyway we need to go flying to lunch somewhere soon as it would be neat to take pictures of the twins together. Mine was 25-1036 serial number. I forgot to check how close together they were on the assembly line. One of mark’s mechanics was finishing up a cylinder replacement when i was there. Where are you based? Quote Link to comment Share on other sites More sharing options...
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