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31 minutes ago, onetrack said:

That's a handy restriction to have, when you're 15 feet off the runway on landing, and the aircraft is getting seriously crossed up, and you need full power instantly to go around.

ha ha  yes. I would think it would be most vulnerable during slow cruise or  descent (lots of airspeed and cooling, low power setting) and going to WOT. I dont know of any verbatim reports on exactly how far into the climb after takeoff when an engine threw a piston- the pilot usually has his hands full.... I do hear a bit of engines throwing pistons 'mid circuit'. 

 

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This has been very interesting to me both for aviation and motorcycle engines. Thanks everyone.

I'm fairly satisfied now that possible reasons for piston throwing in < gen4, which I  beleive that I now have a good handle on, have been substantially reduced in Gen 4.  (piston temps, piston typ

This is something I had no idea about. Bloody interesting.   http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Graphics/Piston%20Assembly.jpg Figure 6- Pisto

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Heat flow is from Hot to cold(er) and the rate is proportional to the temperature difference. With the power max output the same the cooler bore will always have a cooler piston. IF you cut the power, you immediately stop any extra heating. . i've been involved with building many race engines and never  seen or heard of any seizure due to the effect of backing engines off. I've been riding bikes that had pistons fitted too tight and were nipping up with a certain power set and just reducing the throttle usually stops the tightening proceding..Some of this discussion is off the mark. You don't get more seizures when the engine is too cold. You WILL get them if you leave the cowl gills closed on a plane. There's a CERTAIN minimum running clearance you must have or the piston will seize and when that's happening it creates more friction and HEAT extra to normal and that will lock your engine solid as I've had happen on a bike at near full speed and full throttle. Declutching it and letting it roll  the engine freed about  1/2 a Km later by  letting the clutch engage partly to turn it over, which it did but it had no compression as the rings at that stage had been sealed in place by aluminium dragged over the ring lands as well as the skirt. Often there will be some welded to the bore as well when this happens which can be difficult to remove without damaging the bore You CAN'T just hone it. Nev

 

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Good info.

What do you put down (best guess)  the recently-shut-down Jab engine tightening to ? That is a form of backing off, sort of.

 

In the gen3 Jab, I'd Generally I'd be more concerned about my point (a)- - the idle to  WOT condition. now whether those breakages are piston getting too big in the bore, or piston getting too hot, too much work and breaking I dunno.

 

 

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I'm NOT referring to any "W' slot piston behaviour. I think I've made my view consistently clear that they don't work  WELL in aircooled motors. It may do that IF the clearance is not enough. IT may have been tight when it was running. this will show up as skirt "burnishing" or there may be some misalignment  or bad shape  in the original machining. I'd have to look at the piston in question. You can get gudgeon pin bosses not at 90 degrees to the cylinder axis.  Nev

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1 hour ago, RFguy said:

Turbs my GUESS is that there was some heat capacity available beyond the continuous heat duty and this got used up. 

 

In electronics, we have usually high thermal mass 'slugs' under power silicon chips: 

 

IE a sandwich of  chip >> copper slug >> aluminium heatsink.

 

For sustained heat removal , the heat just travels through the lot to the heatsink. But the amount of heat that can be removed per second is limited .

 

For transients and momentary overloads , the high heat capacity of the copper slug can absorb the heat pulse  out of the silicon chip, and stores it for (slower) forwarding onto the heatsink over time.  

 

I gather there is a bit of that going on in said drag strip service, a 2nd order effect. IE you have transient loads that have some heat capacity , and sustained dissipation limits. the piston will heat up fast, and there will be some heat capacity in the conrod, bore walls etc , and once those heat wells are filled up (come up to temperature), the piston temp rise will accelerate. 

 

It's oval track racing, not drag racing with a quarter mile circuit made up of two long straights with a double corner each end (effectively an oval), and covered in about 11 seconds. Acceleration starts  about half way through the first corner, and ends at the end of the straight, so there's a long burst of full power demand followed by a short period of trailing throttle over and over again. We would call that an Intermittent Power Application where the engine gets some relief in the combustion chamber. RPM variance is about 1500 each time.

 

I resolved my problem by cooling the engine with fuel, and went from piston replacements every few races to several years., but I had an advantage in that I was using Methanol, which you can run much richer than petrol before power loss.

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1 hour ago, RFguy said:

... I would think it would be most vulnerable during slow cruise or  descent (lots of airspeed and cooling, low power setting) and going to WOT.

I try to use my cowl flaps to keep CHT’s within a safe band, but to do that I have to close off almost all airflow during idling descent.

 

33 minutes ago, facthunter said:

...You don't get more seizures when the engine is too cold. You WILL get them if you leave the cowl gills closed on a plane.

Nice to Know, Nev. I have sometimes forgotten to open my cowl flaps during a go-around until the CHT gauge starts flashing its red light at me.

 

Thankfully, the cooling process seems to have a time lag of a minute or more: no matter how wide the cowl flaps on climb-out, CHT’s hit my warning limit  of 150C and stay there for a couple of minutes after I’ve leveled off and eased off power to 2750 revs. 

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If you are going to run the later type pistons in the earlier engine you will have to determine an appropriate clearance. I wouldn't even think of not trying to cool the cylinders as well as possible as it's going to need more clearance the hotter the whole thing gets. You've expressed surprise at the figures I've suggest will be in the ballpark  but t hat's the cold figure and if you get it right when the engine's at normal running temps the clearances will be fine. I'd refer to the Cont 0 200 and adjust for bore size (Proportional) and then add  1.5 thou of an inch You should also look at permitted wear limits and that may surprise you. You are going to get more problems trying to keep fits tight than  going the other way. Nev

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Alcohol motors don't usually have problems cooling. the alcohol often freezes the carburetter and the inlet side of an aircooled motor E 85 maybe a fuel for any supercharged Jabiru You will burn more but the motor should have less problems with heat. Nev

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I'm running the standard, slotted Gen3 pistons. No plan on changing them for the solids in this engine. I will run it until I can afford a Gen4 (or used R912 + turbo)  for this bird. 


I wouldnt want a tight Gen3, for me thats a bit of a clearance red flag, intolerance to certain operational situations.

 

Clearances on my engine pistons are at the high end of the permitted clearance, I could not measure any ovality in the cylinder bores.  . So  there's a fair bit of piston rock in the thrust line cold.  Its only 400 hours though. I think its had a good life. (remember- pulled down due to 3 x #2 stuck rings) .

 

Based on all that has been written here by Nev, Turbs, my own research and reading, I'm pretty confident the Gen4 doesnt have the same systematic piston/bore issues.

 

Interesting, the Rotax 912 I have seen installed, no special attempt to cool their deep finned ally cylinder bores . The water cooled heads helps alot- high thermal conductivity ally cylinder barrels  permits the water cooled heads to remove a chunk of cylinder bore heat .

 

 

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Some interestesting reading :

 

Gen3 jab manuals- Between the April 2020 manual (-20)and the October (-22)   manual some of the permitted clearances changed 

 

The build tolerances  >> changed to 
cylinder bore : 97.61 - 97.67mm  >> 97.70 MAX

piston over  skirt 97.48 - 97.54 >> 97.45 minimum

clearance to bore

Split skirt pistons : 0.08 -0.13  EARLY GEN 4

Slotted skirt pistons : 0.13 - 0.17

HD piston (Gen4 and retofit Gen3) : 0.19 -0.23   CURRENT GEN4 and GEN3 retrofitable.

  changed  in -22 manual to : >> 0.08 - 0.18mm. no reference to HD piston. only slotted skirt or split skirt.

 

GEN4 overhaul manual : 

cylinder bore : 97.54   - 97.62   (tight tol ! - if it wears I gather that's the Nikasil gone ????)

piston over  skirt 97.48 - 97.58

piston skirt clearance : 0.05 - 0.1mm

 

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Photos I was sent last year. Gen 4 pistons in a Gen 3 engine after approx  3 hours running. Engine now has non-Jab forged pistons with no issues.

IMG_4535.JPG

IMG_4542.JPG

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OUCH

Yeah they have a Gen3 retrofit specific  solid skirt piston now.  195+gst each.

Produced since December 2020 .

 

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Insufficient clearance by the look of it. Some forged pistons need extra clearance just because of the alloy used. 

 Nikasil  bores don't wear much. The hone pattern on the cyls shown doesn't look the best. Jabiru get a good finish but the hone pattern could be a bit coarser to hold more oil. Honing steel is not easy. Nev

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