Resonance!

[Bleve97]

Interesting article from latest Sport Pilot (May 2016) on the Jabiru engine issue. tl;dr summary : resonance exacerbated by frequent heating/cooling cycles causing fatigue failure of through bolts.

 

https://issuu.com/raaus/docs/sport_pilot_57_may_2016/42

 

 

[bexrbetter]

Great report.

 

 

[jetjr]

It is a good report, Rod was trying to tell me all this last June. Was over my head at the time.

 

It is a big admission really regarding reliability of hyd lifter models. Goes to show a mid production change in design should be properly analysed.

 

Also indicates proper nanlysis on failures should be done, we are up to version 4 or 5 in throughbolt saga, finally some science behind this one.

 

Still means full rebuild for Jabiru owners to setup new bolts

 

Not surprising to me the 50% non adhereence to SB's in returned engines

 

Is there a rpm where this particular frequency occurs or is it not that simple?

 

 

[Bruce Tuncks]

I had a glider where a through-bolt in the main wheel failed in fatigue. It could only have been from the daily (for 20 years) temperature cycle. Wow what a freak thing, thought I as I changed the bolt for a new one.

 

They were socket-bolts which came with the German glider.

 

A few weeks later, another bolt let go. Then I heard of an identical glider interstate which had the problem too.

 

Steel through-bolts in an aluminium case have quite a big stress change due to the different expansion rates, and an engine obviously has much more temperature change than a glider wheel.

 

 

[Hargraves]

Exactly Bruce. The thermal expansion properties of aluminium alloys is and has all ways been vastly greater than steel alloys, and remains the eliphant in the room from the start. At least thats what I was shown during my apprenticeship when building and repairing aluminium and steel warships.

 

 

[jetjr]

Yes but same bolts used, even smaller than current versions, and with the thin nuts, loctite and other evils, in the solid lifter models. They didnt have many problems.

 

The real problem, Jabiru claims, is the resonance which is fixed by upgrading bolts, nuts and washers. Not the thermal expansion.

 

The thermal expansion problem is still there even after upgrades so not sure how big a part it actually plays.

 

 

[deadstick]

Hi, just installed a set of the new 3/8 through bolts in a customers 2200, the machines groove at the centre is the harmonic mod.

 

 

 

[Hargraves]

That is a very interesting piece of engineering development and i do hope its a winner for all jab engine owners. But i still can,t get my head around the point that other certified horizontally opposed aircraft engine manufactures use cast iron air cooled cylinders dispite the obvious weigh penalty incured and continue to manufacture their new engines the same way. It,s easy to have a critique in hindsite and I in no way rubbish all of the work and effort thats been done by jab but i still think liquid cooled alloy cylinders or cast iron aircooled ones would be the final fix of the base design. Cheers Hargraves

 

 

[David Isaac]

weight ... weight ... weight and weight. Jabiru have had a serious weight advantage but at what appears great cost.

 

Lycoming and Continental in particular are not as constrained by the weight issues that RAA aircraft are and they have the advantages of $MMs in investments and research in engine design and decades of experience that Jabiru don't.

 

So Jabiru have to learn the hard way when they depart from aeroengine conventions I guess.

 

Sounds like a simple analysis, but I think we all agree it is far from that.

 

 

[Hargraves]

Your quite right David it is just a simple (old mug storch driver opinion) ONLY, but i have to disagree in that the differance between the two alloys or ferrous casts is that simple. And, to add a little lightness to it,

 

it,s quite possible that the theory behind an aluminium cricket bat is very sound engineering but they did'nt work out in practice due possibly to too much resonance as well. Cheers Hargraves

 

 

[David Isaac]

Your quite right David it is just a simple (old mug storch driver opinion) ONLY, but i have to disagree in that the differance between the two alloys or ferrous casts is that simple. And, to add a little lightness to it,it,s quite possible that the theory behind an aluminium cricket bat is very sound engineering but they did'nt work out in practice due possibly to too much resonance as well. Cheers Hargraves

I actually agree with you Hargraves, I was implying that one of the reasons J may have moved away from sensible convention was weight and they probably didn't have the money or maybe the inclination to do the research.

 

 

[Hargraves]

I,m fairly sure that beauracratic weight restrictions would have been the driving force behind those design decisions and i,m very sure that those restrictions on our fledgling aircraft industry designers have every bit as much cedibility as 300' ceilings and no road overflights in days of yore. Good weather lately up here sir. So i,m going night night

 

To catch some early air in the morning. Cheers again

 

 

[geoffreywh]

sorry mate Lycomings and Continentals use "turned from solid" steel cylinders. NOT cast iron................. BTW I have seen recently a 2200 flat four that was free to turn when cold but extremely tight when hot. felt like it had siezed but with extra puff the prop could be moved. Fine again when cold....Thermal expansion and thru' bolts too strong? (7/16) what else would do that?

 

 

[Hargraves]

Thanks for that Geoff that makes even more sense as well. I don,t think theirs an australian pilot out their that would not want to see the problem finally nailed once and for good for every ones piece of mind,theirs enough to do staying safe as is without any other question marks i think. Re the semi seziure it sounds like a typical cold seizure encountered by very well cooled very high preformance engines but without the afforsaid cooling. As a guess (only) i would look at ring gap settings being not to specs and critically tight as a starting point at least. But just a guess after all. Cheers Hargraves

 

 

[planesmaker]

Engine being tight when warm is a result of crankcase fretting. Time to send it back to Bundaberg ! How many hours?

 

 

[jetjr]

With through bolt replacement instructions you could easy see excess clamp on crank.

 

Some method using spring tension on prop to ensure no over tightening

 

 

[planesmaker]

Jetjr, Bit difficult if it turns freely cold and tight when hot, we had the same thing happen here, when pulled down the cases had fretted, like I said back to Bundaberg.

 

 

[jetjr]

Could be fretting for sure

 

It soes the similar thing if case split and assembled poorly.

 

Tight when hot is one of the issues Camit have worked on and fixed

 

 

[Bruce Tuncks]

My understanding of tighter when hot was that the aluminium piston was expanding in the steel sleeve. I thought recent pistons had more cold clearance to deal with this problem. To me, the tightness when hot should be an unnecessary extra heat source due to the extra friction. In fact, I've wondered about changing to smaller pistons. Do they do this at Bundaberg these days?

 

I'm not disputing about fretting but I don't see why this should cause tightening when hot. Methinks the steel crankshaft in the aluminium block should get looser with heat, just the opposite of the piston and sleeve. I had an exhaust valve seat fall out from heat loosening.

 

And Deadstick, that 3/8 bolt looks great.

 

 

[geoffreywh]

the engine is 500 hrs from new. Absolutely certain the c/shaft is being pinched. pistons are not involved. Also zero end float on the crank. ( Crank pinched by thrust bearings... possible? ) How does c/case fretting happen with all those tight dowels? I have to remove heads for valve job. Or was it remove valves for head job? I'm now confused.

 

As far as "Back to Bundaburg" goes. How will that help , it just came from there!................

 

My theory (for what it's worth) is that the new 7/16" bolts with 40 odd foot pounds of torque exert more crush on the crankcase (less stretch than the thinner bolts). When the case heats up and expands it is squeezing the crank. However, why that should not start happening during the warranty period is a mystery.

 

 

[Hargraves]

Jabarax maybe eh Geoff ?

 

 

[planesmaker]

If it has fretting on c/case halves, only place to get it fixed would be Bundaberg ! 500 hrs ? Not a good run but not unusual.

 

 

[Bruce Tuncks]

The lack of fore and aft play on the crank does indicate the crank is being squeezed. I'm puzzled too, so please tell us what is found. Maybe a slipper bearing has failed somehow.

 

Is there any external sign ( black powder on the join line ) to indicate fretting?

 

The only fretting engine I have known was real tight when cold. It was made well before the latest dowels. It was tight hot too, but I think the hot tightness was caused by the pistons.

 

 

[facthunter]

Motors becoming tight when hot won't do it for long. Some Continental NEW barrels had near to not enough piston skirt clearance and did burnish the surfaces during early running in Darwin on one engine. at least The Equivalent of CASA at the time wrote about in the crash comic. A good engine when shut down will bounce back off compression a few times. It wouldn't take much mainbearing friction to produce enough heat to destroy the bearing fairly quickly. They always need positive clearance to allow enough oil to pass through to cool the bearings. Nev

 

 

[PKnight]

I have had two Jabiru engines, a 2200 and my current 3300 which has 980 hours on it. Both these engines have been significantly tighter when hot; so what is going on.

 

My apologies to all those who know this but I'd like to get the physics bit out of the way first. All metals expand when hot but at different rates. Aluminium alloys expand at approximately twice the rate of iron and steel based alloys. The forces generated by these expansions are enormous. So as the temperature increases a steel shaft in an aluminium bearing (crank shaft situation) should get looser as the aluminium bearing will expand at a greater rate than the steel shaft. Conversely an aluminium shaft (piston) in a steel cylinder will get tighter as the temperature increases and the aluminium expands at a greater rate than the steel that is containing it. The increase in size are very small unless the temperature increase is significant, for aluminium an increase of 100 deg C will give a percentage increase in the component of about 0.0023%. This would be enough to make something that was already tight into very tight but it is still a small change.

 

Sorry about the lecture but I started to look at the behavior of my engine (3300 No A1336) as it got hotter. First bit of information is that the engine rotates freely when cold and has 6 good compressions. If I take the plugs out I can almost whizz the prop round by hand, it is only constrained by the need to put energy into opening the valve springs. When hot, the engine is stiff. First test was to look at the possibility of movement in the crank. I could still detect movement when pulling the prop backward and forward. This shows that there is still some end float and the crank is not squeezed in its bearings as a result of expansion. This is exactly what you would expect of a steel shaft in an aluminium crankcase. I appreciate that you could generate additional compression on the crank as a result of the through bolts squeezing the crankcase halves together but that was not what I was seeing and I think I would expect a much tighter cold engine for this effect to show. I cannot comment on the possibility of fretting between the crankcase halves contributing to the effect as I have not experienced it.

 

So the most likely source of the 'stiff when hot' effect has to be the differential expansion of the aluminium pistons in the steel cylinders but I don't think it is straight forward. I was wondering about how much power was being lost by the need to move these stiff pistons in the cylinders when the engine is running. I don't think it is much as I initially thought as I noticed that immediately after stopping the engine it is not that tight. If you move the prop immediately after shut down (very carefully on a hot engine) there is stiffness, certainly more than with a cold engine but it is not excessive. However if you wait 15 to 30 minutes and try to move the prop I found that the stiffness had increased significantly. I initially thought that this might have been caused by the oil draining away from the cylinders but I don't think that is what is happening. I think the excess stiffness when hot is caused by the cylinder cooling at a more rapid rate than the piston that is inside it. So you not only have the effect of an aluminium piston expanding in a steel cylinder you also have, after a few minutes, a differential temperature as the piston is staying hotter (and so larger) as the cylinder shrinks more as it gets cooler. After about 45 minutes to 60 minutes everything seems to have stabilised and the engine will rotate more easily as presumably the piston and the cylinder temperatures have both reduced and equalised.

 

I assume that having a small element of expansion in a piston over a cylinder is a good thing as it helps to seal the piston against the cylinder wall and so avoid blow-by and keeps the compressions high. Whether the current Jabiru configuration has got the balance correct between ensuring a good seal and an unduly tight engine when hot I simply don't know but from my own experience I am confident that the stiffness is piston rather than crankshaft based and it is increased as a result of the differential cooling between the cylinder and the piston on shut down.

 

I would be interested to know if others experience with their engines mirrors my own.

 

Peter