Another through Bolt gone

[Vev]

Hi Phil,

 

I can't comment about your experience in your industry .. however in my industry, being the oil biz, most products and basically all engine lubes are made to meet or exceed an agreed spec and tested to industry agreed test programmes.

 

There are some smaller oil or additive companies that do make some claims that are hard to pin down as they are not measured against recognised test programmes... some work some don't.

 

Whilst I can sense your scepticism ... a formal recommendation by a major oil company (not a sales rep or some good intentioned retailer) is underpinned with millions of dollars and thousands of hours of testing. There may well be better products out there that may work well or even better, however it's a game of Russian Roulette to try something not recommended, approved or tested .. believe me I have seen the other side of the "when things go wrong".

 

If I was given a clean sheet to design a new lube for a specific aircraft engine I suspect it would look a little different, however if you wanted the same lube to be backward compatible with other products and tolerate top ups and work in 85% of all aero engines and also meet the OEM performance requirements, you may not end up with something not a lot different. Often you can improve the performance of a lube very simply but then make it fail in another area when you chemically push it over the edge ... there is a lot of balance to get a lube to meet all the performance requirements.... I often see this happen when people put their own additives into a lube and unwittingly damage another performance characteristic.

 

Whilst the perception that aero lubes have not progressed is not true either ... new additives, better more stable base oil and better manufacturing and test programmes has improved old formulations a long way.

 

As for testing, suggesting an air cooled motorcycle caught in traffic test as a good proof of product performance is not something I would want to use to risk anyones life on for an aero lube. Testing is done by dedicated people following exact recognised processes which gets repeated over and over again before any field tested can be considered ... bwt, this also includes test progrmmes to make sure motorcycles lubes can tolerate traffic jams, albeit we don't park motorcycles in traffic but we do simulate it at first then do the field trials in the real world. However just making a motorcycle lubes not thermally crack in traffic is just one performance issue ... making it shear stable to work in the gearbox, not allow the clutch to slip, clean the engine, hold soot in suspension to get caught in the filter, not turn into foam, provide corrosion protection, absorb acids, maintain viscosity, give proper anti-wear protection etc etc are just a few and all are important.

 

I agree there is always room to improve and develop new technology ... this is evolutionary process and people are always looking at the next new piece of technology, it will come, but it takes many years to get this over the line safely.

 

In terms of aero engine lubes selection for myself ... I only trust what is approved and recommended by major oil companies and the OEMs... however I will select a multigrade over a mono grade if it is approved.

 

Multigrades can do all that a monos can but a monos can't multitask as well.

 

Cheers

 

Vev

 

 

[Head in the clouds]

More thoughts on bolts -

 

There was a thread I read which was started by OME around last September where he hinted at having found something that might have to do with bolt failures and adoption of certain 'to be revealed later' procedures would possibly fix the bolt snapping problem. Well it was a bunch of intrigue that didn't come to anything in the end, no information was forthcoming. But there were a couple of hints, the word 'torque' used in place of talk, and 25hrs mentioned etc.

 

What I read into it was that something was discovered at a 100hrly that wasn't right and something done at 25hrs might be presumed to put the problem to rights. My guess is that the through bolts were found to be undertorqued at the 100 hourly and possibly the suggestion was to re-torque the bolts each 25 hrs. Well I might be guessing completely wrongly of course but since there was no information promulgated eventually, then I think there's a good chance that I might be right because if the bolts are found to be losing their torque there's a really major problem that most certainly won't be fixed by re-torquing the bolts regularly, in fact that action is likely to make the bolts fail much more quickly.

 

The thermal expansion of aluminium alloys like those used for the crankcase halves (6061T6 or 7075T651 or similar) is around 0.000023mm per degree C temp rise per mm of length at temps between 20*C and 100*C. It's not much but it's significant because it's almost exactly double the amount of the thermal expansion of the steel that the bolt is made of.

 

So if you torque your bolts up at 20*C and the engine is running at 100*C oil/case temperature then the case will have expanded about .25mm more than the bolts, so the bolts have been forcibly stretched by that amount.

 

That's perfectly OK as long as the bolts have been torqued well below what it takes to get them to the top of their elastic region, but if they're cold torqued to the top of the elastic region then any further force applied (by the thermal expansion for example) will take the bolts into their yield region and permanently stretch them. This does two things - first it shows up as undertorqued bolts when the engine has cooled down again, second it initiates cracking at any stress risers which might be microscopic but the the crack will still begin.

 

So, you re-torque the bolts to the top of their elastic region and go flying again, the bolts yield again, the crack gets bigger, the bolts show up under-torqued when cold again, so you re-torque them....

 

It might appear that there is no solution except using bolts made from an alloy that has similar thermal expansion rates to the aluminium but there are actually several solutions. If you had the cash you could use Titanium bolts, they're very expensive and actually have a thermal expansion that is much less than the steel bolts but their elasticity is far greater so they are happy to stretch while the aly expands, and maintain their tension, returning to the original length as the engine cools.

 

Anyway, that's for the millionaires, for the rest of us - well first we have to consider what torques the factory has specified, and most likely they have specified it correctly, but just in case... We need the bolts to be at their rated torque while at the operating temperature of the engine, so we either need to know how much torque is required to stretch the bolts per millimetre within their elastic zone, then know how much they will be stretched by the aly's thermal expansion, and torque them by that much less when cold, or we torque them to their rated torque when the engine is at operating temperature.

 

If we torque them while hot we cannot go wrong in terms of over-torquing them, so we prevent the risk of taking them into the yield zone. If they start to yield the load required to continue the yielding is much less than it was to start the yielding, so the cracking and stretching will propagate very quickly, and thus far we've only considered the effects of the stretching due to temperature increase, now add the effects of the combustion loads and you can imagine what these bolts are going through if they are over-torqued. Now consider the average person's attitude toward torquing, they bring it up to the required torque and give it just a little more 'to be sure'. Under-torquing by a little is far preferable to over-torquing where through bolts are concerned. In fact the only way you would consider them to be not tight enough is if the oil is leaking, either hot or cold.

 

Just thoughts.

 

 

[jetjr]

Sounds like a few posting here should read old posts on Jabiru engine problems and development. Must be thousands of posts on the same topics and many of the proposals here have been answered.

 

- Short through bolts inc picture, this was the quick and easy fix for STRIPPED through bolts, when they were being reused at overhaul, the threads could strip. Thicker nuts supposably gave more threads strength - BS but what the hell. Threads can be stripped even when assembling to spec torque.

 

- ALL Jabiru through bolts are rolled thread, always have been Im told. Larger, stronger through bolts are out there if you want them.

 

- As said above broken bolts is a symptom not the cause. If you use stronger bolts the forces will go elsewhere.

 

- Even when longer through bolt mod done, front and rear STUDS are left in place, these can only be removed at full case strip down.

 

- Solid lifter models didnt break bolts, hydraulic do. Why?

 

- Oils, currently new spec oil is being tested to replace all oil recommended. Manuals and documents will be altered to suit. Done in conjuction with Shell.

 

- 38 degree limit is on the Airframe NOT the engine. To do with freshly cured FRP and softening based on temperatures at construction. Older models had 40 deg limit, older still had no limit. FRP hardens over time and limit isnt as critical as they age I am led to believe.

 

- Camit build the engines to Jabiru spec and is done so to known standard spec and tolerances. No doubt cost is a key driver of Jabiru spec. No doubt some spec should be tightened up like through bolt treads.

 

- 8 cyl engine has been discontinued as there was no market and other similar performance engines are cheaper. Let alone they had some technical issues which remain unsorted Id say.

 

- By running the brand down and getting CASA to "close them down" would benefit who? Certainly not the majority of owners with satisfactory performing engines and aircraft.

 

Edit- re torque, theres a whole debate on dry vs lubed vs Loctited though bolts too. Benefits and problems with all.

 

Overtemps and fuel distribution are a well known problem and have been since day one. Before EFI most automotive engines had similar issues. Everytime Motz or others have had problems the question is asked regarding the EGT and CHT on ALL cylinders and rarely can the answers be given. Pretty basic EMS systems can alarm (even students cant ignore) and log data so real problems may be found.

 

I know of some very well trained mechanics and LAME/L2 who do not know the Jabiru very well. Some even smart enough to say so and suggest you get someone with currency and experience work on them despite their training and qualifications.

 

 

[Sapphire]

In car engines even torque is essential because you have to maintain a seal over a large area. The amount of torque is not super critical as I will increase the torque settings depending on circumstances. In an aircooled a/c engine there is no compression seal and only an o-ring to contain oil splash. The hold down bolts just keep the cyl. head attached and aligned on the block. If the torque is a bit less or more that is not problem as long as the head stays attached and does not move. An issue of breaking bolts should be addressed with stronger bolts. Any issue of extra weight is insignificant to one of cyl. head flying off.

 

 

[Head in the clouds]

........ Threads can be stripped even when assembling to spec torque.....If you use stronger bolts the forces will go elsewhere....

Your first statement is incorrect if the bolts are of decent quality.

 

The second statement is wrong, the forces can't go elsewhere, the only thing that can happen is the bolts breaking. We're dealing with resolved forces here, the bolts' clamping force opposing the case halves' separating force. There is no possible internal lockup unless pistons were hitting heads or valves, or unless valve springs were reaching a fully compressed condition. From what people have posted previously it would seem that the quality of the bolts themselves and/or the torquing specs or procedures would be in question. One thing that doesn't seem to have been considered here - I don't read posts from folks talking about oil leaks. Now if something was locking up under some conditions, to do with the lifters for example, then the internal forces trying to separate the halves would be quite sufficient to stretch the bolts, at the time, enough to distort the case and allow oil to leak. If there's no leaking then there's no massive internal force, in which case my early suggestion of vibration of the bolts could be more likely the culprit.

 

I re-read all of the old threads that I could find (more than 100) and don't think I have duplicated anything previously submitted, so I was just trying to be pro-active and come up with some suggestions about why the problems are happening and what might be able to be done about it.

 

 

[slartibartfast]

- Even when longer through bolt mod done, front and rear STUDS are left in place, these can only be removed at full case strip down.

When I did my through bolts I did the studs too. I had to because one of the two nuts which stripped themselves during the 25 hours after putting new nuts (old style because this was a week before the AD for the 12 point nuts so of course they didn't give me the new ones - or even mention the upcoming mod) on was on a stud.

 

Two of the studs came out fairly easily with heat on the crankcase and locked nuts on the stud.

 

The other two were a bitch. Had to drive splines onto them to get enough purchase.

 

It was all done without a full strip though.

 

Just for information of others who may want to try. I didn't like the idea of leaving the old studs in place.

 

The original problem was stripping of nuts when rebuilding. Calibrated wrench, but two nuts stripped anyway. An expert had a look and said that the nuts didn't match the bolts. On checking 25 hours after putting the new set on successfully, I found 2 had stripped themselves. Must have been a gnats member away from engine failure. The crankcase was only holding together through race memory.

 

 

[jetjr]

HITClouds, I did mention some spec should be tightened, the through bolt thread is what I meant. They are too loose I think but they do fall within design spec.

 

Plenty of other things can let go if detonation is the cause, as Sapphire mentioned the normal fores on through bolts should be fairly limited. As such they could easily be the weakest point.

 

As the problem didnt show itself until more recent hydraulic lifter designs it would indicate some connection with valves and timing/clearances but poor mixture distribution and high EGT/lean running is known to be a problem yet it seems few are measuring all cylinders.

 

Its taken lots of stuffing around to get mine even close at all rpm. I still have one which goes very high upon reducing rpm - like 2900 down to 2600 rpm sees 750+ degrees if you let it go un-managed. Do this for 30-40 circuits/day without attention and problems (including detonation) are likely. EMS would see the problem on first cycle.

 

Sending this data log back to Jabiru would be more useful than angry calls.

 

The problem Slarti has discussed is very real but totally different to bolt breaking problem. Better threads needed without doubt

 

I got new bolts nuts and free install at Bundy. Studs were not changed out, I dont like it but not worth problems to remove them.

 

 

[jakej]

Overtemps and fuel distribution are a well known problem and have been since day one. Before EFI most automotive engines had similar issues. Everytime Motz or others have had problems the question is asked regarding the EGT and CHT on ALL cylinders and rarely can the answers be given. Pretty basic EMS systems can alarm (even students cant ignore) and log data so real problems may be found.

 

I know of some very well trained mechanics and LAME/L2 who do not know the Jabiru very well. Some even smart enough to say so and suggest you get someone with currency and experience work on them despite their training and qualifications.

 

jetjr,

 

If temps are so important, & I believe they are, then I wonder why so many factory Jabbys have only 2 of each EGT/CHT probes fitted.

 

Jake J

 

 

[motzartmerv]

If temps are so important, & I believe they are, then I wonder why so many factory Jabbys have only 2 of each EGT/CHT probes fitted.

Jake J

Most ive seen only have one CHT and no EGT.

Some options had the two CHT. But your stock standard only has one (or at least used to)

 

 

[boingk]

Cht on hottest cylinder if only one fitted, but really not that hard to fit four. Hell even my plane has full cht and egt. Very useful for leaning at altitude not to mention diagnosing any problems.

 

- Cheers

 

 

[facthunter]

But you have a manual leaning carb boingk. Also the continental has an updraft and a "cross" shaped manifold which probably distributes the mixture more evenly. Jabiru have tried to get a relatively streamlined manifold to distribute mixtures evenly, which is a difficult task. Lean mixtures detonate easier than rich ones. Nev

 

 

[Dieselten]

As I recall, when my Jabiru engine broke a through-bolt we recovered the broken section with the nut still attached. The break occurred one thread inside the nut and examination of the broken surface revealed a series of "tide-marks" indicating repeated stress-cycles causing embrittlement and progressive brittle-fracture before the final section failed through plastic deformation. The brittle fracture area was a dull grey (Martensite?) consistent with this failure-mode, the plastic-deformation section was also typical. About all we could really tell was the break occurred at a thread, presumably due to a stress-riser which had developed. Not knowing whether the threads were rolled or cut we were not able to make much more of a conclusion than that. We certainly couldn't come to any conclusion about why or how the stress-riser developed, which surely must be at the heart of the problem. That in itself was frustrating because with the broken part of the bolt we felt we might have had a chance of finding what the problem was.

 

An enquiry was made to a metallurgist at the local steelworks as to the possibility of a probing metallurgical examination of the failed bolt inside the nut but the upshot was the sample was too small for the sort of examination they could perform.

 

Pragmatism set in and the engine was boxed up and returned to Jabiru for a full rebuild. Navel-gazing about why a bolt broke is one thing, getting an aircraft back on cross-hire was entirely another and the commercial imperative came first. As a result the broken end of the bolt has never been metallurgically examined. All we really know is progressive brittle-fracture preceded plastic-deformation and ultimate failure. What was annoying was there were no signs of impending failure in the form of oil-leaks etc.

 

My view is that given time Jabiru will get these engines sorted, and the combination of engine and airframe will deservedly become an aviation classic. The airframe is just about un-killable, but the engines have a tendency to be "fragile". In spite of my trials and tribulations I have stuck with my J160C because it is a factory-built, CASA-certified, aircraft (not LSA) and therefore a change or modification can be approved by a LAME, independently of Jabiru. In these days of the uncertainties of LSA, this is comforting.

 

 

[jetjr]

Overtemps and fuel distribution are a well known problem and have been since day one. Before EFI most automotive engines had similar issues. Everytime Motz or others have had problems the question is asked regarding the EGT and CHT on ALL cylinders and rarely can the answers be given. Pretty basic EMS systems can alarm (even students cant ignore) and log data so real problems may be found.I know of some very well trained mechanics and LAME/L2 who do not know the Jabiru very well. Some even smart enough to say so and suggest you get someone with currency and experience work on them despite their training and qualifications.

jetjr,

 

If temps are so important, & I believe they are, then I wonder why so many factory Jabbys have only 2 of each EGT/CHT probes fitted.

 

Jake J

I agree, with what I know now, no way 2 is enough. Mine came with 1x CHT only.

 

2 CHT and EGT AND a Dynon seems crazy. There is a school of thought that too much information makes some people worry too much!!

 

I have mentioned before to owners and Jabiru that I feel full 4/6 cyl Dynon EMS (or similar) should come std. After an incident, download and review data. Powerful tool for both owner and Jabiru when it comes to problems. A couple ive seen from factory WITH full Dynon EMS didnt have logging turned on.

 

High EGT even changes cylinder position at different rpm. Mine is No 2 in cruise, No 5 in descent. CHT are fairly constant but in mine number 2 is hottest. Probe was @ No 6 which was 50 deg C cooler.

 

Jabirus are quite easy to richen up - use the carb heat. All air is filtered anyway so no loss.

 

 

[facthunter]

There is a significant power loss. ( due to the temp of the intake air). I've seen a couple of go rounds with it still on and not real safe. Having all that monitoring going on is probably somwhat distracting. Nev

 

 

[jetjr]

I disagree, particularly on 3300, power loss is hardly noticable., Jabiru agree it isn't much and it's ok to leave it on all the time if needed. Take off or landing would be an exception.

 

You don't have to watch the monitor, it will alarm or change colour if something's wrong

 

 

[motzartmerv]

Jabiru agree it isn't much and it's ok to leave it on all the time if needed.

Sounds like a good reason NOT to ...:)...

 

 

[Phil Perry]

Hi Phil,I can't comment about your experience in your industry .. however in my industry, being the oil biz, most products and basically all engine lubes are made to meet or exceed an agreed spec and tested to industry agreed test programmes.

 

There are some smaller oil or additive companies that do make some claims that are hard to pin down as they are not measured against recognised test programmes... some work some don't.

 

Whilst I can sense your scepticism ... a formal recommendation by a major oil company (not a sales rep or some good intentioned retailer) is underpinned with millions of dollars and thousands of hours of testing. There may well be better products out there that may work well or even better, however it's a game of Russian Roulette to try something not recommended, approved or tested .. believe me I have seen the other side of the "when things go wrong".

 

If I was given a clean sheet to design a new lube for a specific aircraft engine I suspect it would look a little different, however if you wanted the same lube to be backward compatible with other products and tolerate top ups and work in 85% of all aero engines and also meet the OEM performance requirements, you may not end up with something not a lot different. Often you can improve the performance of a lube very simply but then make it fail in another area when you chemically push it over the edge ... there is a lot of balance to get a lube to meet all the performance requirements.... I often see this happen when people put their own additives into a lube and unwittingly damage another performance characteristic.

 

Whilst the perception that aero lubes have not progressed is not true either ... new additives, better more stable base oil and better manufacturing and test programmes has improved old formulations a long way.

 

As for testing, suggesting an air cooled motorcycle caught in traffic test as a good proof of product performance is not something I would want to use to risk anyones life on for an aero lube. Testing is done by dedicated people following exact recognised processes which gets repeated over and over again before any field tested can be considered ... bwt, this also includes test progrmmes to make sure motorcycles lubes can tolerate traffic jams, albeit we don't park motorcycles in traffic but we do simulate it at first then do the field trials in the real world. However just making a motorcycle lubes not thermally crack in traffic is just one performance issue ... making it shear stable to work in the gearbox, not allow the clutch to slip, clean the engine, hold soot in suspension to get caught in the filter, not turn into foam, provide corrosion protection, absorb acids, maintain viscosity, give proper anti-wear protection etc etc are just a few and all are important.

 

I agree there is always room to improve and develop new technology ... this is evolutionary process and people are always looking at the next new piece of technology, it will come, but it takes many years to get this over the line safely.

 

In terms of aero engine lubes selection for myself ... I only trust what is approved and recommended by major oil companies and the OEMs... however I will select a multigrade over a mono grade if it is approved.

 

Multigrades can do all that a monos can but a monos can't multitask as well.

 

Cheers

 

Vev

Hi Vev, and thanks for that,. . . .

 

One of my long standing aviator friends; Ian Mitchell, who worked for 12 years with Albright and Wilson oil development & technology, explained most of what you just posted to a Club meeting some years ago, he now works with ( as well as oils )researching and developing long chain molecule products ( bloody shampoo - as he calls it. . . ) for another similar development company, . . . . and we have this discussion about "LUBES" on a regular basis at our airfield when engines go bung after running the makers' favourite oils . . . . so I am au fait with the rationale which you have posted for our eddification ( very clearly put as well IMHO ) and please don't be offended by my comments, which were not intended to be in any way derisory. The point I really made was that we have seen, ad nauseum, statements from various oil suppliers, some recommended by manufacturers, to be suitable for our operations incidentally, case in point,. . . Morrisons Mineral Oil used to be one of the most popular oils used in two stroke Microlight engines ( R-337 Robin 440, R447 / 462 / 532 / 582 for some years, even though you had to scrape a crapload of carbon deposits from pistons after around fifrty hours of using the stuff.

 

Guess what their advertising tagline was ??? "You can be ASSURED. . blah blah blah. . .research and testing . . blah blah blah. . " Of course the process of technology is evolutionary, but usually driven by the market, ie, high - time users of the product. Therefore, surely to suggest that alternatives which offer "Apparent Advantages" in other, perhaps slightly similar operations. . . should never be tried when for one reason or another the user is patently unhappy with the recommended product is at the very least debatable, however many millions of whatever currency has been allegedly injected into the product in question.

 

I guess, as you point out, if you want engine insurance, then you HAVE to go with the maker's guidelines, but how the hell you would police that, is, as has often been said,. . .another story.

 

 

[turboplanner]

It's very important to understand that Oil Companies and their specialists are only half the equation, experts as they may be.

 

The other half are the designers and builders of the engine.

 

So taking the advice of the oil supplier without the specific approval of the engine manufacturer can leave you at risk of rapidly destroying an engine.

 

You might think that after 150 years of designing internal combustion engines we would not have any problem producing a perfect product straight off the production line.

 

But the moving targets of changing materials and performance requirements continually cause design challenges, and we always fail somewhere.

 

The new design may have a localised hot spot due to forging design, or an oil gallery restriction which shows up in bench testing.

 

The cost of fixing it may run into millions of dollars, and even then there's a risk of just shifting the problem to another area.

 

A way out of the two problems above may be to specify a high temperature resistant oil for the hot spot, or a very low viscosity oil to get through the gallery bottleneck - neither being a good marketing ploy given that human nature says that many owners don't care about adhering to instructions, but a legitimate action nevertheless.

 

Not surprisingly the manufacture doesn't press release the fact that he has an engine with a hotspot or gallery problem, so the oil companies don't know about the specific issue, so the owner takes a big risk just using their recommendation which, reasonably is often based on engines of various makes and models operating in certain environments.

 

Frequently the special oil supplied by the engine manufacturer is more expensive, so human nature says people will buy cheaper products, or get advice they want to hear ("I've been using it for a month, it's great!" rather than "I've bench tested a hundred engines out to and got a 97% pass to 1500 hours")

 

Edit: low viscosity inlieu of high - sorry Boink, dislexic moment

 

What follows that is a succession of failures, often of the same component(s)

 

So given Recreational Aircraft are cheap unless you have to sink $20,000 a time into a succession of rebuilds (assuming you've survived the forced landings) when it switches to a horrific cost above GA aircraft, it pays to rigidly adhere to the engine manufacturer's commitment on oil specifications and fuel specifications.

 

It's been interesting to read of the major blunders people have been making because they didn't understand the differences between Aircraft and Automotive octane rating specifications - oils have much the same traps for the amateurs.

 

 

[boingk]

Well summarised, turbo.

 

One thing I'd be careful of especially is the recommendation by manufacturers of motorcycle oils - generally these are specified for their antiwear gear additives designed for particularly high stress environments found either in a motorcycle gearbox... or aeroplane PSRU. Rotec specify Shell VX 10W-40 for both their R2800 and R3600 engines because of this, for example.

 

Also might point out that you said you may use high viscosity oil to squeeze through a gallery bottleneck... you'd want to use *low* viscosity oil (ie 10W-40 instead of 20W-50) in that case. Sure you meant to say that but thought I'd clear it up anyways.

 

As for testing without proof of concept... I thought thats what we were doing whilst piloting many of our aircraft? Many designs are being flown that have been either user designed or at the very least not factory backed. I am in no way condoning people just rush out and use any oil in their engine, but I suppose you get my drift.

 

On petrol... modern engines tend to be more sensetive than older style lower compression engines. You'd be mad to run anything less than 98 Mogas in a Rotax 912 for example due to their high compression and high specific power output. My aircraft on the other hand is quite happy with 95 Mogas due to its Continental A-65's low max rpm (2300), low compression ratio (6.3:1!) and low specific power output (23hp/litre).

 

Cheers - boingk

 

 

[Camel]

Turbo, I would like to point out that throughout motor car and motor cycle developement most of the serious advancement has been made through factory run and sponsored racing teams and private racing teams. The greatest example the Bathurst Mt Panorama production race from way back. In aircraft terms I.m sure we have to look at flying schools to put aircraft to the test but they are not to be modified and our experimental class is the next best thing to put things to the test.

 

 

[turboplanner]

The difference is though that when the bright idea fails in a cloud of smoke in the race car you roll to the side and get out another 15 grand for the next meeting, rather than drop down like a piece of floating paper.

 

Also in most cases the goalposts for aircraft engines are reliability rather than increased power, so you are changing a different group of specifications.

 

 

[boingk]

True, but more powerful engines are definitely a possibility. Hell, look at motorcycles - I ride bikes every day that are putting out 120hp per litre. I ride them day in, day out and have yet to have anything go wrong.

 

The engine in my Minicab is putting out 23hp per litre. Politely, thats sod all in the way of engine stresses.

 

You can definitely do more to develop engines, but you must beware of developing them past a certain specification. Most engines will give a 10~25% hp increase quite readily but going over that you may start to run into the limits of the engine, mechanically and reliably speaking. You can certainly squeeze 50hp out of a Briggs & Stratton, for example, but there won't be much left except for the engine case when you're done!

 

CHeers - boingk

 

 

[steve gordon]

Thanks for the info Maj.I had a look at some of the conversation in older threads.

 

All graded production bolts have rolled threads, it's the fastest way to make them, only ungraded cheap rubbish bolts for garden fences and the like have cut threads, and cheap versions of threaded rod do too. But AN, Grade 5, Grade 8, Class 8.8/s and socket head cap screws (SHCS) always have rolled threads, as do the nuts associated with them, the nuts' threads are formed with a 'rollform' tool which looks like a triangular section tap but isn't sharp, it sort of smears the threads into shape.

 

In the case of the Jab bolts, were they actually making their own bolts? That would be plain crazy unless they had a need for some weird shaped stud and even then it'd be better to get a fastener maker to produce them.

 

I saw that some people were talking about up-sizing their bolts to 7/16 from the original 3/8. Can that be done without boring the holes for the larger bolts?

 

I also read that someone who had considered just replacing the nuts with the long series 12 siders decided instead to also replace the bolts with the longer ones, because the threads had deformed 'like a christmas tree'. Thread failure of that nature indicates very poor manufacturing QA because the thread rolling die must have been very much out of adjustment, and that indicates that the bolts were from a very poor quality supplier. For aircraft engine use there is absolutely no excuse for not using individually tested bolts, they don't cost very much more than the cheap ones.

 

Without doubt the best value tested bolts are made by Unbrako and their strength is far superior to almost anything else on the market. Their surface finish is also tested individually to ensure that they don't have any sharp microscopic flaws which could act as a stress riser, eventually resulting in cracking.

 

American Grade 5 bolts have a tensile strength of about 120,000 psi which is similar to metric Class 8.8/s structural bolts. American Grade 8s are about 150,000 psi tensile and Unbrakos test out at between 180,000 and 190,000 psi, so you can see they are significantly stronger and also tested against flaws. Yield strengths are about 75% of the tensiles mentioned.

 

I guess only privately operated Jabs could run the better bolts, flying schools would have to stick with the manufacturer's supply...?

[steve gordon]

in reply to the issues of the through bolts breaking in my experience (mining engineering major shutdown project manager) concurs with a couple of points:

 

1/ Rolled threads are the norm in Grade 8.8(standard high tensile bolts)

 

2/ High tensile bolts should be tension only once and to a wet( lubricated value) or a dry value, over tightening will lead to failure.

 

3/ Cracks propogate from points such as shap edges ie machined threads under high levels of vibration.

 

4/ Ultra critical bolts are tightened using a bolt stretcher this ensures no torsional loading on the bolt and ensures the correct force is applied.

 

Iam not claiming to have the answer and are looking at purchasing a Jabiru and have a problem understanding why so many have engines with differing hrs to airframe hours for relatively new aircraft this suggests that there is problem with engine quality to me a student pilot. however this makes me think twice before purchasing a Jabriu

 

 

[turboplanner]

What you are saying is quite correct Steve, but the failures haven't been limited to these points, or in some proven cases, amateur assembly.

 

I certainly would not use nuts that only seated on the base of the bolt thread, not the tang, but some failures don't even point to that.

 

The material and diameter seems OK.

 

It looks, without metallurgy testing as if something well above compression force is occuring.

 

Some of us thought of carb ice chunks, others manifold design, others detonation etc.