Jabiru Prop failure

[Dafydd Llewellyn]

Your words - so is it simple or not? I side with Ed Heinemann, particularly the 'simplicate' bit, I've never found good reason to complicate something that works just fine as it is. And I spend my days rectifying structural engineers' unnecessary over-complication of just about everything they put their mind to, so that certainly keeps me skeptical.

 

Most people mellow as they age Dafydd.

Chaque un a son gout

 

 

[Guest Andys@coffs]

A bunch of washers and slightly longer bolts is to my mind hardly complex. The benefits to me outweigh the downsides...which to be honest I struggle to see.

 

The reason I asked the original question which started this whole debate was, that its been my experience that while I check as required by the maintenance schedule, I find no adjustment has been required. That said its a sample size of one and in the same location for the time in question so hardly statistically relevant.

 

Jabiru didn't start this way they started as you guys are talking and yet they changed because it wasn't working (assumption by me) because I cant imagine J who are very cost conscious doing something that adds to the costbase and delivers no benefit..... Up until now the assumption by me has been that the washers were to compensate for using packing crates as props. I now know that is not the case entirely.

 

Andy

 

 

[Head in the clouds]

Andy, leaving the washers out of the discussion for now because they weren't on the OP's prop that failed, and also because we all know that wooden props have been used without them for as long as man has been flying. There's obviously nothing wrong with them but we do know they're just an option.

 

So, considering this specific failure, I just went back and had another very close look at the pictures that were posted and they do tell a story. The story might well be different according to the observer so I'll just describe my story and happily allow that others will see it differently.

 

The first picture shows - the front face of the glass/resin sheathed hub, I'm assuming epoxy. Spanwise fracturing on the side of the hub which at first glance look like compression fractures but then I notice that they are not purely spanwise oriented, in fact they're all curved or angled, so does that make them shear fractures? If it was shear it would be because the sides of the hub sheathing is acting as a web between the 'flanges' on the front and rear faces of the hub. So effectively the inner hub face/flange sheathing is being twisted relative to the outer hub sheath facing/flange. And that's unlikely unless the bond between the sheathing faces and the wood had broken down. So the fracturing could be due to fore and aft flew of the blades but that would have to be very large and that also seems unlikely unless some of the bolts had broken. Also there is significant fracturing of the area immediately adjacent to the bolt holes. It looks to me to be more like hammering fracturing than crushing, and why would the sheathing crush anyway? Surely the wood should be softer than the resin/glass so the wood should crush before the sheathing.

 

Picture two shows the curved nature of the fracturing, I can't get much more from that pic except that the front face is to the top of the pic and v.v. The edges of the face at the bottom of the pic appears to have more damage than the other one.

 

Picture three - this face shows much more damage than the other, as if it was working against the drive flange of the engine, and the sheathing is very fractured and does appear to have separated from the timber, so shear fracturing of the sides is a possibility. The bolt holes are enlarged to accept the drive spigots (?) so this is definitely the engine side of the prop. Why then does the right hand blade look like the top surface of the airfoil is nearest the camera instead of the lower surface? That would make it a pusher prop, but not for a J160, so it must be an illusion?? But looking again at the first picture that blade looks like the lower surface of the blade. Are we being had? Is this a prop off a seaplane (pusher) that's flipped and had a water prop strike?

 

Anyway, assuming it's an illusion (opinions folks?) then the next thing I'd ask is why sheath the prop hub in the first place? if you want to keep moisture out it's well established that polyurethane paint is the way to go, and that's what most timber prop manufacturers use and recommend, it can be clear rather than pigmented if that's your preference. Sheathing is usually only applied to the outer 20% or so of each blade, to reduce FO damage.

 

Nonetheless it would appear that the sheathing has not particularly prevented the prop's timber heart from being clamped - the edges of the sheathing have 'given' and allowed the drive flange and prop plate to form an impression at some stage - so the amount of damage to the engine face of the sheathing looks as if there has been a lot of movement there which can only happen with loose bolts. Does the maintenance log show when the bolts were last checked? Had the prop been off recently and someone forgot to torque them? Is this an example of my theory described in the last para of post #72?

 

 

[Guest Andys@coffs]

Andy, leaving the washers out of the discussion for now because they weren't on the OP's prop that failed

Do we know that for sure? The link I posted was J's maintenance manual for 160C's so I think they should have been......

 

http://www.jabiru.net.au/Manuals/Aircraft%20Technical/JTM001-1_Generic_Tech_Rev1_signed.pdf page 226. It only shows the washer's version not the earlier non washers version

 

Andy

 

 

[Oscar]

I think you miss the point again - I saw the test performed, it was not a document, it was a real, practical test conducted in Berwick, Victoria, and for a completely unrelated purpose, interestingly. As far as I know it was not documented it was done to prove a point, just like this one. I have told you how to conduct this very simple test and I have told you the results that I saw from the test I saw conducted. You say "I personally rely on scientifically justifiable testing" when in fact you quote a paper related to theory of 'large structures'. If you want results from testing, then test it! It's very simple to do. If you want to accelerate the test then heat the water. If you want to accelerate the (semi-permeable membrane) test further, then use rock salt instead of balsa to increase the concentration difference across the membrane. This is basic schoolboy stuff that yields accurate real-world results Oscar, not 'unsupported assertions'.

I see neither profit nor point from debating this further. There is any amount of credible information on the relative permeability of epoxy resins: Google: epoxy resin water permeability . None that I have seen supports your contention that 'epoxy is one of the worst'. Those people who wish to acquaint themselves with the information readily available can easily do so, and hopefully will.

 

 

[AVOCET]

I guess this posts almost done ,

 

My wife's French , always handy for a quick translation .

 

I'm a pattern maker and had a fine furniture buisiness for 25 yeas in bundaberg , (no I'm not giving my résumé )

 

I've had a lot to do with timber and jabiru props and the kiln dried hoop pine that is used is not a constant product , seasonal growth diferance from year to year , trees that have been stressed in storms many years before their cut down ,can have quite differant carracturistics in the final plank , even planks from different trees & differnt parts of the tree can effect warpage ,

 

I've seen hoop boards that are planed flat and then days or weeks later look like a propellor ,. The way jab used to plane their wood was to put rough sawn timber into a thickneser

 

And then glue the laminates together , . The correct way is to plane one side flat on a plannerand then use the thickneser , this way the board has a better chance of staying flat but is very "time " consuming .

 

Timber is a fickled thing and takes a keen eye to decide what planks to use .

 

Props that end up warping on there own have more than Likely timber that going to warp no matter what .

 

Many a good looking plank carnt be used for making furniture or props

 

With the jab props it's been the luck of the draw

 

Cheers mike

 

 

[Dafydd Llewellyn]

I guess this posts almost done ,My wife's French , always handy for a quick translation .

I'm a pattern maker and had a fine furniture buisiness for 25 yeas in bundaberg , (no I'm not giving my résumé )

 

I've had a lot to do with timber and jabiru props and the kiln dried hoop pine that is used is not a constant product , seasonal growth diferance from year to year , trees that have been stressed in storms many years before their cut down ,can have quite differant carracturistics in the final plank , even planks from different trees & differnt parts of the tree can effect warpage ,

 

I've seen hoop boards that are planed flat and then days or weeks later look like a propellor ,. The way jab used to plane their wood was to put rough sawn timber into a thickneser

 

And then glue the laminates together , . The correct way is to plane one side flat on a plannerand then use the thickneser , this way the board has a better chance of staying flat but is very "time " consuming .

 

Timber is a fickled thing and takes a keen eye to decide what planks to use .

 

Props that end up warping on there own have more than Likely timber that going to warp no matter what .

 

Many a good looking plank carnt be used for making furniture or props

 

With the jab props it's been the luck of the draw

 

Cheers mike

Yes, timber is an inherently variable product. However, there are some requirements in CAO 108.29 in regard to grain slope, moisture content, density, brittleness (IZOD test) and visible defects. Jabiru props are, so far as I am aware, made to comply with CAO 108.28 and 108.29. So it's not quite open slather. One can, of course, go better than those requirements - but they are what CASA requires . . .

 

 

[AVOCET]

Does that casa requirement cover the dressing of the timber ? I've found that's where some if not most of the trouble starts & then the glue up .

 

.the people that do the dressing Probly don't even know who casa is .

 

Cheers mike

 

 

[facthunter]

People who work with wood should be capable of checking for shakes, cross grain and other defects, or they shouldn't be doing it . Especially with anything structural. Laminating a prop enables better quality control of the timber. I think structural failures of Jab props are not that common. The dimensional accuracy could sometimes be better, but that's a cost thing. Nev

 

 

[Dafydd Llewellyn]

Does that casa requirement cover the dressing of the timber ? I've found that's where some if not most of the trouble starts & then the glue up ..the people that do the dressing Probly don't even know who casa is .

Cheers mike

See http://www.comlaw.gov.au/Details/F2007L04903

 

 

[Dafydd Llewellyn]

Does that casa requirement cover the dressing of the timber ? I've found that's where some if not most of the trouble starts & then the glue up ..the people that do the dressing Probly don't even know who casa is .

Cheers mike

You also need to look at CAO 108.28 - See http://www.comlaw.gov.au/Details/F2007L04873

 

 

[Dafydd Llewellyn]

Does that casa requirement cover the dressing of the timber ? I've found that's where some if not most of the trouble starts & then the glue up ..the people that do the dressing Probly don't even know who casa is .

Cheers mike

And please pass my apologies to your wife for my mis-use of the French - it should be "a chaqu'un son gout".

 

 

[Dafydd Llewellyn]

A lot of very good information which ultimately could save lives, well done everyone and thanks for posting.Isn't this a fantastic forum.

 

Alan.

I hope it has shed some light on to the whole business. I do trust it is understood by everybody that adding Belleville washers to a certificated aircraft propeller installation is a major modification that requires formal approval, either under CASR 21M (was CAR 35) or from the manufacturer, in the case of an LSA aircraft. I notice that it is becoming somewhat popular for derivatives of the RUTAN pusher types in the U.S. Experimental area; the Seabird Seeker example is a certificated GA aircraft - and possibly the first certificated GA aircraft since the Tiger Moth era to have Belleville washers incorporated in its certificated design. So whilst it's straightforward engineering, if you understand the maths involved, and anybody can apply it to their experimental aircraft, that's not the case for anything else (just yet, anyway).

 

 

[austerflyer]

Have come across this interesting discussion re wooden props.

 

There was a mention of a problem with wooden sesenich props fitted to Jabs.

 

Have one were spigot and bolt holes are tight and the two holes on prop blade axis

 

do not line up.

 

Any one else had this problem.

 

Chris Harrison

 

 

[Jabiru Phil]

Have come across this interesting discussion re wooden props.There was a mention of a problem with wooden sesenich props fitted to Jabs.

Have one were spigot and bolt holes are tight and the two holes on prop blade axis

 

do not line up.

 

Any one else had this problem.

 

Chris Harrison

Look up Jab website, they had an article about this.

 

Seem to remember them mentioning your problem.

 

Phil.

 

 

[facthunter]

Sensenich are a very reliable prop manufacturer. What do they have to say about it? Nev

 

 

[jetjr]

Only issue I heard of was due to very thin trailing edges in these, was cracking

 

Be aware Jabiru supplied Sensenich were a little bit different to factory supplied ones I think

 

Was a recent issue with newer composite ones but was related to incorrect assembly. Two different hole sizes in backing plate, can be installed in wrong position.

 

 

[Oscar]

Yes, the Jab. prop. hub has a set of holes at 100mm diameter and a set at 4" diameter.

 

 

[facthunter]

Well that would be to ensure there is only 2 possible positions for the prop to be installed. NO? Nev

 

 

[jetjr]

Cant recall, theres an SB about it

 

Newest hubs use just four bolts, if the backing plate is aligned on wrong holes of spinner backng plate (smaller) the hub itself clamps on fibreglass, stress, vibration and crack forms

 

 

[powerandpassion]

Apologies for bringing up this old thread, but a fascinating discussion. I feel as if I have gone down to the cellar and brought out a fine old bottle of wine, worth uncorking ! My interest is in 1930’s era timber propellors, as an example, fixed pitch propellors used on the Hawker Australian Demon, 500 HP RR Kestrel engine, used by the RAAF from 1935 to 1943. These propellors were originally supplied from the UK in Honduran Mahogony, by the 30’s representing the best possible material, and all were coated with doped Irish linen to create a moisture barrier, with brass leading edges. Old photos show Demons left in the open with canvas propellor covers, if not hangared. From 1928 all RAF aircraft were decreed to be made from metal, the great airship projects R100 and R101 were funded by HM government to allow aircraft manufacturers the stimulus to tool up for metal construction and retrain their carpenter workforce to metal fitting. The driver was the extension of aircraft service life by moving away from the destructive shrink-swell cycle of wood. In 1940 Australia was forced, from lack of UK supply, to make it’s own propellors, selecting Queensland Maple after exhaustive testing as the best material. Most antique Australian propellors that remain are QM, very rarely do you see a Honduran Mahogony example in a museum. The shrink-swell cycle was revisited in the 1940’s with Australian Mosquito production, when the CSIR subjected Mosquito wings to climate tests, from Townsville to Fisherman’s Bend, to resolve structural analysis. These CSIR reports make interesting reading : I would only hangar a Mosquito where good coffee and laneway art can be found, or otherwise a humidity managed hangar elsewhere, if longevity is an issue. For the Demon, the clamping plate was aluminium on a nickel chromium alloy hub, so the hypothesis is that the OEM figured a performance benefit from this arrangement, perhaps related to crush? I have not done a hardness test on the aluminium relative to the doped hub, but an old Demon prop I have shows no crush or fretting on the doped hub face. I figure the old timers had tens of thousands of hours of practical experience, and there is always a reason for something. The hub bolts are hollow, and I understand this was one method of ensuring even heat treatment, whereas a solid bolt might have a tough case and softer core, or uneven performance. It seems like the hub, bolts and prop were a system, with careful and complimentary material selection, aided by simple habit of avoiding moisture or condensation in hangaring or picketing. The Ferrodo discs were used with Aluminium Fairey Reed type fixed pitch propellors on Avro Ansons, I see, so far, no evidence of their use for timber props. Fixed pitch timber props made it from 100HP to 1,000HP application, and beyond with various fine veneered and compressed designs, particularly German Jablo types, protected by bakelite type finishes. The issue with Queensland Maple is most of it was clear felled in the 50’s and 60’s to create dairying land in Queensland, and what remains of native stands is now locked away in national parks. Only a limited volume of what might be cut, if at all, will present as even grained, consistent material suitable for propellor making. It is all a rapidly receding art, with generational knowledge lost as ‘old men die and libraries burn’. One benefit of the internet is to post, share and accidentally record information, in forums like these. 

[Old Koreelah]

Your post resonated with me, P&P. Elsewhere in this forum is a debate about qualities of Australian timbers. My ears pricked up because my dad came from a family heavily into the management of our timber resources, although he joined the ranks of those dairy farmers who cleared our best forests.

 

As posted by others, Australia developed massive expertise in the qualities of our timbers, but much of that has been lost. Members of my dad’s family certainly contributed to recording much knowledge, but they’ve have taken their experience with them.

[Bruce Tuncks]

Powerandpassion, I was very interested in the story about testing qld maple for mosquito props in 1940. This shows far more thought than I thought went into the mosquito project.

What do you think went wrong? I always reckoned it was that Australia didn't have the skills to make the joints close enough for the non-epoxy glues they had then, but there is obviously more to the story than that.

Gosh, a squadron of mosquito planes in 1941 would have been great, alas it was not to be.

[powerandpassion]

Bruce, to clarify, QM was used for fixed pitch propellors, not Mosquito props, which were aluminium. Australian timber for Mosquito production that was used  was Coachwood veneer for British Standard V3 ply. Other timbers like Mountain Ash for high strength structural reinforcing and Bollywood as a balsa alternative were examined, but ultimately Canadian spruce and Ecuadorian balsa used, as substitution would have called for dimensional changes for which there was ‘no time’ for in 1942-3. The ‘issue’ with Australian Mosquito production were delays consequent to poor glueing practice at wing subcontractor GM Pagewood. Once this was discovered deHavillands embedded their inspectors at Pagewood to resolve the issue, but existing wings already mated to Beale Piano factory made fuselages had to be recalled and rectified, and by the time this was resolved the Pacific War had moved on, and the government, anxious to preserve budget, cut back production. Postwar, dH glued up timber fuselaged Vampire jets with no issues. Resorcinol type glues (Beetle) were used on both designs, so the early casein based glues subject to microbial attack were not the issue. Poor gluing practice at Pagewood was put down to the GM ethos of speed in assembly, poor joint preparation and poor ‘glue habits’, ie, not following instructions, probably from a workforce used to putting together metal fabrications. GM Pagewood did assemble dH Tiger Moths successfully, but these were structurally steel tube framed fuselages with timber spar wings, probably more forgiving than something meant to travel at 600km/h with two 1,000 HP engines strapped on, like a Mosquito wing. If you want to start a conspiracy theory, GM was the owner of North American Aviation, (NAA) which made the B-25 Mitchell twin engined aircraft, which might have been considered a competitor to the Mosquito. NAA did licence the P-51 Mustang fighter and later Sabre Jet to Commonwealth Aircraft Corporation, so perhaps some slick haired NAA salesman ‘encouraged’ poor glueing in Pagewood, but this would be unlikely.