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Pindan

Jabiru 240 damage repair

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5. There is no design work in a repair. You simply have to reinstate what was there. If you use the correct materials, the correct scarf angles, the correct surface preparation and the correct alignment then you will have the same strength as the original.

If your aunt had balls, she'd be your uncle, but knowing how to make that happen is a different thing.

 

If the damage can be cut back to the original sub assembly points, and reassembly is then the same as original production, then maybe no design work is needed. If that can't happen then the scarfing is where engineering design work is needed, because you have to calculate the load requirement of the join, and ensure you produce a bond to match that strength. You may need more strength than the original.

 

We know it's possible to sew a pig valve into a human heart, we just need the heart surgeon's decades or so of training to do it, so firstly there's a skill level required; not as much as the surgeon of course, but several weeks training to get a structural grade bond - a bit like learning do do a structural weld. I agree that, IF, you do all these things correctly there's not an issue, but you don't see many people these days building the boats and car bodies which entertained us in the past.

 

7. And just for the record, I fly a kit-built Jabiru ( firewall and lots of other stuff all done by me) and a Libelle which was repaired from a $500 write-off.So far, they have flown for many years without falling apart.

If there is a bond-strength issue, that's no guarantee; it will fail when its strength capacity is exceeded, which may not be until some turbulence is hit, or the aircraft controls are misused.

 

Then the failure is instant, and will leave you watching it unravel in disbelief. I've had a few structural failures in machinery and buildings, and afterwards the cause is so obvious.

 

With composites it's better to play it safe and pay for the engineering and skill set.

 

 

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much clipped5. There is no design work in a repair. You simply have to reinstate what was there. If you use the correct materials, the correct scarf angles, the correct surface preparation and the correct alignment then you will have the same strength as the original. Maybe a little heavier as your layup will probably have a bit more resin than the original....

Whilst I do not disagree that full repairs with equal strength to the original is not possible BUT the third sentence in the paragraph above is directly conflicting with the first - ANY repair in composites that involves scarfing into an existing part IS a design exercise AND the layups and orientation to obtain an acceptable repair is NOT often just replicating the orientation of the existing damaged composites and the best scarf position may not even be at the edge of the existing damage.

 

Not at all denying that repair is not possible just pointing out that the repair of a composite part is a design exercise. However, removal and bonding in a whole new part from the manufacturer would not be design just replace as no existing molded part is being repaired.

 

 

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It depends upon the part.

 

I used to hold a DoT (predecessor to CASA, for aircraft regulation) Authorisation for 'minor' repairs and approved modifications to 'glass gliders (also a C of A Inspectors Authority). Now, this was 'back in the day' - 1978, to be precise - so my 'approved' expertise would not be relevant to higher-tech. composite structures ( e.g. c/f, vac-bagged or infused laminates, post-cured structures, aramid-fibre laminates, vinylester laminates). However, it IS relevant to the materials and techniques used by Jabiru.

 

As it happens, I grew up spending a lot of my time 'playing' in the workshop of one of the finest luthiers of the C20, so I was taught quite a lot about woodwork, as well. Competent scarfing of a join in wood is considerably more difficult and demanding than for a simple composite lay-up (though ironically, the optimum scarf angle of 12:1 is the same, in most cases.) For wood, you need a drum-sander and a scarfing jig, so the join is an extremely accurate fit and you have not 'polished-off' the grain ends which prevents proper glue penetration. THEN, you need uniformly distributed clamping pressure over the entire length of the scarf. I cannot think of a way for this to be possible in a main bulkhead on a built Jab. fuselage without some extremely sophisticated techniques involving bespoke clamping structures pulled together from the front and back-side by torque-limited clamp arrangements (which WOULD need design input).

 

With a laminate repair, IF you know what you are doing, you can prepare the scarf to the 'old' laminate with an air-tool mini angle grinder by hand - the input laminate lay-up will conform to any minor irregularities.

 

BUT - I stress - If you know what you are doing. I've watched the guys at Aircraft Composites Australia ( Aircraft Composites Australia ) at work, and they are artists by comparison to just about all of us lesser mortals (me included). Seriously, it's like watching brain surgery vs. your local GP stitching up your cut from falling through a barb-wire fence.

 

This has been a most interesting thread - to me anyway, who is returning a written-off Jab. - with now many modifications, I can't help myself from seeing things I would like to do differently - to the air. I salute all who have contributed for refraining from the 'Jab bashing' milieu that pervaded this site previously.

 

The OP asked for advice, and I think much has been supplied, all in good intent. However, the range of opinions has made me think: How do you - if you're not very well conversant to the subject in hand - judge what is 'expert' opinion? I know that I have extreme difficulties making such judgements for areas ( such as Share portfolio management) where I do not even know what questions to ask..

 

Probably a subject for a discrete thread.

 

 

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The answer to your question ...........How do you - if you're not very well conversant to the subject in hand - judge what is 'expert' opinion? is actually quite simple......

 

You start with the first person you can find with any knowledge of the subject and ask you Initial "uninformed possabley stupid question" you get an answer, and you gain knowledge of more pertinant questions to ask. Once you have asked as many people as you can and no new questions raise their head you start to sort through all the answers you have gained from the process and usually when you get recurring advice as to the solution of the questions this is the "expert" opinion that you should follow. .......i hope.......hehe

 

I.......am still asking questions

 

Oscar open the yellow pages , ring 10 portfolio managers and tell them you have 100k you want to invest where should they put it for you. If a few f them reckon Pindan's Jab repair business is the way to go .........post it over please......haha

 

 

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Guest RobT
A Composites engineer?

Composite engineer is a hijacked word meaning tradesman who works with composites. Like licensed aircraft maintenance engineer = aircraft mechanic.

 

Aerospace engineers and or mechanical engineers with composite knowledge are the ones who do the initial design and calculations for composite strength.

 

So a composite engineer is not the one to ask and answer engineering questions you have to go further up the pay grade. We have one here who you are arguing with.

 

Then the failure is instant, and will leave you watching it unravel in disbelief. I've had a few structural failures in machinery and buildings, and afterwards the cause is so obvious.

and your qualifications are?

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Composite engineer is a hijacked word meaning tradesman who works with composites. Like licensed aircraft maintenance engineer = aircraft mechanic.Aerospace engineers and or mechanical engineers with composite knowledge are the ones who do the initial design and calculations for composite strength.

 

So a composite engineer is not the one to ask and answer engineering questions you have to go further up the pay grade. We have one here who you are arguing with.

 

and your qualifications are?

A Composites engineer is certainly not a hijacked word, and is not a tradesman; they have their own string of descriptions - laminators etc.

 

Composites involve a detailed knowledge of plastics, primarily thermoplastics, an a knowledge of the unique techniques, such as bonding, pressure and vaccum impregnation, and a knowledge of the different characteritics o chpped strand mat, rovings, woven roviings, carbon fibre etc.

 

A mechanical engineer isn't trained in those things and it quickly shows.

 

Just for the Edenhope gallery, I'm qualified as a draftsman with 8 years design experience in structural composites from tankers to semi trailers to elevating platform beams, including repairs.

 

 

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Some designs exhibit an almost total lack of structures knowledge. It's obvious just by looking it the primary structure. Inevitable result of that is excess weight or reduced strength. A jabiru airframe must be regarded as a fairly good application of a relatively simple composite not at the high end of performance or cost. This didn't happen by accident. It works.

 

Lots of very damaged Jabs are rebuilt and if done at the factory are reliable. That's the obvious best path under normal circumstances. To maintain the original structural integrity, to me the original load and stress parameters must be known and adhered to. Making one part overstrong (or too rigid) may overload another part in a stressed structure. A triangulated steel truss is easy to repair, relatively. You can easily calculate the loads in various members and replace with a similarly performing part and it's relatively inflexible. Flexibility is a far more difficult structure to deal with. Nev

 

 

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Some designs exhibit an almost total lack of structures knowledge. It's obvious just by looking it the primary structure. Inevitable result of that is excess weight or reduced strength. A jabiru airframe must be regarded as a fairly good application of a relatively simple composite not at the high end of performance or cost. This didn't happen by accident. It works.Lots of very damaged Jabs are rebuilt and if done at the factory are reliable. That's the obvious best path under normal circumstances. To maintain the original structural integrity, to me the original load and stress parameters must be known and adhered to. Making one part overstrong (or too rigid) may overload another part in a stressed structure. A triangulated steel truss is easy to repair, relatively. You can easily calculate the loads in various members and replace with a similarly performing part and it's relatively inflexible. Flexibility is a far more difficult structure to deal with. Nev

Every comment here is absolutely spot-on and very, very worth noting..

 

I would add a couple of (sub)-points.

 

Inevitable result of that is excess weight or reduced strength, and/or the introduction of high-stress points into the adjoining structure. Examples of the latter include the mainspar carry-through on the early Foxbat A22, the u/c outer attachment on the Tecnam Echo, (same fault on some Citabrias).

 

Making one part overstrong (or too rigid) may overload another part. Basically the same as above, with the added danger in a repair situation, that the resultant failure of another area in the load-path chain may present an unanticipated structure-critical failure point that could have catastrophic results where a failure in the original structure may have been relatively benign. Remember: in the testing phase, the aircraft is 'original', so the stress-chain for testing is 'original' manufacture. A repair that introduces non-standard stress concentrations HAS NOT BEEN TESTED..

 

A triangulated steel truss is easy to repair, relatively. Yes, this is completely true ( though getting access to replacement of a bent/fractured tube in a steel truss structure may be a PITA). In a truss structure, the load paths are entirely direct and the material composition is (almost) always 4130 tube - so any good LAME authorised aircraft welder can cut out the damage and replace it with same-spec. tube and it will not alter the integrity of the structure.

 

As a further point: when training for my DoT authorisation as a minor repairer and CoA inspector, one of the points drilled into us was to appreciate and inspect for the 'collateral' damage (though it wasn't called that) that can occur. Example: damage to a wingtip from contact with say a fence-post or tree in a difficult outlanding. You have a dented/fractured tip. What is the first thing you go to look for?

 

The answer is: the wing attachment points at the fuselage. The tip may be somewhat damaged - but the entire kinetic energy at the moment of damage transferred to the wing attachment points happened some 7+ metres away from the attachment points (for a standard-class glider). The loads on the wing attachment points are HUGE - and also not 'design loads'.

 

It's not very useful if you just repair the tip damage and the wing falls off next flight. Tends to be a bit of a downer for the day.

 

My point here is: aircraft are designed to very close tolerances for strength/weight. Repairing them is not quite the same proposition as - say - repairing the cracks on an FJ45 chassis.

 

 

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Guest RobT

A composites draftsman? See what I did there.

 

A Composites engineer is certainly not a hijacked word, and is not a tradesman; they have their own string of descriptions - laminators etc.

All trades have there own string of descriptions.

 

Google Composite engineering degree hmmmm cannot do one what wait could be a hijacked word.

 

A true Composite engineer one who can do the structural calcs is someone with an aerospace or mechanical engineering degree and then a composite endorsement there is no grey area here.

 

Ergo Bruce.

 

 

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Guest RobT
Lots of very damaged Jabs are rebuilt and if done at the factory are reliable. Nev

So some pimply faced minimum wage apprentice is going to do a better job than some who's life depends on it being right?

 

There is zero evidence that a factory is required to do this repair. If that was true there would be heaps of composite aircraft falling out the sky because they where not repaired at the factory. I don't know of any sailplane in oz that has been sent back for factory repair i know heaps that have major repair work and fly around with zero issues. There are many many examples of repaired and/or amateur built composite aircraft flying around and not having bits fall off.

 

You made a bold statement can you back it up with facts?

 

 

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You can do your own research. I didn't say the factory is "REQUIRED" to do the repair. Insurance often uses the factory which has always been competitive with airframes and engines as far as cost goes and they put their name on it a s well as having the "bits" and jigs available. Nev

 

 

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Well Pindan, if you are planning to sell that fuselage, please let us know in time for me to make an offer. Then I could be looking out for some damaged wings....

 

On re-reading your first posting, it seems you are thinking of repairing the undercarriage leg itself, as well as the firewall and fuselage.

 

I have to say that regarding the leg, I agree with some of the guys here who are arguing about how hard some repairs are, and I would personally buy a new leg from Jabiru. The freight would not be much on a single leg.

 

 

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There are two great references to use:

 

1. " petite plastic plane patch primer" by Ursula Hanle.

 

2. "GFA Basic Sailplane Engineering"

 

They are both available online. Ursula Hanle was married to Eugen Hanle, who owned Glasflugel. But Ursula had her own sailplane factory, and she has written a great repair manual for fiberglass ( NOT carbon fiber, thank goodness the Jabiru has none of that stuff in it.)

 

The GFA manual is quite big and very good. I reckon is is the best of its type in the world.

 

My recommendation is to print these documents out and read them every day and every night for several weeks.

 

 

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As opposed to repairs according to what some bloke at the pub, whose mate once made a surfboard, told me?

 

There is nothing wrong with using 'text-books' as a source of knowledge and a way to gain reliable information. 'Word of mouth' advice - unless absolutely and definitely coming from an expert - can be unreliable - and there is also the chance of mis-communication, as in the 'I know you heard what I said but I wonder if you understood what I meant?' syndrome.

 

For those who do not have the background or the opportunity to confer with (or preferably, be trained by!) an 'expert' in the field, a reliable text will at least provide a basis on which to evaluate word-of-mouth information as to its value. We used to have, on this site, a self-proclaimed 'expert', some of whose information was so wrong as to be dangerous. I've heard some 'hangar experts' proclaim - with the certainty of bringing tablets down from the Mount - complete nonsense.

 

 

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Academically that is correct; in rural terms you can study how removing the testes from a bull should be done at a certain age, and the optimum method is to use a sharp knife, making a transverse cut across the sac, and extracting each testicle in a precise way. The problem comes when someone says; “ “There’s the bull”, and you are expected to finish up with a pair of balls in your hands.

 

 

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Hi Bruce

 

I did purchase a new u/c leg as the original one was damaged on the inboard bolt hole. Anything that could be purchased new and replaced has been purchased even a new rudder cable incase it got shocked.

 

Ursula no speaka da good english but she has a lot of good info in the book, thanks for the link.

 

 

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I don't know anything about a link to Ursula. Sure you have not got intercepted by the Russian Mafia?

 

Anyway , the uc leg was where the cautionary comments about stiffness of repairs etc were valid.

 

Now if you follow the procedures in those references I posted the rest of the repairs will be ok. Remember that with a

 

uc, the failure will happen on the ground and this is not too bad. Well better than the wings falling off in flight.

 

Also bear in mind that Turbs has a point, and that is how you need more than a theoretical knowledge. So keep posting as you work.

 

 

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Sorry Pindan I can be too suspicious for my own good. Yep she sure writes with a strong German accent huh. Imagine being married to a woman like her, with his n hers glider factories. I actually flew a Salto glider she made.

 

But as you say, lots of good stuff about glass repairs.

 

 

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I am finding this all very interesting and will love to know the eventual result. Overall it makes me happier with my tin can (CH701) It certainly sounds like one sometimes (thank goodness for active noise reduction headsets) but is much, much simpler to repair.

 

 

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