Jabiru Prop failure

[Jabiru Phil]

I am extremely happy with my new Jab/ Bolly prop. Only done a few hours but soooo much smoother

 

Only wish I had installed before my WA trip recently to give a more comprehensive report

 

I mentioned in another post that the CFI that did my BFR the other day said that it was much smoother than the wooden prop on the club 230

 

Interested to hear from any other converts

 

Phil

 

 

[deadstick]

Cant be fitted to a C model jab, would try one for sure if it could.

 

 

[deadstick]

I am extremely happy with my new Jab/ Bolly prop. Only done a few hours but soooo much smootherOnly wish I had installed before my WA trip recently to give a more comprehensive report

I mentioned in another post that the CFI that did my BFR the other day said that it was much smoother than the wooden prop on the club 230

 

Interested to hear from any other converts

 

Phil

Phil,

 

out of interest whats the pitch angle set on your prop?

 

 

[facthunter]

You won't feel torsional vibration so smoothness is not an indicator. OK, Any weight added to either end of the crank will change things. How I won't guess. This whole consideration is quite complex. The P&W R-2000's I did thousands of hours behind had tumbling counterweights and still had three (3) rev ranges to strictly avoid. You didn't go there even for a short time. Straight through on the pitch levers. That RPM considered unuseable. While I know the Jab is less complex the same principle applies to all engines. Flat fours are known crankshaft breakers. ( Volkswagen, Borgward Jowett all did it) I've worked on all of them.... ALL engines DRIVE from the flywheel end. ( some crude marine ones excepted due to the drive shaft angle) but they rev very slowly.. Nev

 

 

[gandalph]

Something don't sound right to me...Jab doing their own funny thing again, against normal aviation practise.............Maj...

Good old Maj, If there's a conclusion to be jumped to - he's your man. So glad he's nominated for the board. At least , if he gets elected, we can be sure the board won't get bogged down long drawn out discussions backed by research and knowledge. They'll be able to shoot from the hip. Maj for Pres! Oh wait, I've just had deja Vu moment........

 

 

[Guest Maj Millard]

Hey don't get on my case, I've never had any wood prop failures !!................Maj...

 

 

[Oscar]

ll new to me and specific to Jab props as I see it. I ran wood props for years on a selection of different engines including opposed 4 cyl 4 strokes, never heard of using washers like that before, and all my props suffered no damage like those on the Jabs. In fact I still have them and they are in perfect shape.

If you don't know what a Beleville washer is, and what its function is, then your credentials as a fount of knowledge is seriously degraded. The use of the Beleville pack and the resultant torque figures for the 2200 props was determined by an engineer who obviously knows the effect of torque pulses and the characteristics of wood in the presence of varying conditions of moisture content and relative humidity of which you are totally, and noisily, completely ignorant.

 

 

[Guest Maj Millard]

I'm very familiar with the Belevelle washer spanker, however I've never seen them used to keep the torque on a wood, or any other prop for that matter, and that is not what they were intended for. And obviously if you review the photos that deadstick posted, you'll see that they don't work very well in that application either.

 

Probabily the same engineer that came up with that idea, did the rest of the engine also .........Maj...

 

 

[ruffasguts]

I'm very familiar with the Belevelle washer spanker, however I've never seen them used to keep the torque on a wood, or any other prop for that matter, and that is not what they were intended for. And obviously if you review the photos that deadstick posted, you'll see that they don't work very well in that application either.Probabily the same engineer that came up with that idea, did the rest of the engine also .........Maj...

\Belevelle washer have been used on wooden props for many years ,gipsy engines for one, use them

 

so if it was the same engineer, may be he got the engine right as well

 

Mick W

 

 

[Old Koreelah]

You won't feel torsional vibration so smoothness is not an indicator. OK, Any weight added to either end of the crank will change things. How I won't guess. This whole consideration is quite complex... Nev

Thanks for your reply, Nev I wasn't seeking to reduce vibration, but to add a little more flywheel effect with the larger spinner assembly, in the hope of reducing flexing of the crank and prop. Looks like this is an area way beyond my rudimentary understanding, so my efforts are in the experimental realm, and I will have to wear the costs of failure.

 

 

[Jabiru Phil]

Phil,out of interest whats the pitch angle set on your prop?

Factory set at 52 but ground adjustable. The original was 53.

 

Happy to leave where they reccomend

 

Phil.

 

 

[Dafydd Llewellyn]

If I may be permitted to spoil a really good stoush with some facts - because there is some really, really bad advice and misinformation on this thread - here are a few pieces of factual information:

 

Firstly, wood propellers should be driven, not by spigots (or bolts) engaging holes in the timber, but by friction at the drive face (or faces - the late-model Gipsy Major hub had a splined front face in the - rather forlorn - hope of achieving a friction drive on both faces.) This was covered quite well, decades ago, in an article in the EAA Magazine, Sport Aviation, written by Rose & Bristol, two engineers from Sensenich (it's too large to attach to this post). The Gipsy installation also used a friction-enhancing device (called a "Ferodo disc") which was essentially a disc of heavy canvas soaked in Bakelite monomer (as far as I can discover).

 

Attempting to drive a wood propeller by spigots or bolts that are a close fit in the propeller timber, is guaranteed to cause splitting, due to change in moisture content - and is in any case prohibited by CAO 108.28.5.4, which requires 2 to 2.5 mm clearance on the bolt. Some propellers (notably Hoffmann FP wood types) use spigot drive, but usually into a plywood "drive ring" on the back of the propeller hub. The basis of this is that the "splitting" stress due to shrinkage of the hub is confined to a small region adjacent to the ply ring, rather than affecting the full thickness of the propeller hub timber. Obviously, this raises the question of the reliability of the joint between the ply ring and the propeller hub timber - so this form is inferior to the "friction drive" approach, in my view.

 

To successfully drive the propeller by friction, firstly the propeller drive flange and the mating propeller drive surface must be sufficiently large; (Rose & Bristol provide the basic data for that) and secondly, the necessary clamping force must be maintained. Beleville washers are the principal way to maintain the clamping force; but they MUST be used in precisely the correct manner. This has to be established for each propeller installation and may vary from one to another; there is no "fits all" way. All this was well & truly thrashed out for the thousands of Tiger Moths that were used in WW2 under the Empire Pilot's Training Scheme. So please will people stop spouting bloody dangerous nonsense?

 

The permissible timbers for wood propeller construction in Australia are specified in CAO 108.29 - and the list includes Hoop Pine (without reference to its sexual status); and it specifies the quality assurance tests required.

 

The CSIRO book "the mechanical properties of 174 Australian Timbers" (Bolza & Kloot) give the following typical crushing strengths for some of the typical propeller timbers: Coachwood 873 psi; Sassafras 955 psi; Hoop pine 929 psi; QLD Maple 876 psi. So Hoop Pine is up there with the best of them, and is notably superior to Mountain Ash (725 psi).

 

Attached is the wood propeller installation which I designed for a FAR 23 aircraft, which is currently running a Lycoming IO-390 (210 HP/4 cylinder) - tho NOT on the original Lycoming flange. It uses 3 groups of 3 Beleville washers on each bolt - count 'em; that's 72 washers in all. This installation has 20 years of satisfactory operational history, including in Iraq, where the temperature gets to over 50C - and it's working in the hot air outlet of a pusher installation. It does NOT require frequent bolt tension checking. RTFM! One size does NOT fit all!

 

Beleville washers come in a wide range; if they are specified for your aircraft, make sure you use the correct part number.

 

The Gipsy installation used - to my recollection - two Beleville washers per bolt; however it is important to NOT tighten Beleville washers sufficiently to fully flatten them; doing so will overstress them and they lose effectiveness. The installation needs to be designed to prevent that.

 

The Jabiru propellers of my experience were normally sheathed in fibreglass, because that increased the torsional stiffness of the blades and raises the critical speed for blade tip flutter; this was demonstrated by running specimen propellers up to 10% overspeed on a static rig; (that requires at least 150% of the nominal engine power).

 

I cannot see sufficient detail in the photos of the failed propeller to express an opinion - but it looks to me as though the drive face has been sanded off before the photos were taken, because the imprint of the drive flange is not visible. I'd wager it was starting to show evidence of charring at the bearing surface, which is a dead give-away that the propeller bolt tension had slackened off - this will cause hammering on the bolt, which will very quickly split the propeller. From what I can see, this failure is typical of loose bolts.

 

The good news is, firstly, if the propeller installation is properly designed, and properly maintained, it will not come loose. Secondly, if it does, the hub fails through the centre and both blades go - whereas with most other forms of propellers, one blade departs and the remaining one takes the engine with it.

 

The propeller IS the flywheel. ON NO ACCOUNT add flywheel mass is the opposite end of the crankshaft.

 

Pendulum counterweights are used to tune-out torsional resonant frequencies within the crankshaft - and as these are affected by the propeller, they also affect propeller blade stresses, which are critical for propellers with metal blades. They are not normally critical for wood propellers; there are plently of aircraft with 180 HP Lycoming 0-360 engines (very few of which have pendulum counterweights) with fixed-pitch wood propellers.

 

So would the bar-room engineers kindly shut up? Your bad advice is likely to kill somebody.

 

 

 

[aj_richo]

So why does my new Sensenich wooden prop come pre-drilled for spigots?

 

 

[Guest Maj Millard]

And my Allsize for a Lightwing or Storch ?..................

 

 

[Head in the clouds]

It uses 3 groups of 3 Beleville washers on each bolt - count 'em; that's 72 washers in all.....

Couldn't see any Belleville washers on that pic, but you can here -

 

 

 

 

 

[Dafydd Llewellyn]

So why does my new Sensenich wooden prop come pre-drilled for spigots?

Presumably because the American manufacturers are used to standard SAE propeller flanges, which are designed for metal props, which use spigots.

 

 

[Dafydd Llewellyn]

Couldn't see any Belleville [ATTACH=full]22952[/ATTACH] washers on that pic, but you can here -[ATTACH]22949[/ATTACH][ATTACH]22950[/ATTACH][ATTACH]22951[/ATTACH]

No, you can't see the Belleville washers on the Seeker installation, because they are inside housings that prevent them from being over-tightened. The arrangement you show gives the maximum accommodation for swelling and shrinkage of the hub, but the smallest clamping force. The arrangement of the washers in the Seeker was

 

((()))((( because it took 8 lots of three washers in parallel to supply sufficient clamping pressure; and three sets of three washers to give sufficient travel to accommodate the swelling and shrinkage of the hub. One can only work over about 25% of the total compression travel of the washers; tighten them too far & they lose their "spring"; tighten them too little and you do not get sufficient clamping force to drive the propeller. It's a fairly critical piece of engineering design, and each installation has to be treated separately. NOT something to be designed in the backyard.

 

 

[Head in the clouds]

No, you can't see the Belleville washers on the Seeker installation, because they are inside housings that prevent them from being over-tightened. The arrangement you show gives the maximum accommodation for swelling and shrinkage of the hub, but the smallest clamping force. The arrangement of the washers in the Seeker was((()))((( because it took 8 lots of three washers in parallel to supply sufficient clamping pressure; and three sets of three washers to give sufficient travel to accommodate the swelling and shrinkage of the hub. One can only work over about 25% of the total compression travel of the washers; tighten them too far & they lose their "spring"; tighten them too little and you do not get sufficient clamping force to drive the propeller. It's a fairly critical piece of engineering design, and each installation has to be treated separately. NOT something to be designed in the backyard.

Your comment makes Belleville washers seem like some sort of complex mystery but there's nothing more complicated about designing for the use of them than there is in designing a simple electrical circuit. If you can do the latter in your backyard you can most certainly do the former. All the information for deflection forces and travel limits is available from each supplier of disc-spring (generic) washers so all you need to know is how much clamping force you require and the minimum amount of travel to suit your application, then work out the appropriate series and parallel combination. See typical charts/graphs below.

 

If you've installed a sleeve over the washers on the Seeker, to prevent over-tightening the washers, doesn't that prevent any expansion movement of the timber/prop plate? Or is the sleeve still loose when the correct force on the washers has been achieved? If you use the sleeve length to determine the compression force then wouldn't you have to fully saturate/humidify the prop at the time of installation and only be using the washers to compensate for prop hub shrinkage?

 

EDIT - if anyone's interested in the 'deflection' image, right click and 'open in a new tab' or you can't read it.

 

 

 

 

[Dafydd Llewellyn]

Your comment makes Belleville washers seem like some sort of complex mystery but there's nothing more complicated about designing for the use of them than there is in designing a simple electrical circuit. If you can do the latter in your backyard you can most certainly do the former. All the information for deflection forces and travel limits is available from each supplier of disc-spring (generic) washers so all you need to know is how much clamping force you require and the minimum amount of travel to suit your application, then work out the appropriate series and parallel combination. See typical charts/graphs below.If you've installed a sleeve over the washers on the Seeker, to prevent over-tightening the washers, doesn't that prevent any expansion movement of the timber/prop plate? Or is the sleeve still loose when the correct force on the washers has been achieved? If you use the sleeve length to determine the compression force then wouldn't you have to fully saturate/humidify the prop at the time of installation and only be using the washers to compensate for prop hub shrinkage?

 

EDIT - if anyone's interested in the 'deflection' image, right click and 'open in a new tab' or you can't read it.

 

If you read the article by Rose & Bristol - I've managed to upload it, see below - and you know the crushing stress of the propeller timber - you can calculate the maximum torque your flange can transmit, and find the Belleville washer or washers acting in parallel, that give sufficient force at about 75% deflection. Then if you work back down the force curve for the washer, to about 50% deflection, you can calculate the minimum torque. It needs to have about 50% safety margin on the peak instantaneous engine torque, which you can estimate from FAR 23.361 - twice the mean torque, for a four-cylinder engine. If you cannot achieve this margin with the existing flange, the flange is too small. (The flange on the Seeker is considerably larger than the standard SAE flange on the Lycoming).

 

Then work out the change in thickness of the propeller hub from about 5% moisture content to, say, 18% moisture content, and that will tell you what total travel you need for the washers between 75% deflection and 50% deflection, which will tell you how many groups of washers you need in series to maintain sufficient clamping force over that range of moisture content.

 

Now, make yourself a little test rig, and test each group of washers - set up as you would place them on a single bolt - to ensure they really do give the force/travel curve you expect from the advertising data - we've found they vary considerably from batch to batch. The test rig will tell you what the compressed length of the stack should be, at the maximum load. The housings need to prevent the pack of washers from being compressed beyond that point. You will also know what the length of the stack will be at half-way between the maximum and the minimum load. That will tell you what the clearance needs to be between the outer part of the housing ("sleeve" as you term it) and the plate that forms the end on the inner part of the housing. You install the propeller using calibrated sets of washers and a feeler gauge.

 

No, this is NOT rocket science. Anybody who can understand the formula that R & B give, would be capable of doing this. However, I re-derived their formula, and found an error in it. See if you can find it. That's all the free info I'm prepared to supply.

 

 

 

 

[Guest Maj Millard]

Daffydd, Can you supply me with a complete list of RAA aircraft ( other than of course the Jab and your beloved 'Sunseeker'), that use wood props and are fitted as standard, with the Belleville washers......Maj...

 

 

[Dafydd Llewellyn]

Play nice people, ...moderator

 

 

[Head in the clouds]

If you read the article by Rose & Bristol - I've managed to upload it, see below - and you know the crushing stress of the propeller timber - you can calculate the maximum torque your flange can transmit, and find the Belleville washer or washers acting in parallel, that give sufficient force at about 75% deflection. Then if you work back down the force curve for the washer, to about 50% deflection, you can calculate the minimum torque. It needs to have about 50% safety margin on the peak instantaneous engine torque, which you can estimate from FAR 23.361 - twice the mean torque, for a four-cylinder engine. If you cannot achieve this margin with the existing flange, the flange is too small. (The flange on the Seeker is considerably larger than the standard SAE flange on the Lycoming).Then work out the change in thickness of the propeller hub from about 5% moisture content to, say, 18% moisture content, and that will tell you what total travel you need for the washers between 75% deflection and 50% deflection, which will tell you how many groups of washers you need in series to maintain sufficient clamping force over that range of moisture content.

 

Now, make yourself a little test rig, and test each group of washers - set up as you would place them on a single bolt - to ensure they really do give the force/travel curve you expect from the advertising data - we've found they vary considerably from batch to batch. The test rig will tell you what the compressed length of the stack should be, at the maximum load. The housings need to prevent the pack of washers from being compressed beyond that point. You will also know what the length of the stack will be at half-way between the maximum and the minimum load. That will tell you what the clearance needs to be between the outer part of the housing ("sleeve" as you term it) and the plate that forms the end on the inner part of the housing. You install the propeller using calibrated sets of washers and a feeler gauge.

 

No, this is NOT rocket science. Anybody who can understand the formula that R & B give, would be capable of doing this. However, I re-derived their formula, and found an error in it. See if you can find it. That's all the free info I'm prepared to supply.

Thanks Dafydd - standard engineering practice ...

 

As far as the props go though, selection of timber types to employ, according to those different timbers' varying propensity for water absorption and also proper moisture reduction prior to the use of modern coatings which deeply penetrate the surface of the timber and prevent moisture changes, goes a long way further to preventing these kinds of failures than the use of bolt tension retainers ever will. A now deceased friend (not a prop failure!) was a timber boffin and prop manufacturer and he conducted endless tests on timbers, he mostly chose mountain ash for props, sometimes alternately laminated with birch (I think) but wouldn't ever use hoop pine even though it was more easily available, because he found that hoop pine very readily and rapidly absorbs moisture.

 

The damage seen on the OP's post is typical of crushing resulting in loose bolts once the timber dries and shrinks again, and is unlikely to be found on a prop made of mountain ash, for example. Even though that timber might have a lower crush resistance, it wouldn't absorb moisture as readily in the first place, so it wouldn't swell as much and cause the crush problem in the first place. I think it's important to keep in mind that the vast majority of timber props do not use spring washers of any kind for mounting them. Your Seabird example might well be a good exception to prove the rule, given that the aircraft may be deployed into a humid marine environment but during operations the prop is subject to the drying effects of the engine's cooling exhaust. I would think that attention to the type and condition of the prop's coatings would be especially important for that application.

 

 

[Guest Andys@coffs]

you know if we take a bit of the vitriol away, this thread has been pretty interesting.

 

I've learned a bit. I didn't know that there was a limit to how much deformation the washers should be subject to. Which isn't to say I've deformed mine, after all the J instructions are to torque to a specific value (6ftlbs from memory) I assume that they chose that value knowing that the deformation was within limits.

 

I've also learned that a bit more attention to the hub during inspections wont go astray.

 

So thanks for the info people

 

Andy

 

 

[deadstick]

Daffydd,

 

thanks for the info and I say you may damn well be correct 'ole bean', the scenario you present of the torque slackening off fits the symptoms to a T. the aircraft has not flown a lot of hours since the prop torque check was c/o at 100 hrly but has sat for a while in inclement weather. The drive face hasn't sanded off but compressed like it swelled and pressed the drive flange into it I am suspect that it has since dried out and shrunk causing the torque loss, if the photos were clear you could see the wood has opened up rather than cracked.

 

 

[Dafydd Llewellyn]

Thanks Dafydd - standard engineering practice ...As far as the props go though, selection of timber types to employ, according to those different timbers' varying propensity for water absorption and also proper moisture reduction prior to the use of modern coatings which deeply penetrate the surface of the timber and prevent moisture changes, goes a long way further to preventing these kinds of failures than the use of bolt tension retainers ever will. A now deceased friend (not a prop failure!) was a timber boffin and prop manufacturer and he conducted endless tests on timbers, he mostly chose mountain ash for props, sometimes alternately laminated with birch (I think) but wouldn't ever use hoop pine even though it was more easily available, because he found that hoop pine very readily and rapidly absorbs moisture.

 

The damage seen on the OP's post is typical of crushing resulting in loose bolts once the timber dries and shrinks again, and is unlikely to be found on a prop made of mountain ash, for example. Even though that timber might have a lower crush resistance, it wouldn't absorb moisture as readily in the first place, so it wouldn't swell as much and cause the crush problem in the first place. I think it's important to keep in mind that the vast majority of timber props do not use spring washers of any kind for mounting them. Your Seabird example might well be a good exception to prove the rule, given that the aircraft may be deployed into a humid marine environment but during operations the prop is subject to the drying effects of the engine's cooling exhaust. I would think that attention to the type and condition of the prop's coatings would be especially important for that application.

Well, yes, the majority of timber props do not use any means to compensate for the change in dimensions due to change in moisture content. Once upon a time, the majority of cars used cross-ply tyres, too. However, to my mind a design feature that requires such constant attention as propeller bolt tension, and has such potentially catastrophic results, indicates that the state of the art has not progressed far enough. Your prop manufacturer would not have been Adams, would it? (I forget his first name) - I helped him with the design of a couple of his props, whilst we both worked for de Havs at Bankstown; lovely fellow. I had an Auster with one of his props on it. I have no data to hand on the relative behaviour of Mountain Ash Vs Hoop Pine in regard to their response to the relative humidity of the environment; also there may well be a significant difference between hardwoods and softwoods, due to the different microstructure.

 

The Seabird installation has survived being installed and test-flown at Hervey Bay, then shipped to Iraq and put into service without touching the propeller bolts. I reckon that's the sort of maintenance intensity that should be required for a properly-designed propeller installation. It can be done; however most installations are neanderthal.