Jabiru Prop failure

[deadstick]

Had a prop fail on my J160, the story goes as follows:- CFI reported that there was an intermittent vibe that he noticed over a few flight recently. I cast my eye over everything including prop and could not find any fault, so he took me for a fly, I did notice a change in harmonic frequency when climbing though 1k feet and put it down to a sticky diaphragm in the carb due to the exhaust being a bit darker as well. Anyway after sitting idle for over two weeks due to terrible weather and flooding the aircraft is prepped for flight and started, there is now a massive vibe at idle and across the RPM range so its shut down and I begin looking for the culprit, I was pretty sure it was the propeller as it was a 1 per revolution style of imbalance. Well as the attached pics will show there was major cracking between the prop hub bolt holes and a crack running down the back of the blade, I can only assume the cracks were present before and the wet weather has allowed the wood to soak with water and swell.... I repeat the prop was inspected on the aircraft prior to the bout of wet weather without any indication of cracking...

 

 

 

 

 

[Guest Maj Millard]

Mmmm not a real flash finish...at a rough guess from the photos it looks like the prop has been a bit loose, and allowed to 'work'...........Maj...

 

 

[deadstick]

MM, the prop didn't come loose its like the wood has swelled and cracked, I suspect there may have been a small crack in the composite finish that allowed moisture ingress. there is a strip of delamination straight down the middle of the back ace of one of the blades. I have experienced the delamination before, One prop didn't even last the ground run post install from new before it threw a leading edge and others have delaminated within a few hours of use. I would really like to throw on a

 

Sensenich W62HJ-48G but its a C model Jab so I am stuck with the factory wooden prop it was certified with unless I could afford the coin to have a car35 Eng certify a mod prop but alas the two Jabs I have keep my hand in my pocket as it is LOL

 

 

[AVOCET]

Looks like a send back to the factory job ,

 

IDE be a bit concerned about the fly wheel bolts , Probly worth a look , check tention ,

 

Maybe replace the bolts ,

 

Mike .

 

 

[Guest Maj Millard]

MM, the prop didn't come loose its like the wood has swelled and cracked, I suspect there may have been a small crack in the composite finish that allowed moisture ingress. there is a strip of delamination straight down the middle of the back ace of one of the blades. I have experienced the delamination before, One prop didn't even last the ground run post install from new before it threw a leading edge and others have delaminated within a few hours of use. I would really like to throw on aSensenich W62HJ-48G but its a C model Jab so I am stuck with the factory wooden prop it was certified with unless I could afford the coin to have a car35 Eng certify a mod prop but alas the two Jabs I have keep my hand in my pocket as it is LOL

I 'd say your Probabily correct there, poor finish letting in moisture. I've always finished my wood props in Estapol satin, in the spray can. Always give a very nice and durable finish. The selection of wood is important also . Should be able to handle a little moisture penetration without going to shxx.........Maj......

 

 

[Vev]

I Believe the 2200 engine is very hard on prop/hubs owing to engine pulsing every 180 deg, where the 3300 is every 120 deg and much easier on components. For what it is worth, I experienced a prop failure at take-off in 2010 in a J160. Take-off roll was normal and got to 300' up wind when I experienced a bad vibration, which I instantly recognised to be prop related failure owing to the frequency. I pulled the throttle power back to where there was less vibration and limped back around to land on the strip.

 

On inspection, I could see the back side of both blades had significant cracks at the blade/hub interface. It was clear the blades had coned forward and the tips may have moved as much as 1.5-2" ... I was lucky it didn't toss a blade/s at 300' as I was running out of clear land ahead, that is, if the engine hadn't departed the airframe altogether.

 

I also saw this repeated in a Mogan Cheetah with a 2200 when the prop suffered multiple fractures and lost bits off the blades. In this instance the vibration was so bad it lost the spinner and damaged the engine rubber mounts leaving the engine flopping around on the engine mounting bolts ... not sure how the pilot got it back home. I was following him and watching him loosing altitude, I ended up rushing ahead trying to get students out of the circuit to give him a good shot at a clear runway for a once only landing option.

 

The lesson for me .... I check my prop very carefully each day and don't hesitate to remove a spinner to take a good look if I suspect anything. I also check bolt torque every 25hrs, this may be an excessive regime but it gives me piece of mind.

 

Cheers

 

Vev

 

 

[Guest Andys@coffs]

Do people find the bolts need added tension each time, the current setup with the multitude of scalloped washers on my 230 doesn't seem to change very much from check to check

 

Andy

 

 

[facthunter]

Engine heat dries out the hub, and it shrinks and loosens. More so with long trips. The previous posts bear out the need to investigate vibrations always . The 4 cyl motor is hard on props, and it is more likely to be a harmonic than engine pulses directly. There may be a RPM setting to avoid. ( most engine/prop combinations have them.) Nev

 

 

[Compulsion]

Probably a dumb question but why are the bolts so long needing lots of washers? Why not shorten the bolts to the correct length?

 

 

[Vev]

Engine heat dries out the hub, and it shrinks and loosens. More so with long trips. The previous posts bear out the need to investigate vibrations always . The 4 cyl motor is hard on props, and it is more likely to be a harmonic than engine pulses directly. There may be a RPM setting to avoid. ( most engine/prop combinations have them.) Nev

Hi Nev,

 

In the 2200, I believe the avoidable engine speed is 24-26k rpm ... I also understand that setting up the position of the blade relitive to TDC on number one cylinder is also important too. I picked up these peices of info when I did the engine school at Jab a few years ago, although I cannot substatiate this beyond what had been passed to me by Don at Jab.

 

Cheers

 

Vev

 

 

[Guest Andys@coffs]

As I understand it the washers are a special shape and need to go onto the bolt in a specific way.....|()()()()()()()| like that so that when the nut is tightened down the washers compress some ||||||||| which means that hub shrinkage doesn't immediately result in sloppiness because the washers will decompress some to fill in the gap.

 

It wasn't always that way.......If you read the prop manual on the jabiru site its very specific how its supposed to go together. if you have ignore or ended up with (((((((((( or some other random form then I believe you might be more likely to suffer damage.

 

Andy

 

 

[facthunter]

Vev That is a bigger range than most such figures work on, so I wonder if the work has actually been done properly. My attitude to this is until more work has been done, stick to wooden 2 blade props of conventional design with the 2200. Nev

 

 

[AVOCET]

Do people find the bolts need added tension each time, the current setup with the multitude of scalloped washers on my 230 doesn't seem to change very much from check to checkAndy

I've found with the scalloped washers that they need little attention , they seem to work fine . The old way with the short bolts was a routeen of having to nip up nuts to tourqe every time and eventually crushing the prop hub ,

 

Seemed more of a problem on the four cyl.

 

I've got quite a collection of props with cracks around the hubs , they make nice prop clocks

 

Cheers Mike

 

 

[Guest Maj Millard]

All 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.

 

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

 

 

[AVOCET]

Maj , it's the soft wood , plantation hoop pine

 

I've seen some variation in the hardness in that timber , I planed a couple of cubic meters for the guy who used to make their props years ago . Plus they only run 3 laminations on the 4 cyl & 4 on the 6 cyl

 

, lots of jabs 4/6 running sweet apple , Brent Thompson NZ they use ash and you can tourqe them to 12 ft /lbs and don't give many problems

 

Mike

 

 

[Guest Maj Millard]

Bloody plantation hoop pine......Jesus, I have that as decorative trim on my front door ....couldn't they do any better than that !?........Qantas years ago used Queensland maple to make props exclusively , and you can still view the old jigs in the old Qantas hangar at Longreach. I don't own a prop that has less than five laminations, and the Allsize for the LW has eight (just counted them) and I have a US made prop that has twelve lams, made for a UL in the eighties. We're going backwards here folks....but no doubt fully CASA approved !!......standard aeronautical practise is that there must be sufficient 'crush' maintained on a wood prop between the hub and face place. It must always be there, there should never be any looseness or movement, and regular torque checking of prop bolts is just part of running a wood prop....Maj...

 

 

[AVOCET]

I tried to sell them 12 cubic mts of class A coach wood some years back , but the price they were paying for the hoop was a gift !

 

Same old same old

 

Actually , virgin hoop pine on a large tree is quite hard and approved as aviation timber

 

Mike

 

 

[Guest Maj Millard]

I tried to sell them 12 cubic mts of class A coach wood some years back , but the price they were paying for the hoop was a gift !Same old same old

Actually , virgin hoop pine on a large tree is quite hard and approved as aviation timber

 

Mike

Yes I know and it's rather hard and stringy to work with , probabily why they'd had enough after three laminations !..........Maj...

 

 

[damkia]

I'm wondering if the Jab 2200 would benefit from a bit more weight in the flywheel to even out the pulses.

 

 

[facthunter]

No it wouldn't. It's not a good idea to have a flywheel one end and have a prop the other. That's the problem. Nev

 

 

[Vev]

I wonder how the new fibreglass prop will go over old the wooden one in terms of absorbing the loads and vibration?

 

Cheers

 

Vev

 

 

[facthunter]

Since the Sensenich composite has problems I wouldn't hold a lot of hope. The idea of chucking a blade doesn't impress me at all. Nev

 

 

[deadstick]

I would love to throw a sensenich wood prop on but being a c model j160 Casa has the engineering authority not the factory. Anyone know of a factory built j160-c with anything other than the wood prop?

 

 

[Guest Maj Millard]

Most Lycoming and Continental engines have floating counterweights on the crank balance journals to smooth out pulses. The radial engines have massive counterweights that do the same thing. The little F23 Hirth boxer (2 cyl opposed) that I ran for a while, with both pistons firing together on the out stroke, had the timing staggered about 5 degrees, one cylinder to the other.

 

The Hirth dealer told me if they hadn't done that the engine would have shaken itself to pieces. Some of the large radials will also stagger their timings so not all the the cylinders are firing on the same positions.......You really don't want the crank and prop fighting against each other, those are the two highest stressed items on the aircraft, other than the pilot inside !......Maj...

 

 

[Old Koreelah]

No it wouldn't. It's not a good idea to have a flywheel one end and have a prop the other. That's the problem. Nev

Interesting discussion. I mounted a large spinner on my Jab engine. The weight of it and the mounting plate are meant to help smoothe out the power pulses, as well as the airflow, but someone on this forum cautioned that it would stress the crank. What do you fellas reckon?