Flywheel attachment failure

[Guest Escadrille]

Yenn,I agree and I believe it was in a Jabachat newsletter?

 

 

[Modest Pilot]

If the problem turns out to be the flywheel alloy yielding the bolt tension may not be the fix. The index marks might look O K but the flywheel may still rock. Keyways in the crankshaft in my book are a no-no. Almost a sure way of starting a fatigue crack in high powered aircooled engines!

 

 

[facthunter]

Keyways etc.

 

You can't rely on keys or dowels or bolts in shear. The contact of the face of the flywheel under the pressure of the bolts tension, ( same as the prop flange in principle). is what does the job. The presence of an oil film would not help.. You have the drive gear sandwiched in there as well to help put a bending load on the bolts. Once this assembly starts to move, it won't last long. I cant see the relevence of the 500 hours. Might as well pick a figure....... Something is seen to be done.

 

What load do you think is causing this problem? there is little drive load involved. The cause is torsional harmonics which will peak at critical RPM (s)and not be a problem at others. IF I had a Jab powered plane ( considering also that The Jab. is the only motor that is fretting the blades ( near the hub) of the Sensenich carbon prop. I would FIND the Revs where this is happening and AVOID them . The flogging of the distributor rotor buttons points to it also. Nev

 

 

[assar]

Harmonic rpm range?

 

In the installation manuals, section "Propeller & Spinner" or "Propeller selection" it is mentioned "Do not cruise or climb in the range 2100rpm - 2400rpm".

 

Same in 2200 and 3300 manuals, new and older editions. In the Instruction and maintenance manual I can find no reference to this.

 

Is this a harmonic rpm range? At least I interpreted it so and made it yellow range on the tach.

 

Anyone know more about this?

 

 

[jetboy]

Its not a vibration problem range just an advisory to not run at too light a loading as the engine is not suitable to operate for long periods at low power settings - cylinder to piston rings seal suffers etc.

 

Both NZCAA documentation and Jabiru comment state the bolt issue is related to loose prop or propstrike damage, and the original ADs and SBs issued were two items to be treated at the same time: prop attachment and flywheel bolt repetitive inspections.

 

Two years ago Jabiru substantially changed all the parts in the flywheel stack - except the gear wheel - this latest failure appears to have happened with the new assembly. We are waiting to see how this new development pans out as its an ASTM certified engine and the problem happened to an LSA, operating under the ASTM continued airworthiness system. The rest of us, with the older engines, have mostly used the results of the BMAA and RAANZ investigations, there seems to be less problems occuring with these, and the thru bolts have come into more attention than troubles with the replaced flywheel capscrews.

 

Ralph

 

 

[Modest Pilot]

I presume Jabiru props in N Z are tracked to 3 mm. I feel this is to much for a light engine like the Jab, under 1mm gives a noticable improvement in vibration. On the orginal Hoop pine prop this was hard to get, but along with a dynamic prop balance; people that flew with me remarked that the engine must have been blueprinted. Until more is understood about the flywheel bolts breaking maybe it would be worth working this area.

 

P S: In the end I fitted a Sensenich prop from the factory.

 

 

[facthunter]

2200&3300.

 

Assar, can't see why the two motors would be the same . 4 cyl and 6 cyl motors have totally different balance characteristics. The 6 cyl engine should always be smoother as the pistons give a more constant flywheel effect, and there are more of them, but it has a LONGER crankshaft. (to twist more)...Nev

 

 

[assar]

Facthunter, it looks strange also to me with a common natural frequency for the 4- and 6 cyl engines. Making a pure operational note as “do not cruise or climb” in the propeller selection chapter of the installation manual, with no further enforcement in the operation or limit sections of the op- and maintenance manual is also strange!

 

As we are going to change our present Sensenich carbon ground adjustable prop for a Woodcomp SR3000/2JM (electronic constant speed, 2-blade wood/composite), and this prop have a higher inertia of 0,46 kgm², i let a friend of me, who is specialist in vibration analysis, make some brief calculations based on the dimensions of the 3300 crank, estimated inertia of the flywheel and Jab’s inertia limit of 0,30 kgm² for the prop.

 

He found lowest natural frequency for torsion of the crank to about 580 Hz, lowering to 500 Hz with prop inertia 0,46. Natural bending frequency of the free prop end with 2” extention and the heavier prop was about 380 Hz (assuming same stiffness for extension as for the crank), somewhat lowering due to gyro moments (not calculated).

 

As the max initiation freq of the engine is 165 Hz (3300 rpm, 3 ignitions /rev), we don’t think there should be a torsional resonance problem at all, and for sure the 2100-2400 range can’t be based on torsional natural frequencies. Please observe these calculations are based on some assumptions, intended to find out if the given rpm range could be based on resonance, and then calculate a new range for the heavier prop, and may only be used as an indication.

 

I think the flywheel bolt problems has to originate from loosing friction force, i.e. clamping force. In the old design with steel bolts in steel crank, clamping alu flywheel it’s easy to understand, but the current design where only steel parts are involved, this should’t be a problem, right?

 

Regarding the prop inertia limits of 0,25 for the 2200 and 0,30 for the 3300, my calculations give the same max torsional moment using standard torque curves for even firing 4- versus 6 cyl engine, so the designer seems to have some concerns about max torque in the crank.

 

 

[Modest Pilot]

Assar: If I read your post correctly you are intending to operate with a prop that exceeds the engine manufacturer's inertia limit. If I may I'd like to counsel you to have a re-think. I recall talking to the "wheels" at Bundy about the weight of this prop. Do yourself a favour and give Don a call at Jabiru 61-7-41552811 and get a second opinion. The forces involved if something goes wrong in this area are truly insane! I may be slightly paranoid on this having flown Lockheed Electra's (prop whirl mode from cracked engine mounts was enough to tear the wings off these aircraft) and I also flew a demo flight in a Sportcruiser (Rotax 912) with a Woodcomp similar to the one you describe the day before a prop failure almost ended it's lifespan. The engine mount broke, the firewall bent, fortunately both carbs were ripped off at the rubber mounts and the engine stop a fractions of a second after the event and a successful forced landing accomplished. Not much room for error in this area.

 

 

[ianboag]

Perhaps this should be in another thread about props.

 

There have been a moderate number of Jabs fitted with Airmaster VP props over the years. None of them (that Airmaster know of) ever broke the engine. The Airmaster weighs about 12 kg, but of course it's the rotational inertia that matters - the actual weight of the hub is not really an issue as it's in the middle and makes little contribution to the rotational inertia.

 

Airmaster have been working (for years now) on a 2-blade VP prop using the blades from the Sensenich ground adjustable. The Sensenich ground adjustable prop is approved by Jabiru because Sensenich did a heap of testing on it and they are a big prop manufacturer. I don't know about the inertia of the ground adjustable prop, but a VP one using the same blades would not be very different inertia-wise although it would weigh a whole lot more.

 

I know someone who ran a Woodcomp 3-blade VP on a J200 for a while - I think he took it off when he smacked a blade against something on the ground and broke it off.

 

I never found anyone who run an IvoProp on a Jab engine. They would not be too bad inertia-wise either.

 

IB

 

 

[facthunter]

Torsion and all.

 

there is an AD on one of the carbon Sensenich props when fitted to the Jab. Motor. There is damage to the blade near the huB. There is no other motor that the prop is fitted to where there has been a problem.

 

I agree that the flywheel bolt failure would be due to a clamping situation and have commented previously. You have 3 elements there . the crank. the cam drive gear and then the flywheel. Once the thing STARTS to move it is only a matter of time before the bolts give up.

 

ALL piston engines have a pulsing torque delivery which has a maximum effect at some critical revs. This usually occurrs over a very small range. (less than 50 rpm) and is actually only at a maximum at ONE particular speed.(engine RPM) .

 

There may be more than one speed that is (should be) prohibited on any particular engine and that would only be with a particular propeller fitted. In other words change the prop and the speed would be altered) The Rotax 912 series has a special redrive set-up where most of this is dealt with. If I was running a jabiru I would only use a wooden prop (as Jabiru recommend) or a fixed pitch carbon prop that was approved.

 

The out of balance forces generated when one blade separates are enormous and invariably takes the engine out of its mounts. Serious when a tractor installation is involved but unthinkable when a pusher installation is considered. Nev

 

 

[jetjr]

Jabiru Australia approve no carbon props. Only Sensinich wood and thier own. Wood can absorb some vibration and prevent fatigue.

 

Nearly all composite manufacturers have had blade failures with 4 cyl Jabiru engines.

 

Discussions with Jabiru say 6 cyl has far less "vibration" as there is two more ignitions per rpm, helping to smoth things out.

 

There was mention that some earlier issues with flywheel issues in NZ were related to a prop maker having shaft extensions which werent true, causing severe vibrations which stiff CF props cannot absorb.

 

Interestingly Jab will have thier own moulded plastic (not composite) prop for 4 cyl shortly.

 

 

[ianboag]

The prop thing

 

Jabiru Australia approve no carbon props.

Jaba Chat August 2010 ...

 

The Sensenich 2-bladed ground adjustable carbon propellers are approved worldwide for Amateur-built aircraft .... MT-Propeller make an in-flight adjustable 3 bladed propeller – with blades made out of high-tech timber ... those are the only propellers with the approval of Jabiru Australia ...

And yes, Sensenich have issued an SB about paint cracks at the blade roots.

 

There was mention that some earlier issues with flywheel issues in NZ were related to a prop maker having shaft extensions which werent true, causing severe vibrations which stiff CF props cannot absorb.

There were prop extension problems on at least one Pioneer 300 which lost its flywheel bolts on our field back in 2006. The (non true) 4" prop extension was from Pioneer - the prop was a wooden two blader from GT. The plane now has a 4" prop extension supplied by Jabiru.

 

 

[ianboag]

Airmaster VP prop

 

They now make a 2-bladed one using the Sensenich ground-adjustable blades

 

http://www.airmasterpropellers.com/Products/ProductDescription/tabid/97/agentType/View/PropertyTypeId/30/PropertyID/19/Default.aspx

 

 

[assar]

Thanks for your input guys,,

 

 

I am well aware of these concerns, and has been in discussion with Airmaster, Woodcomp, and GT about this for about 2 years. The Airmaster with Warp square tips I recall is somewhere around 6000 kgcm², GT is 4350 and Woodcomp 4600. I think the Airmaster AP420 using the Sensenich blades will still be well above the 3000kgcm² limit.

 

The MT 3-blade cs prop, which Jabiru have also approved, is about 5000 kgcm²

 

I turned down the ideas of the Airmaster, as I am not very confident in the Sensenich blades, I already have the paintcracks described in their sb after less than 100h, and I am also not happy with the static/lowspeed performance.

 

This info I got from Jiri Holoubek at Woodcomp:

 

“I comunicated this problem with Mr Rodney Stiff from Jabiru company . The Jabiru company allows the propellers with polar moment up to 0.3kg/m2 ( for the Jabiru 3300 ) normally.

 

The on ground adjustable propeller - type SR 200J has the polar moment 0.41kg/m2. Mr Stiff extended the polar moment to 0.5kg/m2 for our propellers. The SR 3000/2JM is 0.46kg/m2”

 

As it is wood/glass blades there is a lot more structural damping than the sensenich carbon blades, same construction as the MT which is approved.

 

Regarding the loads on the crank, there should be no significant difference between a MT and Woodcomp with similar polar moments, but it’s more important with dynamic balance on a heavier prop and I will certainly do a balance run as recommended in latest Jabachat.

 

From the discussion about props in Jabachat, and that they approved the MT after extensive testing, my conclusion is the inertia limit is probably not a real design limit, more that Jab has not enough own test data to approve other than 2 blade wooden props.

 

Anyone know more about this?