J430 Prop Loss French Island - ATSB Report

[turboplanner]

http://www.atsb.gov.au/publications/investigation_reports/2013/aair/ao-2013-046.aspx

[fly_tornado]

Lol, all jabiru's fault

 

 

[RKW]

Where's Billy Goat Gruff when you need him?

 

 

[aj_richo]

At least they got the prop back to hang on the wall... I had one depart once and it was just BBQ starter wood on retrieval.

 

I can still see it rotating down like a helicopter with its big alloy pulley underneath.. as I switched the wildly overspeeding engine off and nosed over for the inevitable forced landing

 

Ah the days of 300' height limits, 2 strokes and unreliable reduction drives..

 

 

[Guest Andys@coffs]

Tony...At least at 300' you wouldn't have time to agonize over which paddock looks best.....in fact not much time to agonize over anything! I had an EFATO at Gawler turning crosswind...about 700' in a trike was back on the ground inside seconds it seemed at the time.......

 

Andy

 

 

[aj_richo]

No you're spot on there Andy , no time at all.. had to point the nose quite steeply at the ground to regain 50kts and flare

 

Luckily it was over the old parachute drop zone at Saltash near Willytown.

 

I had been out over Tanilba Bay 10 minutes previously, that did scare me a bit afterwards lol.

 

 

[Dafydd Llewellyn]

http://www.atsb.gov.au/publications/investigation_reports/2013/aair/ao-2013-046.aspx

Thanks, Turbs, that gets straight to the facts. Much appreciated; I just torqued the bolts on a Jab prop flange.

 

 

[DonRamsay]

Is that problem a feature of wooden props only?

 

 

[Dafydd Llewellyn]

Don, the propeller did not come off the flange; the flange came off the crankshaft. Read the report.

 

 

[DonRamsay]

You mean read the report better . . . . OK.

 

 

[fly_tornado]

You sure about that?

 

 

 

[Dafydd Llewellyn]

ATSB is always careful not to attribute blame; their function is to discover the cause in the interest of increased air safety. The wrangles over blame come in civil court cases afterwards.

 

FT, what is the source of that post? I bet it was NOT ATSB; looks like CASA to me. CASA and ATSB are entirely separate organisations.

 

I'm not convinced that what those documents say is correct; the torque has to be transferred by friction, achieved by the clamping force generated by the bolts. Dowels only come into play after movement occurs due to inadequate friction - i.e. when the thing is already on the way to failure. In an attempt to improve the effectiveness of the dowels, people make them a tight fit - so the clamping force is reduced by the force necessary to push the dowels into their holes. Further, pressing dowels into the end of the crankshaft results in a minute amount of "bulging" of the metal around the dowels (and there isn't much of it!) which reduces the area of contact by which the friction can work to transfer the torque.

 

If I had a -19 Jabiru, of any description - but especially if it used a Jab 3300 - I'd fit Ian Bent's belt-driven alternator - which as well as supplying more electrical power and also being a 3-phase field-regulated device, ALSO acts as a crankshaft torsional damper.

 

 

[jetjr]

There sure is vibrations in there. I think 2200 is worse

 

Can a dynamic prop balance help this type of issue? Must balance more than just the prop

 

Sure made mine run very smoothly

 

Jab manuals go into detail of dowels in flywheel end, yet hardly a mention or picture of the prop flange end with them. Obviously not fitting dowls to rebuild cranks.

 

 

[Dafydd Llewellyn]

JJ, what do you mean by "dynamic prop balance" ? The term "dynamic balancing" means to me, a form of balancing that spins the rotating assembly and measures the reactions by load-cells or accelerometers at the supports. This method is capable of detecting "moment" out-of-balance, which cannot be detected by simply allowing the assembly to find its point of balance when resting on a couple of smooth rolling surfaces ("static balance"). A propeller that is badly out-of-track will produce a "moment" out-of-balance, even though it may be in static balance. If this is what you mean, to do this for more than just the propeller, then I don't see how one could do this except by removing the crankshaft from the engine, bolting the propeller to it, and dynamic-balancing the assembly. (One can dynamic-balance car wheels using the vehicle suspension; but I don't think this can be applied to a complete aircraft engine).

 

If, however, you are referring to a crankshaft torsional damper, such a damper has nothing to do with propeller balance.

 

The crankshaft has a "flywheel" mass at either end - the propeller at the front, and the ring gear assembly at the rear. The crankshaft acts like a stiff spring, connecting them. This system has a natural frequency, and to the extent that this frequency is excited by the firing impulses, the resulting torsional oscillation will add to the loads being carried by the flange attachments at the front and rear of the crankshaft. A torsional damper acts to absorb energy from this torsional oscillation, which tends to reduce the oscillatory torsional stresses , thus helping to reduce the loading on the flange attachments. The belt-driven alternator acts this way, whereas the mass of the Jab alternator magnets adds to the flywheel mass at the rear end of the shaft. I do not know how significant this change may be, but it's in the right direction.

 

 

[jetjr]

Kind of, it is a accelerometer attached rigidly to engine case, measures vibration and with ir crank sensor says where in the rev it occurs.

 

Add or subtract washers to prop hub reduce vibration.

 

Done at full static ground rpm so it is dynamic

 

It really is balancing whole rotating assembly just not sure about these damaging forces talking about here

 

Pretty alarming how much out of balance things are before weights added even though it felt ok to fly

 

 

[Dafydd Llewellyn]

Ta - hadn't come across that technique; sounds no bad idea, but it has nothing to do with crankshaft torsional vibration stresses.

 

 

[jetjr]

I bought a unit and it can be used to track down source of vibration in any situation. In my case large electric pump installations. Can see bearing wear and monitor potential failures if done regulalrly. Same concept of running on condition rather than to failure like most do.

 

Used to track down loose of failed cement mounting bolts

 

Software runs on android phone and is really setup for prop balancing

 

 

[Dafydd Llewellyn]

OK, I'd like to know more about it. What sort of cost for the gear?

 

 

[AVOCET]

Ditto , sounds like a handy tool for the box .

 

Mike.

 

 

[jetjr]

http://www.smartavionics.com/prod-pb.html

 

Around $700 but might be more now

 

Not something you'd buy for fun but he is a approachable guy and very helpful

 

Had a little bracket cut by Ian to fit accelerometer properly to front of Jab engine

 

It works in single axis only but a second sensor can be used for dual axis in say rotorcraft

 

I a actually did first balance with Dynavibe but it cant do much else and is more expensive too. http://www.aircraftspruce.com/catalog/appages/dynavibe.php

 

 

[Guest Andys@coffs]

I bought one about 3 months back (PB3). The accelerometer that came with mine has an X and Y axis sensor so can be used for helicopters as well as normal aircraft props. In my case having the 90 degrees apart dual sensor means I can mount the sensor as best suits the routing of the cable (so long as one of the sensors aligns with the axis needing to be measured) and then choose the most appropriate accelerometer of the two in the software.

 

Mine cost just under $1000 so avoided the whole GST/Duty thing but only just and would depend on the exchange rate of the day...... I didn't have any android phone so I purchased the Samsung 7 inch lite tablet for $199, though I note that phones can be purchased that are suitable for circa $50 plus on ebay. Whether its locked to a provider or unlocked (later being more expensive generally) is irrelevant cause you don't even need to insert a sim into it to use it for this specific task. Given my age is such that eyes are not as good as they once were I couldn't be be stuffed fighting with a 3inch screen when more realestate is generally good.

 

Andy

 

 

[Guest ivan4ilse]

Having been in the condition monitoring / vibration analysis game until 2,5 years ago I am amazed at the prices quoted in the previous posts. Vibration/balancing equipment has traditionally been very expensive. Actually the hardware is cheap its the software that is expensive - although I am speaking industrial stuff......

 

Some more details like suppliers would be welcome if any one has them.

 

Just for interest does any one know what the max amplitudes are for an out of balance prop on a Jab?

 

 

[Dafydd Llewellyn]

The amplitude will be a function of, inter alia, the spring rate of the engine mount and its rubbers - and thus rather variable.