Jabiru Prop out of wack

[Old Koreelah]

I am doing a static balance before I install my prop. I was told not to trust the centre hole (5/8") being exactly central but when I spin my prop on an axle thru this hole it tracks true, equal blade lengths and balanced.

 

When I mount the axle on a pair of crush plates (with 5/8" central holes) mounted either side of the prop the blades track 20mm apart, one blade is almost 2mm longer than the other and the balance is out. It looks like the machine that drilled the six 1/4" holes for the mounting bolts was not quite at 90°.

 

If this is the case I will have to use a file to elongate the front end of each hole 1.6mm towards the longer blade to correct the error, then fill the rest of each hole with epoxy filler.

 

I can't see an alternative; any suggestions?

 

 

[Guest Maj Millard]

Had an Allsize prop recently for a Gazelle, brand new, same rubbish re-center hole ect.....for the money you pay, you'd think they would get it right !

 

 

[jetboy]

My Jabiru prop didnt take long to warp out of crushable tolerance with regard to track and pitch symmetry. Mine was 1.5 degress different pitch between the 2 blades at 75% chord. Neatest way to measure this is with a laser pointer held across the rear of the blade tracing onto the floor, then positition the next blade at same place and compare.

 

the correction according to Jabiru manual is to install a non-epoxy resin f/glass packer under the prop which has been wedge shaped and can take the heat, FWIW.

 

Ralph

 

 

[Old Koreelah]

Thanks fellas for the ideas. My stud pattern sure is 4" Dexter. Only a gorilla could get a 100mmm plate to fit. It's at least 2nd hand so warranty is probably long gone...

 

This arvo I checked the free play in the holes drilled thru prop and crush plates and found enough movement to allow me to tension the prop down with only 7mm of track variation. Perhaps I'm on the right track. I really only need c.0.8mm "slant" on each hole, which I could easily achieve with a chainsaw file. The prop will track true and blades will be close to equal length. The problem is then how to "lock" it there to prevent the sort of movement that Dexter warns about. Using epoxy to back-fill the holes seems the go, but is heat build up in the prop an issue?

 

Thanks for the method of checking pitch, Ralph. I am learning how multi-dimensional the job is! If the back of the prop is packed, eg. with a vinyl ester wedge, the prop bolts would have to bend slightly. Surely this would place worrying loads on these critical bolts? (I'm surprised that 1/4" bolts are employed on such a high-load area.)

 

Lyle

 

 

[Guest Qwerty]

O.K.

 

What do you mean warranty is long gone. I had a similar problem with a different component that I am still chasing up. I don't care how old the prop is. If is hasn't been abused or butchered and it is still how it was manufactured and it is manufactured wrong then the manufacturer should deal with it. If only to avoid me going public and and publishing their attitude and then promoting a sound alternative.

 

I resent being taken. Get on to them and ask them to fix it for you. If they bugger you around explain what the consequences will be. My supplier is about to get just such a call.

 

No more accepting second best treatment I say.

 

Cheers, Qwerty

 

 

[Old Koreelah]

Crickey, I'll have to 'fess up and apologise to Mr. Jabiru's prop makers.

 

I spent heaps of time re-checking measurements of the two home-made crush plates (I'd made them up to facilitate balancing with a 3/8" tube as a shaft.)

 

It looks like the centre holes of each plate was out by a poofteenth. After correcting that I eventually got static balance close to perfect, fitted prop and torqued it, and was amazed to find the tracking within 1mm and the pitch identical!

 

Started engine for the first time and it spins like a turbine!

 

Thanks for replies, people and sorry about the waste of time.

 

Lyle

 

 

[jetboy]

Lyle I noticed you had been planning to shift the bolt holes and backfill them with epoxy: this would not be correct because the Jabiru prop is located soley by the aluminium bushings supplied that fit through the holes in the engine flange and engage with the recess bores in the prop driven face.

 

Ralph

 

 

[Guest Maj Millard]

How much is a poofteenth anyway ??????:confused:

 

 

[dazza 38]

How much is a poofteenth anyway ??????:confused:

I think it is about half a bees dick. Maj

 

 

[GraemeK]

How much is a poofteenth anyway

You know two thirds of five eighths of all?

 

Well, take that and divide by a few thousand .......

 

:hittinghead:

 

 

[Guest Qwerty]

Poofteenth:

 

Official definition; A poofteenth is an amount that is actually significant and will stop things fitting and generally bugger up your day, cost huge sums of money etc, when any reasonable person would have bet their left nut that such an amount would not matter.

 

Note: It is generally accepted that a poofteenth is considerably less that a bee's dick.

 

Source, "Bruce's book of knowledge and tips on brewing and BBQs"

 

:thumb_up:

 

 

[dazza 38]

I will have to get that book qwerty.

 

 

[Guest Qwerty]

Sorry mate, there is only one copy in existance and I don't want to sell it, none of the original print run suvived the great fire of '04 in Bruce's back yard. The entire stock was lost. luckily I had already bought my copy. Even the original manuscrip was lost. That happened in the great blow (also of '04), just before the great fire.

 

If you need to check anything, just post a question in a thread and I'll look it up for you.

 

Cheers, Qwerty

 

 

[dazza 38]

Thanks mate HA HA :thumb_up:

 

 

[Guest Qwerty]

Dazza, It looks like the is some confusion so here is an actual quote from Bruce's book

 

""

 

Bee's dick; It is generally accepted that a bee's dick is considerably more than a poofteenth

 

Poofteenth: Official definition; A poofteenth is an amount that is actually significant and will stop things fitting and generally bugger up your day, cost huge sums of money etc, when any reasonable person would have bet their left nut that such an amount would not matter.

 

Note: It is generally accepted that a poofteenth is considerably less that a bee's dick.

 

F all : Is known in Western Sydney (The only actual known and recorded amount of F all) as "not much".

 

Two thirds of five eighths of F all; equals 0.416 recurring of F all, now F all times 0.416 recurring turnsout to be just a bit more than a poofteenth.

 

Just a bit more; This amount is related to One more should do it, and One more hit, and are all just enough to be too much and cause overflow/overbalance/overstressing/catastrophic failure.

 

One more should do it ; see Just a bit more above

 

One more hit; see Just a bit more above ""

 

I hope that this clears thing up.

 

Cheers, Qwerty:wave:

 

 

[dazza 38]

Clears it up just fine thank you.

 

 

[Admin]

 

 

[Guest Qwerty]

Good point Ian.

 

So Jab props should track within a poofteenth then?

 

 

[Old Koreelah]

Like I said, you sure get an education on this forum!

 

Lyle

 

 

[Guest Maj Millard]

Thanks guys I'm up to speed now, you learn something new everyday !!!!!...................