New Member chasing info on Rotax propellor spacer

[BDA64]

Good morning all.

 

Just chasing any info on spacing out a prop on a Rotax 912uls. Have just purchased a TL 2000 Sting. The existing constant speed prop has to be sent away for repairs so have purchased a Bolly ground adjustable as an interim measure to get in the air. Need to extend the prop out 6 to 10 mm so as to clear the engine cowl.

 

Not knowing Rotax or RAA aircraft that well, can anyone advise the min amount of propellor dowel that needs penetrate the prop hub? The standard dowel from Rotax will give me about 3mm!!!

 

Any info would be greatly appreciated.

 

Thanks

 

Brad A

 

 

[Sav 4352]

Try Michael Coates on 0418 168 665.

 

 

[Head in the clouds]

Good morning all.Just chasing any info on spacing out a prop on a Rotax 912uls. Have just purchased a TL 2000 Sting. The existing constant speed prop has to be sent away for repairs so have purchased a Bolly ground adjustable as an interim measure to get in the air. Need to extend the prop out 6 to 10 mm so as to clear the engine cowl.

Not knowing Rotax or RAA aircraft that well, can anyone advise the min amount of propellor dowel that needs penetrate the prop hub? The standard dowel from Rotax will give me about 3mm!!!

 

Any info would be greatly appreciated.

 

Thanks

 

Brad A

Ermmm, Brad, I think you're walking on thin ice here. I assume that's a factory-built Sting? If so (and/or if you aren't the original builder from plans or a kit) you can't make ANY kind of modifications without factory approvals and the associated documentation. If you do so anyway, you'd be voiding your insurance, the aircraft would be deemed unairworthy and so you'd be flying illegally, for which the penalties can include imprisonment. If the Sting is already available with the fixed pitch prop configuration you propose then it would need to be installed and signed off by an L2/L4 precisely to the factory spec and it sounds like that might be difficult if, in this case, the engine is positioned further aft (or the cowling further forward) to accommodate the CS unit

 

FYI, though, simply adding a spacer plate between the drive hub and the prop hub is NOT how it's done. If you really needed as little as 10mm spacing the problem is quite complex and would probably be best achieved with spacers at the rubber engine mountings but that would also need an engineering order due to the increased bending loads on the mounting bolts, and it's not even possible to achieve if the mount is dynafocal rather than parallel.

 

You can't drive the prop via the ends of the dowels. For one thing there wouldn't be enough bearing surface area, secondly the load applied to the ends of the dowels would be tending them to 'cant' and damage their bearing area in the drive hub and thirdly the dowels themselves are designed and rated to be operating in pure shear at the drive hub/prop hub interface, not to be operating in 'bend'. And fourthly - just try and imagine the effect of the 2700 (x2), or so, torsional vibration (TV) pulses being transmitted through those bending/canting/slipping dowels to your prop hub. You'd have oval holes, chewed out spacer plate and potentially a prop separation through broken prop bolts within minutes ...

 

The proper way to space a prop is to have a spacer plate dowelled and bolted to the drive hub (just like a normal prop hub is) and then the spacer plate carries its own second set of dowels to which the prop hub is fitted and bolted. You can't just knock up one of those, it needs considerable design calculation. I've designed and made a few of them and they're not a five minute affair. IMHO the minimum thickness for the plate would be about 12mm but only if it was made from properly heat-treated carbon steel and that's a challenge, particularly keeping it flat during/after heat-treatment. To achieve the bolting the drive hub dowels and the spacer dowels have to be out-of-phase, so in operation the thrust is trying the pull the spacer into something the shape of a wavy spring washer, so the calcs involve ensuring that the plate is rigid enough to absorb and/or avoid that distortion - quite complex physics once you add the effects of TV.

 

From 25mm or so thick, for the best combination of rigidity and weight the spacer plate is best made from 6061T6 aluminium alloy as a solid disc. Note all spacers, regardless of their length should be made from a solid billet, never built up from fabricated/welded components. From about 50mm long and upwards the spacer can be 'waisted' in the middle to reduce weight, but again, by how much - and how large a central hole may be - is a matter for careful calculation. IIRC the longest spacer for a 912 gearbox is 120mm but that is subject to Rotax's proviso on prop weights/inertal moments. That info is in the Rotax engine installation guide.

 

Hope it helps, but I'd seriously suggest you wait for your prop repairs to be completed.

 

 

[ianboag]

Good morning all.Just chasing any info on spacing out a prop on a Rotax 912uls. Have just purchased a TL 2000 Sting. The existing constant speed prop has to be sent away for repairs so have purchased a Bolly ground adjustable as an interim measure to get in the air. Need to extend the prop out 6 to 10 mm so as to clear the engine cowl.

Not knowing Rotax or RAA aircraft that well, can anyone advise the min amount of propellor dowel that needs penetrate the prop hub? The standard dowel from Rotax will give me about 3mm!!!

 

Any info would be greatly appreciated.

 

Thanks

 

Brad A

Warp Drive sell what they call "extensions" and BTW I thought the dowels were basically just locators - and the torque is transmitted by friction. Shades of the old Jab flywheel bolt controversies ....

 

 

[Head in the clouds]

Warp Drive sell what they call "extensions" and BTW I thought the dowels were basically just locators - and the torque is transmitted by friction. Shades of the old Jab flywheel bolt controversies ....

Timber prop hubs are driven by friction but metal hubs have precision drilled and reamed holes for the dowels to aid the drive as well as the centring.

 

It's doubtful that enough clamping pressure would be gained to drive a metal prop hub since the metal-to-metal friction coefficient is so low.

 

And - add a loose spacer plate into the equation, providing two metal-metal interfaces, and the situation deteriorates further.

 

 

[ianboag]

Timber prop hubs are driven by friction but metal hubs have precision drilled and reamed holes for the dowels to aid the drive as well as the centring.It's doubtful that enough clamping pressure would be gained to drive a metal prop hub since the metal-to-metal friction coefficient is so low.

 

And - add a loose spacer plate into the equation, providing two metal-metal interfaces, and the situation deteriorates further.

Fair comment. Surely though if the dowels are doing the heavy lifting they would need to be an interference fit. If not, then they could move with the dire consequences you correctly describe.

 

 

[Head in the clouds]

Fair comment. Surely though if the dowels are doing the heavy lifting they would need to be an interference fit. If not, then they could move with the dire consequences you correctly describe.

Yes, they are an interference fit into the drive flange and a size-on-size fit into the prop hub.

 

Rotax engines provide a further benefit because the internals of both types of gearbox provide a buffer between the TVs and the final drive flange. It's the TVs that are by far the worst with regard to the drive/driven slippage, rather than the torque alone.

 

 

[ianboag]

Yes, they are an interference fit into the drive flange and a size-on-size fit into the prop hub.Rotax engines provide a further benefit because the internals of both types of gearbox provide a buffer between the TVs and the final drive flange. It's the TVs that are by far the worst with regard to the drive/driven slippage, rather than the torque alone.

I went through this when I put a 912 into my J160. The engine end lugs that have an internal thread are a press fit - the ones without an internal thread are not. As you correctly state the prop end lugs are not a press fit.

 

All as supplied by national Rotax agent. Go figure ....

 

This is a fairly standard clamp joint. If the faces of a clamp joint move the tiniest bit then the dowels - bolts - whatever eventually snap from bending fatigue

 

Yes, they are an interference fit into the drive flange and a size-on-size fit into the prop hub.Rotax engines provide a further benefit because the internals of both types of gearbox provide a buffer between the TVs and the final drive flange. It's the TVs that are by far the worst with regard to the drive/driven slippage, rather than the torque alone.

[ianboag]

None of the lugs on my 912ULS/Warp flange/prop are an interference fit.

 

I agree about the TVs, but not about the shear-driven nature of the joint. The dogs are not in shear, no matter how short they are. If the joint moves - at all - it will eventually break from fatigue because the pins(bolts) get bent back and forth. The reason the joint does not move is that friction clamps it. Dogs that are anything other than an interference fit do not do any driving, so it doesn't matter (within reason) how short they are, as long as they locate the prop correctly.

 

There's a Wikipedia page that explains bolted (clamp) joints. If the faces of the joint can move - be it by ever such a tiddly amount - the bolts will break. A 12 mm spacer that reduces the dog engagement from 15mm to 4mm is not a safety issue. If you torque the bolts up to whatever the Bolly people say, the joint will not move.

 

FWIW I understand that Cessnas have just one dog and a locating spigot in the centre of the prop flange. The spigot locates the prop centrally and the dog puts the prop in the right crank angle position. It's a clamp joint ... work all done by friction.

 

I realise there is paperwork that makes 24 aircraft sacred in Oz. That might slow one down. No insurance - non-airworthy aircraft - 20 years in jail is all serious stuff. The physical realities are that the Bolly would be a good 3kg lighter than the CS unit is is standing in for. As explained above, 3mm of penetration is plenty to locate it on the flange/spacer so the clamp from suitably torqued up bolts can hold it. The CG would move back some because the prop is lighter. What is hard about this? We have a guy on field who flies a Sting that came with a ground-adjustable and it works fine.

 

Here in NZ for a microlight, one would write up the mod, do a revised W&B then submit it to the technical officer of a Part 149 organisation (we have three of them). When that is signed off (generally not an issue and does not cost anything significant) it goes to CAA who charge $250 and approve it. Scary stuff I suppose. No Engineering Order. No design engineer involved.

 

With such a sloppy attitude, one might expect modified microlights to be falling out of the NZ sky all over the place. Bound to start happening real soon .... I just haven't heard of a single case yet where a microlight crashed/failed because of a poorly done mod.

 

I am not a LAME or an aeronautical engineer, so we can cheerfully agree to differ on this.

 

Cheers

 

IB

 

 

[Head in the clouds]

I am not a LAME or an aeronautical engineer, so we can cheerfully agree to differ on this.

Good idea ...

 

A few points to ponder though -

 

Since the Rotax has a centring spigot for concentricity, your suggestion must be that the 'dogs' or dowels aren't doing anything at all, so why are they there? I haven't found anything that's on a Rotax for no reason.

 

I did point out the much reduced TV issue due to either/both of the 'clutch' arrangements in the Rotax geardrives which would make the drive flange/prop hub interface much less likely to slip but I'd not be confident enough in that to remove the dowels and go for a fly - would you? If you wouldn't either then I'd guess you'd agree they must be there for a reason, and if so I'm surprised you're happy to encourage an admitted newbie to fly his with improperly engaged dowels.

 

My C172 didn't have anything like you describe for mounting the prop.

 

I'm not a LAME or AE either so I'll happily differ with you on this too

 

There's a Wikipedia page that explains ....

 

Seriously though - I never was saying it's not a clamped-plate drive, that's obvious, but the dowels do serve a very definite purpose even if they're not solely resolving the TVs. Without the dowels the bolts would cant in the opposite direction to prop rotation under the combined effects of TV and torque. The dowels prevent this cant exceeding a certain value and they require bearing surface area to achieve that (the prop hub is aly after all, not steel). Using a 12mm spacer plate and allowing only 3mm of dowel engagement reduces that bsa by 80% which would allow the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, allowing the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, allowing the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, allowing the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, allowing the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, allowing the bolts to cant further, stretching the bolts, bending the bolts, wearing the prop hub further, ... you get the drift.

 

 

[ianboag]

It's probably time we put this dispute between two ignorant chaps to bed ... :-) Given that this is not my plane and not yours we have to accept that free advice is worth what the Sting man is paying for it.

 

I very much doubt that the Rotax slipper clutch is there to take up regular torsional wobbling by having the plates move relative to each other. I always assumed that the purpose is to prevent a prop strike from trashing the rest of the drive train. I had a prop strike once and watched the blades sort of stutter as the ends touched the runway. Engine kept going - it was all a bit novel. The gearbox box was professionally stripped, the crank was checked and there were no problems. My Cessna story was second hand ... sorry.

 

But ....

 

In a clamped joint done properly, the clamped surfaces do not move relative to each other AT ALL. They are "dowelled" by the zillions of surface imperfections in the two mating faces. So the bolts that provide the clamp force are not in shear - never - not even "hardly ever". So no cant - no bend - no bearing surface - no wear - .....

 

I'm done. My late father once observed that some conversations are like two people playing tennis where the ball gets hit and returned, and some are like golf where two players just bash away at their own ball. :-)

 

 

[Head in the clouds]

I'm done. My late father once observed that some conversations are like two people playing tennis where the ball gets hit and returned, and some are like golf where two players just bash away at their own ball. :-)

Hmm ... that doesn't make anything any clearer at all - does that make you a tennis player or a golfer?

 

Ian, with the greatest respect (well you have to have respect for someone who puts a Rotax into a Jab), you don't seem to be aware of the 'innards' of your engine. You can find the 'llustrated Parts Catalog' online if you like and see page 3 of Section 72-10-00. Assuming one doesn't have one of the very earliest gearboxes it will have the 'With Clutch' shown there. I have not personally disassembled one of these 'boxes but I was told that that particular clutch is akin to a pair of wavy washers engaged with each other and they are constantly working while the engine is running, thereby absorbing the TV. You will probably be aware that other gearboxes that look outwardly similar to the Rotax 'box usually use a rubber donut arrangement for the same purpose.

 

Contrarily, the 'slipper clutch', as you describe it above, is a wholly different thing altogether, and is also on many later engines (i.e. as well as - being 2 clutches in total - I do seem to recall when I bought mine it was an option to have the second clutch), and is the one you refer to which is there for the protection of the crankshaft in event of a prop-strike. It is not in the gearbox, it is in the crankcase on the output end of the crankshaft. It is a multi-plate clutch similar to a motorcycle clutch and can be seen in exploded view on page 10 of Section 72-10-00.

 

Incidentally, and in case it is of any interest to folks ... I'm told that using avgas deposits so much lead oxide and other 'detritus' into the engine oil that the life of that oil-immersed clutch is almost halved from the usual 1000hrs to around 600hrs (this info from one of my most respected friends who has in excess of 10,000hrs flying behind Rotax in his flying school). It's worth considering that if that clutch wasn't ordinarily slipping then it wouldn't wear regardless of what's in the oil, I would have thought ...

 

Hope it helps.

 

 

[ianboag]

Hmm ... that doesn't make anything any clearer at all - does that make you a tennis player or a golfer? Ian, with the greatest respect (well you have to have respect for someone who puts a Rotax into a Jab)Hope it helps.

I think we have both been playing golf .... or croquet ... or some other bang-your-own-ball activity. Not much meeting of minds.

 

The Rotax used paperwork from an EO ... done by a Llewellyn even.

 

 

[SDQDI]

You fellas and your dodgy advice are dangerous! Any backwards farmer could set you both straight, you can’t just whip up a dodgy extension it needs to be done properly and then welded on just to be sure. If it is only the 80hp rotax half a dozen tacks and a smidge of silastic would be fine.

 

CAUTION!!!!!!!!!!! This post is tongue in cheek and anyone who takes it for real advice is........ well dumb beyond belief I guess:whistling:

 

 

[johnspulsar]

Good morning all.Just chasing any info on spacing out a prop on a Rotax 912uls. Have just purchased a TL 2000 Sting. The existing constant speed prop has to be sent away for repairs so have purchased a Bolly ground adjustable as an interim measure to get in the air. Need to extend the prop out 6 to 10 mm so as to clear the engine cowl.

Not knowing Rotax or RAA aircraft that well, can anyone advise the min amount of propellor dowel that needs penetrate the prop hub? The standard dowel from Rotax will give me about 3mm!!!

 

Any info would be greatly appreciated.

 

Thanks

 

Brad A

Hi Brad, I looked at this aircraft when it was for sale last year but you beat me to it. I'm still interested in it, if you decide you want to unload it, let me know and we could do a deal.

John.