NB what torque** would people recommend for these bolts? Please do not refer to the factory documentation
I’m afraid the only one who can tell you that is the propeller manufacturer. The bolts will most probably overtighten a carbon propeller (I would assume, without ever having torqued a carbon propeller). A assume the bolts are used to hold the propeller.
Jan, most propeller manufacturers have a web presence or some other way to get technical info and I would recommend looking for that data. Propeller installation data for a third party propeller on a Rotax does not have to come from the airframe manufacturer. That said, as a last resort the typical installation torque for an M8 x 1.0 or M8 x 1.25 bolt of class 8.8 is 25 n-m. Bolt Torque Table
If you want to account for the additional friction of the Nylock nuts, measure the torque needed to freely rotate the nut on the bolt, and add this to the installation torque specified by the table. Propeller bolts on certified aircraft are typically locked with safety wire, i.e. not with lock nuts. One reason is that thread locking masks the torque being applied to the assembly at installation. For the same reason unplated bolts and unlubricated bolts often have slightly different torque values in an installation torque table.
Make sure washers fit the bolt diameter properly. Also note that when dynamic balancing a propeller, the mass of a washer is enough to make a measurable difference in propeller balance, so each propeller bolt needs to be the same length and use the same number of washers.
most propeller manufacturers have a web presence
This one most certainly hasn’t – if he had, nobody would be buying from Duc and the rest of them – his prices are 40-60% of general market…
to account for the additional friction of the Nylock nuts
Cheese us, that’s a point I hadn’t realised. And yes, I did have observed most propeller are secured by safety wire, only I hadn’t thought any further. Stupid me!
So yes, I will have to revert to the generic bolt torque table – thanks very much! Alternatively I’ll ask the airframe manufacturer, but communications aren’t always very smooth.
But perhaps the most important, which I ought to have mentioned before: the bolts that I am now struggling with do no more than fix the complete propeller to the Rotax engine flange – with a 40 mm spacer between, plus an alu disc that carries the spinner. As it is, the alu carrier disc has been found to be botched so it is left out for the time being. I could also leave out the spacer, it is only there to keep the spinner away from the engine cowl, but that would open a whole new can of worms regarding weight and balance, I can’t be bothered now.
The complete propeller (two half-shells and three blades) is assembled with another six bolts, one on each side of each blade, and these six have been carefully left as they were.
Illustration with the spinner carrier in place, it is the inner six bolts that are of concern now:
http://i.imgur.com/gMKJs8E.jpg
I could also leave out the spacer, it is only there to keep the spinner away from the engine cowl, but that would open a whole new can of worms regarding weight and balance, I can’t be bothered now.
I would not advise leaving out the 40-mm propeller spacer or changing anything else that affects the geometry and mass distribution of the rotating assembly. Small changes can substantially change the vibration response of the system. Leaving out the spinner back plate is probably OK.
What I meant was that if you torque the M8 bolts according to spec, this could be way too much for the propeller. You may brake it or worse, too much tension will be applied to the propeller and this will weaken it and it breaks in flight. Only the propeller manufacturer can tell you the exact torque to use.
be very very careful with the bolt torques. In your case the hub is said to be carbon composite. There must be a lot more of those carbon hubs for the Rotax and I´d look at these to compare numbers. The hub in your photo looks hollow, are there distance tubes inside to prevent the bolts crushing the case ? And speaking of high tensile: I do hope the nyloc nuts are 8.8 as well ??? Myself I never fit nylocs but use Loctite in different grades, except where wire or tabs are specified.
Vic
