multi-bolt props on petrol engines

[Terry Masters]

I have a new 40 cc petrol engine which is designed for a four bolt fixing of the prop with a central spigot of 10 mm dia. I hear reports of the bolts shearing and also the prop bursting where the bolt holes are drilled through the hub. Do I, must I bite the bullet and modify the fixing to one central bolt/nut combination or should the designed fixing be sound as long as the bolt holes are drilled correctly ? If the latter then what would be the best orientation of the holes relative to the prop blades?
Terry Masters 2888

[andywynn]

Hi Terry,
I use them all the time they are fine. Bolts will shear if over tightened and props will break up if the wrong type is used. On the latter, there was a spate a few years ago of the small petrol engines (40cc size) people decided to use plastic props (Dynathrust if the memory is right) to save them breaking if the plane nosed over. The plastic prop dooesn't have a solid boss hence the prop broke up. For the better petrol engines there are jigs avaliable to help drill the prop accuratly (ZDZ, DA 3W etc). The important thing to remember is to drill the prop (with a jig) from the back. Also if you are using a wooden prop don't use the centre bolt as well, as you tighten the multi bolts, then tighten the centre bolts and it compressses the hub and muti bolt becomes loose and so it continues.

Hope this helps

Andy wynn

[Terry Masters]

Thankyou Andy for the response. I will proceed with the four bolt system as designed and bear in mind your experience and advice.
regards,
Terry Masters.

[Nick Reeves 3055]

Bolts will shear if over tightened and props will break up if the wrong type is used.

how tight should they be then?? i had two out of six shear on me on sunday. i am using a wooden prop at the moment but would a carbon prop help reduce the risk of shearing them again??

cheers

nick

[andywynn]

Bolts will shear if over tightened and props will break up if the wrong type is used.

how tight should they be then?? i had two out of six shear on me on sunday. i am using a wooden prop at the moment but would a carbon prop help reduce the risk of shearing them again??

cheers

nick

Hi Nick,
It should say in the instructions, but if not then a standard size Allen key and a good tightness. Hard to descibe you know when yon know ( i alway put WD40 on the bolt so you don't get false feedback), just remember to tighten opposite bolts ( 1 o clock/ 7 o clock. 2 o clock 8 o clock etc). Carbon props don't compress so give a better indication on being tight, also give better performance. If the plane is not prone to nosing over then i would recomend a carbon otherwise it gets expensive!!!!

Hope that helps

Andy

[Martin Waller]

I have used the six bolt fixing on many engines from 60cc to 100cc and never had a problem. If only 3 of the six are used then they will break (happened to two of my friends I witnessed first hand both times).

If high tensile steel 5mm cap head bolts are used they will not break with over tightening they will strip the aluminium prop boss thread. They will break if not tightened enough as prop moves backwards and forwards and the cycling prop momentum variation will break the bolts. With wood prop when new and for first 10 or so flights check tighness after each flying session, eventually the wooden prop will stop compressing so checking need not be so regular. With allen key apply firm pressure at 4" say about 3 to 5 foot.lbs of torque. Too much and you will keep compressing the wood.

Carbon props do not compress anything like to the same extent so are easier in this regard. I do not think there is a problem with either wood or carbon. I think the plastic props are dangerous on powerfull petrol engines so have never used them. For wood I use Metz and carbon Beila and Mejelik. (sory if spell incorrectly). I believe Beila is carbon covered wood.

I always used spray grease on the prop bolts to reduce the wear on the prop driver aluminium treads as these eventually strip if the prop is removed and refitted regularly. it also stops the bolts binding if left on for a long time when again the aluminium thread will pick up on the steel bolts and end up worn.

Martin Waller LMA 1237