Prop Bolts

[Diablo]

Here you go on the DC-10 crash:
http://en.wikipedia.org/wiki/America...nes_Flight_191

[vsnap]

Ah heck drill a 1/16th hole in the rear retaining pin and put a cotter or similar pin in it so if in fact the a nylon bolt breaks or a steel one backs out your safe. If you grab a wing after the flight and it seems a little loose guess what? I know it works...costs less then a nickel a side and takes 15min max. But then it may add 10 seconds to assembly at the field...

[rcign]

Didn't read the whole thread, but
A single 3/8x24 grade 8 prop bolt will hold ANY prop on ANY engine used on Giant Scale airplanes...
We run 30hp 289cc Herbrandson twin cylinder drone engines and some Aerrow 200cc twin methanol racing engines on 230+ mph 90" span Unlimited race planes...
21x27 carbon prop, 9400 rpm on the 289 and a 21x25 carbon prop, 10,000 rpm on the Aerrow..
ONE 3/8X24 grade 8 prop bolt with two 1/4x20 anti rotation bolts, 1 1/2 inch bolt pattern...The hub is tapered and the prop bolt holds the hub on the taper and the prop on the hub, perfect fit, no key needed....
We once forgot to check the bolts between heats...The 1/4x20 bolts loosened and the prop started to rotate, shearing the bolts off...The counterclockwise rotation TIGHTENED the 3/8x24 prop bolt, the prop did not come off...
There is no reason to use six small bolts to hold a prop on when a single 3/8 bolt works better...
I have re worked some 3W 106 twins and G62 singles by cutting the threads of the end of the crank and drilling and tapping the crank for a 3/8 prop bolt...This works with the one center bolt and two 5mm anti rotation bolts..No more sheared bolts, the prop will not come off even if they did...
The prop bolt can be tapped for the spinner bolt, or a threaded stud and special nut from Tru Turn can be used...

[BAS]

I agree with you Ralph 100% i have never had a problem with the single bolt prop style .The 6 bolt is a big pain in the ---- for sure .

[BAS]

By the way Ralph a friend of mine Roger from Des Moines said he would like to send a 445 in for ign update and i believe he talked to you a few days ago. I am encouraging him to do that for sure. Have a good day Ralph

[dick hanson]

C'mon now --- of course a center bolt will work
Now look at the application:
1. carbon fibre props little if any compression of the prop hub
2. drive pins -(bolts ) -once loose they sheared- (why not use countersunk hardened drive pins ala industrial drive pins?)
3. the prop overtightened due to reverse rotation
Having seen spinners collapse inward from this last problem (at the price of present spinners - I dunno---------
A little more history:
Back when we played with alot of fourcycle glo setups - the real problem (still on some ) was backfiring -loosening the props
OS Engines first added small drive pins - On wooden props - these could and DID split props throwing wood shrapnel everywhere I got a leg cut from a friend's engine which threw a blade a low rpm. The fast reversal on some engines pulled the spinner bolt in - ruining spinners and sometimes collapsing the lightened backplates into the front of fibreglass fuselages - ripping them.
The commercial gas engine guys looked at all of this and decided that a multi bolt -drilled right - addressed the problem in the best manner for the model public - all applications considered
You can argue this all day long but nevertheless that's how the world at large decided to handle it
The application of any setup is the key -----------
In the case of the racers - a glass prop - a single large bolt and PROPER drive pins - looks good to me too.
Having seen how the general run of modelers drills things and adds pins etc., I will stick with the 6 bolt hub for glass and wood props .

[airbusmech]

I just want to throw something in here.Why is it that manufacters don't heli coil there Aluminum hubs.Tourqing a steel bolt into aluminum is a recipe for failure always.Aluminum holes always will let the tension off the steel bolts if they were ever left slightly loose.Then you are done no matter how tight you make them they will have to be re-tightened.When working on full size aircraft the bolts are installed in the same manner then we lockwire them and were done till it's time to change the prop again.One of the things to always check is to make sure there is no gap between the backplate and prop hub face and that you have the correct grip length of bolt for the prop you are using.I think engine and prop companys should give out proper bolt info for prop engine combos.

Just my two cents.

[dick hanson]

you are not wrong-
One point I try to make to guys using the multi bolt-is that all bolts should drop easily thru entire stack and the stack should easily fit flush
The hubs on most model engines appears to be 6061T6 -or similar grade --If it could be harder ( also harder to machine , unfortunately) the whole thing would be better . Simply upping the size of the bolts is also a good idea . 4mm works -but I sure prefer 5mm-

[jamiem]

So if the bolts are only there for clamping pressure, would it not be better to drill the holes slightly over-sized to eliminate any forces other than tension?

[dick hanson]

I always do
The mfgs setup a drill jig which is a slide fit - and if all is perfect - just fine
Frankly I don't see the need for doing it this way
The problems generated are not worth the "neat" fit - which helps supposedly prevent shifting if th e bolts are loose
for one - on wooden props the compressing of the wood causes the holes to become smaller and make bolt installation worse.
also if a prop is drilled from the front - the rear surface may not be parallel and the bolts will simply not align.
I run a larger size drill thru the prop only (using a drill press of course)after initial jig drilling and this has never caused a problem.
If you are paranoid about a bigger hole -- don't be - next sizes up in a fractional index are just fine for 4mm or 5 mm.
I finish drilling a prop then drop all six bolts thru front washer - prop spinner backplate AND into the jig -(a stack)- if all fall cleanly thru - the prop will easily and quickly screw into place in the hub.

[jamiem]

Glad to hear that. I don't think I have EVER drilled a prop so well that ALL of the bolts will just drop through.At least not without using the next larger size drill bit. I use a jig and drill from the rear of the prop and, after the first hole is drilled, I slide a bolt through the jig and into the hole to keep it all lined up. I still havent had any luck with things lining up properly, so I re-drill the holes with the next larger drill bit. Is this the correct method?

[KrisW]

Now that we are back on Prop bolts again. . . .

Yes, that works fine, Jamie. I pilot two holes completely through the prop, short-drill the other 4, then use the two thru-holes to line up the drill jig on the other side, and drillthe other 4 holes from that direction, then chase the holes with the next 1/64th larger drill bit. They line up square in both front and rear that way.