Fluidity -- I understand - also I understand "fluid" contests -
and if you read the unedited comments preceeding -you can see where the fluid got out of hand .
but hey - I am still nice and dry -
The same types of arguments were common for me when working on accident litigation.
nudge nudge wink wink -know wot I mean?

Dick, you are neither the Grand Poobah, nor the moral conscience of Modelling. I suggest you learn to stay out of threads that do not concern you. In this case you basically ruined a perfectly good discussion with your misguided attempts at "correcting" something that was already correct. In this case YOU were wrong.

No one appreciates it when you do that. The "hint hint, nudge wink" is aimed squarely at YOU. I suggest you think about it. A lot of people know as much about these thing, and probably a lot more, than you do. They deserve their say as well, without you constantly trying to correct them over nuance or because your opinion is more important to you.

I shall make it a point not to rain on your parades

It's not a parade, Dick.

But, back to the subject at hand, the "test prop" for this little idea will be two 3-bladed mejzlik props bolted in tandem to this adapter hub. Any slippage at either the hub or at the joint between the two props, will be problematic at best, and might cause severe damage to the props or engine. Therefore, this 6-bolt hub idea is especially helpful in proving, once and for all, whether it is the compression of the props from the bolts, and associated friction against the hub, or if the bolts are actually the last word in what prevents the propellors from spinning on the output shaft.

Please study the photos closely. There is no reinforcing CF or fiberglass at the hubs joint, only a thin layer of thick CA between the two props to hold them in alignment until the bolts and center stud are fully tight. After that, it's up to the bolts to prevent the props from getting out of phase and remain properly centered.

I couldnt bear to stay out of this anymore, Dick is getting flamed, and it aint right.

1. Kris, you're wrong. 6 bolt pattern is for clamping, period. As mentioned previously you cant get enough clamping force with one large nut with large threads. 6 tight thread screws make it easy to accomplish a lot of force. Therefore, when the 6 bolts are tightened securely, it does not slip - due to the clamping. Ever notice what happens to a single bolt if it gets loose? It breaks, thats what. Why? because its not designed to handle side loaded stress, its designed to clamp - period.

2. CAP was talking to BOTH of you. You rain on parades yourself Kris. Once again, you assumed, and were wrong, I for one appreciate everything Dick has posted on this thread.

3. If I were a surgeon looking at that dual prop setup, I'd be thinking about the new boat I was going to buy when you showed up in the emergency room. Please dont attempt to run that thing in public, no sense in hurting anyone but yourself.

Hmmmm..I guess the single 3/8-24 grade 8 bolt on my 32 hp 289 race engine should have broken instead of just holding the prop on when the two 1/4-20 thru bolts sheared.The bolt goes all the way thru the hub into the steel crank.It CLAMPS the prop on the hub, the 1/4-20s keep it from slipping..You can put a lotta torque on a 3/8 bolt....When the carbon prop compressed they ALL got loose, the single bolt still held the prop on..The data logger on the engine showed that it was turning a 27 pitch two blade prop 9000 rpm at the time, a pretty good test.........A grade 8 bolt has a minimum tensile strength of 150,000 lbs...It would be hard to even break one with a sledge hammer, much less shear it off....When a single bolt without anti rotation bolts or pins comes loose the prop just comes off.....Only single bolt I ever saw break was a grade 5 I had tapped for a 10-32 spinner bolt, the head came off where it was thinner under the hex....Never used a grade 5 again for a prop bolt.....[8D
FWIW...At our races we run G62s, GT80s, Aerrow 200s, Herbrandson 289s, and a few others..Not a 6 bolt hub in the bunch, 64 planes at the last race...We don't want all those extra holes in the props...the GT80s are turning 11,500 rpm, the Aerrow 200s about 10,500 and making 180 degree turns at 200+ mph....You think a 20 hp Giant Scale engine turning 6000 rpm puts more stress on a prop bolt/s than that ? Hardly ..[8D]

OK...this is all I gotta say, and I'm done.

Ralph, those 1/4x20's did a great job of stopping rotation right up untill they sheared! I think you are proving Dicks point. Nevertheless you are comparing apples and oranges, a prop/hub design for High RPM racers vs the std garden variety that turns at 6k. Also, in which direction was that grade 8 bolt tested for that 150,000lbs of tensile strength? I'm betting I could easily break one with a sledge hammer from the side, but if I pound from the top I'll not dent it. Bolts are not made to withstand shear forces, they are made to withstand clamping pressure - period <dot>
The whole point is the reason for the 6 bolt was safety. The average Joe that clamps a prop on a 6 bolt hub is going to be more successful in achieving enough torque to prevent rotation than with a single bolt hub and a torque wrench. Just go look at all the "I sheared my prop bolts" threads and se how successful it has been.... lol

I wonder where the shear force comes from when the bolt is in the center of the prop...Our race plane hit the ground at about 200 mph when the spinner came off and hit the prop, breaking off one blade....The engine was completely destroyed, both cylinders torn off, the crankcase demolished, one carb broken completely in half, both pistons broken and the con rods bent back at a 45 degree angle..Only useable parts left were the wrist pins and bearings and one carb...The 3/8 bolt in the center of the hub was still intact and tight....FWIW..There was no mention of a single bolt being sheared off in the "bolts sheared" posts here on RCU...Mostly 6 bolt 3W and ZDZ engines....
The "average joe" would do well to check his 6 bolt prop hubs more often....Or leave 'em loose, I get $20 to fix the 3W sheared bolt hubs

All right, Rat. .YOU explain what stops a prop from spinning on a perfectly smooth prop hub. It's certainly NOT friction. And it's not the bite of a knurled surface either Ever wonder why lawn mower blades are pinned? Ever wonder why automotive crankshafts use really BIG bolts on flywheels. It's sure not fiction keeping them from spinning. Oh, and how about where the torque convertor bolts to the flywheel? Friction? Nope, bolts. One last thing. .How do you keep a BME from spinning the prop? Simple .. anti-rotation PINS, which are nothing more than 6-32 threaded bolts.

The last thing you want keeping a driven surface from spinning on the output of an engine is a "clamped" surface. That's an accident waiting to happen. What you want is a firm, mechanical joint where the two pieces lock together, be it with interlocking teeth or splines, thru-bolts, or hardened pins, or even a keyway in some applications. Relying on clamping force alone is dangerous, which is why critical applications always use a mechanically locked joint that has zero chance of slipping. I guess that's why high horsepower aircraft engines have splined shafts, as well as bolts.

And, as RCIGN has pointe dout, very obviously, a single-bolt will alwasyhold the prop in place, but having anti-rotation elements such as hard bolts or pins, is the only way to make SURE it does not move on the hub.

If it's good enough for RCIGN (and all the speed maniacs otu there), with all his experience, I'd say it's good enough for just about anyone. (hard to believe he and I are agreeing for a change. )

I agree with Kris on this one - I hate it when that happens! :^) That last example using splined drive shafts best illustrates the point - while it's a little like comparing apples to oranges, in higher torque applications, a pure clamping/friction interface is riskier than using a mechanical locking device in conjunction with a torqued bolt providing clamping force. I'm an aircraft mech myself - In full size aviation, big HP/torque applications indeed use a spline drive which provides anti rotation and engagement, while a big "jeezus" nut clamps it together. While the two gas engines I have (60cc) use a single bolt arrangement, in the big 100cc plus applications I'd sure want both the 6 circumferential bolt arrangement and a center bolt too! Very nice setup Ralph/RCign manufactured for Kris. That is the sort of thing IMO that should come stock on them big iron engines. I'd hate to be a manufacturer in these litigious times - most everything can fail if improperly assembled or used. A big prop departing amongst bystanders would be the worst kind of "PR". Look at the fallout from that recent helicopter training incident...

Just for kicks I put a 3/8 bolt in my vise with the threads flush with the top of the jaws..I beat on it with a 4 lb sledge from the side. It broke when it was bent over 45 degrees and the weak point at the threads fractured..Then I put another in the vise and used a 1 inch steel block against it flush, to try to shear it off. The bolt was dented but would not even bend...You can't shear off a 3/8 bolt with a prop, be it wood or carbon, because there is no shear force against the bolt, just tension..The shear is on the other bolts, if the center bolt and all the others get loose...Even then the prop usually stays on the engine....
.

according toa few calculations I just did, the TENSILE strength of a 3/8" Grade 8 (180,000 psi or better) before it is supposed to break is approximately 19,880 pounds, or TEN TONS!!!

Using a 1mm fracture line on that same 3/8 bolt would equate to 1/10 of the tensile strength (3/8 versus 1mm is about 10:1 ratio), still requiring a TON of instantaneous sheer force. Seems to me that any wooden or composite propellor is going to disintegrate long before that bolt shears, unless (as in the case of 6-bolt hubs) the bolt loosens and then is subjected to cyclical bending loads for aperiod of time, which will weaken and stretch the metal to the point of catastrophic failure in short order.

This brings up at least three germaine points.. First, the prop must never slide or move, and retention by mechanical coupling, rather than friction, is the most desired means of achieving this.. Second, the bolts must be tight enough to never be moved sideways by the propellor, which in turn will ensure that the propellor never moves, and they must be strong enough to support the sideways loading of the prop so that even if something comes loose they will stop rotation of the propellor. Thus the first and second points reinforce and support eachother. Third, a non-shear loaded main clamping bolt is a necessity to ensure against losing the prop, and to act as additional clamping force, which will further reduce the chance of prop slippage or cyclical back and forth movement of the prop (which will overstress the 6-bolt pattern leading to fatigue and failure of the bolts)

I guess that pretty much sums it up .

OK, you're right.

All of my engines over 50 cc have as a factory setup ---
a large central, flanged prop locating, shaft (10mm) with locking nuts, drilled for 5mm spinner attaching bolt.
Six, clamping bolts - in 4mm and in 5mm dia. , sizes
Kept tight - is a very good setup.
Honestly -I can't think of a better one.
This sounds a lot like the setup just described.

Kris,
you can have my LSAT books when I'm done

The thought of becoming a Lawyer is intriguing, but repugnant, Dave, Thanks anyway

LMAO

Sorry but I had to laugh.