Charlie wrote:

... They are called 'drive lugs'. They are called
drive lugs because they are what drives the prop.

With metal props, you're absolutely correct. However, with wooden
props, it is in fact the friction of the face of the prop hub on the
mounting flange that drives the propeller. The wood is not strong
enough to withstand the torque pulses (especially in the larger
engines) if only the small area in the drive lug holes was taking the
force.

Sensenich, Catto, Hertzler, and all wooden prop manufacturers are
clear on this point, and talk about just how much compressive pressure
is required to assure safe operation. Sensenich claims (in a paper
downloadable on their website) that it's about 600 psi. If the drive
lugs were doing the torque transmission, then the pressure would be
meaningless, and all you'd need is a good fit on the lugs and some
safety wire so the bolts didn't come off.

It starts wobbling, which wallows out the holes the drive lugs run
in, which allows the prop face to move against the face of the
flange, creating the hangar tales about charred wood caused by
inadequate clamping force.

Not hangar tales at all. If the pressure is too low, then a small
amount of relative motion can occur, which will then wallow out the
holes - not from wobbling, but from relative rotational motion.

You can read about my experience with this issue at:

http://www.cozybuilders.org/Desert_Center/

and:

http://www.cozybuilders.org/Oshkosh_Presentations/

I'm currrently using bellevilles (as the other poster mentioned,
although I disagree with some of his setup) on my prop, and have a
number of other canard flyers that are using my general setup on 1/2"
and 3/8" bolt installations.

To analyze this, you can approach it from multiple directions.
Shear strength of wood fiber just isn't enough to handle the kinds
of loads we are talking about.

This is just incorrect, as I've pointed out before - the wooden prop
manufacturers themselves state clearly that it's the friction of the
hub on the mounting flange that drives the prop - NOT the drive lugs.
Again, with a metal prop, you're absolutely correct that it's the
drive lugs.

... Or, look at those drive lugs. If
it's friction preventing prop rotation on the flange, why do they
bother to do the extra machining on the flange, make 6 extra parts
(and weight), and counter-bore the prop bolt holes to accept these
extra parts?

Because the engine flange manufacturer has no idea whether you're
going to use a metal or wooden prop - this way, you can use either
one. If the lugs weren't there, you could ONLY use a wood prop.

The reason for the drive lugs is to provide a machined precision
fit to the counter-bores in the prop, and to provide a larger
'working surface' in the wood to take the load.

Nope. For metal, you're correct. But for wood, it's the friction.
On the wooden props, the conterbores are there because the drive lugs
are there, but not to take the loads.

... You can see the
same thing in the spar attach points of highly stressed wood wings,
where there are metal bushings for the attach bolts.

Wing attach points tend not to take huge torque/force pulses that are
substantially larger than the average load that the bearing surface
needs to withstand. It's not an appropriate analogy.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2008 http://www.mdzeitlin.com/Marc/