Stealth Pilot wrote:
On Sat, 22 Nov 2008 00:22:49 -0800 (PST), "
wrote:

To All:


selecting goood propeller wood...

almost any wood has the tensile strength to keep a prop's blades on.

wooden props are bolted to the front of the aircraft with , typically,
6 bolts. those bolts hold the prop on to the prop driver by
compression and it is the friction generated between the prop and
driver that makes it do its stuff.

what I have found is that the arbitrating factor in whether a wood is
suitable as a prop wood is the compression strength of the hub.

on my O-200 powered Tailwind the wood prop is held on by a crush plate
and bolts torqued up to 120 inch pounds. the wood needs to be able to
withstand that compression without deforming or crushing.

also wood, no matter how well finsihed, will absorb some moisture in
wet weather and swell slightly, and conversely in dry weather it will
lose some moisture and shrink slightly.

the way to maintain the compression without having to continually
retorque the bolts is to use a sufficient stack of the conical
tempered spring washers called Belleville Washers. these have a known
compressive force and you stack them together to get the compression
needed. my prop uses 8 washers each bolt with 4 washers facing cone
up and 4 washers facing cone down toward each other to generate the
compression needed.
with these I need retorque my bolts only annually.

ol' snake unda the verandah.

(the belleville's are stacked like this...
^
^
^
^
V
V
V
V
with a bolt through the middle)

I know that this is going to inspire more controversy than a political
statement, but here goes:

All that stuff above is absolutely correct, *except* the opening
premise. Notice those big cylinders that all the bolts go through in the
flange? The ones that extend into enlarged holes in the wood hub of the
prop? They are called 'drive lugs'. They are called drive lugs because
they are what drives the prop.

If the prop bolts get loose, the same thing happens to a prop that
happens to the wheels on your car if you let the nuts get loose. 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.

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. 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? Centering
could be accomplished with a raised center ring on the flange.

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. 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.

Charlie