On Aug 22, 2:55 am, "Morgans" wrote:

I'll bet that you structural engineer friends are not experienced with wood
props, and their failure modes. It seems to be their own unique circumstance.
It has been found that the props fail, not the bolts.
--
Jim in NC

Jim,

I share your skepticism. As a structural engineer, I am also curious
about the statement that friction is only considered when it works
against you. It's not clear to me whether these were aircraft
structural engineers or otherwise, so I'll have to give Bud the
benefit of the doubt. However, Chris Heintz, designer of the Zenith
aircraft, that has stated that the reason fatigue isn't much of a
problem for the rivets in the aircraft skin is because the friction
between the joined surfaces typically carries the cyclic loads from
engine vibration (See "Riveted Joints", Chris Heintz, P.E.). I won't
speak to use in aircraft, but in general construction friction is
often considered a working part of the structure.

In fact, there are many instances in steel structures where service
loads are transmitted purely by static friction - moment connections,
end restraints for slender columns, connections with slotted/oversized
holes to facilitate assembly. Bearing/shear of the bolts is obviously
checked, but day-to-day the loads in those structures are transmitted
via static friction between the members. By design. AISC references
these as slip-critical connections. HSFG (High Strength Friction Grip)
is another term. Due to construction methods and tolerances, those
connections may only have one bolt out of the whole group that is
technically "bearing", maybe none. My point is that friction as a
mechanism for transferring loads to a wood prop is not really all that
unique or unusual as an engineering concept.

To address an earlier part of the thread, however, I wouldn't count on
friction for a wood-spar attach fitting. The fittings are often made
from thin material. Out-of-plane bending prevents the fitting from
developing much friction away from the bolt holes. And you have
humidity changes constantly modifying your wood dimensions. Tried-and-
true phenolic bushings, match drilled and reamed to the fittings, cost
about a dollar per hole. In the plane I'm building, that is less than
$50, so it was an easy choice to make.

Another statement that doesn't sit well was the reasoning that a pre-
tensioned bolt has better fatigue characteristics because metal
fatigues less when the stress cycle is all in tension as compared to
stress reversal. This is a clear misunderstanding of the factors in
play. Study the S-N diagrams of these materials and you will see that
increasing the mean stress decreases the fatigue life for a given
stress cycle amplitude. The reason some pre-tensioned bolted
connections (esp. shear) have better fatigue characteristics is
because the cyclic portion of the load is transfered via friction. The
bolt actually experiences a drastically reduced or eliminated cyclic
stress, thereby extending it's fatigue life even though the mean
stress of the bolt is much higher. Tension connections see improvement
through a different mechanism, but the result is the same - reduced
cyclic stress in the bolt and increased fatigue life.

Matt, P.E.