On Aug 24, 1:38 pm, Gunny wrote:
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.

Thanks for your comments, and before I put my comments on this
thread to bed, I want to say that the OP asked a question or two that
have been answered very well, and that is what this group is for. Even
I have learned things about working with wood. Anyone reading this
thread looking for info will find the correct way to construct a wing
joint.
As to whether or not the engineers I talked to were aircraft
engineers, most definately they are.
As to some of your comments, I need to clarify some things. If
you are a civil engineer that deals with steel structures, and you
have design and analysis standards that use friction to qualify
structure, then that is your way to do it. I don't recall seeing a
major building , bridge, etc, that wasn't either riveted or welded
together, but I don't know for sure. So I will take your word for it.
I stated in my first post that friction existed and carried load, but
simply that for aerospace structures it is never counted on to carry
load. You only consider friction when it works against you. That I
know is true. In your statements about why using friction in the wood
spar joint is not a good idea, I think you have begun to uncover some
of the reasons why it is true. Since most airframes are thin shell
material, most of these reasons apply just as well to metal as wood.
As to the statement that I clearly don't understand the factors
involved, you clearly do not understand what I said, the nature of
preloaded bolts, or even the S-n curves themselves. Improved fatigue
life due to preloading has nothing to do with friction. Friction may
improve fatigue life in the real world by spreading load over a larger
area, but the benefit of preloading on fatigue life is due primarily
to an effect that exists even if no friction is present at all. Why
you think I need it pointed out that higher stress levels result in
shorter fatigue life is puzzling. Of course the higher the load you
place on a structure, the fewer cycles it will survive before failure.
What is hard to understand about that? What you apparently don't
understand is what constitutes a load cycle, how much is the load, and
what preload does to that. Preloading the bolt reduces the cyclic load
that it sees, since the load in a preloaded bolt only increases about
10% until the applied load exceeds the preload. When the prop bolts
are allowed to lose their preload, the full applied load becomes the
amount of cyclic load that causes fatigue. This is best demonstrated
by giving an example. Take two identical bolts, having a breaking
strength of 5,000 lbs each, and preload one to 2000 lbs, and none to
the other. If we now begin to subject both bolts to the same cyclic
loading of 1500 lbs, where the applied load is increased from 0 up to
1500 and then reduced to zero again, the bolt with the 2000 lb preload
will see a cyclic load of only about 150 lbs, whereas the un-preloaded
bolt will see a cyclic load of 1500 lbs, and will obviously fail much
sooner. Same bolts, same loads. The meaning of this is that if you
keep the prop bolts properly preloaded or torqued as it is, then BOTH
the bolts and the prop hub see a much smaller cyclic fatigue load than
if you allow them to become loose, thereby greatly increasing the
cyclic load that they see, and increasing likelyhood of failure.
As for S-n curves, there are more than one type. The one
relating to what I am talking about are the ones that show S vs N for
different stress ratios. The stress ratio is the fraction equivalent
of the maximum to minimum load. For example, something that is loaded
in tension to 25000 psi, followed by being loaded in compression to
25000 psi back and forth, will have a ratio of -1.0 ( +25000 tension/
-25000 compression). Something loaded to 25000 psi tension that is
reduced to 10000 psi tension and back and forth will have a stress
ratio of .4 (10000 tension/ 25000 tension). The S-n curves show that
the amount of cyclic load that structure loaded with a ratio of -1
will fail far sooner than one with a ratio of .4, even though the
maximum stress level is the same. You can look in Mil-Hnbk-5 or
elsewhere for S-n curves to verify that.
The best book to explain all this is "Mechanical Engineering
Design" by Joseph Edward Shigley, Professor at the University of
Michigan, chapter 8, "Design of Screws, Fasteners, and Connections".
It is THE most widely used text on the subject in the top engineering
schools of the country, and has been for many years.

Regards,
Bud
M.S. Aerospace Engineering