On Aug 22, 4:21 pm, "Cy Galley" wrote:
The big reason for the "drive lugs" is to remove the nuts from the airstream
in back of the prop and reduce the drag the produce. If they are the only
drive mechanism, the recessed holes will quickly be beat out. If they did
do the job, you could have one center bolt. Many props such as used on A-65
don't even have drive lugs but use bolts straight thru.

--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A 46 Year Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot

"Morgans" wrote in message

...





So you are correct that friction is used in tranferring torque
in
wooden props. They also use counterbored drive bushings to transfer
the torque. I'd be willing to bet that the bushings transfer most of
it, but that is only a guess.

Thanks.
--
Jim in NC

I spent some time talking today at lunch with some friends who are
structural engineers, about this issue of friction delivering torque
to the prop. They said that if the strength of the attachment of the
prop to the flange had been determined through experience, then
eliminating the friction load path for the engine torque to be
tranmitted to the prop could result in failure of the junction of the
flange and prop, but that it wasn't likely.

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- Hide quoted text -

- Show quoted text -

I talked some more to my structures friends. They said that wood
reacts to cyclic stress in much the same manner as metal. It is also
an elastic material (unless overloaded) and it also will fatigue more
quickly when cycled back and forth from tension to compression than it
will from repeated tension or compression alone. So the same
preloading to improve the fatigue life applies to wood as it does to
metal. Lower preload results in a lower fatigue life.
So the "drive" lugs are really low drag lugs? I doubt it very
much. I've read alot about airplanes, and this is the first I've heard
on that one. The most highly stressed part on the whole airplane is
the propellar attachment. Any design utilized here must take care of
that issue first and foremost. The drive lugs are better than plain
nuts and bolts for at least two reasons. First, counterboring the back
of the wooden prop hub to allow the insertion of the lugs results in a
larger diameter hole in the wood, therefore a larger bearing surface
which improves the load capacity. Second, it utilizes the fact that
for elastic materials, the bearing strength is usually about 3 times
as much as the shear strength, and the drive lugs use the bearing
strength and any friction between the crankshaft flange and the back
face of the prop will test the shear strength of the wood. A
crankshaft flange 6" in diameter will have a little more than 28 sq in
of contact area. For a prop this size, the hub should be at least 4 in
thick. with a 3/4" dia drive lug, this will give you about 18 to 20 sq
in of bearing area. Given that the only place where friction will be
great due to bolt tension will be close to the bolts, as the slight
warping of both flange and wood, comparing the two scenarios easily
shows that using the bearing strength of the wood with the drive
bushings is far stronger, and much more reliable.
As to some props such as C-65 cont having no drive lugs, this is
not surprising on old, low powered engines. I have installed a prop on
an old Cub, and one of the things that you had to be careful about,
was making sure the bolts had a slight drive fit to them through the
wood. This is really the same setup as as bushings, just that the
bolts bear against the wood instead of the bushings. More modern
designs with higher horsepower (Katana, etc) use drive bushings to
make the holes larger and and a more precise fit. All higher
horsepower modern props use drive bushings. There is a reason for
that. It is because it is a better way of doing a very critical joint.
One of the people I talked to is a Boeing Technical Fellow, the
highest level an engineer can achieve there. He said that in all his
years, he has only heard of one instance where friction was used to
qualify a structural joint. It was for some trivial thing like an
adjustment slot for a secondary structure part (like the slot for
adjusting the belt tension on some alternators).
I hope that homebuilders that read RAH seeking help will take
Fortunat1's advice from the building manuals he quoted, and realize
that thinking that you can rely on friction to hold the wing joint
together may get you killed. This is even more true of propellars,
where the cyclic loads are severe and constant.

Regards,
Bud