Prop torque
I have a three-blade ELIPPSE prop made by Craig on my Lancair; it is fiberglass over wood. I typically torque my prop bolts to 25 lb-ft. BUT, I use Belleville spring washers under the bolt heads to maintain the clamping pressure, since the prop is driven by friction, not the so-called drive lugs. Klaus has cautioned me about too much torque, since it can crush the wood fibers. Those washers are the best thing you can do for your prop, especially if you live in an area subject to temperature and/or moisture extremes. Desert climates with high heat, low moisture days can dry out the moisture in the blades, so that when the day cools, the prop has shrunken slightly. Shrunken prop, less clamping. That is where the washers shine. They will maintain the clamping force over as much as .04", depending on washer stacking. Vance Jaqua before his death wrote an informative article about his analysis of their use which I've copied below. Sorry, the graphics never came through:
Paul Lipps (an innovative prop designer referenced in a previous issue
of Contact Magazine) contacted me after reading the treatise on bolt
preload. He indicated that he was employing an assembly of spring
washers for installation of his composite over wood propeller He
solicited my comments on this approach, and suggested that this would
be a good basis for some analysis and an informative article. Since I
had previously of some reference to this practice, I agreed with him on
both counts, and started searching out reference materials
It is always a source of amazement for me, how things which are very
simple on the surface, have a very complex nature when examined
closely. Propeller bolts are just such an enigma. For a metal prop,
things are virtually that simple. Just follow the usual bolt practice,
and torque them up just short of yield, and the just keep them from
backing out. For a wood (or primarily wood core composite) propeller,
the "wicket" gets stickier than the proverbial tar baby. The crushing
strength, and "crushing" modulus of most woods used are relatively
modest, and the usual bolt torque tension loads would severely damage
the wood structure. This is further aggravated by the dimensional
changes of the wood as moisture is absorbed and released.
PROPELLER LOADS
Just how tightly do we have to secure a propeller? What are the forces
and loads that are trying to take the prop off your airplane? The first
thing you think of is the thrust forces that are pulling (or pushing)
the plane around the sky. These loads are the least of our problems.
Another, more troubling load is the gyroscopic precession as the plane
direction is changed by pitch and yaw. With the lighter weight wood
props, this is seldom a serious problem. However a big metal constant
speed prop can react a load of as much as 600 ft lbs with a yaw rate of
one radian per second. This can be a serious problem if one engages in
violent aerobatics. The fabled Lomchavok uses this precession force to
turn the stalled airplane end over end (and broken crankshafts have
been know to occur).
The big need is for the clamping force, which acts much like the
clutch disk in a manual transmission automobile. Although the classic
prop hub has the drive lugs, the primary drive force is still this
"clutch" action. If it were not for this friction, the prop would
cyclically slip back and forth in the hub. The situation is further
aggravated by the large displacement four cylinder engines typically
used in aircraft. With only two power pulses per turn the peak torque
values are higher than the rated steady values, and are actually
cyclically reversed twice each turn. Once the shrinkage has reduced the
preload, the cycling can induce alternating slippage at the flange
face. The resulting heat further dries the wood, and a totally charred
prop hub can result.
I have personally seen the result of just such a scenario. The
Continental IO-240 has a small prop flange designed for metal
propellers, aggravating this situation. On a flight to a local fly-in,
engine roughness was noted as the destination approached. An expedited
landing was initiated without problems, but as the engine was cut the
propeller looseness could be visually seen. The hub of the wood prop
was charred, and delaminated. A replacement was borrowed for the trip
home, and the damaged prop now hangs on the wall as a visual reminder.
This application normally employs a 4-inch prop extension, and an
extension transitioning from the Continental hub to an S.A.E. number 2
flange was mandated for all subsequent installations.
The preload on a wood propeller must be moderated to avoid a crushing
failure of the wood. The crushing strength of wood varies with species
and density, ranging from about 1700 psi for maple down to about 840
psi for spruce. The rather aptly named "crush plate" for most props has
about 18 square inches in bearing. Most of the wood varieties selected
for propellers are on the high end of this range. Staying a bit below
the high end at a target value of 1000psi, this would equate to a total
clamping force about just under 18,000 lbs, or about 3000 pounds force
for each of the six bolts. As stated in a previous article on
preloading bolts, and as a general truism, determining preload on a
bolt using a torque wrench is a very inexact measure. You might at
first think that this is a relatively simple treatment of the analogy
to driving a force up the inclined ramp representing the pitch of the
thread. Sorry! No cigar. The component of the effective ramp angle is
so obscured by the other friction forces, that it is totally ignored in
the usual prediction, As most of you are aware, the coefficient of
friction varies widely with surface finish, and degree of lubrication,
as well as the properties of the two materials in rubbing contact. The
usual assumption in this case is smooth steel to steel, lightly
lubricated. Lightly lubricated generally means that you wiped off any
visible liquid, but did not clean with any degreaser, which is about
what you would do to avoid rust. Torqueing a bolt involves at least two
surfaces turning against friction. The thread , of course, and the
washer face of the bolt. The thread friction has a multiplier because
of the vee angle of the thread, which is a much larger driver than the
lead angle of the thread. Lumping all these forces and coefficients
together for a 0.3 to 0.4 at the radius "arm" at the washer face of the
bolt gives us a pretty good WAG estimate. The attached table of
suggested torque value for the different classes of bolts in automotive
use, is probably targeting about 75 percent of the allowable yield
strength in the thread roots. These would also be typical of the values
used for metal props, but would vigorously crush a wood prop.
con't next submission.
But do check it. These props have a lot of good features, but a down side is the torque must be checked more often than just at the annual condition inspection. 
