Shear Pins for Propellors?

The shear pin would fail at idle. The propeller on an airplane
is the engine's flywheel, and its inertia carries the crankshaft
between power strokes and smoothes its rotation. There's a
reciprocating force between the prop and crank flange, therefore, and
ANY looseness between the two will result in forces that will fail
attaching hardware. There have been cases of six 7/16" prop bolts
breaking when they weren't tight enough. There have been cases of
charring of wooden props when they weren't tight enough; the friction
created by small amounts of movement between the prop and flange heats
the wood. Props use six or eight substantial bolts for a reason: to
keep the thing on there tight. A shear pin in the system requires
looseness in the drive, and it just won't work. This isn't a boat ot
tractor. Imagine the damage when the flywheel of an outboard motor,
auto or tractor engine is a little loose.
Further, a sudden stop of the prop also pulls the crank radially
as the weight of the prop at its free end tries to keep moving. The
crank gets bent anyway, or the case fails, or engine mounts are
damaged. Any forward movement bends the prop backwards, too.

Dan

wrote:

The shear pin would fail at idle.

How about this one. The prop would have a clutch plate on the front with a hole
in the center of both prop and plate large enough to fit over the end of the
crank. The prop/clutch assembly would slip over the end of the shaft and another
clutch plate assembly would bolt onto the end of the shaft. The latter would
have pivoting weights to increase pressure on the clutch as rpm rises (much like
some older CS props have to adjust blade angles). When the engine is producing
little power (as it would during landing), the prop would only be loosely
connected to the engine. Add power, and the clutch engages more and more
completely, both from the pressure added by the weight mechanism and by the
thrust pushing the prop forward.

This system would also allow the prop to windmill without attempting to turn the
engine in cases of engine failure.

You'd have to thread the inside of the crankshaft end and perhaps have to
rethink the current method of driving CS props with engine oil, but I doubt that
the problems are unsolvable.

George Patterson
Whosoever bloweth not his own horn, the same shall remain unblown.

This system would also allow the prop to windmill without attempting
to turn the
engine in cases of engine failure.

So what do we do for a flywheel?

Dan

On Fri, 1 Apr 2005 at 02:32:55 in message
.net, Rich Lemert
wrote:
This morning, while driving past the airport on my way to work, the
thought occured to me that a shear pin could be used to protect
airplane engines (at least partially) from prop strikes. My
understanding is that the props on light singles (at least) are
connected directly to the engine's drive shaft

It occurs to me that a shear pin could only work effectively against a
rotational stoppage. Since a normal prop either fits over a crankshaft
end or is bolted to a plate, a shear pin could do little or nothing.
What loading cases would you propose for the shear pin failure?

In a minor accident which way are the prop tips usual bent?

I somehow feel that the possibility of a prop detaching in flight should
not be encouraged. :-)

--
David CL Francis