Shear Pins for Propellors?

"Rich Lemert" wrote in message
link.net...
Back when I was growing up on the farm, we used several powered
implements that received their power from the tractor through a power
take-off assembly. Somewhere on the device's power train, between the
PTO take-off and the "business end" of the apparatus, there was always
either a belt-drive or a coupling fitted with a "shear" pin. Both of
these systems were intended to protect that tractor (and the
implement)
by failing if the implement bit off more than it could handle.

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.

Recognizing that this issue is driven as much by regulation as by
anything, I'm wondering if there would be any benefit to using shear
pins in these systems. Would it reduce the need for a complete (or
partial) tear-down after a prop strike? Would there be any benefit for
more complex propeller arrangements?

Any thoughts?

Rich Lemert

Sounds to me like an idea worth pursuing. Could have two shear pins,
with visually checkable integrity during preflight inspection, to
counter the problem of one failing.

John Lowry
Flight Physics

"John T Lowry" wrote in message
k.net...

"Rich Lemert" wrote in message
link.net...
Back when I was growing up on the farm, we used several powered
implements that received their power from the tractor through a power
take-off assembly. Somewhere on the device's power train, between the
PTO take-off and the "business end" of the apparatus, there was always
either a belt-drive or a coupling fitted with a "shear" pin. Both of
these systems were intended to protect that tractor (and the
implement)
by failing if the implement bit off more than it could handle.

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.

Recognizing that this issue is driven as much by regulation as by
anything, I'm wondering if there would be any benefit to using shear
pins in these systems. Would it reduce the need for a complete (or
partial) tear-down after a prop strike? Would there be any benefit for
more complex propeller arrangements?

Any thoughts?

Rich Lemert

Sounds to me like an idea worth pursuing. Could have two shear pins,
with visually checkable integrity during preflight inspection, to
counter the problem of one failing.

John Lowry
Flight Physics


Now you have added two additional points of failure.

Now you have added two additional points of failure.

I'm sure Lycoming would be glad to pursue this, especially in light of their
recent failures in court.

Not.

Too bad, it's got merit.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Dave Stadt wrote:

"John T Lowry" wrote in message
k.net...

Sounds to me like an idea worth pursuing. Could have two shear pins,
with visually checkable integrity during preflight inspection, to
counter the problem of one failing.

John Lowry
Flight Physics



Now you have added two additional points of failure.


Just to play devil's advocate (1): I would think that the pins could
be designed to have a mean time between failures at least as long as
that of the engines themselves.

Just to play devil's advocate (2): Would failure of a shear pin
be qualitatively any different than any other engine failure?

Just to play devil's advocate (3): Logic would suggest that a
propellor shaft shear pin failure would most likely occur during
take-off, since that's when the engine is delivering the most power.
Just how much torque is the propellor applying to the shaft at this
time?

(Disclaimer: I really do understand everyone's concern about adding
potential failure points to the system.)

Rich Lemert

A 300-hp direct-drive engine is producing 583 lb-ft of torque at 2700
rpm.

On Fri, 1 Apr 2005 at 13:06:26 in message
t, John T Lowry
wrote:

Sounds to me like an idea worth pursuing. Could have two shear pins,
with visually checkable integrity during preflight inspection, to
counter the problem of one failing.

Good idea but when both were in place the shear failure load would be
twice as strong?
--
David CL Francis