Shear Pins for Propellors?

The propellors on outboard motors for boats have (or at least always
used to have) shear pins.

The argument against using them in airplanes on safety grounds seems
pretty compelling, however. Boats keep floating even if the shear pin
shears . . .

In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power (maybe not the most likely scenario but quite possible).
The prop makes contact with something and the pin shears. At that very instant, the engine is set to high power but now has
no load on it. With no load, the engine will speed up a lot - way past redline. A likely outcome would be pistons,
connecting rods, and parts of cylinders/heads flying out in all directions, possibly followed by fire - considerably more
damage than from a normal prop strike!

Eric Law

"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

In article ,
EL wrote:
In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power
(maybe not the most likely scenario but quite possible).
The prop makes contact with something and the pin shears. At that very
instant, the engine is set to high power but now has
no load on it. With no load, the engine will speed up a lot - way past
redline. A likely outcome would be pistons,
connecting rods, and parts of cylinders/heads flying out in all
directions, possibly followed by fire - considerably more
damage than from a normal prop strike!


The engine's ECU should take care of that. (grin


--
Eduardo K. |
| "World domination, now"
http://e.nn.cl | Linus Torvalds

Heh heh...

I once had a car where the rev-limit function was built-in to the fuel pump relay and it had a design quirk. Once the RPM
limit was exceeded, the fuel would be cut off but only after a short time delay. So the effective limit was 6100 plus
however many RPM you could get in about 1/2 second!

Eric Law


"Eduardo K." wrote in message ...
In article ,
EL wrote:
In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power
(maybe not the most likely scenario but quite possible).
The prop makes contact with something and the pin shears. At that very
instant, the engine is set to high power but now has
no load on it. With no load, the engine will speed up a lot - way past
redline. A likely outcome would be pistons,
connecting rods, and parts of cylinders/heads flying out in all
directions, possibly followed by fire - considerably more
damage than from a normal prop strike!


The engine's ECU should take care of that. (grin


--
Eduardo K. |
| "World domination, now"
http://e.nn.cl | Linus Torvalds

Dale wrote:
In article .net,
Rich Lemert wrote:


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?



I think the best protection for the engine would be to not land gearup,
taxi into super soft stuff, or do any of the other things that cause
prop strikes. G


Agreed, but then again I plan on asking for some spin training when I
get back into lessons again.

Rich Lemert

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.

EL wrote:

In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power (maybe not the most likely scenario but quite possible).
The prop makes contact with something and the pin shears. At that very instant, the engine is set to high power but now has
no load on it. With no load, the engine will speed up a lot - way past redline. A likely outcome would be pistons,
connecting rods, and parts of cylinders/heads flying out in all directions, possibly followed by fire - considerably more
damage than from a normal prop strike!


This is something that I hadn't considered (and I suspect a lot of
others haven't, either). In fact, I think this answer is more compelling
than the primary answer I'm getting of "unnecessary potential failure
point", especially since you can probably design the MTBF for a shear
pin system to be at least as long as that of the engine itself, whereas
I don't see there being a simple way to implement an over-speed
prevention system.

Rich Lemert

Rich Lemert wrote:

EL wrote:

In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power
(maybe not the most likely scenario but quite possible). The prop
makes contact with something and the pin shears. At that very
instant, the engine is set to high power but now has no load on it.
With no load, the engine will speed up a lot - way past redline. A
likely outcome would be pistons, connecting rods, and parts of
cylinders/heads flying out in all directions, possibly followed by
fire - considerably more damage than from a normal prop strike!


This is something that I hadn't considered (and I suspect a lot of
others haven't, either). In fact, I think this answer is more compelling
than the primary answer I'm getting of "unnecessary potential failure
point", especially since you can probably design the MTBF for a shear
pin system to be at least as long as that of the engine itself, whereas
I don't see there being a simple way to implement an over-speed
prevention system.

Electronic rev limiters such as cars have had for years now... But that
is too modern a technology to be suitable in an airplane engine. :-)

Matt

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