Sunday, November 18, 2007

Prop Strikes and Crank Failure

The GO-435 crank-breakage incident that I described before may or may not have been related to a prop strike that the plane in question was known to have had (indeed, the Safety Board found it was not), but it makes you think. This is one of those cases where you can't necessarily detect prop strike damage by "dialing out the flange." The engine had prop reduction gearing. On a GO- or GTSIO-series engine, it's unlikely you're ever going to bend the crankshaft by striking something with the propeller.

But crank bending is not the only type of damage that occurs in prop strikes. It's true that in low-rpm incidents, bending is the most likely kind of damage. But in high-rpm/high-power incidents, you have torsional overstress to worry about. (If the crank has counterweights, you also have to worry about counterweight-slamming.) If you've ever been to an engine shop and seen broken cranks from prop-strike engines, you know that the break is seldom at right angles through the crank. More often, the crack opens up at a diagonal angle and the crank almost seems to "unwind" in a helical fashion as the crack propagates. (This makes sense, since most aircraft cranks are manufactured as twisted forgings.) The crack is usually somewhere between the oil slinger and the first conrod throw.

You have to remember that the crankshaft in a Continental or Lycoming engine is nitride-hardened, which makes the surface glass-hard (and glasslike in brittleness). If a prop strike opens up a crank in the nitride layer, it will almost certainly propagate to failure. Maybe not right away, but eventually. Obviously, you can't detect cracks in the nitride layer by dialing the prop flange. A damaged crank can be perfectly straight.

When a prop strike happens, all the attention seems to go to the crank. But again, there are other worries, such as crankcase cracking. A cast-aluminum crankcase is nowhere near as strong as a steel crankshaft, obviously. Yet it's expected to hold the crank in place even as the crank is coming to a sudden stop. Newton's third law applies.

You do remember Newton's third law?

1 comment:

Alec Myers said...

The reason for a spiral fracture is nothing to do with twisted forgings.

A torsional stress resolves to two 'compressive' stresses each running at 90 degrees to each other along oppositely directed helical paths through the solid. One is a compressive stress, and the other is a tensile stress.

Most solids are strong in compression and fail in tension, and a crankshaft is no exception. The material fails because the tensile stress exceeds the limit, and the resulting fracture surface is at right angles to that tension - hence helical.

You will get the same pattern if you twist to failure any rod-shaped homogeneous solid - a pretzel stick, a stick of blackboard chalk, a cheese-whatsit corn snack, a femur (your own or someone else's) or a crankshaft. Try it - preferably not with the femur.

I strongly recommend a pair of books by J.E. Gordon called "Structures: or why things don't fall down" and "The new science of strong materials, or why you don't fall through the floor" for more information.