Prop Strikes

Incidentally, what's a "stress riser?"

A stress riser is any flaw in a structural piece that concentrates
the stresses through that area to the point that failure might occur. A
common example is cutting glass. The "cutter" doesn't cut; its small
roller causes a shallow crack in the glass that will allow you to break
the glass cleanly when it's bent. On a propeller a nick intereferes
with the lines of force in the blade, causing them to have to bend
around the nick and so concentrating them below the damage. Their
concentration can start the propeller cracking. The blade undergoes
huge G forces outward, thrust forces forward, and drag forces
chordwise; a prop is often the most heavily stressed part on the whole
airplane, and I often see chewed-up props on otherwise cared-for
airplanes.
Owners don't understand the risks. A prop that throws a foot or so
of blade is liable to tear the engine out of the mounts before the
pilot can get it stopped, and guess what happens to the CG when about
300 pounds of engine and prop leave a 172? The airplane can't even
glide.
I demonstrate the stress riser phenomenon to my class using
strips of light aluminum flashing. The students try (unsuccessfully) to
tear a piece of the flashing. Then I file a tiny nick in the edge, and
it tears easily. A second piece with a nick dressed out becomes
impossible to tear.

Dan

a prop is often the most heavily stressed part on the whole
airplane,

From an engineering standpoint -

A light plane propeller, whether wood or aluminum, is about 1/8 inch
larger in diameter at cruise than when standing still due to
centrifugal acceleration. Consider also that a prop is an essential
non-redundant monolithic structure, which if aluminum, is made of a
material (2024-T3) that has good tensile, but mediocre fracture
toughness properties. In operation it is subject to very high-cycle
bending fatigue due to torsional resonances.

Fracture toughness is a measure of the crack propagation resistance of
a material. As a fracture toughness example, compare the tensile
strength of cellophane vs shrinkwrap with and without a tiny transverse
slot cut into it.
Props are highly stressed and must not be treated casually.

Consider also that a prop is an e=ADssential
non-redundant monolithic structure, which if aluminum, is ma=ADde of a

material (2024-T3) that has good tensile, but mediocre fract=ADure
toughness properties.

Lightplane props are universally made of 2025T6, according to my
Sensenich prop manual. 2024T3 is used mostly for aircraft skins. The
2025 lacks the manganese and magnesium of the 2024, but has silicon
that the 2024 doesn't have. The yield and tensile strengths of 2025T6
are a bit less than 2024T3, but the metal isn't as hard, likely
reducing crack tendencies somewhat. The copper is still present, making
the prop corrosion-prone, and in Canada, at least, we have to remove
the prop, strip it and inspect it for corrosion at least every five
years. Corrosion pits can be as bad as nicks for starting cracks.
Dan

Lightplane props are universally made of 2025T6,

The drawing I saw was 2024 (& I recalled T3) but it was not a Sensenich
drawing.

I never heard of 2025 before, though since they are a forged blank,
they could create any alloy........ I originally thought they used
2017.