Prop Went Bye Bye

[Motorman]

Had something funny happen yesterday. Was flying a Foxberg CG Buster with Fox 35. I do the final stages of break in in the air and the engine was still a little tight so I had it set to break in and out 2 stroke 4 stroke while flying straight and level. Almost at the end of the tank, half of one of the prop blades just broke off. The vibration foamed the fuel in the tank and the engine quit in 1/4 lap with no other damage. It was an APC 9-6 sport prop in good condition. It had a couple days of sport flying and one contest on it. It was very windy and it let loose going down wind. I have to think it was some weird vibration that did it. Ever had that happen?

MM

[Andrew Hathaway]

No, but I've seen more than a few APC props with faint white stress cracks in the blades after a mishap.  The prop still looks ok unless you look closely.  The stress cracks can be hard to spot and become more visible when flexing the blade.  I can't imagine a Fox 35 stressing an APC prop enough to make it fail, but than I don't really like APC props on Fox 35s.

[Motorman]

I'm very familiar with the white stress lines and mottling on APC's. There was nothing like that showing at preflight and it was a clean break just past the thick part of the blade.

It made me try another prop that turned out to be faster but dang it's also an APC lol.

MM

[Elwyn Aud]

I was afraid I was going to read about a crankshaft failing and the prop taking off on its own. I have heard about someone having a bad prop failure where the engine kept running and the horrible vibration destroyed everything.

[Dick Pacini]

Back in '75, I was practicing for the NATS and my plane had a fat run.  I set it down (trike gear) and yelled to my buddy to rag the prop when it came around.  He did as instructed and tossed a red shop towel into the prop as the plane went by.

Instantly, the entire nose broke off as well as the prop.  It lay there in a screaming shaft run for a few seconds before the OS Max 35 seized up. 

[Geoff Goodworth]

I had a white stress line develop on an APC prop that I was using to run in a Brodak 40. Before using it on the B40, the prop was new, never flown, never crashed.

We spotted the problem before the prop broke but it was the first time I've seen that happen and it spooked me.

[Dick Pacini]

I remember boiling nylon props years ago to anneal them a bit.  I don't know if it made a difference, but it was modeling folklore at the time.

[Motorman]

I use to fly a racing event called Warbird. WW2 planes with Fox Mk7 combat engines and smokin' K&B 5.8s. They had a rule plastic props only.

APC 9-5 sport prop at 115mph blew a blade off. First thing that happened a big chunk of the fuselage where the canopy was broke off and went flying like an ejection seat, funny stuff. Then lots of breaks in weird places. After that I made a fiberglass copy in light gray that matched the APC color perfectly. Shortly after they opened up the prop rules for safety.

I guess when they call it a sport prop they're not kidding.

MM

[Phil Krankowski]

I have a stack of 1/2a top flite white nylon props I won't run...as far as I am concerned they are "rotten" plastic the same way nylon monofiliment fishing line goes rotten.  I tried boiling a few and it did no good.  1 exploded after the engine started running, 3 others broke while propping over.  The blades seemed supple and flexible still, when bent the "easy" way, but when bent the "hard" way they shatter.

I realize this is not the same as a supposedly new glass filled nylon prop that has a good pigment load.

The main thing about boiling nylon is to add moisture into the matrix which imparts more flexibility.  "DAM" or "Dry As Molded" parts are often quite brittle.  This is why baggies of nylon parts often have a very small amount of water sealed in.  With added moisture the nylon is more resistant to fracture from deformation, although it take less force to deform. 

In the case of my props I suspect it was a combination of age and UV exposure along with dubious storage conditions.  The Top flite props that were stored in my parents dark, insulated, semi-climate controlled, usually somewhat humid attic have been fine, but the props I bought on ebay were not.  The white nylon prop has not been produced by Top Flite in about 20 years.

Phil

[Brett Buck]

I have to think it was some weird vibration that did it. Ever had that happen?

   I haven't seen an APC come apart. But I have seen standing waves form in Fox 35 props from vibration and I can at least imagine that it eventually fatigued and broke off. After the first time I saw the waves form, I stopped using plastic props on Foxes.

     You were very lucky that the engine quit before it shook everything into confetti. But *very carefully check* the entire airplane for latent damage.

    Brett

[Bill Heher]

It doesn't take much to induce a stress fracture in a prop, a tiny inclusion or micro bubble when molded an be enough. Once a stress riser is present, a cyclical loadin/ unloading can cause it to propagate quickly, resulting in failure.

For a while we had to pull the blades off the PW 120 powered Dash -8s every 100 hrs and have NDT inspections done on the blade root. Tiny points of corrosion in the taper bore where they packed lead wool to balance the blade weight would start a stress riser that could lead to a blade Fracture / Liberation in flight- Not Good! Took them months to get all the blades back to overhaul, but once certified by OEM, the 100 hr inspection went away. We got really tired of pulling props and blades every night.

The defect came to light when an EMB-120 spit a blade, the resultant severe vibration tore the engine gearbox/ prop from the rest of the engine before the crew could shut it down. The prop/ gearbox got lodged between the nacelle and fuselage causing severe drag and yaw. They tried toput it down in a field, but hit some trees about 1/2 mile short. No survivors, and one passenger was a congressman or something. EMB fleet grounded within a couple days, AD came out in record time, and subsequently spread to other aircraft with same basic blade design.

Don't miss that aspect of my former career one bit, your sphincter gets tight every time you do a post install vibration / Power Assurance run......