Prop failure....... Have you had one??

[sweetpea]

I had something happen a few years back. Either the prop let go, the prop bolts let go or the spinner bolt let go. What I do know is that the motor departed the plane through the cowl. The cowl was shredded. Part of the prop went through a wing and cut it in half upto the wingtube (tube was only thing holding the wing together). Spinner was shredded (alum). I hit my throttle kill and landed the plane just short of the runway. AS I came in to land I noticed the motor was hanging below the plane. I had put dental floss on the ignition battery connection and the the motor was hanging from the ignintion. Ended up screwing my ignition as well but the motor ended up fine after I removed to sheared prop bolts.

 

[Alky6]

Under the variable forces and circumstances that can cause prop failure it is mostly recognized that wood is the better choice simply for safety reasons (actually insurance hoping liability would be reduced) but that was in due consideration that at the time when engines rarely exceeded 2.5 hp .
Now lets take Terry's DLE 85. 8.5 hp . 4X's the driving force to potentially send that splinter over supersonic speed. Or better yet 18 hp of brute force not just to only send a piece on its way but on its way with authority! (kinetic energy and kenny from south park).
Why does it matter if it is wood, carbon, composite or aluminum???. And it was a material ( wood ) failure causing all the havoc on Terry's plane. Now lets take an even more dense material such as aluminum. Does this same type of incident happen??? I think the insurance odds if studied would say NO! In regards to prop strikes. Is not aluminum just more likely to bend??? (You can make aluminum only so hard with silicon). It is still going to bend and not necessarily snap off like a hardened steel that is brittle.
When it comes to human flesh it's a game of rock paper scissors. This video is graphic but the message is "Being Safe" around propellers and highly "aware" of potential danger. Not "more safe" propellers. I would show a training film I have from my Navy days but it WAY to graphic. This one makes a "typical" point of appeal...... Oh!.....I group props and bullets in the same safety category.lol
Remember when the first female surgeon general said "We need to make bullets more safe" LMAO.

Interesting topic Dimes. Some things to consider - most revolve around inspection and fatigue failures and as I am sure you know this... Wood has an infinite life as long as the yield stresses are not exceeded. Steel has an infinite life if the stresses do not exceed 50% of yield. With aluminum, there is no point of infinite life, at some point the aluminum will crack and fail and it is not that predictable. It appears that a portion of Terry's wood prop had fibers that exceeded the yield strength. Interesting thing about wood, so many unknowns and non-uniformity that places the yield strengths all over the place. Just a bad piece of wood located in the wrong spot. Looking at the National Design Standards for wood, it is crazy how much the allowable yield strengths change depending upon the grading of the wood. IMHO, that is what makes laminated props much more attractive. Lower probability that a lousy piece of wood will be in the wrong location as the veneers are easier to inspect than a solid piece...

 

[bigblueswope]

Interesting info @alky. Thanks for sharing.

 

[Snoopy1]

The bolt failing interests me in my business the last 10 years and an increasing amount of high grade bolts are failing. Some of the bolts were traced back to China and when the testing was done the oil that they were using to ship the bolts was from old transformers and we all know what that is. We changed our buying practice and bought all our bolts from a company called Umbraco, all these bolts US made and certified and all our problems went away.

 

[49dimes]

Keeping in mind "insurance" mostly decides what we get to do but the no metal prop rule IMO is way out dated. Even though I do not fly scale it would be nice to do something that you do not have to have as "static display" but functional .

This prop would be very nice on this plane.....

 

[Jetpainter]

If I ever get around to painting my project airplane, the prop and spinner will both be painted chrome. It actually looks more like polished aluminum which is perfect.

 

[Terryscustom]

I do replace my prop bolts every 3 times they are removed or re-tightened. After seeing a few broken one year I decided that was cheap insurance since they stretch every time you tighten them. You can get a box of 25 at McMaster Carr for less than 6 bolts at the hardware store. I also got in the habit of using stainless nordlok washers, they last pretty much forever and are really nice two piece lock washers.

 

[Bill Vargas]

Hey Terry, what grade bolts are you using?

 

[Terryscustom]

Looks like class 12.9 is standard. http://www.mcmaster.com/#91290a264/=x1xz0u

From the website:
The standard among high-strength fasteners, these screws are stronger than Grade 8 steel screws. They have a minimum tensile strength of 170,000 psi. and a minimum Rockwell hardness of C37, unless noted.Length is measured from under the head.

Metric screws have a Class 5g6g thread fit. They meet DIN 912/ISO 4762.

Black Oxide—Screws have been heat-treated forhardness, which results in a dark surface color.

 

[stangflyer]

I do replace my prop bolts every 3 times they are removed or re-tightened. After seeing a few broken one year I decided that was cheap insurance since they stretch every time you tighten them. You can get a box of 25 at McMaster Carr for less than 6 bolts at the hardware store. I also got in the habit of using stainless nordlok washers, they last pretty much forever and are really nice two piece lock washers.

I also like the Nord-Lock washers since you got me hooked on them. My guess is on the Sukhoi, had I not had them on the bolts...the four that did not break would have either loosened and departed from the assembly or would have sheared right along with the other two. That would not have been a pretty outcome for a maiden. Come to think of it, would not have been good for any amount of flights. Now I make it a religious practice to check my prop bolts for proper torque every single time I go to the field. Not one of my planes is Nord-Lock free. Even using them on the exhaust headers. Yup, no more droppy down headers from loose bolts.