Propeller failures

Metalog · May 12, 2020

I work with failure analysis.
My anafi fell, (luckily less than 4 meters high) two propellers broke, nothing more.
I analyzed one of the fractured propellers, the only one I found, and found that I had a fracture caused by a fatigue mechanism. I inspected the other 6 propellers, not yet fractured, and found that two of them were already with fatigue cracks. I am putting some photos, showing where the crack nucleation occurs, to facilitate the periodic inspection of other pilots.

Jagerbomb52 · May 12, 2020

Yes very common. I have had one break on me that resulted in my Anafi spending a winter out doors in a pine tree till I found it in the spring. There is a document out someplace in the forum about this common problem.

Liger 1956 · May 12, 2020

@Metalog That is very interesting. The crack propogation appears to be from a corner stress raiser but why that particular position. Is it from a moulding mark? The "beach marks" also do not appear to have extended very far before the material fractured. I don't know whther you can estimate the number of cycles to failure. I would have thought that there would have been a greater factor of safety built in. Have you looked at any unused propellers to see whether the cracks are caused by manufacture or appear during use? Am I also correct in assuming that the propellers are genuine Parrot ones?

Jagerbomb52 · May 12, 2020

Safety Bulletin For Props

Some how I missed this back in September.

parrotpilots.com

Metalog · May 12, 2020

Liger 1956 said:
@Metalog That is very interesting. The crack propogation appears to be from a corner stress raiser but why that particular position. Is it from a moulding mark? The "beach marks" also do not appear to have extended very far before the material fractured. I don't know whther you can estimate the number of cycles to failure. I would have thought that there would have been a greater factor of safety built in. Have you looked at any unused propellers to see whether the cracks are caused by manufacture or appear during use? Am I also correct in assuming that the propellers are genuine Parrot ones?

Metalog · May 12, 2020

Apparently the cracks did not appear in pre-existing flaws in the propellers, they appeared in the corner of the hole, two of them, in the propellers not yet fractured, on the opposite side of the propeller blade. In the fractured propeller, the crack started at about 90 degrees from the blade. I inserted a philips wrench in the hole, which has a tapered neck, causing the crack to open, and I could verify that, in both cases, the cracks had already reached about half the height of the tube. I was unable to count the cycles, due to the pandemic I am without access to SEM. These analyzes I did at home, with my stereomicroscope. The propellers are original, I have been using them since June 2019, almost a year. I will now inspect every 3 months. Now, knowing where the cracks are born, detection is easier. Thanks for the comments.

Rocky the FS · May 12, 2020

These are really great photos! They will help us all a lot in inspection of our hubs, as they show just how small and insignificant the cracks appear at first. One needs a loupe and a really bright light to see them. In the aviation industry, we used a UV dye penetrant and a black light to see miniscule cracks like this....I wonder if this technique could be adapted to plastic.

I think I'm going to inspect my hubs before the next flight! I replaced some last summer, but like an idiot I didn't write down which ones and now I can't remember. ?

Metalog · May 12, 2020

274/5000

The photos were taken in a stereomicroscope, with incident light. Then I realized that the cracks, in non-fractured parts, are very visible using a light through the helix tube, like the cell phone light. With this lighting I found cracks in two more propellers.

Metalog · May 18, 2020

I found another way to detect the fatigue cracks in the propellers.
Using a blue laser. The laser light passes through the propeller material and is interrupted when there is a crack.

Liger 1956 · May 18, 2020

Aren't Blue lasers dangerous to eyes?
Do you have a larger picture of your setup?

Metalog · May 18, 2020

Never point a laser beam directly at the eyes.
I took this photo with the stereomicroscope, held the laser in one hand and the piece in the other. I kept rotating the propeller tube until I found the best position to see the crack. All the cracks I found were in the same position, so after finding the first one it was easier to find the others.

Jagerbomb52 · May 18, 2020

Metalog said:
Never point a laser beam directly at the eyes.
I took this photo with the stereomicroscope, held the laser in one hand and the piece in the other. I kept rotating the propeller tube until I found the best position to see the crack. All the cracks I found were in the same position, so after finding the first one it was easier to find the others.

Note to myself.........add stereoscopic microscope and blue laser light to my flight bag and check list

Liger 1956 · May 18, 2020

Metalog said:
All the cracks I found were in the same position, so after finding the first one it was easier to find the others.

Looking at your photo I assume that the crack is approximately at right angles to the bore of the tube on the side opposite the propeller. Were these new or used propellers and if used how many flights had they completed? Do you know how deep the crack is?

Metalog · May 18, 2020

The cracks position was exactly as you described. The propellers were original, they came with the drone. 134 flights were made with a total of 14 hours and 44 minutes.
The smallest of the cracks had covered more than half the height of the tube.Based on these numbers, I assume that the propellers must be replaced with 10 hours of flight time.

elrojo14 · Aug 2, 2020

I have had propellers break off of my Anafi three times now. The last one yesterday was a fatal blow as it hit a rock and broke an arm. I am going to start a regular propeller replacement schedule. How long do you think I should go before I replace them all?

Metalog · Aug 3, 2020

The failure in my Anafi occurred with 14 hours of flight. The propellers were original. I replaced it with new propellers, also original, and I intend to change them with 10 hours of use

AnafiPilot1 · Aug 22, 2020

Since this thread was started I have been inspecting all the props I replace - around 16 individual props (so far) and I have not seen any cracks like this, or any other cracks near or around the hub area.

I have never had a prop fail on me, but I have had many (well the 16) where the drone starts sounding very raspy in flight.

The cause of this for me has been by little chips appearing on the edges of the props, I suspect from hitting insects and maybe the odd contact between props on start up.

Jagerbomb52 · Aug 22, 2020

To avoid the chips either hand launch or use a landing pad of some sort. Avoid tall grassy and gravel roads. For over 6 months now I have been making sure my props are lined up with the trailing side against the hubs. Not sure if this last tip helps but I have not had any more chips on my props since doing this.
Hopefully the picture will explain how I place my props before start up.

jaja6009 · Aug 22, 2020

Agustine said:
To avoid the chips either hand launch or use a landing pad of some sort. Avoid tall grassy and gravel roads. For over 6 months now I have been making sure my props are lined up with the trailing side against the hubs. Not sure if this last tip helps but I have not had any more chips on my props sine doing this.
Hopefully the picture will explain how I place my props before start up.

View attachment 4704

Good points. I will be adding this to my Anafi Pre Flight Check and routines.