Beware of worn out prop hubs

[Paleomanjim]

I have just over 160 miles on my Mini now. Around a month ago I heard a noise similar to cicada's when descending to land. It lasted for around 1 second causing the whole drone to noticeably twitch and repeated several times. I looked things over, especially the props, and could not find the cause and the issue seemed to go away. Yesterday the issue returned and I flew it around for several minutes to see what might me the cause. The drone flew perfectly when climbing but when descending fast it twitched violently. After landing I noticed some black circular marks on the drone body near the front. It was obvious that the left front propeller was dropping down over 1/2" and contacting the drone body! I placed the drone on a table and measured each of the 8 blades to see how much free play the blades have up and down as measured at the tip. Most were around 1/8" free play, some slightly more. But one blade on the left front had well over 1/4" free play. The screw holding the blade was still tight, not loose at all, but the plastic propeller hub was severely worn out allowing the free play. This was clearly the issue and I replaced the propeller. The drone flys perfect once again.
On take off or climbing wings on planes or helicopters can be seen to flex upward as they pull the airframe up. On landing or descent wings tend to droop. The same is true for drone propellers. They flex up when ascending and droop downward when descending. The Mavic mini props are very flexible and this combined with the 1/8" of freeplay allows props to flex up and down a lot. So much so that wear over time can allow the props to contact the body of the airframe. It is easy to imagine a severe case of this bringing the drone down! It takes a lot of miles to cause this, but apparently 160 miles is enough. So for those high mileage drone fliers out there I'd suggest measuring the amount of free play on your props at rest and consider replacing blades that have too much free play.

 

[igor011080]

Is it because of the photo or the tips of your props don't seem to be all the same color?

 

[Starz]

Some reading material.

Pay attention! (I didn’t)

Check your props and see if they are doing a bit of self trimming. If you have ever heard a clicking noise that comes from nowhere, you may now have an answer. I never put the two together until someone else brought it up. I’m not thinking this is a pilot error issue at all other than the fact...

mavicpilots.com

 

[Gagey52]

Certainly don’t have that many miles on the mini, but something to watch out for, thanks

 

[scro]

Was the cause of excessive free play on one blade established?

A one-off event could have plastically deformed the hub and ovalised the mounting hole. This would be independent of amount of distance or time the aircraft has clocked up, and would fit with the observation that one blade was a lot worse than the others. I'm not saying this is the cause, but to have 1 blade significantly worse than the others indicates that something other than expected wear is going on.

 

[Paleomanjim]

Yep, not sure of the cause of one blade being more worn than the others. But after measuring the others several of them were slightly worn as well, although I did not replace them as they are not a problem yet. Also, the new replaced blade has less up and down movement than any of the others, so there does seem to be wear on the blades over time. Pic shows how I measure propeller tip up and down free play...Maybe others can share their measurements to see if there is a trend with those having high mileage/hours

 

[igor011080]

Yep, not sure of the cause of one blade being more worn than the others. But after measuring the others several of them were slightly worn as well, although I did not replace them as they are not a problem yet. Also, the new replaced blade has less up and down movement than any of the others, so there does seem to be wear on the blades over time. Pic shows how I measure propeller tip up and down free play...Maybe others can share their measurements to see if there is a trend with those having high mileage/hours

Are those original DJI blades? That's what i meant to ask with my first post.

 

[Deleted member 103366]

I doubt they are original, all DJI MM props seem to have silver tips.
I would also have concerns about seemingly swapping just one blade

 

[igor011080]

I doubt they are original, all DJI MM props seem to have silver tips.
I would also have concerns about seemingly swapping just one blade

That's my point since last Tuesday and the reason I asked about the different colors of the props. No need to bother with all the measurements and the flying up and down... But unfortunately the pilot doesn't seem willing to give that specific info.

 

[Paleomanjim]

That's my point since last Tuesday and the reason I asked about the different colors of the props. No need to bother with all the measurements and the flying up and down... But unfortunately the pilot doesn't seem willing to give that specific info.

Skip the measurements if you like. Intent here was to share an issue that could bring down any Mavic Mini with high mileage. Based on Post #3 above it looks like this issue was previously identified and others have had issues with props contacting the body of the drone. I had not viewed that post before so was not award that the problem had been previously identified. I purchased by drone used about 6 months ago, had 2 previous flights on it with those props on it. I replaced the left rear props after striking gravel on a landing early on and splitting the end of one prop. Is this issue due to the different colored props? Not likely. See post #3 from Start above and click on the link.

 

[Paleomanjim]

Skip the measurements if you like. Intent here was to share an issue that could bring down any Mavic Mini with high mileage. Based on Post #3 above it looks like this issue was previously identified and others have had issues with props contacting the body of the drone. I had not viewed that post before so was not aware that the problem had been previously identified. I purchased by drone used about 6 months ago, had 2 previous flights on it with those props on it. I replaced the left rear props after striking gravel on a landing early on and splitting the end of one prop. Is this issue due to the different colored props? Not likely. See post #3 above and click on the link.

 

[igor011080]

Skip the measurements if you like. Intent here was to share an issue that could bring down any Mavic Mini with high mileage. Based on Post #3 above it looks like this issue was previously identified and others have had issues with props contacting the body of the drone. I had not viewed that post before so was not award that the problem had been previously identified. I purchased by drone used about 6 months ago, had 2 previous flights on it with those props on it. I replaced the left rear props after striking gravel on a landing early on and splitting the end of one prop. Is this issue due to the different colored props? Not likely. See post #3 from Start above and click on the link.

I took the time to read all of it. And frankly, there's no conclusive sign of an inherent design failure (check the post #120). It's more about a propeller storage issue, a screw not straight, a bent arm... In your case, the damage on the body looks significantly more (the black trace) than what I've seen in the other thread, so I'm more concerned about the quality of a non DJI original propeller and/or improperly stored blades (with all due respect, this can happen to anyone). Propellers holder for me is a big no as I think the less anything touch the blades, the better it is, I always use the Fly More Combo case to store the drone. For the record, I fly the mini since last December, 280 miles roughly always in S-mode, changed the propellers only once because I cut some high lemongrass in the garden (not on purpose!)
So, back to your original post, the mileage may be one of the reason you had this issue but, in my opinion, not the only one and not the main reason.