Myth of self tightening propellers

[Shelipso]

Hi guys, just want to share an experience with the self tightening props. I have a quad and use DJI E600 propulsion system with NAZA M V2 on it. It flies (well was flying) really steadily and I have been very happy with the setup. That was till I had a crash from a few meters above the ground a few days ago. I was testing my system after a change in the frame. I had it mostly in hover and was playing with the throttle when suddenly one of the props flew away. Being a quad, it crashed like a stone and broke the frame. Lesson learnt was to tighten the props before flying harder and do not trust the self tightening badge.

PS: I later found that the ESC was faulty and am now considering if it was suffering from a jumpy rev. Could this cause the prop to come off?

 

[SleepyC]

Always crank down the props. The self tightening aspect is just there to keep the torque from assisting in loosening the prop nut. I know some of the PR showed different, but never trust PR.

 

[Gary Seven]

This makes no sense to me....

 

[Shelipso]

Well I am trying to find an explanation too! But at least I know I should properly tighten these props before flying and I am changing the ESC with a new one.

 

[Gary Seven]

Well I am trying to find an explanation too! But at least I know I should properly tighten these props before flying and I am changing the ESC with a new one.

I should have elaborated a bit. The DJI self tightening props are designed to do just that...self tighten. The threads on the CW prop posts are the opposite of the threading on the CCW posts. Hence, when you spin the motors up they should tighten up the props accordingly. At least, that's what I've always understood and my experience (so far with about 50 flights of my F450) up to now has been without any prop fly-off incidents.

BTW, I always notice when I go to remove the props after a flight that they are much tighter on the post than they were before the flight. Anyway, that was the source of the previous post above.

 

[Shelipso]

Thanks Gary. I was expecting the same but will be a bit more careful next time. If you recall recently DJI updated their A2 firmware twice. In the first release they gave it a bit higher idle rev and I think one reason I heard (multirotor podcast) was to get those big heavy props tighten before take off. Similarly, my guess was that if motor revs high but suddenly breaks or slows down due to a faulty ESC, the rotational inertia could loosen up the prop.

 

[Old Man]

Similarly, my guess was that if motor revs high but suddenly breaks or slows down due to a faulty ESC, the rotational inertia could loosen up the prop.

If things are working right such an action should make it tighter. FYI, some of the older RC gas two strokes used a single propeller nut to retain some pretty large and heavy (24"-28") wood propellers but the rotational and thread relationships of the arrangement made for a prop nut that became tighter over time. They didn't ever toss a prop until and unless the engine suddenly ran backwards. That was not common but did happen because a two stroke can be timed to run in either direction and mistakes with ignition timing happened from time to time.

That's why I agree with Gary Seven that something seems odd with your experience.

 

[Gary Seven]

If things are working right such an action should make it tighter. FYI, some of the older RC gas two strokes used a single propeller nut to retain some pretty large and heavy (24"-28") wood propellers but the rotational and thread relationships of the arrangement made for a prop nut that became tighter over time. They didn't ever toss a prop until and unless the engine suddenly ran backwards. That was not common but did happen because a two stroke can be timed to run in either direction and mistakes with ignition timing happened from time to time.

That's why I agree with Gary Seven that something seems odd with your experience.

OMG. I would NOT want to be standing tangent to a freakin 24" prop when it comes flying off a gas powered RC plane. I mean, geez! Good story, BTW.

 

[Old Man]

Thanks, unfortunately experience is what generates a great many safety considerations. Those concerns/rules/policies always happen after the accident happened. From historic experience I'm very leery of being close to anything swinging 27" and up propellers, especially ones made in blade halves. Carbon is strong but two m3 screws just isn't enough for that kind of centrifugal force IMO. That's another tangent though so I'll leave it alone.

 

[Gary Seven]

Thanks, unfortunately experience is what generates a great many safety considerations. Those concerns/rules/policies always happen after the accident happened. From historic experience I'm very leery of being close to anything swinging 27" and up propellers, especially ones made in blade halves. Carbon is strong but two m3 screws just isn't enough for that kind of centrifugal force IMO. That's another tangent though so I'll leave it alone.

Ha ha! Pun intended??

 

[Old Man]

Pure accident

 

[Shelipso]

Thx guys. Wish I had a video of what I saw! Single prop went flying up. Here is a link to last 20 seconds of the fpv video. You can see the motor on the ground without the prop. The prop is intact.