I'd like to preface this post with this statement: I love my (lab's) Udrones Hexa C, but I feel that this simple fix would save a lot of grief for Udrones flyers.
So! I have noticed that the Udrones Hexa C that I use has a nasty habit of vibrating its motor mounting screws loose. Twice before, I have landed my hexa only to find that one of the motors was dangling by one screw, because the other had vibrated loose. Well this time it finally happened! During the landing sequence from 80m, the rear left motor came unscrewed and hit the arm, flying off.
Here's the sequence of events:
18:25:45: Hexa is returning to launch. I notice the it sounds rattle-y. I've heard that sound before, it means that a motor has lost a screw. Since it was already about to land, I let it continue. Landing sequence commences at 80m above home.
18:26:06: Motor comes off at 55 meters. At this point, the copter stays airborn but cannot stay in one spot. Missing one motor, it yaws at a good rate, making manual flight very very difficult. I can't compensate for the yaw, because it makes the copter tip over.
18:26:18: At this point the height reading in the tlog says that the copter very nearly touched the ground. I can assure you it did not, I was trying to get control of it in the air, no lower than 20m at the very least. At this point I was deathly afraid of the copter landing on a roof, or much worse: the pool. I attempted to stear it towards the safest place I knews, the forest.
18:26:37: Not quite reaching the forest, the copter is flying over the tennis courts at a frightening clip. The telemetry says it was underground at this point, but I assure you it was about 20-30m up.
18:26:46: At this point I am still trying to hit the forest but barely miss it. At this point I am running through the tennis courts. I see that the copter is going towards a medium sized road. Under no circumstance do I want the copter to crash anywhere near moving cars, it would be dangerous to everyone involved. So I lowered the throttle until the copter entered a descent.
18:27:01: From my point of view, the copter vanishes behind a stand of trees, still descending. I give it two seconds, and then cut the throttle completely.
18:27:04: Impact. From that point, I dash over and am immensely relieved to see that the copter landed upright on a grassy field with nothing broken beyond the missing motor and a chipped prop. I went back to the launch site and recovered the top half of the motor, complete with propeller still attached. The bottom half of the motor was still dangling from the copter, its mount cracked.
The moral of the story is: apply threadlock to your motor mounts, and check them before every flight! Otherwise the screws will vibrate loose and possibly detach your motor. I urge Udrones to add threadlock to the motor mounting screws of their ARF copters.
Comments
Thanks for the advice, I dislike how high the collets sit on the shaft. However I don't have the tools to safely trim those shafts. How did you trim yours?
Now, if you want the ultimate in reliability, you use safety wire. That would be fun!
Using set screws to hold shafts, even with threadlock, is also a horrible thing to do. It's cheap and easy, but again, is not a "best practice" by far.
I did an internship a long time ago where I learned a lot about bolted joints. Bolting failures? How about this: A 3/8" bolt. Cost: $8,000/each. "What?!" you say? Well, somebody counterfeited those bolts. And one of them failed, resulting in a chain reaction where they all failed. The failure resulted in $1,000,000 damage to a Destroyer, and 6 months in dock for repairs.
R_Lefebvre- You are very right! The split ring washers on the ARF Udrones were not reliable.
Michael- Good! I'd hoped my post would help at least one person who needed threadlock.
You did the right thing as a pilot. I'm going to put thread lock on all my screws now!
The split spring "lock washers" do not really work. I know they are used very widely in US industry, but they have been shown to not be very reliable. You will never find a lockwasher like that on an aircraft or other highly engineered machines.
Use threadlock if you can't use a Nylock nut in the application.
Great, thanks for the feedback everyone.
I like the point C , I use nail polish for skid marks. Nail polish was also used in days when we did not know about thread locks and bright color nail polish was the fix for thread locks :)
A - yes, threadlock belongs on all threads where metal touches metal. Use red for stuff you want to stay put forever (will only come lose if heated) and blue for things you want to unmount in the future without too much effort.
B- reduce vibration, where it is created! Thus reducing the chance of parts breaking and coming loose. Balance your motors first (plenty of tips on Youtube and this forum, I personally do the ziptie method), then your props, then mount props and check vibration to be sure.
C- a good way to check screws (or really any relative motion) visually is to apply skidmarks. After mounting everything where it is supposed to be, apply a thin line with a laquer pen across the boundary of the two parts that can move relative to each other. For screws that means a line from the head onto the material that the head sits in.
If the screw moves, the line is broken. The human eye can detect offsets of less than 1/10 mm easily.
D- I know that things are handled differently in the US but DON'T fly in residential areas! Trying that in Germany with a Hexacopter that size has a high likelyhood to have the cops on site the second time you do it. For good reason, too. If your aircraft goes out of control, you don't know who may get hurt. You fly across tennis courts? what do you think happens with people that get hit by a spinning prop? if you want more OUMPF to that imagination imagine your kids being hit. What do you think happens with you after that? Unless you are 1000% sure of your skills and the technical soundness of your aircraft and you maybe even have a permit, don't put other people at danger. Go to a sportsfield at least and use a second person to watch your flying scene.
Good luck with your next flight! Check everything thorougly before flying again. You had a crash and things may not look loose or bent or cracked but may be upon closer inspection.
Greetings from Germany, Otto
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