I recently installed a 3DR APM 2.5 on my well worn AD-6 heavy lifter, during the tuning process I noticed it was having issues with yaw stability so I decided it was time for a bench session to give the old bird a thorough examination. This particular frame hasn't seen a lot of use recently, the heavy lifting duties being handled by a Cinestar frame lately. The AD-6 has had many different flight controllers in its time from Mk to WKM, Hoverfly, Naza, the current APM 2.5, and for a short time even a Multiwii Pro board, its been around and had its share of "hard landings".
The main issue I found on close inspection was this...
For those of you familiar with the Droidworx AD6 series you know this is the top half of the motor mount and the missing chunk is how it locates the motor to the arm by a small pin that sticks down from the center. That missing pin should seat here...
The problem here is twofold, one the Delrin the part is made from is somewhat prone to cracking easily when it takes an impact, and as it ages Delrin tends to creep and deform, especially if it's held tightly in compression as the motor mounts are. End result is if you crash it can break the pin out of the mount as you see here, or over time the mount deforms under constant compression and as the sides get pulled down by the mounting bolts the pin gets pulled up out of the hole. End result either way is the motor no longer has any way to keep it located square to the frame so in the vertical plane it can be off in either direction from straight up and that can have a BIG effect on how steady it flys on the yaw axis, exactly the symptom I was seeing, not surprisingly because all of the 6 motor mounts had a problem with either deformation or missing pins from impact damage.
If you've ever priced replacement motor mounts for one of these frames you know they're a bit pricey, swapping out all six wasn't a viable option for an aging frame that doesn't see enough use to warrant the expense. Still, something had to be done because it simply wasn't ever going to fly right without having the motors locked onto the arm so they aren't leaning one way or the other. Here's the solution I came up with...
Taking off the bottom halves of the motor mounts I chucked each one into my lathe and using a drill chuck in the tailstock I drilled a hole in the exact center of the mount half with the appropriate bit for tapping the hole with 3mm threads...
Once that was done I swapped the drill bit for a 3mm tap and leaving the tailstock free to slide on the bed I rotated the chuck by hand, allowing the tap to start threading the hole in perfect alignment. Once the threads were started I took the tap and mount half out of the lathe and finished the threads by hand...
With the drilling and tapping completed the next step is to screw in a plastic 3mm screw that becomes the new locating pin for the motor mount...
Now the last step of the repair is to remove the through bolt from the inner boom clamp on the AD-6 frame and loosen the front clamp so the boom can rotate. Turn the boom 180 degrees so the motor mount locating hole is now on the bottom side of the boom where the new locating pin in the lower mount half can be inserted into to lock the motor in place. By using a plastic bolt I'm assured that if the motor hits the dirt in a hard landing it can shear, allowing the mount to move without damaging the carbon fiber boom as a metal bolt would.
When I got to boom number 6 I found yet another problem, at some point that arm had apparrently taken a decent hit and the inner boom clamp had lost its top half, gone, not even a broken chunk remaining. Not having a spare on hand and certainly not worth the shipping expense for that one little item I dug out a piece of round Delrin stock and made up a suitable replacement, boring out the center hole on the lathe and then cutting the flats in my mill...
Repairs completed it was time for reassembly. By the time I finished the heavy rain had abated enough to give it a test flight, HUGE improvement, the unpredictable yawing is gone now that all the motors are perfectly vertical and locked in place. Now once the rain goes away I can charge up some batteries and get on with dialing in the PID settings not having to worry about trying to tune around a mechanically unstable setup. This old bird has got life left in it yet and will be seeing duty carrying a Hero 3 on an RCTimer brushless gimbal!
Ken
The main issue I found on close inspection was this...
For those of you familiar with the Droidworx AD6 series you know this is the top half of the motor mount and the missing chunk is how it locates the motor to the arm by a small pin that sticks down from the center. That missing pin should seat here...
The problem here is twofold, one the Delrin the part is made from is somewhat prone to cracking easily when it takes an impact, and as it ages Delrin tends to creep and deform, especially if it's held tightly in compression as the motor mounts are. End result is if you crash it can break the pin out of the mount as you see here, or over time the mount deforms under constant compression and as the sides get pulled down by the mounting bolts the pin gets pulled up out of the hole. End result either way is the motor no longer has any way to keep it located square to the frame so in the vertical plane it can be off in either direction from straight up and that can have a BIG effect on how steady it flys on the yaw axis, exactly the symptom I was seeing, not surprisingly because all of the 6 motor mounts had a problem with either deformation or missing pins from impact damage.
If you've ever priced replacement motor mounts for one of these frames you know they're a bit pricey, swapping out all six wasn't a viable option for an aging frame that doesn't see enough use to warrant the expense. Still, something had to be done because it simply wasn't ever going to fly right without having the motors locked onto the arm so they aren't leaning one way or the other. Here's the solution I came up with...
Taking off the bottom halves of the motor mounts I chucked each one into my lathe and using a drill chuck in the tailstock I drilled a hole in the exact center of the mount half with the appropriate bit for tapping the hole with 3mm threads...
Once that was done I swapped the drill bit for a 3mm tap and leaving the tailstock free to slide on the bed I rotated the chuck by hand, allowing the tap to start threading the hole in perfect alignment. Once the threads were started I took the tap and mount half out of the lathe and finished the threads by hand...
With the drilling and tapping completed the next step is to screw in a plastic 3mm screw that becomes the new locating pin for the motor mount...
Now the last step of the repair is to remove the through bolt from the inner boom clamp on the AD-6 frame and loosen the front clamp so the boom can rotate. Turn the boom 180 degrees so the motor mount locating hole is now on the bottom side of the boom where the new locating pin in the lower mount half can be inserted into to lock the motor in place. By using a plastic bolt I'm assured that if the motor hits the dirt in a hard landing it can shear, allowing the mount to move without damaging the carbon fiber boom as a metal bolt would.
When I got to boom number 6 I found yet another problem, at some point that arm had apparrently taken a decent hit and the inner boom clamp had lost its top half, gone, not even a broken chunk remaining. Not having a spare on hand and certainly not worth the shipping expense for that one little item I dug out a piece of round Delrin stock and made up a suitable replacement, boring out the center hole on the lathe and then cutting the flats in my mill...
Repairs completed it was time for reassembly. By the time I finished the heavy rain had abated enough to give it a test flight, HUGE improvement, the unpredictable yawing is gone now that all the motors are perfectly vertical and locked in place. Now once the rain goes away I can charge up some batteries and get on with dialing in the PID settings not having to worry about trying to tune around a mechanically unstable setup. This old bird has got life left in it yet and will be seeing duty carrying a Hero 3 on an RCTimer brushless gimbal!
Ken
