Arm Extension Kits for DJI Frames
by Bosnianbill, published
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This kit upgrades ANY multirotor that uses the DJI-style arms. It is a drop-in swap for the stock arms and you’ll notice many improvements. Specifically, the kit:
- Allows you to install ANY length or diameter arms. The kits are available for 8,10,12 and 16mm CF tubing.
- Can use ANY motor – even large, low KV pancake motors.
- Can mount ANY size propeller, simply by varying the arm length.
- Stabilizes your multirotors – Better aerial photography!
- Lightens your quad (the kit is less than 100g), which increases your flight time significantly.
- Is stiffer than the stock DJI arms.
- Is tougher than the stock arms.
- Is cheaper & easier to repair after “gravity-related incidents” than stock DJI arms.
Here is what you’ll need:
- Arm adapters
- Motor mounts
- Carbon Fiber tube
- M4 x 20mm screws (4 for [email protected] Home Depot)
- M4 Lock Nuts (the one with the nylon bushing), (4 for $0.64 @ Home Depot)
Here’s how to upgrade your multirotor.
- Remove the propellers.
- Unplug the ESCs.
- Remove the two bottom screws from each of the arms.
- Remove the four top screws from each arm and set it aside. Repeat on all arms.
- Cut your carbon tubes to the desired length.
- Insert the M4 screw and nuts into the arm adapters and motor mounts but don’t torque them down yet.
- Slip fit the arm adapters and motor mounts onto the tubes. They will be snug.
- Tighten the arm adapter screw until you cannot turn the tube. DO NOT OVERTIGHTEN!!! It just needs to be snug. There is NO rotational stress on this part so PLEASE don’t go crazy with tightening this screw. Nothing good will come of it. You’ll either crush the carbon tube or break the motor mount. There is a wide variation in CF tube dimensions but the gap is wide enough to accommodate them all. Please do NOT tighten the screw until the gap is closed. If you cannot spin the arm adapter, then it is tight enough.
- Slip on the motor mount but don’t tighten it yet.
- Lay the arm on a flat surface to make sure the motor mount is level with the top of the arm adapter. Once it is, snug the screw until the mount will not turn. Again, please DO NOT OVERTIGHTEN. Snug is good. (Too tight and you’ll get pissed at yourself for stripping or breaking something. Just avoid the pain and don’t gorilla-tighten the screws…).
- Install the arm into your frame. Use the original SHORT screws on the top and they will cut their own threads into the ABS and it will act like a lock nut. In almost 120 hours of flying these I have NEVER had a screw back out of the ABS. Again, just snug the screws up and DO NOT OVERTIGHTEN. Yes, you are strong and I am SURE you can strip the plastic, but please don’t do it. Snug is good.
- Use original LONG screws on the bottom two legs. The long screws will add additional strength to the legs and, like the top screws, will never back out or vibrate loose. Snug is your friend. Over-tightening will end up something broken or stripped.
- As you install each arm, zip tie your ESC to the arm to keep it from flopping around and stressing the soldered joints.
- Once all the arms are mounted, double check that the motor mounts are still perfectly level. This is critical on all CF-armed multirotors. Close is not good enough. If they are not nearly perfect then your yaw control will suffer and it will not give you a stable hover.
- Mount the motors. The mounts accommodate everything from 18mm to 50mm motors. Cheap insurance: Use Loctite on the motor mounting screws. (I have not tried every single motor on the market but have tried KDE Direct, Tiger, Multistar, Sunnysky and DJI and they all mount while the motor mount is attached to the CF arm. If you have a motor that cannot mount with the tube installed it is no big deal and usually happens with the 16mm tubes and a motor with a narrow mount. Just slide off the motor mount, install the motor, re-install the motor mount and re-level it. Easy). (Pictured is the Sunnysky “Angel” line which has a tiny mounting screw pattern.)
- TIP: The motor mounts have a provision to allow in-tubing wiring. Aesthetically, this is nice and I’ve designed in a “bumper” on the end to run your wires through and protect them. From a practical point of view it is not optimal. Thru-tubing wiring requires more wire, which can add up to 100g of unnecessary weight. It is also harder to maintain and inspect. Finally, it is MUCH more difficult to repair if you have a “gravity-related incident”. With outside wiring zip tied to the arms, you simply unplug everything, replace the broken part (usually the arm), and zip tie it all back together. Five minutes and you are back flying. (Pictured is the Turnigy 16-Pole MultiStar 4225-390Kv pancake motor, which has a wide mounting screw pattern).
- Plug in your ESCs and check the motor rotation direction (STILL with NO PROPELLERS). Once correct, mount your props.
- Go fly. Depending on how wild you got with the arm lengths you may need to adjust your gains. I have 92 hours on 1855 propellers on my 1.7kg 700mm MTM F450 and my gains are: 180 180 130 150 80 80.
When things go wrong…
OK, we all strip screws. It happens & all is not lost. Here are some workarounds I’ve used.
- Instead of the short screw, use the long one in the stripped hole. If you already stripped a long one….
- Put a drop of CA in the stripped hole. Let it sit overnight & it will soften and expand the ABS. The next day, insert a new screw. If that doesn’t work,
- Put a SINGLE drop of acetone in the stripped hole. Be very careful since acetone melts ABS. Let it evaporate and dry overnight. The next day, insert the new screw. If THAT doesn’t work,
- Fill the hole with epoxy. Let it set. Drill a small hole (about 3mm) and thread in the new screw. If THAT doesn’t work,
- Drill the hole deeper, about 15mm. Insert a 12-15mm M4 screw & thread it in. If THAT doesn’t work…
- You are out of luck.
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Arm Extension Kits for DJI Frames by Bosnianbill is licensed under the Creative Commons - Attribution - Share Alike license.
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