Hi guys, I hope this is not a totally inane thing to post about but I thought some people might find my build-log interesting.
David Windestål’s (rcexplorer) build video on flitetest is largely responsible for me having a go at this, I’ve been researching it on and off over the past few months. I decided on a tricopter rather than a quad based on lower part count and thus lower initial investment. As a noob I don’t want to dump a lot of money into this before I learn to crawl.
Tx: Turnigy 9X
Rx: Bundled 8 channel.
ESCs: HK 20A UBEC’s
Flight controller: KK2.1 HC
Servo: HiTEC HS-82MG
props: 9047SF Slow Fly
Battery: Turnigy 4S 3000mAh (I also picked up a Accucell 6 to charge the batteries)
I decided to use the ‘optimised’ variant of the rcexplorer coffin-body frame (http://www.undcon.com/colorex/) making sure to adjust the screw hole positions to allow for the booms I have. I cut the plates from 3mm plastic sheeting - a £1.80 clipboard from Tescos - allowing me to get 4 of them per clipboard; I’m sure any other brand clipboard would work just as well.
Frame is built from 15 * 12mm pine stripwood from B&Q. I couldn’t find anywhere locally that stocked 12mm square stripwood and I don’t have a table saw that would have allowed me to square the booms up.
Note that with the ‘optimised’ design the front booms rotate around the outside screws rather than the inside screws.
I’m working on the yaw mechanism, I had hoped to replicate the rcexplorer design but HobbyKing was out of the landing gear parts in the UK store so I went looking for alternatives and found the Simplecopter design. I could have purchased the parts from the international HobbyKing store but postage costs would have been 10x the cost of the parts…
I’ve built the rotating platform and am testing that today. If all else fails, some friends have a 3D printer, I may experiment with some of the designs I’ve seen on Shapeways if I get a chance to get myself up to speed with the nuances of using the device.
I built a very expensive test rig (see below) after I had soldered bullet connectors to the motors and ESCs to ensure everything was working. Thanks to everyone who contributed to the KK2 thread on the Multirotor UK forum, it was very handy and informative.
I decided to flash the KK2 board after reading a number of reports that the base firmware is barely fit for purpose. I used the version 1.18S1 Pro firmware created by Steveis. I initially attempted to flash using an arduino but the kkMulticopter tool couldn’t detect it under linux. I tried again on windows but the avrdude flash tool didn’t play well with the arduino so I bit the bullet and picked up a USBasp programmer for a couple of quid on ebay. Flashing was thereafter painless with one exception - the aperture for the 6-pin header on the kk2 HC is not large enough for what appears to be a standard-sized plug on my USBasp. I used some servo extensions to ‘bridge the gap’.
I appreciate that the ESCs can be programmed using the beep codes but after a tedious time programming the first one a programming card was promptly purchased. You plug it in, the ESC beeps once and that’s it. For less than £5 it’s a no-brainer.
After messing with the simplecopter design for a bit I was unhappy with how much slop was in the mechanism. I saw a very simple yaw design on the Multirotor UK forum. It was such a good idea I shamelessly pinched it (thanks to user DarrellW for the inspiration). I didn’t have the L hooks he used so I used a bit of clothes hangar wire. It’s pretty stiff wire and should handle a crash fairly well. If it doesn’t there’s plenty more. Here’s what the assembly looks like:
Here’s a quick video of the prototype (warning, the motor sound is quite shrill):
You may notice a little bit of play in the servo - the 15mm wide boom means that the zipties aren’t pulling directly down. I think a few notches on either side of the boom will resolve this. Regardless, I think this is a promising design as the servo directly drives the yaw reducing slop and it’s super cheap to build whilst being pretty structurally sound.
N.B. Whilst I haven’t experimented, it makes sense that the optimal axis of rotation is through the combined centre of mass of the motor and prop. A high angular acceleration in the yaw mechanism may present a risk to the servo in the long-run. I’ll put his theoretically better design on the drawing board for now, though.