SK450 - it's Dead Cat Time

About this time last year, I built myself a SK450 quadcopter to cheaply familiarize myself with the hobby and gain a fun new toy. That project came off very well, and I was pleased with the results for the money spent. This time, I'm upgrading that copter into a "dead cat" configuration, using some new parts and some old ones — but mostly new.

Once upon a time, DJI made a $300 kit drone called the F450. This got knocked off considerably cheaper by Turnigy, and later the frame was copied by Neewer, which makes camera accessories. I bought that one on ebay for twelve bucks. But if you go back to the middle of that story, you find Hobby King offering a dead cat conversion kit for the SK450. Named after a drone literally wrapped in a dead cat due to the shape, this relatively costly set of plates and hardware re-angles the front arms outwards and provides locations to mount a sizable camera and FPV transmitter.

The first time around I used XXD ESCs, and this time I am using "Lighting Power" SimonK firmware ESCs. These may even have the same hardware, and I haven't cut off the shrink tubing to find out. I may do that later, when I get clear heat shrink tubing; it's nice to be able to see if you've burnt a component on an ESC at a glance. I also used a fairly typical Crius power distribution board for large quads. I'm going to use that again temporarily now, but my long-term plan is to effectively make my own power distribution board using copper foil. I originally intended to use copper tape, but it's just too thin. If you want to carry 30A with a single trace, it has to be .5mm thick and 6mm wide (or any other equivalent size) to provide adequate performance. A typical PCB-based PDB made in 2oz copper basically uses an entire side of the board to provide each conductor, and a whole mess of vias to get the power from one side to the other. I will cut .5mm copper foil into 6mm strips, and epoxy them to the lower PCB. The contact points will be fully overlapped for maximum current capacity, and I'll coat the tops completely with solder for corrosion resistance and additional current capacity.

My original flight controller was a KK2.1.5 mini, which actually performed adequately. I'm going to retire it in favor of a Acro Pro SP Racing F3, which is an inexpensive and simple upgraded Naze32. This comes without a compass or barometer; I'm using the compass on my GPS which will let me avoid interference from the motors by mounting it high on a mast (also coming in the mail, eventually) and I'm using a remotely mounted barometer module. Right now I will use a BMP280 because I have it, but I will later upgrade to the HMC. I want to see if I can attach it directly to the flight controller, and need to get a look at it first to decide if I just need to order a separate I2C module. The F3 has lots of I/O for things like LEDs, and hardware for floating point math which cuts the loop time about in half. This means more agility for racing quads, or more stability for larger ones. I'll be adding an assortment of WS2812B LEDs, probably two per arm to start with, running the connections with 28AWG magnet wire which can be epoxied down.

When I built my quad originally, I had no radio equipment. I bought a Hobby King 6 channel (rebranded FlySky) radio and a PWM to PPM encoder, and used that to get PPM into the KK mini. You can only use GPS if you don't use PWM input, so that's how it shook out. Now I am using something much, much fancier: a fully-modded Devo7e which will talk to a OrangeRX R820X DSM2 RX. This is an 8ch PWM/9ch cPPM DSM2 RX with dual satellite connectors and telemetry. Using nothing more than a cable and a thermistor, you can have RSSI, pack voltage, and pack temperature, and these are readily available for under $30 on eBay. I also have a Lemon RX 0008 DSM2 satellite, which pairs nicely when used with the ORX receiver. The Lemon RX 0008 is a true diversity satellite with soldered antennas; the R820X's two antennas are both IPeX connected so that you can switch to different options, like extending out to a RP-SMA connector to be able to use larger antennas. My Walkera Devo7e transmitter has been "fully" modded, with the 2x3 switch mod, 3in1 radio module, diode (tx power) mod, and an upgraded high-efficiency power regulator, but even a completely unaltered unit can speak DSM2 (with telemetry!) with Deviation firmware. It also does DSMX, but that raises the price of both RX and satellite somewhat and is generally considered unnecessary for anyone who does not participate in competitions.1

I got new motors with my ESCs, but they are the same low-quality (albeit functional) A2212/13T 1000kV units that I'm running already. These are adequate for unladen use, but I will probably need to step up to a superior motor if I plan to carry a substantial FPV rig. I am only trying to get 1080p60 recording and CVBS FPV, and this does not seem unrealistic. The ESCs will handle a 4S battery pack (and no more) but the listing said the motors could only handle the current from a 3S pack, which seems probable. I have been flying with 3S 3Ah, but I have 4S, 5Ah packs to move to. These are old packs which have probably lost some capacity, but they should serve me acceptably for the meantime. If you are willing to go with Turnigy's cheapest, you can get 4S 5Ah for $30. It's not clear whether that's a good idea — possibly not. I am seriously contemplating a sheet of Aluminum between the battery and the quad in any case, though the only component I have so far converted into fire and smoke has been a RtF 4in1 22A ESC. The leads came off the board before it could set the landscape on fire.

For the motors I will probably be stepping up to SunnySky X2212 980kV. You can get four of these out of Texas (clearly from one Chinese business with many eBay accounts, but anyhow) for fifty bucks. These were listed in someone else's similar and related build — they too were apparently using a knockoff SK450 frame, and were building out for 4S.

So far I've managed to disassemble the quad and assemble the new frame, and I've plotted out how I'm going to arrange the foil on the board in order to build my PDB. I only wonder if my cheap Kapton-clone tape ("Koptan") will work properly as insulation...

  • 1. The advantage of DSMX is that it spreads the signal across more frequencies within the 2.4GHz band, which enables more users to share the spectrum without noticing any competition.

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