Almost a year ago I bought a secondhand XAircraft X4 quadcopter frame kit from a friend and finally decided it was time to get it on the workbench for building. The goal was to build a mid-sized multirotor with good flight endurance, FPV, GPS on-screen display and the capability to carry a GoPro Hero - without breaking the bank. As the X4 is a traditional ‘X’ shape multirotor and I wanted to avoid having propellers in the GoPro videos, I decided to combine the kit with the HobbyKing SK450 Deadcat conversion kit, which uses the same size arms as the X4. Read on for my photographic build log and thoughts on the SK450 frame.
The SK450 frame conversion kit from HobbyKing’s international warehouse arrived packaged a clear zip-lock plastic bag, which contained the top and bottom plates for the frame. I also ordered some green and red ‘DJI Flamewheel’ clone arms from HobbyKing however did not use these as the XAircraft arms were made from a slightly stiffer material. As I was trying to keep the cost down I chose the HobbyKing KK2 flight controller, which is a no-frills entry level control board.
The complete specifications of the quadcopter were as follows:
- HobbyKing SK450 Deadcat Frame Conversion Kit with XAircraft X4 (flamewheel clone) arms (via HobbyKing)
- 4x Afro 30 amp speed controllers with SimonK firmware (via HobbyKing)
- 4x Sunnysky X2212 980 KV motors (via Radio-C)
- MultiStar 4S 4000mAh Multi-Rotor lithium polymer batteries (via HobbyKing)
- HKPilot Power VI Module with dual UBEC (via HobbyKing)
- HobbyKing KK2.1.5 LCD Flight Control Board (via HobbyKing)
- ImmersionRC EzOSD and 5.8GHz video transmitter (via HobbyKing)
- Sony Effio-V 960H FPV camera (via Tecnic Store)
- 6x HobbyKing coloured LED strips (via HobbyKing)
- FrSky Delta-8 Hitec compatible 2.4GHz control receiver (via HobbyKing)
- Graupner E-Prop 9x5 Propellers (via local dealer)
After laying out all of the parts for this SK450 build, the first step was to attach the four arms to the lower SK450 frame plate. The SK450 conversion kit from HobbyKing does not include assembly screws, so I used the fixtures that came with the XAircraft X4 kit.
With the arms fitted to the lower frame plate I then attached the Sunnysky X2212 motors. I selected the motors, ESCs and propellers based on experiments with the eCalc.ch PropCalc tool which indicated this would be a good power setup using 4S batteries. Using the glassfibre ‘X’ mounting rings that came with the X4 kit, the motors were attached without any problems.
The next step was to fit the four electronic speed controller boards to the four arms. I referred to the KK2 flight controller manual and used a servo tester to spin up each motor to check it was wired correctly before soldering.
Rather than use the bullet connectors that were included with both the Sunnysky motors and Afro speed controllers, I trimmed the motor power cables to custom lengths and permanently soldered the motors to the ESCs. This saves a little weight and removes the possibility of a motor connector becoming disconnected during flight. I also used heat-shrink tubing to insulate and support the solder joints. I bought a large bag of assorted heat-shrink tubing from oomlaut.co.uk.
Here is a close up showing one of the motor arms with the ESC and motor soldering completed. At first I used some spare Turnigy Battery Straps to secure the ESCs to the frame arms, but later decided to use rubber reusable cable ties. I also marked the motor arms to indicate the motor number and direction according to the KK2 instruction manual.
As the standard SK450 Deadcat frame kit does not include an integrated power distribution system, I used the HKPilot Power Module which includes on-board 5V and 12V voltage regulators. Another reason I chose this module was the future possibility of upgrading from the KK2 flight controller to a HKPilot Mega 2.7 flight controller.
Each ESC was wired to the power distribution board, in addition to an XT60 battery connector lead and JST connectors on 5V/12V power supply outputs. I used hot glue over the battery and ESC solder joints to protect these critical connections from vibration.
Next I added a JST breakout cable and six coloured LED strips from HobbyKing. The LEDs are very helpful for orientation when flying line of sight and also in low light conditions. At this point I was starting to wish the frame had an integrated power distribution system due to the large amount of wiring across the base plate.
The LED strips were very bright and visible in day light. I chose red LEDs for the rear facing arms and white LEDs on the front arms.
I could see via the EzOSD system that the six LED strips together pulled a decent amount of current from the power supply and calculated it would reduce total flight time by around 10%.
Two blue LED strips on the underside of the SK450 frame complete the lighting setup.
Here is a final shot of the wiring inside the frame with everything connected before fitting the SK450 top plate.
The quadcopter assembly was completed by screwing the SK450 top plate on to the four arms and mounting the flight controller and radio control receiver. The four ESC control signal leads were connected to the motor output pins on the KK2 board, and the control channels on the receiver were connected to the input pins on the KK2 board.
After some brief test flights to confirm everything was operating as expected I removed the top plate so that the video transmitter, CCD camera, EzOSD GPS module and power sensor could be wired in. The Sony 960H camera was bolted to the frame and then secured in position with hot glue.
The FPV CCD camera overlooks the HD camera mounting area which is not ideal - but the frame has limited options for mounting of cameras.
The 4000mAh MultiStar batteries are mounted underneath the quadcopter using the incredibly strong self adhesive Velcro available from HobbyKing. I was not sure if this would be sufficient to secure the battery in position, but after several flights it seems to work just fine.
This last photo shows the completed SK450 Deadcat multirotor.
After completing the SK450 build, it was then necessary to configure the KK2 flight controller. A major part of this step was to reprogram the KK2 motor layout to appreciate the non symmetrical layout of the Deadcat frame. The HobbyKing Live YouTube channel did a reasonable job of explaining in a special video series how to set up the KK2 board with this frame, but it was lacking some key details and there has been no follow-up from HobbyKing on this. I also tried using motor layout values from several different blogs and forum posts but none of the various settings provided a completely stable and smooth flying experience. In the end I wasted countless hours trying to get the KK2 to play nicely with the SK450 Deadcat frame and never managed to achieve satisfactory performance.
I definitely do not recommend using the KK2 board with the SK450 Deadcat or any other deadcat style frame unless you are a master of proportional integral derivative (PID) mathematics. I understand that the DJI Naza flight controller can fly deadcat frames out of the box, so this might be a better choice but considerably more expensive.
Another problem I experienced with this build was interference lines on the FPV video signal, something I had never had problems with on previous FPV projects. I eventually isolated the problem to the switching power regulators on the HKPilot Power Module board.
Ultimately this was a failed project for the above reasons so I have resolved to use these parts on a Lumenier QAV500 frame with a Naze32 flight controller. This is a standard ‘X’ frame quadcopter that has better camera mounting options and doesn’t require special flight controller tuning. I’ll have a complete build and review of the QAV500 V2 frame posted here in the coming weeks/months.
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