When a friend offered me a HobbyKing Dark Wing, I jumped at the opportunity to build my first quadplane. Being a bit smaller than an X8 and constructed of a fibreglass fuselage with balsa wings, it seemed like a great opportunity to try building something other than a foamy. It was an interesting build but it ended up weighing 6Kg which which was a bit heavy for it's size. So armed with this experience I ordered a Skywalker X8 for part two of my quadplane adventures. If you need slow flying with plenty of lift and long flight time, then the X8 is a good choice.
There are a few build examples of X8 quadplanes from other members that I have watched with interest. The main design decision appears to be whether to add the vertical lift motors on bars attached directly to the wing or to attach the vertical motor support bar to an extra section of wing and insert it between the fuselage and existing wing. I'm sure both systems work fine but I opted for adding wing extension for a few reasons. Having the motor bars attached to a wing extension keeps the wiring attached during transport and the extra wing section adds about 15% more wing area resulting in a bit more lift.
Adding wing sections was by far the most time consuming part of the conversion process. In our first build we put the motor bars underneath the spars and attached them to the spars with small metal straps. This minimised the number of holes drilled in the tube which is always good. In the X8 build we took a much different approach and drilled large holes in the square carbon fibre tubes to allow the spars to pass through. We inserted square aluminium bars in to the carbon fibre tubes to join two lengths of tubes and to add strength where holes were drilled. Although this certainly reduces the strength of the tube, the results showed that the tube was actually much stronger than required.
We have gone through three techniques to make the actual wing extensions. For the DarkWing build we simply hand cut foam sourced from the recycling bin with a hack saw. It was fast and dirty but worked fine as a prototype. We used vinyl wrap to add strength and mask our sins. Needless to say, I don't recommend this technique.
To make the wing extensions for the SkyWalker X8 we made a hot wire foam cutter and sourced EPP foam which worked quite well given it was our first attempt. Using templates attached to the sides of a foam block, it was fairly easy to trace the hot wire along the templates to create the desired wing shape. Cutting little slits in the bottom of the templates to allow the wire in to the centre of the block for cutting out spar cavities was much more challenging. For the next attempt we had the wing extensions cut on a CNC hot wire cutter by the company which supplied the EPP foam.
After attaching wing joiners to both sides of the wing extensions, threading the motor wires and embedding the square carbon fibre motor mount bars, the hard part was done. I should mention that in this design I've done something many will consider questionable. I have put 10mm carbon fibre tubes inside the wing extensions and then cut the normal 8mm spars included with the plane so that they extend only half way in to the 10mm tube. This allows for 8mm spars in the wings to enter the other side of the 10mm wing extension tube creating a join. Some may think that this weakens the spar but my tests show that it's actually quite strong.
With this design you can even remove the wing extensions for travel if required although that will mean having to disconnect the vertical motor wires from the fuselage. It's not necessary for every day travelling but might be handy if packing in to a case such as when shipping by air.
Once all the hardware was assembled then all we needed was to enable all the Q_Plane functions in Ardupilot. It was finally time for the fun part – flight tests. Results ranged from bad to catastrophic with the plane rolling over after take off at the first turn. Voltage levels and current draw were quite suspicious as well. We concluded that there was a possibility of a ground loop in the wiring when we noticed some very hot wires in the power board cable. During these tests the plane would stall because the turn would increase while not responding to commands for opposite roll. Then the vertical motors would engage which allowed for a controlled landing, saving the plane more than once. But after we changed a power module and a bit of wiring around we ran out of luck. Once again the plane stalled on it's first turn and gravity won when the vertical motors did not engage. After a long walk of shame, that was the end of testing. Back to the bench...
We replaced some of the 14 gauge wiring with higher capacity wire and changed some power connections from XT60 to XT90 to ensure that the entire wiring harness was capable of handling 90 amps of current. When the vertical lift motors are in use and the forward motor kicks in to initiate forward flight there is quite a surge of current which needs to be taken in to account. It is possible to run seperate circuits for the forward motor and the vertical motors which is how the DarkWing built earlier was wired up, but I prefer having only one circuit for simplicity.
With our new wing extensions and vertical lift bars replaced (one was broken in the crash) it was back to the field for some testing, tuning and hopefully an automated mission or two.
Flight testing went very well. There were a few minor issues such as a transition that didn't work well. We are looking in to those. So for now things are looking good and we are ready for endurance and auto mission test flights.
A big thank you to my good friend John for spending Saturdays with me in his man cave working all this out. Further thanks to the Canberra UAV team who I'm very lucky to fly with for all their help over the years. And let's not forget a huge ThankYou to the ArduPilot community for the fantastic work that fuels all this fun!
Skywalker X8 black frame kit
4 x 3510-13 700Kv TMotor motors
4 x HobbyKing 40amp Plush ESC
1 x HobbyKing SK3 40xx forward motor
1 x HobbyKing 85amp ESC
4 x 14” RC Timer props ( two CW and two CCW )
1 x 12x8 prop
1 x Pixhawk flight controller
Flight control software - Ardupilot 3.8b8
2 x RFD900 radio modems
1 x 5v BEC for Pixhawk servo rail and backup power
1 x HobbyKing HV power module
2 x 4S 5800maH batteries to power motors
1 x status LED / USB extension for external mounting
XT-90 and XT-60 power connectors
Volo LEDs for motor arms nav lights - optional
Modified Aeronavic multirotor motor mounts (no longer available – substitute as required)
4 x 15x15x500mm square carbon fibre tube – if you can source 1mtr length then no joins required
2 x 12.8x12.8x250mm aluminium square tube (used to join carbon fibre tubes)