Greetings fellow modelers,
I thought I might share some of my experiences with a home built quad frame (X) and an APM2 brain.
Early in December 2011 I was shopping at Cool Components thinking of goodies for xmas and was attracted to the purple brushless 1000kV motors and the text “ideal as an 'outrunner' motor on a Quad copter.” I purchased 4 with the recommended speed controllers and set off on a path of discovery that lead here and to the fine work and shared experiences that is DIY Drones.
I had an idea to build an alloy rebated crossbar as a starting point and not too challenging to construct. I got some 10mm square section anodised alloy tube with 1mm wall thickness at the local hardware DIY store.
Before cutting it up and drilling holes I had the notion to consider the props, the popular 10 x 4.5 were recommended for the outrunners, apparently each being capable of exerting in excess of a kilogram thrust.
Not being certain of any final weight or dimensions and how these might affect performance, I erred toward smallest and as far as possible symmetrical in shape and weight. The frame dimensions are derived from the optimum prop size for the motor at 3S and a gap of around an inch between neighbors. This coincidentally provides an unobstructing central space about the size of a CDROM. All in all the design was a stab in the dark but it felt intuitive.
I cut the square tube and made two lengths at 445mm and rebated each 10x5mm at the centre point, some of the photos show how it went together using stainless fixings. A 1mm aluminium plate 110mm square provides reinforcement and a place to anchor the ESC’s and CDROM deck.
The motors were mounted at 275mm centres which measures up at 389mm across the diagonals.
If I build another, the diagonal distance would increase to 400 to allow the lid of a blank CDROM case to be used as a rain proof cover. ;)
To mount the hardware APM and receiver I glued a couple of old CDROMS together with 5 minute epoxy and very carefully drilled holes that line up the APM2 and four more to match holes I’d made in the plate.
Mounting the APM2.
My local tool shop sells a really nice selection of round thin wall alloy tube. I used some with an inside dia of 3mm. Cutting the tube is easy if it is rolled back and forth on the bench with the edge of a sharp craft knife, paring off lengths as required. I cut a number of these as spacers and used longer stainless screws. The receiver is traditionally mounted using double sided servo tape as are the 20A ESC’s.
I dislike miles of spare cable hanging about, so I set about making up custom fly leads. Shorter ESC, PWM, motor, power and receiver to APM2 leads.
The Servo leads are taken up through the CDROM hole. The ESC power leads are taken through two 8mm holes in the aluminium plate and joined underneath in a pair of Y harnesses, the idea being to fly as little copper as possible.
Thinking about the mounting for the APM2 and I’m not sure if it even matters, but I decided to try and get the gyro chip bang on top of the centre of the quad in the X configuration, not easy when the mounting plate is a CDROM with no hard centre to work from. Although the mass of the APM2 is no longer dead centre the compromise is negligible particularly as eventual battery position makes a significantly bigger difference to c of g given the relative size and weight.
I don’t have a photo to show the legs and battery sling I’ve added, rather hastily I might add, in an effort to get airborne quickly. Some 100mm bent lengths of aluminum strip attached under the frame from the motor mounts. The design of the legs leaves a lot to be desired but worked quick and dirty as you would expect. The problem is the 1Kg mass moving laterally to the ground, if you catch a leg on the turf it’s immediately bent :) I really need to make them more skid like.
The stuff I used
Alloy square tube and various alloy strip – (Wickes)
Alloy plate – (surplus)
Alloy tube round – (Richardsons Ironmongers)
Various stainless 3mm hex screws washers and nylock nuts (Apex Fasteners)
2 x old CDROMS preferably something you want to take flight. (not a greatest hits!)
4 x 1000kV DYS Brushless outrunners (Cool Components)
4 x 20A DYS ESC (Cool Components)
1 x APM2 – (DIY Drones Store)
2 x Turnigy 2200mAh 20 – 30C 3S1P lipo (ebay)
Both batteries are carried together in flight, but so far I haven’t made a harness to parallel wire them. Balancing when doing this for the first time is tedious and very much a manual process with safety concerns. Importantly both batteries can be positioned either side of centre maintaining a central c of g (as near as I can tell).
I use a JR x3810 transmitter upgraded with a Corona-rc 2.4Ghz + receiver.
I also managed to get 6 flight modes working on this radio using the 2 position p-mix switch and 3 position flap / land switch, but had to use 4 programmable mixers with offsets and the AUX3 pot to get there. I managed it using one channel. If you have a JR X3810 or a JR XP8103 here’s one way of doing it.
Configuring the JR3810 is relatively simple providing you have the manual and understand the quirks when setting offsets in the mixers. The trick is to have the switch you are configuring in the on position and then rotate the AUX3 pot to get the desired offset.
Return the AUX3 pot to centre or just to one side to get the best 6 position distribution in the APM2 mode config.
Use programmable mixers 3,4,5 & 6. Each mixer should be set for AUX3->AUX3 mixing, effectively a rate and offset mixed with the pot for each switch’s on position in the mixer. AUX3 is one of the variable pots on the transmitter and is output at chan8 on the receiver. Use this pot to set the midpoint when no switch is in an on position. One upside to this set up is if the pot accidentally gets disturbed only the mid position on the flight switch is affected meaning the middle two modes might not line up with the mode you expect. The rest are unaffected.
Programmable mixer 3: Rate +100, SW: ELE>F, Offset -170
Programmable mixer 4: Rate -98, SW: LAND, Offset +98
Programmable mixer 5: Rate -34, SW: MIX, Offset +54
Programmable mixer 6: Rate +27, SW:MIX, Offset -170
The all up flying weight is exactly 1Kg according to the baking scales. That gives me approx 3Kg of thrust to launch it skyward with ;)
It works happily using the default PID settings with the current version 'AC2.4' delivered from AMP, so far I have flown 6+ flights successfully. Mainly in stabilize as the back garden is suddenly very small. The handling is responsive stable and inspires confidence in one’s ability to sling it about a bit. Indeed slinging it from side to side aggressively is so satisfying and skyward launches straight up are a favorite, so long as you first assess wind-age above fence level. I need to get out a bit more. :)
I have the channel 6 FLAP pot dialed to accept a variable P rate and have notched it up slightly in my most recent test albeit only about 10% with no ill effects and a subtly more responsive attack.
There is one negative characteristic which I need to iron out. When throttling up for liftoff, bringing the revs up slowly brings the forward facing corners off first and the resulting ground effect wants to tip the quad over. Alternatively If I stab the throttle upward the quad launch is stable and comes up with authority. At this point I am holding a very slight forward stick in to maintain attitude and consequent drift. This is hampering stabilize + simple testing where the changing yaw angle moves the direction needing the correction. I hope this can be trimmed out either by a better job of aligning, balancing and leveling or by dancing with fairies.
For the future I plan some wireless telemetry, proper autonomous flight and some FPV while it’s at it. I’d also like to build a hex or octo format for a bit more payload and redundancy.
No doubt I’ll be adding some updates as things progress.