I've been neglecting this blog recently due to various distractions but have several projects I want to write up. Around April I found myself with the urge to build a multicopter. In the end I settled on a tricopter design as it's a little unusual and because the wider angle between the arms allows plenty of clearance to mount a camera without getting the propellors in frame.
To keep costs down and because I enjoy designing things I ended up drawing and laser cutting my own plywood frame. This was loosely based on the folding arms of David Windestål's design, with a lot of modifications to fit the size and shape I wanted and to fit the parts I had chosen.
The arms of my tricopter are cut from 3/8" birch engine bearer stock, as sold in most model shops. I bought 3 12" lengths which I cut off centre to make a set of 7" arms and a set of 5" arms. In the end I was happy with the 7" arms so I've not yet tested the 5" version.
The frame is designed to have two 3mm ply lower plates that the arms are sandwiched between, held together with 2mm machine screws. The screws are tightened so that the arms are held in place by friction when unfolded, but they can still be folded relatively easily without any adjustments or tools.
Following David's pattern, the landing struts and motor plates are attached with cable ties. These hold everything firmly in place but will hopefully give or break before the frame components if too large an impact is applied.
Above the two structural plates of the frame there is a third plate with a large number of cutouts. This sits above the frame on laser cut plywood standoffs and serves to protect the electronics that sit on top of the frame. The KK 2.1 flight controller I'm using has a build in LCD display and buttons for configuration in the field. All of the buttons and the display are accessible through a cutout in the top plate. Additionally the plate helps with cable management, wiring being attached to the frame with cable ties and velcro straps to keep it neat.
Because I went for a tricopter design I required a swivel mount for the rear motor. This consists of a modified motor mount plate with two tabs on the bottom through which an M3 bolt is threaded. A pair of bearing carriers are formed from two layers of ply parts that slot over the rear arm with appropriately sized holes to trap a pair of bearings through which the bolt runs. The rear landing strut was cut with an appropriate cutout for the metal gear servo that moves the rear motor mount. This turned out to be a weak point and is so far the only part I have broken. A new design has been drawn up but has yet to be tried as the replacement rear leg is still going strong after many more landings (with slightly more care).
Electronically the tricopter is fairly simple, though there is a small hack to power the rear servo. Each speed controller includes a voltage regulator which normally powers the other equipment that needs 5 volts. Since there are multiple ESCs in a multicopter, only one of these is required. People cut the 5v wire to prevent the regulators from fighting (probably only necessary with switching regulators). The KK 2.1 board simplifies this as the first ESC connector powers the board and radio reciever while the 5v pin from the other 7 are isolated so no wires need to be cut. This does mean, however, that no power is provided to the servo. To work around this I modified the rear ESC by desoldering the 5v wire and removing it from the connector. I then soldered a 3 pin header to the voltage regulator output to make the 5v and ground lines of an additional connector. The 5v wire from the original connector was attached to the third pin of this and at the other end plugged into the signal pin from the servo output on the KK 2.1 board. This provides a connector on the rear ESC into which the servo could be plugged, providing power and the appropriate signal to drive the servo (see diagram).
Very few changes were required to make the tricopter fly nicely, with the exception of increasing the proportional gain for roll and pitch without which the controls felt quite sluggish.
I have some video from an early test flight shot using a cheap 808 keychain camera. I've since bought a Mobius and added an appropriate mount but haven't had chance to get any footage with the new camera due to weather. The downside of building the tricopter from wood is I don't want to get it wet!
I've made the drawings from which the tricopter was built are available to download and I'd enjoy hearing about it if anyone uses them in a project.