I finally bit the bullet and made a start on building my tricopter. This instructable is for the build of the frame only, which is all that I have completed so far. As I get to the rest of it, more will follow.
I've never built anything like this before, but I've hoovered up all sorts of information from the web and made a start. I plan to use motors and other stuff used by other people who have made machines of a similar size and weight, that way there's a reasonable chance that it will fly - whether I can actually fly it is another story.
If you are planning to copy all or part of this build, please note that I have an impressive stock of spare screws and materials in my workshop, which means that so far this has cost me almost nothing for materials - you may not be as lucky. I also have a number of machines in the workshop that you may not have, especially the lathe and the milling machine. If you don't have access to machines like these, you may want to re-design bits of the frame to suit the materials and the equipment that you do have.
Step 1: Step 1 Design
Step 1 Design
Many of the designs for multicopters on the web are based on bits of wood and lots of zip ties and gaffer tape. While these undoubtedly work, it seems to me that it would be best to build it with as much precision as possible so that the machine is well balanced and the control computer doesn't have to work too hard to keep the thing straight and level. I decided to build the main three arms from some 10mm square alloy tube that I had lying around, and the plan is to run all the wires inside these tubes to keep the whole thing tidy. The length of the arms was determined by the material that I had, I ended up with the front two at around 350mm long and the rear one around 310mm.
I also decided that the landing feet would be best positioned at the ends of the arms to give less chance of tipping over on landing than a helicopter style skid arrangement. However, with all the weight of the batteries in the centre, a hard landing would put a lot of force on the three arms, so I included a bracing rod underneath each arm to make the whole thing more rigid.
Low weight is important, obviously, so all of the materials (apart from fasteners) are all aluminium and fibreglass sheet. The last thing you want is for anything to come undone in mid air, so most of the screws have nyloc nuts to stop them from vibrating loose, and those that screw in to a tapped hole are secured with lock washers and/or thread locking compound.
The part of a tricopter that seems to cause builders the most problems is the tail rotor swivel - there's probably a proper term for this, but the rear motor needs to be able to pitch left and right in order to steer the thing and keep it pointing where you want it to go. Its important to have no slack in this mechanism because it needs to respond instantly when the flight controller decides that its time for a change, and any slack will cause a delay - I've never flown one of these beasts, but I would guess that slack will cause the whole thing to over-correct and cause a potentially fatal oscillation. There are various ingenious ways that others have used to build this part, I used a spare chunk of alloy bored out to take a pair of miniature ball bearings, and the servo to tilt the motor mount is mounted underneath in between the landing feet.