I quickly figured out that the only way you could build a $100 tricopter would be if you were an RC hobbyist with a lot of key parts from another aircraft already. I had the normal assortment of hand tools and electronic tools, but was essentially starting from scratch on the RC front. I ended up spending around $400 on parts plus all the odds and ends you need when you start any new hobby. If you don't have a good soldering iron, heat-gun, and multimeter, add another $100.
David's: RCExplorer Tricopter 2.5 is a popular design for home-built tris. He's got a very detailed build log at the site, and I followed his suggestions when I could. The overall design is fairly simple, and his pictures and parts list are a boon to a newbie like me. However, David is an experienced builder and pilot, and if you're building one of these for the first time (like me) there are some things you should know.
Other stuff to buy
Radio: Get the Turnigy 9x radio. Add a programming interface to it and flash it with ER9x firmware. This requires some fine soldering technique to install the programming interface. If you're not comfortable with that, just use the stock firmware, or buy a drop-in programmer. Word on the street (November, 2012) is a new Turnigy 9xR is about to be released, and it will have better firmware and a programming interface already installed. Watch for it on Hobby King.
Controller: IMPORTANT: Use the KK2 controller available from HobbyKing. $30. Don't use the older ones that David used. Ongoing improvements and development by the software author. Don't be confused by older ones with a V3 or V4. What you want is a "KK2" with the LCD screen. There are more expensive controllers out there that will allow easier flying. But I wanted something I could "fly", not program and watch.
Batteries: Buy at least 2, probably 4 or 6. I get about 10-11 minutes of hover time, and about 8-9 minutes of active flying time to a 2200mAh battery. At first, 20 minutes may be fine for an outing, but later, you'll want at least four. It takes a while to charge the batteries, and it's possible to damage them in a crash.
Battery Charger: I bought this one and am happy with it. If you get a LiPo for your transmitter, you will probably need this harness. Note that most battery chargers do not come with a wall plug/adapter. You may have one lying around (has to be 12 to 15v and at least 2amp or higher), or may be able to convert an old computer power supply. Or just buy one.
Props: When you crash, you break props. Buy a lot of props. In the first few weeks of flying, you'll easily break a couple each time out. I've had bad crashes that broke all three. Buying 24 normal and 12 counter rotating would not be excessive if you've never flown before.
Nylon stand-offs, screws, and nuts. I bought a variety of these in m3 size. You can find collections of them cheap on eBay, or buy from Hobby King. These are necessary for mounting the controller board and protectors.
Variety of wired servo and battery extensions: Getting some wired servo or connector extensions is a good idea. Then when you need a new connector, you just snip them in half.
Changes to the design for N00bs:
Landing gear: If this is your first tri, you are going to land/crash hard frequently. I started with a simple landing gear like on the David's design, but got tired of replacing zip ties them every time I landed hard. I ended up cutting 1/2" rings off a 3" PVC pipe connector, cutting open the circle and using the heat-gun to melt and shape it into an open P shape. (See picture above)
Rear servo mounting: My servo kept wiggling away from the servo horn. I added some double sided tape between the servo and the arm, and another big zip tie to keep the whole thing held together, and the problem is gone. (In the picture above, the extra zip-tie is disconnected. Be careful your zip ties don't restrict the yaw mechanism.
Motor mounting: I added some really thin foam between the motor mount and the arm. Helps keep the motor from wiggling/vibrating out of place. (Yellow stuff in the picture)
Board protection: You will definitely flip your aircraft or otherwise crash in a way that will damage the controller. I put mine in a Gladware snack box, and later upgraded to a better looking box, pictured above. (I used this one but I bought it locally for under $1.) Highly recommended, as it fits the KK2 and the RX perfectly, has a flat mounting surface, and the clear top allows you to see the display. What you're looking for is something that will hold a deck of playing cards.
Board mounting: Sticky pads like David uses aren't working for me. Get some nylon screws, nuts, and stand-offs to more securely mount your board protector and the board itself.
Prop mounting: some people attach the prop between the motor bell and a nylock nut. I use a sandwich: nylock nut/washer/prop/washer/nylock nut. I feel the props are much more secure this way. Check for loose props after crashes and as a pre-flight check. (Picture shows the prop mounting, my old landing gear, and a servo/motor mount that would often wiggle free.)
Wiring: You might want to use a power distribution board instead of soldering everything together. Probably adds weight, and I'm not sure where I'd mount it, but it would make building and modifying later much easier. Also, I find these pre-wired connectors much easier to wire up than just the bullet plugs in David's build.
Battery protection: Again, David's an expert pilot. You're probably not. The battery hanging off the bottom of the tri is vulnerable. I poked holes in a couple batteries before I wrapped them in shrink-tubing. I'd recommend using some of this to protect your batteries. Adds weight, but prevents zip ties or rocks from damaging your battery.
Cockpit/canopy: Add a foam "cockpit" to the top of your board mount. I assume you're learning to fly "Line of Sight" first. You need something at the front of the tri to help you keep your orientation, and to focus on. If you don't, you'll have a tendency to watch the tail rotor, and that's a helicopter flying "don’t" (You need to watch the nose, not the tail.) Mine is cut out of styrofoam and hot-glued to the lid.
Battery Mount: I used a longer piece of wood than David, and used a combination of rubber bands, zip ties, and foam to attach it to the bottom of the tri. (Kind of visible in the cockpit photo)
Lessons learned:
RC connectors, in general, are not keyed. It's possible and easy to plug stuff in wrong/backward and sometimes fry things. These batteries have a ton of power in them, and they're not shy about using it.
Charging: LiPo power sources were new to me. No one tells you how to plug things in. Be careful. This is the kind of thing that is obvious if you're an old-timer, but can be utterly baffling unless someone shows you. These things aren't like niCad chargers. You need to plug in a balance plug and a charging plug. Be careful, and don't just guess at stuff, as plugging things in wrong/upside down can easily damage the battery, your charger, or start a fire.
Balancing: Balance your motors without props. Balance your props on a balancing stand. Then dynamically balance each arm/prop. I use clear tape on the top surface to balance my props, and have had no trouble. If the hub is out of balance, I file some material away on the heavy side.
Zip ties: These things are awesome and cheap. Buy a lot and replace as needed. Watch for ones that stretch but don't break after a crash. When attaching, wedge a thumbnail against the locking mechanism, and use a pliers to pull the free end tight.
Props: Like I said earlier: buy lots and replace when nicked or bent. If I just "trim some grass", I don't replace. But you may want to recheck the balance. When you mount props, make sure they're right-side up, and spinning in the right direction. If props spin backwards, swap two of the wires coming from the ESC to the motor. Check to make sure your props are tight before flying and after crashes.
Learning to Fly:
I'll do this in another post.
