Multirotors
You will need a transmitter to control your multirotor(s). Transmitters vary quite a bit in price and features, one important feature for multirotors is telemetry (so you can see how much battery life you have left before your aircraft plummets to its doom). Also important is the "mode" setup of the transmitter. In the US, Mode 2 is common, with the throttle on the left stick.
- The radio I reccommend is the FrSky Taranis. It's got an amazing feature set (telemetry, voice feedback, MicroSD slot for model memory, highly flexible programming) and a robust RF link system for a relatively low price (around $200): http://www.alofthobbies.com/frsky-taranis-plus.html
- For a bit less ($130) you can go with the Spektrum DX6i, but it doesn't have a lot in the way of features and supports a max of 10 models: http://smile.amazon.com/Spektrum-DX6i-Channel-Transmitter-Only/dp/B00K1P3KYW/
- On the cheap end ($50) is the Turnigy 9XR which has some decent features (runs the same software as the Taranis, but on a slower processor and missing some of the features). Also it doesn't come with an RF module, so you'd have to get one separately (such as the $36 FrSky module here: http://www.alofthobbies.com/xjt.html) http://hobbyking.com/hobbyking/store/__31544__turnigy_9xr_transmitter_mode_2_no_module_.html
You'll also need a receiver for each of your aircraft. Receivers are not cross-compatible so you need to get whatever is made to work with your transmitter. There are some clones of some of the bigger-name brands though which makes them a bit more affordable.
- FrSky has plenty of receivers, these are the newer telemetry units that have some interconnect compatibility with flight controllers (we'll talk about those later), these will work with the Taranis as well as the Turnigy 9XR if it has the FrSky module installed: http://www.alofthobbies.com/radio-gear/frsky-telemetry-system/x-series.html
- Spektrum receivers are pricey but there are clonese available in the LemonRX (http://www.lemon-rx.com/shop/) and OrangeRX (http://hobbyking.com/hobbyking/store/__1796__1119__Radios_Receivers-DSMX_DSM2_Compatible.html) lines.
A flight controller is the device that allows your multirotor to stay stable in flight.
Sensors tell your flight controller what is going on in the world.
- Gyroscope - This is the bare minimum of what you need to keep a multirotor in flight. It senses angular motion along the yaw, pitch, and roll axes and the flight controller uses that data to feed back throttle control in the motors to counter any instability in the aircraft (if it starts to pitch over nose-first, the gyro will detect that, and the controller will feed a bit more throttle to the front motors and a bit less to the rear motors to get it to stabilize). This is enough to maintain the status quo of the aircraft, but you need to do a lot to keep it steady in the air. For more, you need...
- Accelerometer - This detects acceleration forces on the aircraft. The primary purpose is to keep the aircraft level when you center your control sticks. Turns out gravity is an acceleration force, so it knows which way is down. It can also counter some rough wind conditions by detecting lateral or vertical acceleration changes. It can't keep you pointing the same direction though. For that...
- Magnetometer - Detects magnetic fields, including the Earth's. This will do a fairly good job of maintaining heading hold.
- Barometric pressure sensor - Determines the air pressure, which changes with altitude. These can do a decent job of maintaining altitude hold, as long as you have a good chunk of open-cell foam on it to reduce wind buffeting.
- Sonar - Great for short-range distance measurement, usually to the ground. Augments barometric pressure sensors for low-altitude work, helps a lot for landing.
- GPS - Required for absolute position hold. Also enables additional features like return-to-home, loiter, waypoint navigation, etc (depending on the flight controller's features)
You may see sensors referred to in degrees-of-freedom, or DOF. Typically a 3DOF sensor is just a gyro, a 6DOF is a gyro plus accelerometer, 9DOF is gyro+accelerometer+magnetometer, 10DOF is gyro+accelerometer+magnetometer+barometer.
Now, on to the controllers themselves. This list is limited in scope because the flight controller landscape has changed drastically since the last time I looked. For the most part they can be broken up by the processor they run on. Most older ones run on an Atmel AVR (ATMega328P or ATMega32u4), for Arduino environment compatibility. These are 16MHz 8-bit microcontrollers. Newer ones tend to run on 32-bit ARM microcontrollers, running at 72MHz or faster. You're looking at a 10-20x increase in processing power with the ARM units, enabling many more features.
| Controller | DOF | GPS option? | Sonar option? | Software | Processor | Price range |
|---|---|---|---|---|---|---|
| AeroQuad32 | 10 | ? | Yes | Aeroquad (open source) | ARM Cortex-M4 168MHz | $150 |
| ArduPilot Mega | 10 | Yes (RTL/position hold/loiter/waypoint) | Yes | ArduCopter | ATMega328 | $40 |
| PIXHawk | 10 | Yes (RTL/position hold/loiter/waypoint) | Yes | ArduCopter | ARM Cortex-M4 168MHz | $100 |
| CC3D | 6 | Yes (with Cleanflight) | Yes (with Cleanflight) | OpenPilot, Cleanflight | ARM Cortex-M3 72MHz | $25, $25 |
| Acro Naze32 | 6 | Yes (with Cleanflight) | Yes (with Cleanflight) | Baseflight, Cleanflight | ARM Cortex-M3 72MHz | $24, $25, $25 |
| Full Naze32 | 10 | Yes (with Cleanflight) | Yes (with Cleanflight) | Baseflight, Cleanflight | ARM Cortex-M3 72MHz | $50, $53, $53 |
A note on Hobbyking: most of their gear ships from China and can take a while to arrive, and their stock levels are pretty unpredictable. Given the huge explosion of US domestic sources for equipment, I tend to avoid HK these days.