Quadcopter Realities

After a couple of months of dreaming, planning and waiting for parts to ship from China I have a complete quadcopter! It flies and everything. Its a small Diatone fpv250 minus the fpv (first person view) and was actually very easy to put together. I zip tied it together over a long hot saturday at the London Hackspace and have spent the remaining weeks trying to find time and space to fly.

I'll get the boring stuff out of the way, this thing is cheap. It uses the smallest crappiest brushless motors and esc's (electronic speed controllers) from Hobby King. A knock off Spektrum radio reciever and an bargain tiny MultWii based flight control board from Ready to Fly Quads. Total cost with some unessecary extras and some boring stuff like battery chargers and a radio transmitter. About £200 all in. I'll post a detailed list of the parts and what I paid for them at the end but here's a picture of the thing stapped together and ready for tuning.

And here's a video of me flying it one handed and crashing.

Crashing is a thing you have to get used to when building and flying these little airborne drones. It will happen so building a cheap one that you don't mind bashing about is a good idea in my book. There are certain challenges in flying a remote controlled aircraft from a fixed position and that is one of orientation. You need to know which way the thing is pointed because when it's pointed towards you left becomes right and right becomes left. This is alot more confusion than you can imagine and something that takes hours of Hubsan X4 stick time to get used to. I'm certainly a long way off.

Flying

I can hover it alright although that highlights another challenge of DIY quad ownership. PID tuning. Quadcopters are physically possible because of the little computers that comprise their control systems. A quadcopter differs from a regular helicopter in that it alters the torque and power of it's motor's to rotate the three axis you use to fly aircraft; pitch, roll and yaw. If you want to pitch forward you lower the power of the front two motors, if you want to roll left likewise with the left two motors. Yaw is achieved by changing the torque differentials between counter rotating propellers which is very, very clever. If you can't understand this imagine a motor hanging by a string, as it try's to spin it's prop shaft it also spins itself. Without a counter rotational force like friction in a tyre or a tail rotor in a regular helicopter motors will just spin themselves and waste energy. A quadcopter balances the counter rotational torque's (rotational force) or two motors to create stable flight. This is a rubbish explanation but hopefully gives you an idea.

Tuning

This is all frighteningly complicated and is made possible with some very clever electric systems. These are held on the flight control board which uses a PID loop to effectively guess it's way to stable flight. It uses the sensors it has access to which range from at simplest a gyroscope to guess what action will make it do what the pilot asks. It basically asks am I were I want to be? How do I get there? Did I get there? Millions of time's a second. Again this is a grossly simplified explanation but is the essence of what a flight controller does. PID loops need tuning through various differential 'gains', if these aren't honed the quad becomes unstable in flight and wobbles unerringly. I still haven't quite got mine locked in but I'll get there. When I do I'll have a go at explaining what i did. I'm using a Flip 1.5 MultiWii control board from Ready to Fly Quads, at $15 or around £8 it's a bargain and very good at what it does. On larger quads it's got a reputation as a plug and fly board.

Where next?

So mission accomplished? Yes, but I've still got the itch, I've built a beater of a quad. Something I'm very proud of getting to fly but not very happy with the way it looks and is put together (zip ties do not maketh an engineering masterpiece). So what next? Another mini quad, I like small light and fast things and I've just put an order in for a bargain set of motors and esc's from RCtimer. I'm looking for a frame, ideally foldable and possible carbon fibre. I also plan to stick some camera's on it and get in on the latest RC fad. FPV.

Parts List

• Diatone FPV250 nylon frame - £7
• Hobbyking Multistar 1704kv Brushless Motors - £30 (£8 for 4)
• Hobbyking Multistar 12amp ESC's - £24 (£6 each)
• 14 AWG Wire - £2 for two meters
• XT60 Battery Connectors - £2 for a pair
• Flip 1.5 Flight Controller - £8
• Lemon RX Radio Reciever - £4
• Bluetooth Module - £10
• 3 Bladed 5 Inch Props (left and right rotation)- £6 for loads
• Zip Ties - Cheap and buy lots

Other necessary bits if it's your first quad

• Turnigy 9XR Transmitter - £30
• Orange DSM2 Radio Module - £10
• Battery Charger and Power Supply - £20
• 30w Soldering Iron - £20 for a good one