I had an opportunity last week to go to a friend's office and see a quadrotor unmanned aerial vehicle (UAV) he's working on. Very cool.
Here's the little beauty in flight.
And landing on my iPhone.
I have been interested in these projects for a while but thought they were too expensive to 'play with'. However, having a look at HobbyKing it seems they can be built for a couple of hundred dollars. Now you have my interest!
I've been studying the HobbyKing site looking at the options and ratings for various components. However, what HobbyKing isn't helping me with is the selection of these component, althought the forum is good. Maybe I haven't found the HowTo: Component Selection section on the site. In the meantime I have read through this basic guide: http://blog.tkjelectronics.dk/2012/03/quadcopters-how-to-get-started/. This post links to a calculator (that I haven't used yet) to help with selection: http://www.ecalc.ch/xcoptercalc_e.htm
Component selection appears to be a recursive process. The selection of prop/motor/battery is determined by the full weight of the UAV, which you won't know until you've selected the props, motors, battery and all the other components. So it seems to me you need a spreadsheet or calculator that you populate with some nominal set of components and keep checking your criteria until you find a combination that should fly. Factors that seem to be important are:
- Frame size determines maximum prop diameter
- Prop pitch, diameter, motor kv rating and battery voltage determine maximum thrust (lift)
- Thrust must exceed total mass. Hover should be achieved using 30-60% of full thrust.
- Smaller and faster props make for a more aerobatic quad, bigger and slower props are used for 'heavy lift' applications (cameras, etc)
- Battery chemistry must be lithium polymer (LiPo). Nothing else has the power-to-weight ratio required.
- Battery capacity in milli-amp-hours (mAh) determines flight time.
- Battery discharge rating (e.g. 25C) in conjunction with capacity gives maximum current draw from the pack. This must be able to supply all motors at full power.
There are surely more basic principles at play here, but these are the ones that come to mind after the research I've done. If you're wondering what the motor 'kv' rating is, it's a constant that gives motor speed for a particular voltage. A 1000kv motor running on 20 volts spins at 20,000RPM.
I'm learning about quadrotors so that I can specify a parts list to build a prototype. If you see any good resources, please post a comment!
That is all.