Requirements to know:
Camera mounts (multiple?, gimbal?)
Batteries (how many batteries do we need/want? - do we want to fly for the full 30min?)
Grabber attachment - base should probably be at least 20cm diameter
What do we need to carry (sizes and weight of all electronics)
Basic drone information: https://www.dronezon.com/learn-about-drones-quadcopters/how-a-quadcopter-works-with-propellers-and-motors-direction-design-explained/
Motor calculator: https://www.ecalc.ch/xcoptercalc.php
Number of rotors - http://www.dronefromchina.com/new/how-to-choose-the-UAV-from-4-rotor-6-rotor-8-rotor.html
4 rotors:
Cheapest/lightest
Can be smaller
If one rotor breaks then you will most likely crash
6 (and similar for 8):
Can carry more weight
Larger
More redundant so should survive if 1 rotor break
