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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
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Can carry more weight
Larger
More redundant so should survive if 1 rotor breaks
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break
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