One practical advantage is increased ground clearance, reducing the likelihood of a prop strike on landing or take-off.
Aerodynamically, a pusher1 design can be less efficient because the propeller is spinning through the wake of the fuselage - this results in 'lumpy' air, causing vibrations. I don't have exact figures, but I've heard anecdotally that this ...
Expanding on @Kralc's answer:
Here's a study (unfortunately behind a paywall) that I'm going to copy key parts of below:
The hexacopter was mounted on a load cell test stand, and data were collected in the University of Michigan’s 5 × 7 ft wind tunnel over different freestream flow speeds, motor thrust percentages, and hexacopter angles of attack. To ...
The DJI Matrice RTK has rotors on the bottom. I believe this is to avoid impact when flying upward for inspections. (Under side of a bridge for example).
I'm not sure about the aerodynamic effects, but I will say the majority of my near misses (or crashes) are due to upward/tilting into movements, and not downward tilting away.
4-M3 means that you need four screws (on each side), each with a metric M3 thread. M3 is 3mm diameter.
The length of the screw obviously depends on the thickness of the thing you're bolting the motor to, plus a little for the base of the motor.
The amount of bolt in the motor is important. Obviously you need at least a couple of mm so that it grips, and ...
This blog post by Oscar Liang has a table identifying the type of motor that is used for each frame on 4S (other voltages would only affect the KV). Here is a screenshot of it:
And yes, 22XX means any motor with a stator diameter of 22mm and with any height, as it is the stator diameter that will determine the mounting pattern.