As I was replacing a shaft screw on a motor, I thought I'd check the other motor shaft screws. I noticed that none of them were tight, so I tightened them. As a result, the drone flew horribly. So I loosened them up and it flew fine again. What's the rule of thumb as to how tight/loose the shaft screws should be?
There are a few things that may be happening.
Tighter and flying bad screams bad mounting. A prototype I was working on last week had this problem. As I tightened the motor, the c-clip on the base was rubbing on the arm and slowing down. Loosen the screw and it spins faster. To check for this, take the motor off and spin it with your hand and get a feel for how much it will "free spin". If you mount it and this "free spin" decreases then you want to look for friction. Leave the leads disconnected when you are going to do this, as some motors will retain residual electricity and until drained they will spin slow (this is a false positive, so test everything unplugged)
Generally speaking motors are built to be bolted down REALLY TIGHT, so there is probably operator error going on.
Another very common problem is the mounting screw length. There are 2 things I have run into related to mounting motors and screw length. First is the possibility that the screw is too long and is actually hitting something in the internals. If so, look for damage and/or compare amp draw with other motors, watch for higher heat, and be careful. The other more mystical problem is when the screw is too long, somewhat metallic or electromagnetic, and alters the magnetic field of the motor. I have had his happen a few times and it is often not obvious.
Another possibility is that the motor and/or mount is warped. In this case, tightening can further disfigure or warp it and decrease performance. Many elements on a motor should be parallel/perpendicular to other parts and that can help you see if a crash bent something.