I am going to share my experience on voltage drop in flight of copter.
I observed that, voltage going down in start flying of copter. Than I observed that the power cables copter are going heat, and I understand that power of lipo battery going loss in heat.
So I replaced good heavy wire (large diameter wire) in the view of large current should pass properly.
In this way My problem is solved.
Try to use good quality wires with good quality metle. Which will loss less power. If we observe that cables are going heat than we should have to replace wire with high current capacity cable.