When a body experiences unequal external force, it is displaced from its original position. This displacement could be in terms of position, inclination or a combination of both. Depending on the behaviour of the body, it can be either of the three types of systems.
If the body returns to its original position after being displaced, it is said to be a stable system. For example, when you push a swing in the garden, the swing will return to its original position after a few oscillations. Such a system is called as stable system.
If the body does not return to its original position after being displaced, it is said to be an unstable system. For example, if you balance a pen on its tip on a table, it will start to tilt downwards by the slightest application of external force and it will keep on tilting until it completely falls down. The body does not return to its original position but keeps on moving further away. Such a system is an unstable system.
If the body does not return to its original position and does not even keep on moving away from it but stays in the new position, it is said to be neutrally stable system. For example, keep a spherical ball on a horizontal table. Move it by your hand to a new position and leave it. The ball will now neither return to its original position nor will it move further. t will stay in this new position. Such a system is called as neutrally stable system.
A drone without any sensors equipped is a neutrally stable system. When a drone is dropped from a height without applying any force, the angle at which it is dropped will remain same till it touches the ground. Hence, neither the angle goes back to its original inclination nor does it deviate further but remains same. Also, in the case of a drone falling from a height, it is a neutrally stable system when it comes to the angle. However, if you consider the position of the drone, then it becomes an unstable system.
Multi-rotors are neutrally stable system which is stabilized with the help of sensors, micro-controller and motors. Thus, if one is to drop quad-rotor without arming, it would fall in the same orientation as it was left in.