Linearity of the systems

Take into consider the control system which we desire to control several states of it. But some of the states are not linear. For instance, we have a drone and when we increase the voltage of DC Motor above the propeller, and it's getting faster. We can say, there is a linear relation between voltage and speed of the drone. Besides this, when speed is increasing displacement's getting bigger either. This is also a linear relationship. But consider that, when voltage is increasing 1 of my states increases squarely related to voltage and it creates non linearity. To sum up, can we say that due to that 1 non linear state the system we want to analyze is completely non linear system, and for example, instead of using ordinary kalman filter, now we need to implement extended or unscented kalman filter just for that 1 non linear state? More simply, does just one nonlinear state become the system nonlinear completely?
Drone is a very complex example, there are many parameters beyond your control like enteral wind velocity, Air density, hot spot pockets etc. May be in general for non linear systems if you have a mathematical model of the system then it might be easier to play with. If not then try to get maximum samples and then do a curve fitting, which will be a closest fit of at least 99.999% accurate will help.