Severla question about aircraft modelling

Hello Everyone

I am somewhat new to the world of aviation, so I have some questions about mathematical modelling of an aircraft that need to be answered.

Here are my questions:

1-Is a simple mathematical model for an aircraft for pitch, yaw , roll motions can be described as simple levers? (Is roll motion is like a lever hinged from its middle point, while yaw and pitch are like a lever hinged from its end ? )

2-I know that the whole flight is non linear process, but are landing and taking off is more highly non linear than other periods of flight?

3-Does pitch motion affect roll, and roll motion affect the pitch ? (I mean if I design an autopilot controller to control both, do I need an iterative method to tune both, because I can not make test for each independently? )


4-I have noticed that I can make yaw motion in air using either rudder , or using aileron by adverse yawing (is that its name?), so I ask what is the benefit of this redunduncy? Is not one of them enough?


Thanks very much