Generally when most mention coupling, they are talking about a non-pure control issue with the plane, due to the plane design or setup. By non-pure, I mean that if you give one input (say aileron), the plane does more than roll (like yaws too.) This is the coupling, i.e. a pure aileron input imparts a yaw as well as the expected roll, yet the rudder was never moved. As touched on in a prior post, in this example, the yaw is due to one aileron producing more drag (it would be the downward or lift generating aileron that produces more drag, i.e. the process of generating lift causes drag.)

Common coupling issues are ailerons causing yaw, and rudder causing roll and pitch, but other issues can occur. These couplings occur because of plane design.

Some coupling is desired in some models. For example, if you have dihedral, rudder input will cause a roll in the same direction, by design. In that case, the coupling might actually be desired as it helps coordinate turns, i.e. left rudder would also cause a little left roll.

However, a lot of coupling is not desired. A standard sport aerobatic model might have some very noticeable coupling issues. Even the best competition aerobatic models often have some very minor coupling issues. In the first example where ailerons cause yaw due to excess drag on the downward aileron. The yaw is actually in the opposite direction as the roll, i.e. a left aileron roll would cause right yaw. As you can imagine, this is generally not desired. There are several ways to fix that specific issue, such as decreasing just the downward travel of the ailerons (called aileron differential.) This can be done either mechanically or with a mix on the radio. But you could also setup a radio mix that applied a little rudder when you moved ailerons such that it offsets the yaw from moving the ailerons.

So, when you hear pilots talk about coupling, mixes, knife edge mixes, aileron differential, and the like, most likely they are discussing unwanted coupling effects and the mixes and/or setups to used to eliminated these issues such that the plane is “honest” to the control inputs.

Hope that helps. Cheers.