ABSTRACT
We have seen that the horizontal tail plays a critical role in trimming and stabilizing the airplane pitching motion. By judiciously selecting the horizontal tail setting angle it and the horizontal tail volume ratio VH, one can trim at a desired angle of attack, and hence velocity, as well as render the pitching motion (short-period dynamics) stable (in the sense discussed in Chapter 2). We have also seen that any change in the CG location induces a change in the trim and stability. The other way round, by deliberately moving the CG position, one can obtain a different trim velocity in flight, but there are a couple of issues here. Firstly, the new trim comes with a different degree of stability, and, secondly, moving the CG may not be such an easy task (except for hanggliders, perhaps). So, we need a better device that lets us move from one trim state to another, or even make more general manoeuvres in pitch, that does not share the drawbacks of CG movement.
