Aerodynamics: what about pitch stability?!
As I was reading again the ATPL theory about Aerodynamics, in the chapter of Stability & control I kind of made a hold at the chapter about longitudinal stability. I do need some clarity about one specific topic.
First I have to say I'm reading the Oxford ATPL P.O.F. manual. I provide you with a scan of one page in the mentioned chapter to make myself clear.
Herein it states that to increase static longitudinal stability or in other words to have a more effective tailplane, engineers apply longitudinal dihedral between the wing and the tailplane. The tail is at a lower incidence than the wing. It states that by having this, it will generate a greater percentage increase of the tail lift than the wing lift in case of a vertical gust. Regarding the drawing they provide below I can not understand how this in any way could happen?
Thinking about the increase of downwash (as the angle of attack at the wing increase) this would decrease the effective angle of attack at the tail. Secondly, as the drawing shows the tail providing a nose down force for stability (lift upwards), the lower effective aoa would reduce the tail lift and so less static stability would be provided.
Secondly it writes that downwash reduces the longitudinal stability. In case the tailplane provides upwards lift (nose down) for stability, then I do confirm that downwash would reduce the effective aoa, so as reduce the lift.
In case the airplane though has a more aft CG (aft of the wing's AC), this means a nose up correction must be provided by the tailplane, then downwash at the tail would increase the nose up tendency. Am I right or do I somewhere miss the global picture?
Please some advice would be great! Thank you.