OK, yet a fourth criteria. I confess I'm getting utterly confused as to what, exactly, is now being claimed for longitudinal dihedral, and even what it's being defined as.
The thread started with a question:
Longitudinal Dihedral
Old definition: Angle between the angles of incidence of the wing and tailplane. Positive if the wing incidence is greater.
Why would you design longitudinal dihedral into an aircraft?
I fail to see how the latest criteria:
dCL/da /CL < dCLt/da /CLt
relates to that (there's nothing there about tail or wing setting angles, just (indirectly)angles of attack, which aren't the same thing)
More importantly, I'm not sure what the new criteria is supposed to represent. Is it a criteria for stability?
And how does it work for, e.g. CL approaching zero, for a case where pitching moment is non-zero and therefore a Clt is required for trim? How does it apply to a case where the trim tail load is zero? How does it apply in inverted flight, where the lift coefficients on both wing and tail reverse sign?