Talking recently with someone at work about tailplane lift. He reckoned that all aircraft (conventional) had tailplanes that exerted a downforce/negative lift. I had thought that some did, some didn't i.e. C of G could be either ahead of or aft of the mainplane.
For stability in angle of attack, it's only necessary that the proportion change in lift from the tailplane exceeds the proportion change in lift from the mainplane for a small change in AoA at both. If you imagine identical symmetric aerofoils for the mainplane and the tailplane, that criterion only requires that the tailplane is at a
lower AoA than the mainplane, not that it is at a
negative AoA.
For the same loading, the difference in AoA will be smallest for low AoA (high speed) cruise -- after all, you pull back (increase the tailplane lift in a downwards direction) to increase the AoA. But that means that the mainplane AoA is already at its lowest, and if the tailplane AoA has to be even lower, it's likely to be only just positive. I don't know if that ever occurs in practice.