Originally Posted by
Pilot DAR
The position out the wing (the wing station) of the outboard end of the flap corresponds poorly with the outboard end of the horizontal stabilizer. In a slip, the wake off the outboard end of the flap can impinge on the H stab, and blanket part of it, and perhaps induce some elevator motion, which will create unexpected momentary pitch control forces, and some pitching. It's more surprising than a real problem, but it is a thing. It varies with models of the 172, some are worse than others, but it was the cause of the "avoid slips" placard common to most 172's. It's also a thing with C 170B's. It's worthy of understanding, and an imperfection with the design, but it did pass certification, and is manageable. C 150/152 don't suffer, as their flaps don't reach as far out the wing. 18x don't seem bothered by it (I've tried all of them!).
Ah. Thanks. It's good to finally have an explanation for that placard and a warning of what to expect if it's ignored.