Larry Dighera wrote
I know this has been beaten to death, but I just received an article
from AOPA about the C172 and there is a reference to this problem.
Article: "Anatomy of a Success" (The makings of a winner) by Thomas A.
Horne, AOPA Pilot in May, 1992.

What makes the AOPA or Thomas Horne an authority on the subject?

I have posted the following on previous occassions before and still
believe it to be true. It is excerpted from the book, "Cessna, Wings
for the World" by William Thompson. Bill Thompson served as Manager of
Flight Test and Aerodynamics for the Cessna Aircraft Company. Seems as
if he should know the 'real' story.

Bob Moore

With the advent of the large slotted flaps in the C-170, C-180, and C-172 we encountered
a nose down pitch in forward slips with the wing flaps deflected. In some cases it was
severe enough to lift the pilot against his seat belt if he was slow in checking the
motion. For this reason a caution note was placed in most of the owner's manuals under
"Landings" reading "Slips should be avoided with flap settings greater than 30° due to a
downward pitch encountered under certain combinations of airspeed, side-slip angle, and
center of gravity loadings". Since wing-low drift correction in cross-wind landings is
normally performed with a minimum flap setting (for better rudder control) this
limitation did not apply to that maneuver. The cause of the pitching motion is the
transition of a strong wing downwash over the tail in straight flight to a lessened
downwash angle over part of the horizontal tail caused by the influence of a relative
"upwash increment" from the upturned aileron in slipping flight. Although not stated in
the owner's manuals, we privately encouraged flight instructors to explore these effects
at high altitude, and to pass on the information to their students. This phenomenon was
elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware
of its existence and know how to counter-act it if it occurs close to the ground.
When the larger dorsal fin was adopted in the 1972 C-172L, this side-slip pitch
phenomenon was eliminated, but the cautionary placard was retained. In the higher-
powered C-172P and C-R172 the placard was applicable to a mild pitch "pumping" motion
resulting from flap outboard-end vortex impingement on the horizontal tail at some
combinations of side-slip angle, power, and airspeed.