"AnthonyQ" wrote

I was told many years ago that a full rudder slip in a C172
(especially the older models with 40deg flaps), it is possible to
induce a tail stall....not good close to the ground....

From the book "Cessna, Wings for the World" by William Thompson,
Manager-Flight Test and Aerodynamics for the Cessna Aircraft
Company where he also served as an Engineering Test Pilot and
other positions for a total of 28 years.

"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."

This is probably as close to the real story as we will get.

Bob Moore
ATP CFI
PanAm (retired)