Greg Esres wrote

found 2 years (66 and 67 I think it was) that do Prohibit slips
with flaps.

Was it actually in the "Operating Limitations" section, or just
mentioned somewhere in the text of the POH? I'm just wondering if the
pilot who wrote the procedures of the book exceeded his authority in
using the word "prohibited". I couldn't find a "prohibited" placard
on my CD of type certificates for any 172 model.

I think that Greg has hit the nail on the head! I have just returned
from the airport where I retreived the "Owner's Manual" for the 1959
Cessna 172A that I regularly fly. While at the airport, I inspected
the cockpit for a placard pertaining to flaps and slips and found none.
Reviewing Section IV of the "Owner's Manual" titled "Operating
Limitations", which is the only *regulatory* section, I find no reference
to flaps other than the speeds for the flap operating range.
In Section III, "Operating Details", the following appears in a paragraph
titled "LANDING":
"Normal landings are made power off with any flap setting. Slips are
prohibited in full flap approaches because of a downward pitch encountered
under certain combinations of airspeed and sideslip angle."

Section III is not regulatory and I feel, just as Greg, that the
author probably came under the influence of Rick Durden or one of his
predecessors. :-)

I still believe William Thompson's writings on the subject are as close to
the truth as we will discover. Posted below once again.

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.


Bob Moore