12/12/2020
Here’s what Bill Thompson, former Manager of Flight Test & Aerodynamics at Cessna, had to say about the issue of slipping with full flaps in the 172 (Cessna — Wings for The World, by William D. Thompson, Maverick Press, 1991, p. 41):
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 deg. 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 crosswind 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 counteract it if it occurs close to the ground."
