I beleive the concern is/was in some Cessnas Full Flaps and slipping could blank out the rudder and they were placarded with some to that extent.
Slips and Crosswind landings are not the same.
Cessna 172's (and only 172's) have a note in the POH and placard near the flap swich/handle that says to "avoid" slips with flaps extended. The problem is not rudder blanking, but an oscillation which can develop. Personally, after 40 years of slippin 172's with flaps extended, I've never experienced it, but some folks have. If it happens, just center the rudder and it goes away.
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.
The “oscillation” mentioned in this thread is an unrelated phenomenon that Thompson described in newer models in full-flap slips: “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.”
So although the 172L’s larger dorsal apparently solved the pitch-down issue, they kept the cautionary note in the POH because of the latter phenomenon.
Unfortunately Cessna contributed to the “end of the world” fear of slips with flaps, by not explaining the phenomenon in the manuals; and in fact, many earlier C-172 manuals expressly said that slips with full flap were "prohibited." I rummaged through my collection of old Cessna owners manuals:
1958 C-172: “prohibited”
1959 C-175: “prohibited”
1966 C-172F: “prohibited”
1972 C-172L (first year of the big dorsal): “should be avoided”
Ron has correctly pointed out that these old owners manuals aren't the TCDS and don't carry the force of law, but that’s what a lot of us old-timers read back then and remember.
