I've been renewing my homework on the 40 to 30 flap change in the 172's. Quoting from William Thompson's magnificent book "Cessna, Wings for the World", Page 41, with respect to 172's:
"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 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. ........... Although not stated in the owner's manuals, we privately encouraged instructors to explore these effects at high altitude and to pass the information to their students. .....
When the larger dorsal fin was adopted in the 1972 C-172L, this sideslip phenomenon was eliminated, but the cautionary placard retained. In the higher powered C-172P and C-R172 the placard was applicable to mild pitch "pupming" motion resulting from flap outboard end vortex impingment on the horizontal tail at some combinations of sideslip angle, power and airspeed. "
It is noteworthy that some modifications on Cessnas, particularly float installations, result in more restrictive maximum flap use, as pitch effectivness or balked landing climb can be inadequate with 40 flap. I have only ever seen this limits by placard, as opposed to actually rerigging the flap limit switches.
I prefer the 40 flap available in 100 series Cessnas, and use it at all times for landing (which is why I prefer the 150 to the 152).
For my interest, and my being most effective in drafting Flight Manual Supplements, I would be pleased to hear the reasons why some pilots choose to use lesser flap settings for landing. What factors influence the choice of flap setting? Is this generally a result of personal experience, or training carried over?
Some of the modifications I test and approve are better taken off, and landed in more flat attitudes, and I am interested to understand why a pilot might not wish to use full flaps for all landings, so I can consider this in my normal procedures information.