One: I think what was meant by "let the nose drop" was that if you were trimmed for sufficient airspeed before the slip, you won't stall in a slip if you don't apply up elevator force.
I think all airplanes I've flown have required slight forward pressure to maintain sufficient airspeed -- I'll have to experiment today.
Pointing the nose down helps maintain sufficient airspeed as gravity acts as a thrust vector.
Two: I have flown a fair number of airplanes and read many a POH and I've never found one that was truly "prohibited" from slipping in any configuration, nor have I found any scary tendencies slipping them. There are some that cannot be slipped (e.g. the Ercoupes without rudder pedals) but excluding those and the very few other designs that have a landing gear designed to land in a crab, prohibiting slips is akin to prohibiting crosswind landings.
I never said "prohibited." I said the OP should check to see if the POH has a
limitation.
Many airplanes have some warning or caution or suggested limitation. Especially in the case of student pilots, I think it would be wise for them to read the specific POH and heed those sections. While I agree few (if any?) prohibit slips, if the POH adds some qualification it would be best to fly the airplane IAW that advice until such time as the pilot has sufficient altitude and experience to do otherwise.
Three: Some of the 172 (not the 182, 206, or any other piston single Cessna made) flight manuals included language which said to "avoid" slipping with full flaps. AFaIK there was even one year when the wording was changed to include the dreaded word "prohibited" but later versions for the same model year retracted that and in any case the phrase was never in the limitations section or the TCDS which would be necessary if this mode of flight were truly not allowed. Unfortunately the "avoid" suggestion was taken to mean an outright ban by a large group of pilots including way too many CFIs with some apparently inventing symptomatic "reasons" why slips should be avoided ranging from "the airplane will suddenly enter a spin" to "the elevator will be damaged". The only issue that ever existed in relation to slipping a 172 with full flaps is a slight pitch bobble that might be distracting but won't by itself interfere with control and it can be eliminated immediately by simply letting up on the rudder.
I hadn't heard those particular reasons before. The two I've heard have been fuel un-porting (very likely with low fuel in many airplanes) and elevator downwash blanketing (results in nose bob).
You can put me down in the "
teaching student pilots to fly according to the POH" camp of CFIs. Not because the airplane is incapable, but at that
specific level of learning, we're not
expecting test pilot or even commercial pilot knowledge and skill.
While you and I may know that the worst that will happen in a 172N full flaps in a slip is some slight nose bob, that's something we learned at altitude (hopefully). I will certainly
demonstrate this to student pilots (and may in fact let them try it for the sharper, less timid ones), but I don't
expect mastery. If the Cessna model warns against slips longer than 30 seconds, I keep the student to that on real approaches.
Why?
If the student needs a 2 minute slip to lose altitude in order to land, he/she should be going around anyway.
Besides if he's been doing 2 minute slips on final, tries that on the checkride, and the POH suggests no longer than 30 seconds -- I'm guessing there will be issues -- not because the DE is an idiot, but because the applicant is exhibiting poor judgment. The POH has a limitation, the applicant is ignoring that limitation -- what other limits is he ignoring?
While we all know the airplane will keep flying, at the student and private pilot level or training we're not
expecting that level of airplane handling. Eventually that pilot should expand that envelope -- absolutely! -- but CFIs need to know their students and the expected levels of mastery and teach to that.
IF there is a student who shows above average proficiency/ability/skill -- then by all means capitalize on that and if he/she can do more than you (as CFI) can teach, then send him/her to a fellow CFI who can really push that student.
Anyway, all good points so far and worth discussing, even if it's been rehashed before.