As far as I remember (which is not much) not doing forward slips with flaps extended in a Cessna is a recommendation, not a prohibition. I think the wording in the POH was something like, "Forward slips with flaps fully extended not recommended."

makes sense...
 

That makes sense because older 172s have a 40 degree electric flap setting. I flew a C172N with that on it but I do not remember there being a forward slip placard. Time to look for cockpit photos online.... By the way, the 40 degree setting was nearly useless and if it caused spins no wonder they took it away. It added some drag but was hardly worth the delay in retraction time.

By the way, anyone care to comment on whether it is safe or not do forward slips to a crosswind. The wind hits the airplane from the side and you broadside the oncoming wind; is this a recipe for disaster? Generally forward slips are done in winds coming straight down the runway, but they can be used to slow the aircraft in crosswinds by turning away from the wind. Right wind, hard left rudder, turn the airplane so it gets broadsided by the crosswind; pitch down hard and let the right aileron deflect upward to control the path. Obviously this wouldn't be useful for very long due to directional loss, but I wonder how much you run the risk of stalling the left wing since it is now in the turbulent wake of the rest of the airplane. I am leery of testing full forward slip in quartering crosswinds at low altitude because I fear the airplane may whipstall like the dreaded late-turn-to-final scenario. Or, maybe because the angle of attack is low in a forward slip the possibility of one wing stalling is negligible. Comments?

You are correct. I had this same converastion with my instructor today during my pre-solo brief. Here is what I came up with. The direct verbage from page 4-48 of the 172S PIM is "Steep slips with flap settings greater than 20* can cause a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle and center of gravity loadings." Here is another quote from page 4-49 of the same PIM, "If flap settings of greater than 20* are used in sideslips with full ruddder deflection some elevator oscillation may be felt at normal approach speeds. However, this does not affect control of the airplane." I beleive it is that last sentance that makes it a recommentation not to perform them as to a prohibation.

Brian

I would suggest complying with the published limitations for your aircraft...read the POH, different cessnas are different, but I do not recall a limitation on slips with partial flaps, only full flaps. With that being said the story about flaps blocking airflow and stalling the rudder is an old wive's tale. The real issue was turbulent airflow causing buffeting, which would probably be harmless, but might scare or confuse an inexperienced pilot.

I've done forward slips with full flaps on a 172 and felt the buffet. It's minor.

The 172S is different. I got checked out in one a few years ago, and I remember seeing that in the POH. Some older Cessnas do have a limitation for full flap slips, as the one I got my private in did. As for the tail stall being an old wives tale...turbulent airflow over an airfoil is the beginnings of a stall, and with a slow enough airspeed, or sudden change in relative wind, it would be very easy for a tail stall to develop. I agree that it can probably be done and has been many times, but the recommendation and / or limitation is based on what can happen, not what will happen every time.

Actually it would not be disregarding a placard. The placard uses the word "avoid" and it implies to "avoid" doing this during landing (aka close to the ground). Think about it this way: I doubt the POH would recommend practicing a power off stall on final approach, but there is obviously nothing wrong with practicing power off stalls at a safe altitude. And yes he should go spin an aircraft (not an arrow) with a qualified instructor. People who avoid learning how their aircraft stalls, spins, tail stalls, etc because of safety concerns are actually less safe as pilots. If recovery technique is not a natural instinct, you'll be dead if and when it happens close to the ground.

I am not saying that you shouldnt go practice these things at safe altitudes like you stated. I am however cautioning against doing something that the aircraft has cited as"either prohibited or should be avoided". It's there on a placard for a reason, and that reason is not for you or a new pilot to go and try and figure out. As far as the "whipstall" on base to final that you are speaking of, it is called a cross controlled stall, and has nothing to do with flap settings or changes of relative wing. As far as on stalls and so forth, your focus should be more on the recognition of the impending stall and not so much the recovery, although the recovery is important. The main focus however should be on how detect the oncoming stall so that student never has to recover from an actual one.

Hehehe, reminds me of a "Lesson" I learned. Full foward slip in a 172N(1979 model). I had in the 40-degree's of flap(Oh yeah, loved that). I had a very nice buffet, and eventually it created a nuetral elevator feel in which I had no elevator authority. It was really weird, but as soon as I relaxed the rudder, all returned to normal.

And yeah, everyone should know how and practice forward slips in my opinion. Starting with them in the practice area and moving towards doing them on landing. Either way, use a straight-line reference(runway, road, power lines, etc...) to get used to how much rudder/aileron mixing you need to maintain a straight course.

The N model only had 30 degrees of flaps. I've owned both.