The POH strongly recommends not slipping the AC with full flaps...why?
It seems natural in a crosswind approach to allow the horizontal lift component to cancel the airmass movement across the centerline.
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$\begingroup$ Fastest edit ever! I predict a massive response from our two resident sideslip experts, complete with breakdown of all forces, real and imaginary. Plus the effect on the ball. (I wont name either of them... ;) $\endgroup$– Michael HallMay 7, 2020 at 16:56
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3$\begingroup$ I'm pretty sure this has been answered on ASE. I seem to recall the answer has to do with the turbulent wake on the horizontal tail and a resulting loss of pitch stability. Not sure why this would be different in a slip than in non-slipping flight though. Have experienced a weirdness in pitch control when slipping with full flaps though, esp in the versions where flaps went to 40 degrees. Can't recall whether said weirdness was a nose-drop tendency, or just a marked reduction in yoke pressure per unit change in elevator position away from trim, or what exactly. $\endgroup$– quiet flyerMay 7, 2020 at 16:56
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$\begingroup$ @Bianfable -- Thanks for the edit, seeing as a Cessna 172 is not a military cargo aircraft. $\endgroup$– quiet flyerMay 7, 2020 at 16:57
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2$\begingroup$ The use of full flaps can blank the airflow going over the tail, resulting in loss of pitch control; not good when at low altitude coming in for a landing. $\endgroup$– CrossRoadsMay 7, 2020 at 17:25
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$\begingroup$ @CrossRoads -- that could be an answer. Yes I seem to recall that in the situation referenced in my comment, larger-than-normal movements of the yoke were needed to control the aircraft's pitch attitude. $\endgroup$– quiet flyerMay 7, 2020 at 19:42
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2 Answers
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The basic problem is that sideslip induces a cross flow over the fuselage that can increase the local negative AOA and flow disruptions beyond that already caused by the turbulence and downwash being generated by the flaps. The stabilizer surface on the "lee" side of this cross flow may get enough flow disruption to be felt in the elevator, maybe accompanied by a bit of pitch bobbing at some extreme condition that maximizes the tail loading, like side slipping at forward C of G with large flap angles. In spite of its very powerful flaps, the 172's issue is fairly benign (or there would be a prohibition against using slips with flap as opposed to an advisory).
This can be a significantly worse problem with stabilator or all-flying tails that operate at larger negative AOAs than fixed stabilizer tails, as Cessna Cardinal owners are well familiar with. The Cardinal required slots to be added to the stabilator inboard leading edge to fend off tail root stall during landings, and resulting broken nose gears, during the first year of production, and that was just doing regular landings with maybe a bit of slip due to crosswind (the '68 Cardinal I once owned had the field mod slots added under the "Cardinal Rule" service bulletin - Cessna sent teams out to modify every aircraft pre production cut-in in 1968, somewhere around 500 a/c, and you can slip them with flap all you want).
The stabilator equipped Thorp T-18 homebuilt had to have a flap limitation added due to stabilator stall in sideslip (John Thorp did some testing after an incident in the 60s, discovering that the stabilator could stall with full flap, forward CG and sideslip, causing a nasty pitch over, and advised owners not to use full flap for landing).
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There could be several reasons for such recommendation, but specifically for C172, its POH explains it as follows (section 'Normal Landing'):
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.
