The subject  of slipping, specifically with high wing Cessna's; entertains a lot of discussion. 

Many years ago a fellow named Gene Seibel rallied a T-shirt design on rec.aviation.student. 
It was supposed to mirror the theme of "Runs with Scissors."    

I still have the shirt in my pajama drawer.  I guess it was appropriate for last years polar bear plunge, 
at MapleLag, in Detroit Lakes Minnesota.   A careful observer will note that I didn't plunge very deep that year. 
It is a yearly tradition, but the level of committment varies.   That is another story ...


 

The debate has always fascinated me. Perhaps becuase I have flown older taildraggers, or perhaps because
I am invariably too high and too fast.    My first exposure to the "controversy" was with an FAA Safety Inspector.

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I have mild CP and walk with a limp.  It affects the strength and dexterity with which I can apply my left foot.

As such I required a Statement of Demonstrated Ability in order to receive an unrestricted third class medical
and this required a flight test with the Milwaukee FSDO just prior to completing my private pilot training.
My second class waiver was awarded some years later based on operational experience alone;  perhaps 
I will detail the process in a later entry. 

The flight test  was fairly straightforward and the inspector simply asked for some of the practical test  maneuvers; all the while pointing out that if I had been properly organized this could have accomplished the flight test.  Perhaps the only unusual element was an "oscillatory" stall performed by pinning the control yoke full back and applying rudder corrections while the airplane stalled and recovered in succession.

As you can imagine, retiring my anxiety about the medical barrier was a large hurdle in my early training.
So when the examiner seemed satisfied, I was more than eager to point the Cessna 150 at MKE and tie it
down before some kind of fumble. 

At approximately 3000ft AGL over the field, and headed the wrong direction out to sea; the tower controller volunteers runway 7 with the sporty "can you make it work?"    I picked up the microphone and began to squeek "Student pilot would like a long slow series of gentle vectors around" when the nice lady from the FAA snatches the microphone,  responds "yes this will work."   She barks to me, "your flying is fine, and I am late for dinner."  Her arrplane.

As we curve around out of the sky she sets up a full control deflection side slip and asks "has your instructor ever talked to you about slipping with flaps?"   I recited the mantra and she admonished that the placard 
reads "extended slips are to be _avoided_"  It is not a formal limitation and they are not forbidden. She indicated that it was a valuable tool to adjust the approach angle in a power off landing (or in this case, increase descent rate to make up time when late for dinner). 

I watched her honk us down out of the sky and accomplish a very nice landing.   It may not have been very dramatic but I was quite early in my flying career and caught up in the event.

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The following paragraph is copied from the book "Cessna, Wings for the World" written by
William D. Thompson, an Engineering Test Pilot and later Manager of Flight Test and Aerodynamics at the Cessna Aircraft Co.


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° 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 cross-wind 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 counter-act it if it occurs close to the ground.

When the larger dorsal fin was adopted in the 1972 C-172L, this side-slip pitch phenomenon was eliminated, but the cautionary placard was retained.  In the higher-powered C-172P and C-R172 the placard was applicable to 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.

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The following link points to an online video from Ron Wanttaja, www.BowersFlyBaby.com.   It displays the slip seen from a digital movie camera mounted to the landing strut. 



                                      

                                                                   Fly Baby Slip to Landing

Finally this is a video from the ground of a slip in a glider:

                              
                                                               

                                                           YouTube, slip to a landing in a glider