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5 Answers

Flaps and Turns

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Why I cannot give Flaps on turns and why I cannot sideslip the aircraft with flaps?


Thanks in advance.


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5 Answers

  1. Chris Carlson on May 07, 2013

    Some Cessna 172s and maybe other models I’m not aware of prohibit slips with full flaps because it ‘shadows’ the elevator and may cause a tail stall, which on a final approach will likely be your last stall.

    Putting flaps down when in a turn is just a safety precaution incase you have only one flap go down or retract, which would cause asymmetric lift and drag, and make for an ugly situation.

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  2. Sam Dawson on May 08, 2013

    Not prohibited in 172s (except- I think for 1 model that may actually have the statement prohibited). It is “Not recommended”.

    Also, the reason cited by Chris for the recommendation against slipping a 172 with flaps is false.
    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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  3. Chris Carlson on May 08, 2013

    Thank you, Sam.

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  4. Sam Dawson on May 08, 2013

    I’m sorry if I came across as harsh in my previous answer. I was just attempting to correct what is a common perception.

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  5. Chris Carlson on May 08, 2013

    No no, I am very glad you gave me reference.

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