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  • Q810430

    When flaps are extended whilst maintaining straight and level flight at constant IAS, the lift coefficient will:

    (A) Eventually remain the same.

    (B) First decrease and then increase.

    (C) Eventually increase.

    (D) Eventually decrease.

    Accepted answer is A.

    The explanation justifies this by stating that, for lift to remain constant, all variables in 1/2*CL*S*rho*v^2 must remain constant, and therefore CL remains constant.

    Whilst I would indeed agree with this if everything else was kept constant, I am not entirely satisfied that S would remain constant with the extention of flap. Some flap types do not change S, granted, but certainly some do (Fowler etc). Therefore I'm not sure the question can be answered as-is.

    If S did increase, then CL would have to reudce (answer D).

    Can anyone clear this up? Cheers!

  • #2
    What you say is correct. But, in the introductory notes to EASA PoF Learning Objectives, it states that (in the exam):

    Normally, it should be assumed that the effect of a variable under review is the only variation that needs to be addressed, unless specifically stated otherwise.

    In this question, the only variable under review is the lift coefficient. It does not specifically state that the surface area is increasing.

    Our experience in practice is that, if we are supposed to consider Fowler flaps, the question will specifically say so. Otherwise, we should assume that we are being asked about the most simple, basic, plain flaps.


    • #3
      The "S" in the lift equation is not necessarily the actual wing area, it is a "reference" wing area which is fixed for a given aircraft type and does not change with flap deployment.

      When winglets were retro fitted to the Boeing 737-800 this changed the actual wing area, but the reference wing area for all Boeing 737 600 though 900 is 1340 sq ft and that did not change.