How do the different types of slotted flaps (Single, double, and triple as well as a non-slotted one just for comparison) differ aerodynamically? I read in Joe Sutter's book 747 that a double-slotted flap design for the 747 had 8 knots higher speed than a triple-slotted one. How does that work?

  • $\begingroup$ which speed does the book refer to? stall speed? cruise speed? second-segment climb speed? $\endgroup$
    – costrom
    Feb 22, 2016 at 23:56
  • $\begingroup$ @mins Low Speeds $\endgroup$ Feb 23, 2016 at 0:41
  • $\begingroup$ There is a comparison of flap type efficiencies in this answer and a smaller set, but with full lift graphs in this one. $\endgroup$
    – Jan Hudec
    Feb 24, 2016 at 7:47
  • $\begingroup$ @JanHudec They compare the different types of flaps, I just want the different variations of the slotted flap. $\endgroup$ Feb 24, 2016 at 12:59
  • $\begingroup$ Related: (1953) study. $\endgroup$
    – mins
    Mar 2, 2016 at 8:47

1 Answer 1


The high energy air that passes through the slots prevents boundary layer separation over the top surface of the flap. Though, this increases drag but it improves the stall characteristics of the airfoil. In other words, we can have higher maximum lift coefficient (Cl_max) by having higher maximum angle of attack (alpha_max). So, higher the number of slots higher Cl_max one could reach, there should be a limit. In case of a fixed weight of an aircraft,a higher lift coefficient implies that one can fly at lower speed before it stalls. A comparison between different types of flaps (up to double slotted)can be found in the book Aircraft Design and Performance by Anderson.

Source: The same book.

  • 1
    $\begingroup$ just one detail: More slots will not necessarily give a higher maximum AoA, but a higher distance between the zero-lift AoA and the maximum AoA. More slots translate into more camber and a more negative zero-lift AoA. Also, each slot allows to stretch out the flaps more, so the Fowler effect of an increased surface area is increased. $\endgroup$ Feb 27, 2016 at 20:09
  • $\begingroup$ There is no high energy air in slots,only if you have external source like air pump...total pressure cant be higher then in freestream $\endgroup$
    – 22flower
    Oct 3, 2021 at 18:00
  • $\begingroup$ @JurgenM Total energy cannot be higher in the free stream, but it can be lower - in the boundary layer. Slots help in restoring this BL energy. While the energy of the air in the slots is not higher than the total energy, it is relatively higher when compared to that of the BL, and that's what he meant with "high energy air". $\endgroup$ Dec 21, 2022 at 9:38

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