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Flaps can significantly increase the lift for a wing, is all that extra lift added upstream where the spars sit? I ask this question because the flaps in airliners seem to be only connected by a few links and actuators.

enter image description here
Source: dreamstime.com

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  • $\begingroup$ It seems like the title question and the accepted answer are not in agreement. The accepted answer pulls from the body of the question. This results in a confusing situation for readers. @Mridul, would you mind editing the title so that it reflects the question in the body? $\endgroup$ Sep 7 at 22:59
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You've asked:

  • Is all that extra lift added upstream where the spars sit?

What if I told you flaps decrease the bending moment on the spars? And the Airbus A350 takes advantage of that in cruise? Interesting? Read on:

enter image description here

As shown above, the higher the flap angle ($\delta$), the lower the bending moment. The reason is really simple:

In steady flight – i.e. a plane climbing at a constant rate, cruising, or stabilized on approach on the glide slope – whether flaps are deployed or not, lift = weight. (If you account for the thrust, "A climbing aircraft needs less aerodynamic lift than in horizontal flight, not more.")

Put another way: just like how the angle of attack is increased as the plane slows down to maintain the lift, deploying flaps does the same – they allow the total lift to be maintained at a slower speed.

Since most flaps are partial and don't cover the full span, and with the total lift maintained: the flap section creates more lift, and the outboard section less lift; with less outboard lift, the bending moment is reduced.


And from an Airbus paper on the A350:

[Differential flap setting] can be applied in early cruise phases to shift the center of lift more inboard and by that reducing the wing root bending moment, which can be transferred into a structural weight saving.

If you watch closely and listen carefully to the hydraulics, you can see it in action here (around 00:21).

enter image description here

Regarding the connection, it's a lot more than small links; see above how they attach to a spar on a Boeing 777.


Sources of images:

Airbus paper:

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – fooot
    Sep 5 at 22:16
  • $\begingroup$ @ymb1 Thanks for the detailed answer. Another point that bothers me is the slotted flap is acting as an airfoil on its own. Is the increased lift coef due to the flap acting as a mini wing and experiencing loads like an isolated wing of the same chord or is the increase due to delayed separation because of the slot and flap doesn't actually bear the bulk aerodynamic loads. $\endgroup$
    – Mridul
    Sep 6 at 15:32
  • $\begingroup$ @Mridul: You're very welcome. The increased lift coefficient is due to both things you mention. I recommend checking out this answer by Peter Kämpf where he compares plain and slotted flaps and links to a NACA report. Pay attention to his explanation on how to interpret the graphs. $\endgroup$
    – ymb1
    Sep 6 at 15:44

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