Is all the extra lift from flaps added upstream where the spars sit?

Question

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.

Source: dreamstime.com

Answer 1

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:

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.

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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).

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

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Sources of images:

• Fujii, Kanata, et al. "Numerical study on adaptive wing structure using leading and trailing edge flaps for reduction of bending moment." JAXA Special Publication: Proceedings of the First International Symposium on Flutter and its Application. 2016.
• Zaccai, D. Design Framework for Trailing Edge High-Lift Systems: A Knowledge Based Engineering Application. Diss. Delft University of Technology, 2014.

Airbus paper:

• Strüber, Henning. "The aerodynamic design of the A350 XWB-900 high lift system." 29th international congress of the aeronautical sciences. 2014.