What Flaps on an Airplane Do: They Don’t Just Push Upward

Slotted flaps, hinged or tracked, add a separate source of lift.

Friday, February 28, 2025 at 08:59 AM ET

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Flaps, or more properly 'trailing edge high lift devices,' have two functions—they increase drag and enable the wing to sustain lift at a lower speed. [FLYING Magazine]

Key Takeaways:

Flaps primarily function to generate additional lift at lower speeds, enabling better control during approach rather than just increasing drag.
The article refutes the common "equal transit time" Bernoulli explanation for lift, asserting that lift fundamentally stems from the wing's angle of attack, which shifts the stagnation point and alters airflow to create suction over the entire upper surface.
Various flap designs, like split and plain flaps, work by manipulating airflow and the stagnation point, while slotted flaps further boost lift by creating additional "mini-wings" with their own suction peaks.

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Flaps, or more properly “trailing edge high lift devices,” have two functions—they increase drag and enable the wing to sustain lift at a lower speed.

The purpose of increasing drag is not so much to steepen the approach as to require you to use some power to maintain a desired approach angle. If you carry a moderate amount of power during the approach, you can adjust the aim point with small throttle movements. Airplanes that approach at idle power, as ones with low-wing loadings like old taildraggers do, have to use the somewhat less delicate forward slip to steepen the approach.

Peter Garrison

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, ""Technicalities"" and ""Aftermath,"" with FLYING readers.