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 (trailing edge high lift devices) primarily increase lift to enable slower flight, with their drag-increasing function being secondary for approach control.
The article refutes the common Bernoulli explanation for lift (air traveling a longer distance over the wing) as flawed, stating that angle of attack is fundamentally responsible for generating lift.
Lift is generated by a wing's angle of attack, which shifts the stagnation point, causing air to veer upward and accelerate over the leading edge, creating increased pressure below and suction above the wing.
Different flap types enhance lift by manipulating airflow; plain/split flaps shift the stagnation point to increase suction over the entire wing, while slotted flaps create additional mini-wings and 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.