Aircraft Wings: How Flaps Work

Aircraft Wings: How Flaps Work

Here is an article to demystify the aerodynamics of flaps on an airplane

These Project Air articles about Simple Aerodynamics break down the basic parts of aerodynamics that you need for the RC airplane hobby. We’ve covered wings and lift already, so now let’s talk about how to slow down airplanes. Let’s dive right in!

To understand how flaps work, firstly, you have to understand how a wing creates lift. Make sure to read this previous article on wings, airfoils and the key principles of lift!


Air reacts with an airfoil to increase the lift of a wing. It's quite intuitive to see why this is: just like a standard aircraft wing, a deflected flap pushes air downward. This has the effect of pushing the wing in the opposite direction - up! This is the Newtonian understanding of how lift works.

When a flap is introduced into the system (refer to the diagram below), the airflow is further deflected downward as expected. This increases the force exerted on the wing's underside, thereby generating additional lift. Along with this lift, there is an increase in drag due to the airflow on the upper surface of the wing detaching at the trailing edge and forming a swirling vortex.


Issues with Aircraft Flaps

Sometimes, fitting flaps to an airplane can be a rather bad idea... For instance, with a high wing plane, where the wing is above the CG (centre of gravity), the increased drag of the flap can pitch the plane upwards as it rotates around this point.

On the flipside (literally), Low wing planes have the opposite problem. Lowering flaps and increasing drag below the center of gravity might cause the plane to pitch down.

This can be counteracted, though, through adjusting your RC transmitter settings. By setting up mixing, you can program the elevator to counteract the flaps' effect, balancing the plane out. This way, you don't need to worry about it. Pretty cool, right?

Flaps are extremely helpful for slowing your aircraft down for landing, enabling you to fly from smaller airstrips. They can also be used to an extent on takeoff, providing more lift so you can take off in a smaller space at a lower airspeed.


I hope this article helped dispel some of the myths around flaps and provided a practice explanation of how they work. If you'd like to see an article on another Simple Aerodynamics topic, comment below.

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