Aerodynamics Simplified: How Flaps Work

by FliteTest | June 27, 2018 | (9) Posted in Tips

Here's a super simple explanation of how flaps work on an aircraft - that's why we call it aerodynamics simplified after all!

This series of articles is all about simplifying the most essential components of aerodynamics that you'll need to know about in the RC hobby - whilst avoiding confusing you. Following on from wings and lift, this one is all about how we slow down airplanes using. Okay, let's get right into the explanation.

To really understand the principles of lift. If you haven't already, make sure to read the previous article on wings, airfoils and the key principles of lift!

When it comes to looking at how air interacts with an airfoil, it's fairly straightforward to see how a flap does its job. On the one hand, you can see that the airflow is deflected downwards slightly as it leaves the back end of the wing. The lower pressure and newtonian force of air pushing up against the wing underside helps to create the lift we spoke of in the previous article. You can imagine that if this edge of the wing was angled downwards to a greater extent, the lift air would be deflected more. This is what a flap does. 

When you add a flap into the mix (see diagram below), as predicted, the air is deflected downwards even more. This exerts a greater force on the underside of the wing with has the reaction of creating more lift. As well as more lift, you also get more drag. This is because the air going over the top surface of the wing falls off the back and creates a swirling vortex. 

There are sometimes problems with putting flaps on airplanes. If you have a high wing aircraft, with the wing above the center of gravity, increased drag from the flap can pitch the plane upwards as it pivots around the CG. 

On the other side of the coin, Low wing airplanes have the opposite problem. Deploying flaps and inducing drag below the CG can cause the plane to pitch downwards. With both of these situations, the issue can be fixed with some mixing on your transmitter. Essentially, you can tell your elevator to compensate for the flaps by adding some trim to counteract the effect. This way, you don't have to worry about it. Neat hu?

The reason that flaps exist on airplanes is very simple: they allow you to fly slower. They are especially helpful on takeoff and landing. With flaps, you can fly at a lower airspeed with the same amount of lift you would have without flaps at a higher airspeed. When you come in for a landing, flaps will allow you to descend in a shorter distance without trading your altitude for airspeed. 

Hopefully this article has helped to explain how flaps work and why they're often great things to have. If you have any comments, additions or suggestions, leave a comment below to benefit others!

Article by James Whomsley

Editor of

Instagram @jameswhomsley


Kotze101 on June 27, 2018
Lift from a wing (or a rotor blade) is created from the angle between the chord line and the relative airflow - the angle of attack (AoA). The chord line is a line from the center of the leading edge to the end of the trailing edge. By lowering the flaps the chord line angle changes thus increasing the AoA and increasing the lift...
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CptCrazyFingers on June 27, 2018
While that is true, it’s not always the case. A standard airfoil is capable of producing lift at a zero angle of attack and even up to a few degrees below zero, depending on camber of course. This is because of the difference in pressure between the top and bottom surfaces of a wing. This is what Bernoulli’s theory is about. Because the air over the wing has farther to travel, it will travel faster to try and meet the air going under the wing at the trailing edge. This results in a lower pressure on the top of the wing which results in a net, upward, force on the wings.
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Jackson T on June 27, 2018
They also increase the curvature of the aerofoil (the camber), that would create more lift even without increasing the AoA.
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CptCrazyFingers on June 27, 2018
Exactly, flaps actually change the shape of the airfoil. Using something called thin airfoil theory, you can imagine that the aerodynamics of an airfoil can be calculated by using the shape of it’s camber line. When flaps are lowered, they essentially change the shapes of the chord and camber lines. The change to the chord line results in greater lift because it increases the effective AoA. The change to the camber line results in greater lift because the lowered flaps create a longer path of travel for the airflow on top of the wing which increases the pressure differential between the top and bottom surfaces.
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Abouttime67 on July 4, 2018
What is the correlation of flaps and ailerons? Both on the wings yet different functions? I love your simple graphics and basic explanations... very useful for teaching my grandchildren. Any more articles in this series?
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CptCrazyFingers on July 5, 2018
Flaps change the airfoil geometry in order to change the overall lift characteristics evenly between both wings. Ailerons produce uneven lift (ideally, opposite and equal lift) between both wings in order to create a moment about the roll axis (basically, to make the aircraft roll). Flaps need to be close to the center of pressure, the point where all the aerodynamic forces can be balanced. Any force acting off of the center of pressure will produce a moment (torque) on the airframe.
This is exactly why the ailerons, elevator, and rudder are as far from the center of pressure as possible. Toque = force x distance.
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Razor7177 on July 4, 2018

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Abouttime67 on July 9, 2018
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Aerodynamics Simplified: How Flaps Work