Aerodynamics Simplified: How Flaps Work

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 FliteTest.com

James@whomsley.net

www.youtube.com/projectairaviation

Instagram @jameswhomsley