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How Do Flaps Work?

Boldmethod

What actually happens when you lower your flaps? You increase lift and induced drag for any given angle-of-attack. Here's why it happens.

It All Starts With One Equation: The Lift Equation.

Different phases of flight require different wings, so you use flaps to alter the shape of your airfoil to get the performance you need. Lowering flaps on your aircraft increases your aircraft's lift, as well as induced drag. Here's how it relates back to the lift equation (don't worry, we'll make this easy!).

Let's use an example of landing your plane.

If everything in your formula remains constant while you reduce your airspeed (velocity) for landing, the amount of lift you produce will decrease.

As you enter the traffic pattern and land, you slow your plane from cruise speed to landing speed. But as you slow down, you don't want to lose too much lift and eventually stall, so something has to change. In order to compensate for your lower velocity, you change the wing's shape (and possibly surface area) by adding flaps. Understanding when to use flaps is really just about knowing how to 'balance' the equation.

How Do Flaps Give You Extra Lift?

It comes down to two primary changes in your wing: camber and chord.

First, let's start with camber. What is camber?

Camber is a line that follows the midpoint between the upper and lower surfaces of your airfoil. Wings with higher camber look more curved when viewed from the side.

Generally, as you increase your wing's camber, the coefficient of lift increases as well. This is because as your airfoil moves through the air, it can redirect more air (flow turning) causing your coefficient of lift to increase.

As a result, your total lift increases.

Your airfoil might already have some camber built into its shape, but as you lower your flaps, you increase its camber even more.

Chord Line

With your flaps retracted, your chord line is a straight line from the leading edge to the trailing edge of the wing. But when you lower your flaps, this chord line changes, altering the characteristics of your wing.

With many flap designs (like slotted flaps), when you lower your flaps, you'll see the wing expanding. This means you're also increasing the wing's chord length. Expanding your chord line's length also increases the wing's surface area, leading to more overall lift production.

Thinking back to the lift equation, increasing the surface area of your wing will cause your lift to increase.

Reduced Stall Speed With Flaps

Extending flaps reduces your aircraft's stall speed for a fairly simple reason. Because your wing creates more lift with the flaps down, you don't need as much angle-of-attack to balance the four forces of flight.

And because you can fly at a lower angle-of-attack with flaps extended, your stall speed will be lower as well.

Lift And Drag

Extending flaps increases drag as well, which, for the most part, is a good thing.

Any time lift is produced, drag is created as well. This type of drag is called induced drag. Normally you'd think drag is a bad thing, but you can use induced drag to your advantage.

One major advantage flaps provide you with is the ability to make a steeper descent at the same airspeed.

Putting It All Together

Lowering flaps changes the camber and chord of your wing, increasing lift and induced drag.

So the next time you're on a flight, not only can you tell your passengers what flaps are, you can tell them exactly how they work as well.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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