Airplane Flaps – What are They, and How Do They Work?

Anyone who has flown on an airliner and looked out over the wing during flight is probably familiar with flaps.

They come out of the wing, a sure sign that you’re slowing down for landing. The wing grows and changes shape in fascinating ways.

But after watching this you may be asking yourself, what do flaps do on a plane?

Not all planes have flaps, but most do. And while the flaps on our Piper or Cessna might not be as big or fancy as the flaps on that Airbus, they perform the same functions.

Here’s a look at what they’re for and how they work. This article is part of our flying basics article series covering the basic parts of an airplane.

What are Airplane Flaps?

Before we descend into the details of flaps, it’s important to make one distinction.

Flaps are an example of a secondary flight control. Unlike primary flight controls–the elevator, rudder, and ailerons–secondary controls are not responsible for controlling the airplane’s motion around an axis of flight.

Instead, the pilot uses secondary flight controls (like flaps) to modify some flight characteristics of the plane.

The purpose of the plane flaps is to change the shape of the wing so that it can make more lift at lower speeds. This enables pilots to approach runways at steeper descent angles and helps them avoid obstacles.

Flaps are a type of high-lift device.

Flaps on a plane are attached to the wing’s trailing edge near the root. How they extend depends on the plane, but the most common method is with an electric motor.

The pilot activates the motor with a cockpit switch, and the motor moves a linkage that extends the flaps on both sides of the plane. 

To activate the flaps, the pilot has a control in the cockpit. The control placement depends on the plane, but it is generally mounted on the panel and has a handle that looks like a flap.

The switch usually has detents (notches) built in to allow the pilot to set certain plane flap settings.

Some planes refer to these settings in degrees, while others mark them in positions like “Flaps 1” or “Flaps Approach.”

Piper Archers have a unique system for their flaps.

There is a handle between the front seats that resembles a parking hand brake in a car.

When the handle is flat on the floor, the flaps are up. Bringing the handle up lowers the flaps with three detents (10, 25, and 40 degrees). The system is completely manual, with no motors and nothing to fail. 

Dangers of Differential Flaps

It’s important to know how your plane is rigged. For example, many low-wing planes have the two flaps, one left and one right, welded together on a single strut.

Move the strut, and both flaps will go down the same amount. If the motor dies, the flaps won’t move up or down, but they will be where ever they are equally on both sides of the plane. 

But some planes, Cessna 172s, for example, can’t have a single spar tying the two sides together.

The bar would extend through the cabin.

So instead, Cessnas use a system of pulleys, cables, and bellcranks to move the wing flaps down, all of which are actuated by an electric motor. The problem with this system is that, should one pulley get stuck or a cable break, it is possible for one flap to extend without the other. 

That could lead to a dangerous situation known as asymmetric flap extension. The lowered flap will make more lift, and the airplane will begin to roll in the direction of the higher flap.

Flaps are powerful, so if corrective action isn’t taken promptly, it is possible that the ailerons might not have enough authority to overpower the flaps. 

What do you do?

First, learn your plane’s systems and know if asymmetric flap extension are even possible.

If it is, use caution every time you put the flaps down. Pay attention to the roll of the plane, and be ready to correct should there be any indication of a roll you didn’t ask for.

If the plane does start to roll, hold level with the ailerons while retracting the flaps to their previous position. If you can’t get the flaps up, leave them set as evenly as possible.

Types of Flaps

There are four types of trailing edge flaps that you’ll see on airplanes. 

• Plain flaps
• Split flaps
• Slotted flaps
• Fowler flaps

Plain Flaps

As their name implies, plain flaps are the simplest of the bunch. They resemble ailerons, except they are on the wing root and move downward together.

Since these aircraft flaps work by increasing the wing’s angle of attack, induced drag is increased as they are lowered. Of course, there is an increase in lift, too, but mostly there’s drag. 

Split Flaps

Split flaps are less common on small planes but can be found on many light twins. On a split flap, only the lower part of the wing moves.

The upper camber of the wing remains fixed to the trailing edge. The results of a split flap are very similar to a plain flap–a little more lift but a lot more drag. 

Slotted Flaps

Slotted flaps look like plain flaps but with one important difference. When the flap extends, a slot is exposed that allows air to travel over the upper surface of the flap surface.

That makes them very effective at high angles of attack, and it adds a considerable amount of lift compared to other flaps. 

Slotted flaps are one of the most popular flaps used on light aircraft. They’re relatively easy to design and provide a big boost in performance regarding soft or short field landing and takeoff capabilities. For example, Cessna 172s have very distinctively slotted flaps. 

Fowler Flaps

Those fancy flaps you see on airliners that make their wings double in size? Those are Fowler flaps. These flaps extend not only downward but outwards as well, making the wing’s surface area much larger. A bigger wing generates more lift and slow speeds. 

Think about the enormous speed difference airliners experience between takeoff, landing, and normal cruise. At cruise speed, the wing needs to be as small as possible.

There’s plenty of air flowing over it to lift, and any extra wing area will only introduce drag.

But that same airliner cannot land at 300 knots. So to slow down and approach runways at a reasonable speed, the wing shape needs to change to have enough lift and good low-speed handling characteristics.  

How Do Plane Flaps Work?

Now, with a little background about what they are and how the pilot uses them, what are the aerodynamics that makes flaps do their thing? Here’s the short version, but if you want the full briefing open up “Chapter 6: Flight Controls” in the FAA’s Pilot’s Handbook of Aeronautical Knowledge. 

Aircraft flaps work by changing the chord line of the wing. You’ll remember that the chord line is an imaginary line drawn from the leading edge to the trailing edge.

When you put the flaps down, the trailing edge moves down, changing the chord line.

The chord line is important because it is half of what makes the angle of attack (AOA). The AOA is the angle between the chord line and the relative wind. So by changing the chord line, the pilot increases the AOA, which makes more lift. 

But it isn’t all good news. As the AOA increases, so does induced drag. So while flaps add some lift and allow the pilot to fly at lower airspeeds, they also add drag.

Some flap designs add more drag than others, which is one of the main reasons it’s important to get a feel for how a new-to-you plane flies during slow flight. Some may require more power than others to overcome excess drag at slow speeds.

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