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Flaps help your wing adapt to your current phase of flight. Are you taking off or landing? Extend your flaps to increase lift and fly at slower speeds. Are you cruising at altitude? Retract them to reduce lift and drag.
But how exactly do flaps they work? To put it simply, flaps increase the camber (and sometimes the area) of your wing. By increasing the camber of your wing, you also increase the amount of lift your wing can produce. With flaps down your wing can produce more lift at the same, and even slower speeds, than when your flaps are retracted.
Using flaps gives you three distinct advantages in your plane:
Not all flaps are created equal. In fact, there are 4 primary flap designs, and each of them have advantages and disadvantages. Here's how they work.
The most simple flap is the plain flap. Plain flaps hinge to the back of the wing, and they pivot down when you extend the them. However, they're fairly limited in the amount of lift they can create. That's because as air moves over the wing, it loses energy and starts to separate from the wing. By extending flaps, the airflow separation is even more pronounced, creating a large wake behind the wing.
But you can use that wake to your advantage. The drag created by the wake lets you fly a steeper descent to landing without increasing your airspeed.
Next up are split flaps, which deflect from the lower surface of the wing. Split flaps produce slightly more lift than plain flaps, but like their plain counterparts, they also produce a lot of drag.
Split flaps are pretty uncommon these days, but you can find them on the wings of several warbirds at your local airshow.
Slotted flaps are the most commonly used flaps today, and they can be found on both small and large aircraft. What makes them so special? Two things:
By opening a slot between the wing and the flap, high pressure air from the bottom of the wing flows through the slot into the upper surface. This adds energy to the wing's boundary layer, delays airflow separation, and produces less drag. The result? Lots of additional lift, without the excessive drag.
When you need serious lift, you need serious flaps, and Fowlers are here to make it happen. Fowler flaps increase the area of your wing by extending out on rails or tracks. Fowler flaps often have a series of slots to add energy to the airflow as well - they're called slotted-Fowler flaps.
In the first stages of a Fowler flap's extension, there's a large increase in lift, but little increase in drag, making the setting ideal for takeoff in an airliner. As they continue to extend, the flaps move downward more and more, creating a little more lift, but a lot more drag.
So there you have it. The next time someone asks you about flaps, not only can you list off the 4 types, you can tell them how each of the flaps actually work.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.