Flaps are very useful for giving an airplane good handling characteristics at low speed. They are vitally important for giving fast planes the ability to go slow for takeoff and landing.
There are 6 types of flaps
- Plain Flaps
- Split Flaps
- Slotted Flaps
- Fowler Flaps
A simple hinge at the rear of the wing is used to make plain flaps. They are easy to design but they can’t provide much lift before the drag increases very significantly.
Since increasing drag is one of the purposes of a flap the performance is not too bad for landing. When taking off though, drag is not desired.
The reason this flap has so much drag is because the air from above the wing tends to separate and become turbulent when it has to flow downwards at such a steep angle. Think of it like a car on the highway having to make a sharp turn to stay on the road. This airflow separation is like that car spinning out because it can’t make the turn.
Turbulent airflow separation above the wing reduces lift.
The split flap surface is actually below the wing and pushes down out of the bottom. It does provide some lift but it creates a lot of drag and is almost like a speed brake under the wing.
This is the most common type of flap because it is relatively simple to design and build but provides a huge benefit over the plain flap. The slot created when the flap extends allows air to flow from under the wing.
This airflow provides a cushion for the air from on top of the wing to keep it from separating. It also flows along the top of the flap surface. These two benefits combine to create a lot of extra lift.
Fowler flaps are complex but provide a lot of benefit. They act like slotted flaps opening up a channel for air, but they also slide outwards as depicted below. By sliding out from the wing they greatly increase wing area.
Put simply, they make the wing bigger. A bigger wing means lots of lift!
Most modern airliners use double or triple slotted fowler flaps. These have several flap surfaces that extend out from each other as the flaps are lowered creating a much larger wing with several slots for air to pass through.
This modification to the leading edge of the wing provides a channel of air that is pushed up over the wing and channeled towards the upper surface. At low speeds, this simple fixed device can increase the critical angle of attack (the angle at which the wing stalls). This means that the plane can fly much slower without stalling.
Slats are another type of leading edge flap. They slide down at low speeds and provide a large increase in lift like the fixed slot. The benefit of the slat is that it is retractable and won’t create extra drag at high speeds because it slides up into the wing surface.