Flaps are secondary control surfaces on the trailing edge of the wing that increase both lift and drag. When extended, flaps increase the aerofoil’s camber, change the chord line, and increase the angle of attack.
The simple flap extends downwards at the trailing edge of the wing. The air is deflected further downwards so the total reaction of the wing is increased, which increases both lift and drag.
The split flap is hinged below the wing and when extended does not change the upper wing surface. Split flaps create more drag than the simple flap as they act more like an airbrake.
The slotted flap has a gap that opens between the wing and the flap when extended. This allows higher pressure air below the wing to pass through to the top of the wing, where it remains attached to the top surface and deflects downwards. As with all flaps, both lift and drag are increased but the slot allows for a greater increase in lift than the simple flap.
The fowler flap extends rearwards as well as down to increase the surface area of the wing. The first stages of the fowler flap extend rearwards with only a small downwards deflection. This increases lift without adding too much drag – a good setting for takeoff. As the fowler flap extends further down in the later stages, drag is increased to help during the approach to land.
All flap types create a greater increase in drag than lift, though the ratio of lift to drag depends on the degree of extension.
The increase in lift at small flap settings can be beneficial in many circumstances despite the greater increase in drag. For example, during takeoff the increase in lift at small flap settings will allow the aircraft to become airborne at a slower speed, so you will be able to safely use a shorter runway. Since the increase in drag was also relatively small, climb performance after takeoff is only slightly reduced.
Larger flaps settings are typically reserved for the final approach to land since the large drag helps to keep airspeed low during descent and the extra lift allows the pilot to touch down at a slower speed.
The use of flap in level flight allows the pilot to adopt a lower nose attitude while maintaining the same amount of lift. This gives the pilot better forward visibility at low airspeeds, improving safety in poor weather conditions (such as haze).
Flaps have a maximum speed at which they can be extended, known as the Flap Extension Speed (VFE). This is shown by the white arc on the airspeed indicator. You must not extend the flaps unless you are below VFE.
For some aircraft VFE will be higher than the typical cruising speed, however many aircraft need to be slowed down before flap is extended.
It may be the case that your aircraft has different maximum speeds for different flap settings. For example, 10° of flap can be extended at 100 knots but 30° cannot be extended unless you are below 90 knots. Make sure to check the airspeed limitations in your aircraft flight manual.
Slats are leading edge devices that increase lift at lower airspeeds, such as during takeoff, initial climb, approach, and landing. The slats extend forwards from the leading edge of the wing, increasing the wing’s camber and surface area.
Slats are mostly used in larger transport aircraft with faster cruising speeds and are not typically found on training aircraft.