Production of lift is dependent primarily on airspeed, angle of attack and aerofoil design. A fundamental component of aerofoil design is the camber. Camber is defined as the convexity of the curve of an aerofoil from the leading edge to the trailing edge.
Flaps are used during takeoff and landing phases of flight and help to improve aircraft performance.
- Flaps increase the airofoil camber, resulting in a significant increase in the coefficient of lift.
- Flaps greatly increase drag and move the center of pressure aft on the airofoil, resulting in a nose-down pitching moment. It allows you to fly a steeper descent angle to the runway.
- When used during takeoff, flaps trade runway distance for climb rate: using flaps reduces ground roll but also reduces the climb rate.
- Flaps may be fully extended for landing to give the aircraft a lower stall speed so the approach to landing can be flown more slowly, which also allows the aircraft to land in a shorter distance.
Types of flaps
There are many different flap designs and configurations in use. Large aircraft sometimes incorporate more than one type, utilising different flap designs on the inboard and outboard sections of the wing. Let's talk about most popular configurations.
The rear portion of the wing aerofoil rotates downwards on a simple hinge arrangement mounted at the front of the flap. It increases the airofoil camber, resulting in a significant increase in the coefficient of lift at a given angle of attack. One of the main reasons this type of flap is not used very often is that other, more advanced flaps are now available.
Example: Cessna A185
The split flap is deflected from the lower surface of the airofoil and produces a slightly greater increase in lift than the plain flap. More drag is created because of the turbulent air pattern produced behind the airofoil. When fully extended, both plain and split flaps produce high drag with little additional lift. If you visit an airshow and look around at the older warbird planes they usually feature, you might see some split flaps. Otherwise, they are not very common these days.
Example: Douglas DC-3
Similar to a plain flap but incorporates a gap between the flap and the wing to force high pressure air from below the wing over the upper surface of the flap. This helps reduce boundary layer separation and allows the airflow over the flap to remain laminar.
Example: Cessna 172
A split flap that slides rearwards level for a distance prior to hinging downwards. It thereby first increases chord (and wing surface area) and then increases camber. This produces a flap which can optimise both takeoff (partial extension for optimal lift) and landing (full extension for optimal lift and drag) performance. This type of flap or one of its variations is perfect for very large jets that need a lot of lift, but a limited amount of drag.
Example: Boeing B-52
Slotted Fowler flap
This design improves the performance of the fowler flap by incorporating the boundary layer energising features of the slotted flap.
Example: Boeing 747
- Pilot’s Handbook of Aeronautical Knowledge by FAA - Chapter 6 Flight controls
- SKYbrary library - Flaps
- 531824 - Creation
DATE OF SUBMISSION
- 12:51, 23 February 2021
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