Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 84

The flaps are the most common high‐lift devices used on aircraft, and their contribution to the amount of lift an airfoil can produce will be discussed in more detail in Chapter 4. For our discussion here, we will review their location on the aircraft, as well as the basic flap designs on aircraft today.

Flaps for this discussion are considered to be on the trailing edge of the wing, usually inboard close to the fuselage, and are referred to as trailing edge flaps. These surfaces contribute to the camber of the wing airfoil in most cases, as well as to the area of the wing in other cases. By increasing the angle of attack of the wing, and in some cases the area of the wing, they allow the aircraft to fly at lower speeds, which may be needed for takeoff and landing, or for situations requiring increased maneuverability. Flaps increase the lift as well as the drag, and it is generally considered that 15° or less of flaps produce more lift than drag, and any flap setting over 15° drag exceeds lift. For this discussion, we will look at four types of flaps: plain, split, slotted, and Fowler; and as you can see, there are designs where the benefit of one design is combined with another (Figure 3.8).

The plain flap is the simplest design, and illustrates the advantages and purpose of flaps. As the flaps are deployed, the camber of the wing increases, and the lift coefficient (C_L) increases accordingly with the angle of attack. The lift coefficient will be discussed at length in Chapter 4. The farther the flaps are deployed, the greater the lift and the resulting drag. A split flap is deployed from underneath the wing, and results in more drag initially than the plain flap due to the disruption of the flow of air around the bottom and top of the wing.

When the flaps are slotted, at high angles of attack high energy air is allowed to move through the slot and energize the air on top of the deployed flap. This allows for an increase in C_L at lower speeds, allowing an aircraft to operate out of shorter landing strips or with obstacles surrounding the airport. The highly energized air also delays boundary layer separation, which lowers the stalling speed, improving performance at slow speeds. More on this topic will be discussed throughout this textbook.

Fowler flaps are commonly found on larger transport category aircraft, as they are heavier than the other flap designs and incorporate more complex systems to operate. Fowler flaps slide out and back from the wing, which offers the benefit of not only increasing the camber of the wing but also of the wing area. Fowler flaps also double as slotted flaps in that they allow higher‐energy air from beneath the wing to flow over the deployed flap area. Cessna high‐wing, single‐engine aircraft are the best example of the use of Fowler flaps on light aircraft.