Flight Theory and Aerodynamics. Joseph R. Badick
the stick fore and aft and sensitivity is increased. This leads to greater chances of the pilot overcontrolling the aircraft; so, components like an antiservo tab and balance weight are added to reduce the sensitivity.
Some larger aircraft incorporate an adjustable horizontal stabilizer controlled by a jackscrew through a wheel in the cockpit or a motor. Though an elevator is still located on the trailing edge, the usually fixed horizontal stabilizer is adjustable, in this case allowing the pilot to move the stabilizer to reduce control pressures on the stick (Figure 3.6).
Rudder
The rudder controls yaw, or movement of the aircraft about its vertical axis. Similar to the smaller, movable elevator attached to the trailing edge of the horizontal stabilizer, the rudder is attached to the rear of the fixed vertical stabilizer. As with other components we discussed, the rudder is connected to the rudder pedals in the cockpit via various mechanical linkages; pressing on the left or right rudder pedal moves the rudder left or right, respectively. Most of the time, the rudder is used to maintain coordinated flight, especially when banking the aircraft. Moving the rudder creates a larger camber on the vertical stabilizer, which in turn creates a greater sideways force. This force causes the nose of the aircraft to move left and right, or yaw, around the vertical axis (Figure 3.7).
Canard
The canard design dates back to the glider designs of the Wright Flyer, and can still be found on a few modern civilian and military aircraft. Though few were made, the Beechcraft Starship is a great example of an aircraft with a canard design. Most canard designs incorporate a horizontal stabilizer in front of the main wings, providing lift for the nose instead of the previously presented aft horizontal stabilizer which provides tail‐down force.
Figure 3.6 Adjustable horizontal stabilizer.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Figure 3.7 Rudder movement.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Secondary Flight Controls
Secondary flight control systems usually consist of wing flaps, leading edge devices, spoilers, and trim systems. These controls often support or supplement the primary controls, and their importance to understanding aerodynamic principles cannot be overstated.
Flaps
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 (CL) 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 CL 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.
Leading Edge Devices
Leading edge devices are discussed in more depth in Chapter 4; so, only a brief introduction will be presented here. As with trailing edge flaps, leading edge devices like movable slots, leading edge flaps (Krueger flaps), and leading edge cuffs work to increase the CL over a wing, allowing the aircraft to fly at slower speeds for takeoff and landing. In some cases, the camber of the wing is also increased, which allows for an increase in the angle of attack and increased performance. As with trailing edge flaps, the first setting results in more lift with little increase in drag, while fully deployed settings usually result in mostly drag with little lift.
Figure 3.8 Common flap designs.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Figure 3.9 Ground spoilers deployed.
Source: U.S. Department of Transportation Federal Aviation Administration (2016b).
Spoilers and Speed Brakes
Spoilers are used on many different types of aircraft as high‐drag devices used to increase drag and reduce lift. They “spoil” the smooth air around a wing, and may be used in flight, or on the ground when landing or during a rejected takeoff.