Flight Controls

increase the wing camber, but allows a higher maximum C L because the stall is delayed until the wing reaches a greater AOA. Movable slats consist of leading edge segments that move on tracks. At low angles of attack, each slat is held flush against the wing’s leading edge by the high pressure that forms at the wing’s leading edge. As the AOA increases, the high- pressure area moves aft below the lower surface of the wing, allowing the slats to move forward. Some slats, however, are pilot operated and can be deployed at any AOA. Opening a slat allows the air below the wing to flow over the wing’s upper surface, delaying airflow separation. Leading edge flaps, like trailing edge flaps, are used to Figure 6-19. Spoilers reduce lift and increase drag during descent increase both C L-MAX and the camber of the wings. This type and landing. of leading edge device is frequently used in conjunction with trailing edge flaps and can reduce the nose-down pitching variables. Trim systems are used to relieve the pilot of the movement produced by the latter. As is true with trailing edge need to maintain constant pressure on the flight controls, and flaps, a small increment of leading edge flaps increases lift usually consist of flight deck controls and small hinged devices to a much greater extent than drag. As flaps are extended, attached to the trailing edge of one or more of the primary flight drag increases at a greater rate than lift. control surfaces. Designed to help minimize a pilot’s workload, trim systems aerodynamically assist movement and position of Leading edge cuffs, like leading edge flaps and trailing edge the flight control surface to which they are attached. Common flaps are used to increase both C L-MAX and the camber of types of trim systems include trim tabs, balance tabs, antiservo the wings. Unlike leading edge flaps and trailing edge flaps, tabs, ground adjustable tabs, and an adjustable stabilizer. leading edge cuffs are fixed aerodynamic devices. In most cases, leading edge cuffs extend the leading edge down and Trim Tabs forward. This causes the airflow to attach better to the upper The most common installation on small aircraft is a single surface of the wing at higher angles of attack, thus lowering trim tab attached to the trailing edge of the elevator. Most trim an aircraft’s stall speed. The fixed nature of leading edge cuffs tabs are manually operated by a small, vertically mounted extracts a penalty in maximum cruise airspeed, but recent control wheel. However, a trim crank may be found in some advances in design and technology have reduced this penalty. aircraft. The flight deck control includes a trim tab position indicator. Placing the trim control in the full nose-down Spoilers position moves the trim tab to its full up position. With Found on some fixed-wing aircraft, high drag devices called the trim tab up and into the airstream, the airflow over the spoilers are deployed from the wings to spoil the smooth horizontal tail surface tends to force the trailing edge of the airflow, reducing lift and increasing drag. On gliders, spoilers elevator down. This causes the tail of the aircraft to move are most often used to control rate of descent for accurate up and the nose to move down. [Figure 6-20] landings. On other aircraft, spoilers are often used for roll control, an advantage of which is the elimination of adverse If the trim tab is set to the full nose-up position, the tab moves yaw. To turn right, for example, the spoiler on the right wing to its full down position. In this case, the air flowing under the is raised, destroying some of the lift and creating more drag horizontal tail surface hits the tab and forces the trailing edge on the right. The right wing drops, and the aircraft banks of the elevator up, reducing the elevator’s AOA. This causes and yaws to the right. Deploying spoilers on both wings at the tail of the aircraft to move down and the nose to move up. the same time allows the aircraft to descend without gaining speed. Spoilers are also deployed to help reduce ground roll In spite of the opposing directional movement of the trim after landing. By destroying lift, they transfer weight to the tab and the elevator, control of trim is natural to a pilot. If wheels, improving braking effectiveness. [Figure 6-19] the pilot needs to exert constant back pressure on a control column, the need for nose-up trim is indicated. The normal Trim Systems trim procedure is to continue trimming until the aircraft is Although an aircraft can be operated throughout a wide range balanced and the nose-heavy condition is no longer apparent. of attitudes, airspeeds, and power settings, it can be designed to Pilots normally establish the desired power, pitch attitude, fly hands-off within only a very limited combination of these and configuration first, and then trim the aircraft to relieve 6-10