Primary and Auxiliary Groups Explained
The flight control surfaces are hinged or movable airfoils designed to change the attitude of the aircraft during flight. These surfaces may be divided into three groups, usually referred to as the primary group, secondary group, and auxiliary group.
The primary group includes the ailerons, elevators, and rudder. These surfaces are used for moving the aircraft about its three axes.
The ailerons and elevators are generally operated from the cockpit by a control stick on single engine aircraft and by a wheel and yoke assembly on multiengine aircraft. The rudder is operated by foot pedals on all types of aircraft.
Included in the secondary group are the trim tabs and spring tabs. Trim tabs are small airfoils recessed into the trailing edges of the primary control surfaces. The purpose of trim tabs is to enable the pilot to trim out any unbalanced condition which may exist during flight, without exerting any pressure on the primary controls. Each trim tab is hinged to its parent primary control surface, but is operated by an independent control.
Spring tabs are similar in appearance to trim tabs, but serve an entirely different purpose. Spring tabs are used to aid the pilot in moving the primary control surfaces.
Included in the auxiliary group of flight control surfaces are the wing flaps, spoilers, speed brakes, slats, leading edge flaps and slots.
The auxiliary groups may be divided into two subgroups. Those whose primary purpose is lift augmenting and those whose primary purpose is lift decreasing. In the first group are the flaps, both trailing edge and leading edge (slats), and slots. The lift decreasing devices are speed brakes and spoilers.
The trailing edge airfoils (flaps) increase the wing area thereby increasing lift on takeoff and decrease the speed during landing. These airfoils are retractable and fair into the wing contour. Others are simply a portion of the lower skin which extends into the airstream thereby slowing the aircraft.
Leading edge flaps are airfoils extended from and retracted into the leading edge of the wing. Some installations create a slot (an opening between the extended airfoil and the leading edge). The flap (termed slat by some manufacturers) and slot create additional lift at the slower speeds of takeoff and landing. Other installations have permanent slots built in the leading edge of the wing. At cruising speeds, the trailing edge and leading edge flaps (slats) are retracted into the wing proper.
Lift decreasing devices are the speed brakes (spoilers). In some installations, there are two types of spoilers. The ground spoiler is extended only after the aircraft is on the ground thereby assisting in the braking action.
The flight spoiler assists in lateral control by being extended whenever the aileron on that wing is rotated up. When actuated as speed brakes, the spoiler panels on both wings raise up - the panel on the "up" aileron wing raising more than the panel on the down aileron side. This provides speed brake operation and later control simultaneously.
Slats are movable control surfaces attached to the leading edges of the wings. When the slat is closed, it forms the leading edge of the wing. When in the open position (extended forward), a slot is created between the slat and the wing leading edge. At low airspeeds this increases lift and improves handling characteristics, allowing the aircraft to be controlled at airspeeds below the otherwise normal landing speed.