Aircraft control systems are classified as primary and secondary. The primary control systems consist of those which are necessary for the safe flight of an aircraft, such as ailerons, elevator and rudder. Secondary control systems improve the performance feature of the aircraft or reduce the control forces by the pilots. Examples of secondary control systems are wing flaps and trim systems.The ailerons are located on the outboard trailing edges of both wings and are used to ro tate the aircraft around the longitudinal or roll axis. These surfaces move in opposite direc tions from each other and are controlled by rotation of the control wheel. When the control wheel is rotated to the left, the right aileron moves down and the left aileron moves up. This causes the airplane to roll to the left. When the control wheel is turned to the right, and op posite aileron movement results and the airplane rolls to the right.The pitch motion around the lateral or pitch axis is controlled by elevator moved by fore and aft movement of the control wheel. A change in position of the elevators modifies the camber of the airfoil which increases or decreases lift. When the pilot wants to raise the air craft nose, back pressure is applied on the control wheel. This moves the elevator up, result ing in a negative angle of attack. This causes the tail to move down and the nose of the air craft to move up. The rudder moves the aircraft about the vertical, or yaw axis resulting in a motion known as yaw. The movement of the rudder is to the left or to the right and is con trolled by the movement of the rudder pedal in the cockpit. Pressure applied to the right rud der pedal displaces the rudder to the right. This increases the camber on the left side of the vertical tail surface, creating a low pressure area on the left and a high pressure area on the right side. The resulting pressures cause the tail to move left and the nose of the aircraft to move right. Applying pressure to the left rudder pedal provides an opposite reaction. The rudder is used in conjunction with the ailerons to coordinate turns.2.The control wheel can provide control for the airplane to （ ） ．