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Trim 2010-03-18
Most aircraft have single axis trim for the elevator. Airliners have three-axis trim for the elevator, rudder and ailerons. Trim is used to correct for any forces that might tend to counter your selected flight performance. Trim allows the pilot to relax. A pilot who cannot trim will be an exhausted pilot in a short time. It takes only a couple of flights for a pilot to realize the benefits of trim. The simplest elevator trim uses a wheel, lever, or crank to pull a cable or rod attached to a trim surface bell-crank. Other systems use a jackscrew and rod to set trim. Electric trim is best used for coarse settings. Only the coordination of eye and hand can correctly set fine trim settings. Using the trim control positions the trim and the aircraft for the desired attitude. If an aircraft is improperly rigged trim is not the fix required. An aircraft that consistently flies one wing low need help only a mechanic can give. The aircraft wings have adjustments that can correct problems detected in using trim. The controllable trim tabs are required on all aircraft. It is usually on only one side of the elevators since they are both on the same rod (Cessna). It is hinged and can be moved only by use of a cable system connected to the trim control in the cockpit. The direction the tab moves causes an opposite deflection of the control surface. The ground adjustable trim tab is a small surface on the trailing edge of a control surface, most often the rudder, that can be bent to set control forces at cruise speeds. The trim setting creates the aerodynamic forces required to keep the elevator and the airspeed in the desired position. The three factors affecting trim are the center of gravity, airspeed and configuration (flaps/gear). The passenger load will affect the center of gravity and require unique takeoff and level flight trim settings. Each trim setting has a corresponding speed that the aircraft will seek and hold. If you are holding any pressure on the yoke against the trim setting a moment of distraction will result in an airspeed change. A stabilized approach to landing is difficult, to impossible, if the aircraft is not well trimmed. The less skilled the pilot the more likely he is to neglect proper trim technique and attempt to maintain control by arm and hand pressures. Good technique requires that the pressures felt on the yoke be from pilot applied input. Any pressures applied otherwise are indicative of improper trim. Trim is the power steering of flying. Not using trim is equivalent to being able to turn on/off power steering. Trim makes it possible for the pilot to configure the aircraft to counteract and neutralize the normal nose heavy condition. There is a designed twisting along the longitudinal axis caused by a difference between the weight on the center of gravity and the lift acting through the center of pressure. If the pilot does not trim then control pressure must be held maintain the negative lift value of the horizontal stabilizer and elevator. Trim allows this control pressure to be adjusted for hands-off flight. In a trimmed condition the pilot can feel the control pressures required to a acquire a desired flight attitude. An improperly trimmed aircraft is constantly seeking to relieve any pilot induced control pressure. The original design of the aircraft sets the shape, position, and size of flying surfaces and controls so that in cruise conditions these would provide least resistance and maximum speed. Outside of this condition a trim control was installed to maintain the aircraft stability required for climb, descent, landings and other flight speeds and configurations. On some aircraft the angle of incidence of the horizontal stabilizer can be changed by a trim control. This is more effective and efficient than a trim tab (Mooney). The stabilator is another way (Piper). It is an airfoil that in one piece acts as both stabilizer and elevator. The trim control of the stabilator acts as both a trim and anti-servo tab (power assist). The yoke applies control forces to the tab to move the entire control. No change in trim technique is required in either case. Ideally an aircraft would have a three-axis trim; elevator, rudder, and aileron. Without such trim some aircraft just fly crooked. Fixed tabs on the rudder and adjustment screws on the wings can make semi-permanent or even permanent fixes to the aircraft trimmed condition in level cruise. A pilot can, with low-wing aircraft utilize fuel weight/consumption to adjust the aircraft 'trim'. Passenger seating can also make a difference. 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