This relationship, center of lift behind center of gravity, gives the aircraft a built-in tendency to pitch nosedown. The airplane's tendency to pitch nosedown is countered by downloading on the horizontal stabilizer. The horizontal stabilizer actually flies in a downward direction when the aircraft is in straight-and-level flight. Typically, the horizontal stabilizer carries downloading of approximately 8 percent of the gross weight of the aircraft. As a result, the loading of the aircraft wing in straight-and-level flight is approximately 108 percent of the gross weight of the aircraft.
For example, the wing of a Cessna 152 flying at a maximum gross weight of 1,650 pounds is actually producing approximately 1,782 pounds of lift in straight-and-level flight. Consider an aircraft such as a Boeing 747. At a gross weight of 500,000 pounds, the downloading on the tail would be approximately 40,000 pounds, which is the same as the total lift produced by the main wing of the largest corporate jets.
The center of gravity, gross weight, positions of the wing's leading and trailing edge devices such as flaps, airspeed, and thrust of the aircraft engine or engines all vary in flight. In some cases, fuel consumption can even bring about direct changes in the aerodynamics of the wing. The outboard segment of the wing on a Mitsubishi MU-2 twin-turboprop twists forward slightly when the wing tip tanks are full and untwists as fuel in the tip tanks is used. Experienced MU-2 pilots know that the aircraft flies faster with empty tip tanks because of the lower induced drag of the wing.
To compensate for the changing center of gravity, weight, configuration, speed, and power, aircraft are equipped with trim systems. These systems relieve the pilot from having to exert strong control forces for extended periods of time.
The more sophisticated and larger the aircraft, the more trim controls it is likely to have. Small training aircraft typically need only one type of moveable trim control-pitch. Aerodynamic balance along the longitudinal axis experiences the most dramatic changes due to constant in-flight variations in the center of gravity, center of pressure, and tail loading. As all of these factors vary in flight, control forces also vary significantly. Aircraft pitch-trim systems give the pilot a way to balance these variables and reduce flight control loading to relieve strain on the pilot and the autopilot actuator system.
When power, airspeed, or the position of wing leading and trailing edge devices changes, the pilot needs to change the pitch trim to keep the aerodynamic forces in balance. If we increase power in straight-and-level flight, we also increase airspeed, which, in turn, increases the airflow over all of the aircraft control surfaces. This increase in airspeed increases the lift on the main wing more than it increases the downloading on the tail. This makes the nose of the aircraft tend to pitch up. If we decrease power in level flight, the opposite occurs, and the nose tends to pitch down, requiring elevator pressure to keep the nose level.
Wing flaps are generally the only wing devices (other than ailerons) found on light aircraft. Wing flaps have a significant effect on pitch trim. As we lower the wing flaps, the lift on the main wing increases, which causes a large change in pitch. If we had no pitch trim to neutralize the elevator forces, the pilot would have to apply significant amounts of pressure on the flight controls to maintain level flight, control airspeed, and prevent a pitchup to a stall. This is also true when flaps are retracted and the aircraft transitions from a low-thrust, flaps-down ap-proach configuration to a high-thrust, flaps-up configuration such as you would use to go around.
Aircraft trim systems range from simple metal tabs that can only be adjusted on the ground to complex systems of in-flight adjustable tabs. Ground-adjustable trim tabs are thin metal tabs on the trailing edge of an elevator that are bent by hand, typically by a mechanic, to achieve trim in one specific mode of flight, usually cruise. In any other mode, such as climb, slow flight, or descent, the aircraft will be out of trim.
Simple in-flight adjustable pitch-trim devices are generally cable operated and work in one of several ways. One method is to vary the angle of a small tab attached to the trailing edge of the elevator. Changing the angle of the trim tab changes the aerodynamic neutral point of the elevator. Another pitch-trim design varies the angle of incidence of the horizontal stabilizer via a jack screw attached to the leading edge of that surface. This is the pitch-trim system used on the Piper Cub, for example.
Mooneys employ yet another pitch-trim design. When the pilot moves the pitch-trim control in the cockpit, the entire empennage pivots to change the angle of incidence of the horizontal stabilizer.
Most trainers and other light aircraft have simple ground-adjustable tabs attached to the trailing edge of an aileron and, in some cases, the rudder, to help trim roll and yaw forces. The tabs usually are adjusted for neutral roll and centered ball in the most commonly used cruise-flight power and loading condition. The trim will be off in any other situation, but the pilot likely will notice only a slight increase in the control force needed to maintain the desired attitude.
On more powerful and faster single-engine aircraft, engine torque and asymmetric thrust (P factor) are significant control-force factors. To compensate, manufacturers build in an offset by mounting the engine and/or vertical stabilizer at a slight angle. These modifications contribute to flight-control balance in cruise flight, but result in an out-of-trim condition in all other phases of flight. Three-axis in-flight variable trim control is required to level the aircraft in pitch and roll while eliminating yaw.
Multiengine aircraft require powerful in-flight variable trim systems in all three axes of flight to counteract the strong asymmetric thrust introduced when one engine is not pro- ducing equal thrust.
Aircraft trim systems exist to compensate for aerodynamic imbalances that routinely occur in the aircraft in flight. Trim systems are not designed to be used as primary control devices. They are not quick enough or powerful enough to produce timely re- sponses in aircraft movement, they provide no feedback to the pilot on control inputs made, and they cannot be applied or reversed quickly enough to prevent under- or over-controlling.
Always control the aircraft through the use of the primary flight control system, not the trim system. As the aerodynamic balance of the aircraft changes-for example, when you level off at cruise altitude and indicated airspeed increases-adjust the trim to relieve adverse pressures on the flight controls and maintain coordinated flight. Relieving these pressures provides for easier, more effective aircraft control and makes the aircraft more aerodynamically efficient. If the aircraft is equipped with an autopilot system, trim first before engaging the autopilot.
Regulations require that in-flight adjustable trim systems include a trim position indicator located in full view of the pilot. Aileron and rudder trim systems generally use an indicator with a neutral position that also shows left and right (or up and down) deflections. The elevator trim indicator has the same indications, and also a takeoff range clearly marked, usually with a green or white arc. The aircraft pre-takeoff checklist will also include an entry such as "Trim-Takeoff Position."
An in-flight-adjustable trim system (as opposed to fixed tabs) can be used to control the aircraft if a primary flight control fails, but it re-quires significant planning and an understanding of the limitations of the system. In such an emergency the control response resulting from a trim change will be slow and sluggish, and the inputs and time required for the desired aircraft response will be difficult to judge. Be sure to allow plenty of time, plenty of maneuvering room in the air, and plenty of runway should you ever be faced with this scenario.
I often have my students experiment with the possibility of having to land the airplane using pitch trim only. The experience gives them a good understanding of the limitations of the trim system as a primary control device. It also gives them some idea of what they will face if the situation ever arises.
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