Aircraft generate lift from air moving over the wings, and can pitch up or down with the angle of attack. Control surfaces, such as ailerons, elevators, and rudders, are used to control roll, tilt, and yaw. The horizontal stabilizer creates a pitching moment to force the nose down, and the center of gravity is important for balance. Aerodynamic stall can occur if the angle of attack is too great, and an aircraft may not recover from a stall if the weight is too far back. Ground effect can also affect pitch moment.
Aircraft moving through the air develop lift, or an upward force that exceeds weight, from the air moving over the wings. One way an aircraft moves is when the nose or nose of the aircraft pitches up or down, often referred to as pitching. Pitch moment is a measure of the up and down motion at different angles of the air across the wings, known as the angle of attack.
Most fixed-wing aircraft have two or four wings about halfway down the fuselage, which is the main body of the aircraft. The wings have movable ailerons that move the wings up or down, known as aircraft roll. There is a horizontal stabilizer with a movable elevator panel at the rear or rear of the fuselage to control tilt up or down. The horizontal stabilizer often looks like a smaller wing on each side of the tail in a flat or horizontal position.
A vertical stabilizer with a movable rudder panel is positioned vertically to the horizontal stabilizer to move the nose fore and aft, which is yaw control. All moving surfaces are connected to a pilot control wheel or stick and rudder pedals controlled by the pilot’s feet. The pilot can turn or roll, turn left and right, and ship, or move the nose back and forth with the controls.
If the aircraft drifts up or down by movement in lift, engine power, or weather turbulence, the angle of attack changes due to the airflow across both the wings and the horizontal stabilizer. The horizontal stabilizer is designed as an inverted wing and creates an upward pitching moment to force the nose down. Other parts of the aircraft are trying to push the nose up due to aerodynamic forces, which are effects of air moving across different surfaces.
The forces created by the horizontal stabilizer are often called torque, which is a measure of force multiplied by the distance from a pivot point. The pivot point on the plane is normally the center of gravity, which is an imaginary point where the plane could be lifted and be in perfect balance. Passenger weight, baggage, and fuel will change the CG or CG, and pilots will make calculations to determine that their aircraft will fly within an acceptable CG range.
The pitching moment created by the horizontal stabilizer occurs from a much smaller wing than the main wings. This is possible thanks to the torque calculation. For a desired amount of force, the wing can be smaller because it is further away from the center of gravity. Almost all aircraft have a long tail with horizontal and vertical stabilizers at the tip for this reason.
When the angle of attack gets too great, the air will no longer flow smoothly across the top and bottom of the wing. Turbulence occurs, air no longer flows along the wing, and the wing creates no lift. This is known as an aerodynamic stall and the aircraft can no longer maintain level flight. The CG range is carefully designed and tested by manufacturers so that an aircraft’s nose drops when a stall occurs. This allows the aircraft to gain speed and restore airflow through the wings and tail, and is caused by the aircraft’s designed pitching moment.
If a pilot mistakenly adds too much weight to the rear before flight, an aircraft may not recover from a stall. The horizontal stabilizer cannot develop enough thrust to overcome the excess weight and lower the nose. This is known as the aft or rear CG condition and is very dangerous if not corrected by the pilot.
Pitch moment can also vary from aerodynamic effects that occur close to the ground, called ground effect. Ground effect is caused by changes in the way air moves over and under the wings and affects lift and pitch momentum. This can cause the nose to pitch just before touchdown and contribute to crashes if not understood by the pilot.
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