What Is Pitching Moment?

The pitching moment refers to the moment about the oy axis of the airframe caused by the external force acting on the aircraft. Including the moment generated by the aerodynamic moment and the engine thrust vector due to not passing through the center of mass of the aircraft, also known as longitudinal moment.

The body's moment of inertia is

The aerodynamically induced pitching moment depends on the flight speed, altitude, angle of attack, and elevator deflection angle. In addition, when the aircraft's pitch rate

Change in angle of attack

And elevator yaw rate

When it is not zero, additional pitching moment is also generated, which is called dynamic aerodynamic moment.

The aerodynamic pitching moment can be written as

, Can also be written as the moment coefficient as

.

When the angle of attack increases, its incremental lift acts on the focal point, so the focal point can be interpreted as the point of action of the incremental lift.

Pitching moment	Engine thrust torque		Engine thrust and aircraft center axis do not coincide
	Aerodynamic pitching moment	Zero-Lift Torque	Mainly caused by vertical tail resistance
		Stable torque	Moment of attack
		Operating torque	Torque generated by elevator / flat tail / duck wing / elevator aileron
		Damping moment	Jet lag and pitch angle rate and elevator yaw rate are generated
	Disturbance torque		Generated by the engine rotor or propeller

Pitch moment in steady straight flight

(1) Longitudinal steady straight flight

Longitudinal steady straight flight refers to a flight state in which the vertical plane where the flight speed vector is located coincides with the plane of longitudinal symmetry of the aircraft xOz, and the flight path is a straight line, with the speed of each point on the path always constant.

In this flight mode, you can approximately think

. In this way, the longitudinal moment is only related to the flying speed, altitude, angle of attack and elevator deflection angle.

(2) Effect of drag on pitching moment

Strictly speaking, drag also affects the pitching moment, but generally the line of action of the drag is close to the center of gravity of the aircraft, so it can be ignored that the pitching moment of the aircraft is mainly caused by the lift.

(3) Lift of each part of the aircraft

The sum of the lift of each aircraft component is

, Are the lift generated by the wing, the lift generated by the fuselage, and the lift generated by the tail. The corresponding lift coefficient is

.

The pitching moments during a steady straight flight are: the pitching moments generated by the wings, the pitching moments generated by the fuselage, and the pitching moments generated by the flat tail. Coefficient of pitching moment

.

Pitch moment in any condition

When the aircraft's pitch rate

Change in angle of attack

And elevator yaw rate

When it is not zero, additional pitching moment will be generated. Therefore, the aircraft pitching moment can be expressed as a coefficient as

Among them, the first three terms are static and aerodynamic derivatives, and the last three terms are dynamic and aerodynamic derivatives.

Angle of attack

Is the abscissa,

Is the argument

Draw a family line.

Pitch moment steering lever

When the pitch balance is broken, the pilot moves the joystick to deflect the elevator, so that the horizontal tail produces a moment, which is balanced with the additional moment that changes the angle of attack of the aircraft. This kind of horizontal tail wing moment caused by the pilot deflecting the elevator is called pitching moment. The relationship of the above balance moments can be expressed by the formula as pitching moment = additional pitching moment

As can be seen from the above formula, an important role of the elevator is that when additional pitching moment is generated on the aircraft, it can be used to generate the equivalent pitching torque to maintain the original pitching balance. For example, when an additional pitching moment is generated to force the aircraft to descend, the pilot pulls the lever backwards to make the elevator deflection increase the pitching moment of the horizontal tail. Balanced state.

Pitching moment using adjusting piece

In flight, the pilot generally uses the joystick to deflect the elevator to maintain the balance of the aircraft, but sometimes the elevator adjustment piece can also be used to deflect the elevator to maintain the balance of the aircraft. For example, deflecting the elevator adjustment piece downward for a certain angle forces upward lift on the piece. He forms a torque on the elevator pivot axis, forcing the elevator to rotate upward. After the elevator is deflected, another moment is formed due to the pressure difference between the rudder surface. When the two moments are balanced, the elevator automatically maintains a certain upward deflection angle. This is the same as the pilot bringing the rod backwards, which can keep the aircraft in a pitch balance state. Using this method to maintain balance, the pilot does not need to take the rod for a long time, which can reduce fatigue, so the rod force is zero at this time.

In short, the pitch balance is damaged during flight. If the nose is lifted up automatically, the pilot should push the stick forward or the tilt adjustment piece to tilt the elevator downward. If the nose is tilted automatically, you should pull the lever backwards or the downward adjustment tab to maintain the pitch balance of the aircraft with the help of the horizontal tail moment.