What is the difference between centre of pressure, aerodynamic centre and neutral point? - اسأل الطيار ask pilot

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Wednesday, October 28, 2020

What is the difference between centre of pressure, aerodynamic centre and neutral point?







Center of pressure: the net force of air on a body/surface/wing. The center of pressure changes as an aircraft maneuvers because different parts of the aircraft stick into the airflow as the angle of attack changes. Notice how the center of pressure moves forward as the angle of attack increases (the airflow is right-to-left):

Aerodynamic center: the location at which the moment does not change regardless of the angle of attack. Wings tend to produce a rotational force due to the way they push air around, and this is the location at which the rotational force stays constant at any angle of attack. It is often easier to consider the aerodynamic center for where you want to place your wings and mass because its location is constant, unlike the center of pressure. For now, think of Mtrim as equivalent to the rotational force the elevators must impart in order to achieve level flight. Thus the aerodynamic center does not change with variation in angle of attack. Due to this, the aerodynamic center, rather than the center of pressure is used in the analysis of longitudinal stability.

Neutral Point: the position of center of mass where the aircraft would be neutrally stable. If you put your center of mass behind (aft) of this point, you will get an unstable aircraft that will increasingly turn with any maneuver the pilot makes. At low speeds, this may be tolerable, but computers are needed to fly unstable aircraft at higher speeds.

As the center of gravity is moved forward, the stability of the aircraft increases (as the main wing lift arm is reduced) and the aircraft is statically stable.As the center of gravity is moved aft, the main wing lift moment arm increases and the aircraft stability decreases. The aircraft is said to be statically unstable.

Thus, Center of pressure of an aircraft is the point where the Lift acts.
Aerodynamic center is the point in the wing where the pitching moments are constant. Neutral point it the location of center of gravity where the aircraft is neutrally stable.

Source: FAA Handbook

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