# What is the difference between the centre of pressure and the centroid?

• Kiran Bose
In summary: However, no book I have seen instructs readers to focus on this quantity.All books say, total force (the thrust)= vertical area of the plane X pressure at centroid.
Kiran Bose
[Moderator's note: This thread was created by moving posts from another thread, hence no homework template]

The name "centre of pressure (COP)" indicates representative pressure will act through this point for a vertical plane inside liquid.
If so, then total thrust= vertical area of the plane x pressure at COP.
But this does not match with the books.
All books say, total force (the thrust)= vertical area of the plane X pressure at centroid.
I do not understand this.

Kiran Bose said:
The name "centre of pressure (COP)" indicates representative pressure will act through this point for a vertical plane inside liquid.
If so, then total thrust= vertical area of the plane x pressure at COP.
But this does not match with the books.
All books say, total force (the thrust)= vertical area of the plane X pressure at centroid.
I do not understand this.

Please name some of these books.

Kiran Bose said:
[Moderator's note: This thread was created by moving posts from another thread, hence no homework template]

The name "centre of pressure (COP)" indicates representative pressure will act through this point for a vertical plane inside liquid.
If so, then total thrust= vertical area of the plane x pressure at COP.
But this does not match with the books.
All books say, total force (the thrust)= vertical area of the plane X pressure at centroid.
I do not understand this.
The force on the vertical plane will indeed be equal to the vertical area times the pressure at the centroid. However, if a force of this magnitude were applied on the opposite side of the plate to try to keep it in equilibrium, the plate would not be in equilibrium; it would rotate. This is because the moment balance on the plate (treated as a rigid body) would not be satisfied. In order to satisfy the moment balance, the force would have to be applied somewhat below the centroid (at 1/3 of the way up). This is because the average pressure on the bottom half of the plate is higher than the average pressure on the top half of the plate.

Kiran Bose said:
The name "centre of pressure (COP)" indicates representative pressure will act through this point
Not representative pressure, no.
As Chestermiller posts, a "representative" force acts at the centre of pressure, in the sense that if you were to insert a rigid plate in that vertical plane and apply the right force at that point then equilibrium would be achieved against the forces on the other side of the plate.
If by "representative pressure" you mean total force divided by area then that equals the pressure to be found at the centroid.

## 1. What is thrust?

Thrust is the force that propels an object in a specific direction. In the context of flight and aerodynamics, it is the force generated by engines or propulsion systems that allows an aircraft to overcome drag and maintain its forward motion.

## 2. How is thrust calculated?

Thrust is calculated by multiplying the mass flow rate of the propellant by the exhaust velocity of the propulsion system. This is known as the rocket equation and is a fundamental principle in rocket science. In simpler terms, thrust can also be calculated by measuring the change in momentum of an object over time.

## 3. What is the centre of pressure?

The centre of pressure is the point on an object's surface where the total aerodynamic force is considered to act. In other words, it is the point where the combined effects of lift, drag, and other aerodynamic forces can be represented as a single force acting on the object.

## 4. How does the centre of pressure affect flight?

The location of the centre of pressure plays a crucial role in an aircraft's stability and control. If the centre of pressure is too far behind the centre of gravity, the aircraft may become unstable and difficult to control. If it is too far ahead, the aircraft may become too stable and unresponsive to control inputs.

## 5. How is the centre of pressure determined?

The centre of pressure can be determined experimentally by conducting wind tunnel tests or through computational fluid dynamics simulations. It can also be estimated using mathematical equations based on the shape and size of the object, as well as the angle of attack and airspeed. Additionally, aircraft manufacturers often conduct flight tests to accurately determine the centre of pressure for their specific aircraft models.

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