Aerodynamic Center

[beinghuman]

Hello Guys,

Can anyone please explain what exactly is Aerodynamic center and why and when do we consider this ?

I know it is constant, it never moves and it is approximately 25% in from the tip. However, why do we use it ? Is there a aerodynamic center for the tailplane as well ?

Thanks in advance :)

[fizz57]

It's just a kludge to simplify the maths.

The forces acting on an airfoil can be described by a single force passing through a certain point, known as the centre of pressure. As the angle of attack changes, both the magnitude of the force and the location of the centre of pressure change.

To calculate the dynamics of a rigid body, you need to know the total force (which gives you the linear acceleration) and the total moment (which gives you the rotational acceleration). The moment of a force about a point is the magnitude of the force multiplied by its distance from that point. Clearly, if the centre of pressure moves about, describing the moment of the aerodynamic force is going to be complicated.

However, you can find a point about which this moment is constant: basically, as the force increases it moves closer to to the chosen point so the product remains the same. When you define the pitching moment about this point, your equations do not have to take changes in pitching moment into account and are thus much simpler - in fact, solvable analytically, which was a great plus before computers came around (and still is, in the classroom!)

It is an approximation, valid only for small changes in angle of attack - but that is all you need for the classical stability calculations. However, even when using numerical methods, it is still convenient to use the aerodynamic centre as changes in the moment around this point can be described with fewer terms (it is the point about which the changes are smallest).

Note that "it is constant, it never moves" because it is defined to be so - not because there is any real "physical" reason for it to do so.

When making calculations on a complete airplane, the moment is calculated around the centre of gravity of the plane. The movement of the centre of pressure of the tailplane is usually quite small compared to the distance of the tailplane from the centre of gravity. Thus, although the tailplane certainly has an aerodynamic centre, the concept is not really needed.

[beinghuman]

thanks :) I got it. Appreciate it man.