Aerodynamic Center
 

Studying Principles of Flight and struggling to understand the AC.

Quote; Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis.

But CP moves with AOA? And why is it important to know AC?

If you are studying P of F I assume you are a 'pilot' and not a 'scientist' or 'aeronautical engineer'?

If you are a pilot, you do not need to understand aerodynamic centre.

As far as I remember, the aerodynamic centre is the point on the chord line where clockwise and anti-clockwise aerodynamic moments are balanced.

As you say in your post, this point does not change with angle of attack so provides a constant point of reference.

This may be of interest to an aerodynamicist or designer but is of little use to a pilot.

I generally enjoy the technical side of flying but have never seen the relevance of aerodynamic centre to my understanding of handling techniques.

On the other hand, knowledge of the movement of centre of pressure with angle of attack underpins the understanding of handling during slow flight.

I'm just an aviation enthusiast and got my own 737 NG homecockpit. I'm working through my ATPL CBT from oxford aviation.

Maybe i'm trying to learn more than i need to know.

I'm trying to visualise this AC and what moment or torque are acting on it?

Imagine a straight, constant-chord wing in a wind tunnel. You are looking at it from the side. The airflow is from left to right.

The wing is mounted on a horizontal pole which is fixed to the walls of the tunnel. The wing is free to rotate clockwise and counter-clockwise on the pole.

There are several holes which run span-wise through the wing, enabling the wing to be mounted, with the pole in several different locations along the chord line.

1: The wing is mounted using the front hole, which is just behind the leading edge. The airflow will exert a very strong counter-clockwise moment and the trailing edge of the wing will tend to rise. As it rises, the angle of attack will reduce and the moment will also reduce. Eventually a stable angle will be reached which will be proportional to the weight of the wing and the speed of the airflow.

2: The wing is mounted using the rear-most hole, which is just forward of the trailing edge. Assuming the front of the wing is supported initially at a small positive angle, the airflow will exert a strong clockwise moment and the nose of the wing will tend to rise. As it rises, the angle of attack will increase and the moment will also increase. Eventually the wing may flip over completely, depending on its weight and the speed of the airflow

3: The wing is mounted using a hole that is located at the aerodynamic centre. (About 25% back from the leading edge). Assuming the wing is initially placed at a small positive angle of attack, the clockwise moments and the counter-clockwise moments will balance and the wing will not rotate around the pole. If the angle of attack is changed manually, there will still be no rotation around the pole. The aerodynamic moments are balanced at all angles of attack.

Ah makes sense now and i can see how this fixed AC position and the aerodynamic forces acting in different directions.

You gave examples of different directions of each? Are those made up or the actual turning directions that will happen?

The moments are real for each example given.

The causative forces vary, depending on from which point you want to measure them.

For example, you can close a door in 3 seconds either by using a small force near the handle or a large force near the hinge. The moment (N x m) will be the same in both cases.

However; given that you can chose any point about which the moments may be analysed, and given actual forces of weight, lift, drag, etc. the whole exercise is somewhat arbitrary. It's a bit like the notion of 'centre of gravity'; an arbitrary point at which the mass of a body can be said to act (which may even lie outside the body in question).

A couple of urls might be useful ..

https://ocw.mit.edu/courses/aeronaut...ectre10_cg.pdf

https://www.grc.nasa.gov/www/k-12/airplane/ac.html

Thanks alot i seem to have a better idea now on the AC

Great links cheers

So the moments are the AC and the aerodynamic forces - ie lift that are acting on the wing?

You could say its like a balanced seesaw?