Center of pressure movement due to flaps

[Pugachev Cobra]

Hello folks,

Just about as simple as it gets:

On a fixed-wing aircraft, when flaps are deployed, the center of pressure moves fore or aft of its initial location with flaps retracted?

And why?

The question is geared toward asymmetric airfoils, but if you want to you can also answer about symmetric too.

Thanks!

[eckhard]

Aft, because the lift due to the flap increases the proportion of the total lift that is produced by the aft section of the wing.

However:

The change in pitching moment for the whole aircraft can be nose up, neutral or nose down. This is due to the interaction of the change in downwash over the tail, the increase and vertical displacement of the total drag line and the sweep of the wing, in addition to the increase and aft movement of the lift vector.

[dhc83driver]

One way to think about it is a vector of pressure on the aerofoil. To complicate things it also depends on the type of flaps, but generally flaps increase wing area so the centre of pressure will move aft accordingly. It`s reaction on pitch depends on its vector against centre of gravity. With large fowler / slated flaps you may have the centre of pressure move forward and aft of the centre of gravity during deployment as centre of pressure moves aft, then as the flaps extend to a greater deflection causing drag the centre of pressure moves forward again.

Large aircraft use trim compensation systems to help with the changes. Slats also act in the same way moving the centre of pressure forward.

Design of light aircraft means changes in trim and control loading are kept low.

[NZScion]

CP also tends to move inboard with flap extension, as more of the lift is generated by the inboard portion of the wing.