Centre of Pressure movement

[Golden Rivet]

A question that has been bugging me all afternoon_

Which way does the centre of pressure move when the flaps are lowered ?

One book shows the CofP moving forward with an increase in angle of attack up to a specific AoA and then moving backwards.

Another book shows CofP moving rearwards with an associated pitch down moment.

Can anyone clear this up for me?


Cheers

[Doodlebug]

Hi Golden Rivet

Quote: " Lowering the flaps in flight generally will cause a change in the pitching moment. The direction and degree of the change in pitch depend on the relative original position of the centre of pressure and the centre of gravity. The factors that contribute to this are

1. The increase in lift created by the increased wing area and chamber will lead to a pitch-up moment if the centre of pressure remains in front of the centre of gravity.

2. If the associated rearward movement of the centre of pressure is behind the centre of gravity, then this will produce a nose-down pitch.

3. The flaps will cause an increase in the downwash, and this will reduce the angle of attack of the tailplane, giving a nose-up moment.

4. The increase in drag caused by the flaps will cause a nose-up or nose-down moment depending on whether the flaps are above or below the lateral axis.

The overall change and direction in the pitching moment will depend on which of these effects is predominant. Normally, the increased lift created by extending the wing chord line when the flaps are extended is dominant and will cause a nose-up pitching tendency because the centre of pressure normally remains in front of the centr of gravity."

Hope this helps.

Doodlebug

[Golden Rivet]

thanks for the reply

You've cleared up the query with moments but am I right in thinking that an increase in the angle of attack of an aerofoil will push the centre of pressure forwards ?

If the flaps are extended doesnt the extended chord line result in a greater AoA and thus CoP moving forwards, or does the fact that the chord line is longer giving you a larger pressure distribution push it back?

gr

[Eckhard]

Hi Golden rivet.

In Doodlebug's reply he quoted:

'If the associated rearward movement of the centre of pressure is behind the centre of gravity, then this will produce a nose-down pitch.'

That seems to imply that the extension of flaps will produce a rearward movement of the CofP. However, if the CofP still remains forward of the CofG after this rearward movement, then the increase in the magnitude of the lift vector will produce a pitch up:

'The increase in lift created by the increased wing area and camber will lead to a pitch-up moment if the centre of pressure remains in front of the centre of gravity.'

Although I'm not an expert, I've always worked on the assumption that the CofP moves aft with flap extension. After all, the rear part of the wing will now be producing more lift than it was before the flaps were lowered. The chord line (and therefore the AofA) won't change much for the first few degrees of extension, especially with fowler flaps.

On the high-wing Cessnas, the flaps produce a marked nose up pitch which is due to the greater downwash on the tail and the high position of the flaps raising the drag line and increasing the thrust/drag couple. On the low wing PA-28, the early slab winged 140s used to pitch down very comfortably with application of flap but when they increased the span of the stabilator on the later 'Warrior' and 'Archer', the effect was reversed (for the first stage of flap) due to the increased downwash effect on the larger tail. The last stage gave you the familiar pitch down again.

On a swept-wing jet, you have the extra complication of leading edge devices, but it's been my experience that 737s and 747s have relatively little pitch-trim change with flap (except for flap20 on the 747, which produces a definite pitch up).

On the classic 74s I believe you can selectively deploy outer or inner flaps on the standby system which could give you some pitch control in the event of a control problem, due to their longitudinal separation. The outer flaps will give you a pitch down and the inner ones will give you a pitch up. (We can't do any of that on the -400.)

Hope this helps,

Cheers, E

Having just RTFQ again, I see that I didn't answer part of it!

The CofP will move forward with increasing AofA until the stall AofA is reached, when the CofP will move rapidly aft. This rapid aft movement is what gives the pitch down as the classic symptom of a stall.

Whether or not the flaps increase the effective AofA is a difficult one. After all, if the extension of flap produces a pitch down moment, the AofA will actually reduce.

Regards,

E

Having just RTFQ again, I see that I didn't answer part of it!

The CofP will move forward with increasing AofA until the stall AofA is reached, when the CofP will move rapidly aft. This rapid aft movement is what gives the pitch down as the classic symptom of a stall.

Whether or not the flaps increase the effective AofA is a difficult one. After all, if the extension of flap produces a pitch down moment, the AofA will actually reduce.

Regards,

E