Flaps have three different effects on the flightpath

1.Putting flaps down increases the lift of the wing which makes the plane rise so the tail does its job of pitching the plane up top get it straight into the airflow again. Putting flaps down also alters the relative rigging angle between wing and tail so again causing a nose up pitch.

2.Putting flaps down increases the camber of the wing. Camber causes a wing to pitch nose down, the more camber it has the more strongly it pitches nose down. This effect can be very strong and is the reason why some aircraft pitch nose down and dive when flap is lowered. This is how deltas and tailless aircraft fly, the “flap” has the same effect as an elevator in pitch, down flap is down elevator.

3.Putting flaps down increases the angle of the downwash. If the tail sits in the down wash such as on a high wing, low tail plane like a Cessna, the leverage it exerts can be stronger than the nose down effect from the wing’s camber so it pitches nose up. If the position of the tail relative to the wing is high enough such as a CAP then it feels no change to the downwash and the nose down pitch will dominate.

Wings always create a nose down pitch when flap is lowered, but if the tail is in the downwash it may balance or even overcome that and pitch up.

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