The effect of flap on the wing's various forces is always to make the wing pitch nose down, taking the rest of the aircraft with it. Any wing with camber has a nose down pitching force related to the amount of camber, lowering flap increases the camber and therefore increases the nose down force.

Lowering flap will increase lift in the early stages of flap, in its extreme down stages it barely changes the lift force. The increase of lift will cause the plane to rise, which will cause the tail to pitch the plane nose up, providing a pitching force opposing the effect of camber. This force is transient and is gone when straight flight is resumed.

The tailplane often lies in the flow affected by the downwash of the wing, and the affected airflow can extend to quite some height above the wing. As the flaps are lowered the angle of the downwash increases, so if the tailplane is affected by the downwash its angle of attack is made more negative and it pitches the plane upwards.

So you have a nose down effect from increasing the camber, and perhaps a nose up effect if the tail is in the downwash. Which of these is the more powerful determines whether the plane has a net nose up or nose down effect. High wing low tails such as a Cessna 150 have a pronounced nose up as the tail is firmly in the downwash. Low wing high tails tend to have a nose down effect as the tail has little or no flow in the downwash.

Most model fliers tend to lower the flap at much too high a speed, resulting in a ballooning climb due to the huge increase in lift, often exacerbated by lowering flaps instantly at full servo speed. Once the excess speed has bled off the true character such as a nose down pitch becomes evident, leading to the model porpoising as it balloons upwards then pitches nose down and dives, requiring up trim to stop it. If this happens, learn to slow down before lowering the flap, and use servo slowing to soften the effect as well.

The safe plane is the one that pitches slightly nose down when flap is lowered as this helps to maintain speed against the increase in drag. A plane that pitches up when flap is applied is unsafe as it faces two forms of loss of speed - height gain and more drag.

Harry