This is an interesting one that has been the point of discussion many time on my field.[sm=confused.gif]
On straight wing the simple theory should apply the flap is always behind the MAC and the CG (through which all forces operate) therefore when it is lowered, it acts like an elevator, so down elevator puts the nose down.

The increased lift will make the plane balloon (first 20 deg.) but the lift should not really affect the pitch. The plane should just 'float' more.

On the L39 Which fits all of the 'standard' criteria, when i drop take off flaps the plane becomes more 'floaty' as predicted but the nose goes UP!
If I drop full flap, which not only should produce more nose down but also applies significant drag, (more nose down in theory) the nose comes up even more and I have to push down to stay level.

Over the years I have flown many planes (prop and Jet) high and low wing, with flaps, some follow the rules other work backwards (as the L39).

I to would be interested in a definitive answer on this.
Could it be a down draft or turbulent affect on the stabs?

Paul