ORIGINAL: BMatthews
Frank, pitching up or down or not at all with flaps depends on the flap arrangement as well as the planform of the model. It also depends a lot on if the flaps are full span flaperons or if they are conventional flaps with separate outboard ailerons. And to add to the mix the percentage of the chord used by the flaps will play a part. Oh, and the tail moment length. So all in all the answer is "it depends".
It certainly does depend. Not only on the size and spanwise extent of the flaps, but also on the tail area (as a % of the wing area) and also on the tail height. I made this observation inof my jet column in AMI magazine a while back.
Models like my Composite-ARF Lightning or the Ultra Bandit that Ali Machinchy and Raul Lozano flew at Florida Jets need a little down elevator trim (Ali says 8%) with flaps. Check out the photos and you can see that the low mounted tailplane is in line with the wing. Raise the tail up a bit and you get away with little or no trim change, as has been done on the Tomahawk Futura that Ali and others flew at Florida Jets this year. Place the tail even higher, as on the L-39 Czech trainer, and you need up trim. Malcolm Harris says that on his 1/6 scale L-39 from Global Jet Club he has 3 mm of up elevator with full flap, mixed in proportionally throughout the flap range.
I have seen photos and video of a model called "Tizzy" in the UK that had fixed tail surfaces and was controlled solely by elevons. Up elevon pitched the nose up, I believe. But on other configurations it could pitch the nose down. On some models, like my Falcon 120 the pitch effect of flaps is virtually neutral, so I would never recommend elevons as a primary pitch control without an extensive analysis of their overall effects on all parts of the aircraft.