Matthews decalage example/explanation is a welcome one for me...I've heard of different explanations but his seems easy to understand...I've heard of the "falling leaf" example in the past to illustrate the pitch stability that dacalage angle gives a plane. KHodges brings an interesting point as well...I think that some twin piston planes (maybe the Barrons?) use a differential flap rigging to help with torque effects in landing configuration when torque may pose a problem...on an aborted landing for example. I work on Lears and the flaps are equal...but I think I remember a friend that was rigging a Barrons flaps after installing a motor and the flap rig was a bit different...this was a while ago though...so don't quote me.