Some years ago I remember having a conversation with Mr. Palmer when he came up to Seattle and kindly graced a contest there at the old Boeing Space Center.  I ask about the differential  flaps on the Thunderbird and his comment was something to the effect that the concept looked really good on paper.  Your answer lies in the fact that nobody uses the system today if they want to be competitive.

I remember the old Veco Thunderbird had what they called differential flaps, where one flap moved to a greater degree than the other, thru the use of 2 separate control horns with a y-shaped control rod going to 2 different holes in the horns. My question is was there any advantage? Maybe by doing this it helped keep the wing more level during stunts?

  Actually, it was intended to keep the wing *less level* during stunts. The idea being that you could roll the airplane away from you to get more line tension in inside or outside maneuvers. It does exactly that. The problem is that you have to change from inside to outside very quickly sometimes, and all that rolling around messes you up.

    Brett