Originally Posted by
B2N2
... My reasoning is that at a particular weight at a particular climb speed there is a aerodynamic effect from the upper deck hump that causes some yawing that is not sufficiently dampened by the yaw dampers.
Not a phenomenon from the fuselage. This is a 744? Winglet flow is sensitive to AOA, would think that increasing to proper climb schedule would remove the yaw. Next time, set up a printout of roll and yaw, and the aileron and rudder positions. The lower rudder position will give an idea if the YD is part of the issue. The upper deck length didn't make much difference to the flow, but the extended upper deck was a slightly lower drag but added some weight. The winglets do reduce lateral directional stability, but nothing much to speak of. The plane has a very gentle phugoid, overall, an amazingly nice plane to fly.