Feedback:
The aircraft flew this morning. The flight crew report that the handling problems appears to have been resolved. The cause of the problem would appear to be the offset in the vertical fin. Apart from a few minor snags, the aircraft appears to be acceptable. Although the fin offset was small, it appears to have had a major impact on the handling. I’m very relieved that the problem has now been resolved.
We’ve certainly learned a bit from this exercise.
An interesting side point was the effect of temperature on the aircraft symmetry. The aircrew carried out a few last minute checks before flight, including checking the position of the elevators relative to the sides of the tailcone. For those not familiar with the Dak, the rear tailcone has short horizontal stubs adjacent to the elevators. The check suggested that the one elevator was set at a slightly different angle to the other. Then we realized that the aircraft was standing in the sun, with the vertical fin casting a shadow on the one stub. With the temp at about 30 deg celcius, the one surface was noticeably hotter than the other. We poured water on the stubs, and the elevators then lined up. I guess there’s a reason why we do the rigging checks in the hangar, and very early in the morning ;-)
Thanks to all the forumites who shared their experience. It was certainly reassuring to see that our initial ideas were sound.
Blue skies