I am struggling to set up a plane I just finished. It was originally designed and built as a C/L plane, profile fuse, covered wing, solid tail and stabs. Wing profile is symmetrical airfoil. Has ailerons and elevator but a fixed rudder. It looks like it was built for speed. I put an OS PET on it and that seems to be about the right amount of power for a spirited flight. However, under power the plane wants to climb to such an extent that I have to feed in down beyond my ability to trim down. It's extremely nervous both up/down and left/right, hard to achieve any kind of stable flight. Lots of up/down oscillation. I have adjusted the c of g forward quite a lot and it's still nervous, but not hair-raising as it was to start (the first flight gave me gray hair and ended in a nose dive into terra firma that broke the tail off.) After moving the C of G forward as much as I dare, it's all I can do to get the plane to fly roughly where I want it to go. The other day, while I was battling with this devil plane, the engine suddenly died (I later discovered my fuel tank stopper came out in flight). The plane immediately went into an uncontrollable spin/dive. Fortunately, the hay is coming along so it wasn't destroyed. (On a previous occasion, the electrics mysteriously died and the plane flew itself on a wild ride for about two minutes circling and porpoising before diving into the ground at 45 degrees on full power but that's another story.)

After pondering the possibilities, I came up with two. First, the prop had a touch of down thrust relative to the fuse midline, but a LOT of downthrust relative to the wing airfoil midline, as the wings have a rather high angle of attack on this plane (it was built that way by someone who is probably no longer with us). The second is that there is about 5 degrees of decollage, for the same reason.

I also noted that the plane was built with the nose pointing left, presumably to encourage it to fly in counterclockwise circles. I have already offset this by mounting the engine so it points about 2.5 degrees to the right, relative to the wing centerline. It misbehaves as outlined above, despite what I think is now correct left/right engine alignment.

My question is: which misalignment is most likely to cause the flat spin behavior when the engine dies? I want to adjust one of these angles at a time, not both together. It will be easier to remount the engine to pull straighter relative to the wing airfoil in the vertical plane. Adjusting the decollage will involve taking a slice out of the profile fuse under the stab and regluing it to increase the angle of attack of the horiz stab, or cutting off the horiz stab, filing the fuse, and regluing. I prefer the former as there is some reinforcing cloth glued to the fuse to keep the stab attached.