Todd, if you have wing adjustability, increase wing incidence before you do any thing else. I am talking 1 to 1 1/2 degree positive, not just a couple turns on the screws. Do one thing at a time. Most likely the plane will require a bunch of down trim. That's okay. Observe the knife and start removing some of the negative from the stab. Eventually you will most likely arrive at a stab that is also positive relative to the reference line. The model will look like it is flying with the tail a little high possibly.
If no amount of tinkering with the incidences helps enough, then add a dorsal fin about 1 1/2" wide by about 12" long (triangle actually) from the base of the vert fin, forward. That will improve the character of the knife edge and you should be able to take out about half of themix you are using now.
Observe how the downline is affected. With the wing at about 1 degree pos, and the stab at about 1 degree pos your down line should be pretty close. Check your up line. If the up line pitches to canopy, add a little down thrust about 1 degree. Most likely it will not pitch to gear, but if it does, after an extended vert line, then you may have too much down thrust. But I doubt it.
Make sure you track all your changes so you can return to where you were.
Let me know how you make out
MattK
ORIGINAL: tschmidt
Matt,
Your input is very interesting and greatly appreciated. One of my first "real" pattern planes was a SL1 that I flew for several years. This plane had a very large stab at 36" and was placed pretty high in the fuselage. It was one of the worst snapping/spinning planes I ever flew.
I do have trimming question for you or anyone else that can help.
The wing on the Defiants sets on the thrust line which I know is uncommon in todays designs. The wing has no dihedral and the stab sets 1/2" above the thrust line. What I like about this arrangement is how axial the plane rolls. It does have a slight pitch to the belly on rudder input, but from what I read most planes do. In advanced, I was able to get away with flying it this way, but this is something I'm going to have to correct before flying the new Masters pattern. What do you feel is the best way to correct this problem? Currently my wings are set at -0- incidence and the stabs .5 to .75 negative (leading edge down). Do I need to zero the stab and try a little positive in the wings?
Todd