I'm putting my World models 90" Extra 330L together right now.

The motorbox is tabbed and keyed so that when I build it--there is 3* right thrust and 1* downthrust. That seams completely normal to me. Build it like that and then adjust after I fly the plane.

Someone sent me a PM and told me that all low wing (Extra 330L) aerobats should have 1* UPTHRUST in the firewall. I've never heard of this.

Would anyone care to comment on building 1* engine upthrust into a 90" plane? Doesn't seam right to me.

In theory, the engine thrust line should go through the plane's neutral point, which is the point where all plane air forces can be centred (sigma M = 0)
Thust high located engines have some upthrust (see Catalina), and the lower the engine, the more the engine must be angled down.
Thus high wing trainers with quite low engines must have substantial downthrust, while low wing planes with high engines may be set neutral, or even have some upthrust.
Tests:
Test 1
Fly trimmed level at full throttle. Suddenly cut throttle. If the plane pitches up, there is too much downthrust. If plane continues flight path before the nose drops, thrustline is OK. If plane immediately drops nose (tucks under) there is too little downthrust.
Test 2
pull up plane to vetical at sufficient throttle for towering vertical. If plane tucks under when it slows down, there is too much downthrust, and vice versa.
Test 3
fly slowly at reduced throttle. Apply throttle. When the plane looses altitute before climbing again, there is too much downthrust, and vice versa.

Okay, makes sense. I understand the theory. It makes most sense when you pointed out that most trainers have significant downthrust because the wing is so far above the engine centerline.

I guess, it's trial and error. Fly and trim. Fly and trim.

You think I should set the engine to 0* to start, and trim from there?

Here's the plane:
http://ecsvr.com/abm/shopexd.asp?id=1318

In a new design, I always start out with all angles set to zero. (Thrust, incidence, stab, fusedatum). That is close enough for uneventful maiden flights.
Thrustline and wing incidence are inverse interdependant, but for 3D I would indeed at first stay at zero, because the COG is way back, and the plane should behave the same in all flight positions.
Some side thrust is required to counteract torque and the spiral propwash against the fin.

Thanks for the help. I'll set it to 0*