I would say add the down thrust. The TF GS Corsair I bought used last year had it about 3 degrees built into the engine mount and it flew great. When I mounted the engine in my TF 60 size Corsair I added a couple degrees there and same thing. Maybe I'm just used to having it from the planes I learned on. I taught myself to fly with the Parkzone electric warbirds and they all have down/right thrust. Maybe I have just got used to it being there, but now when I mount the engine or motor I always make sure it has some of both and things usually work out pretty well.