Hi Vermontbadboy. First let me welcome you to RCU and you will find lots of good information here as you already have in this thread. Please don't be offended but your statement above is I think one of the key points here. It seems that perhaps your full scale experience clouded your judgement of you ability to handle the test flight. I hope this was not your first RC flight at the time. This is quite common that you can believe. I have spent most of my life in full scale flight training and now in my retirement RC training as well as mentoring folks in the hobby.

Ok this factor above is what I believe was ninety percent of the problem but it seems you moved on continuing flying RC and since learned a lot. That's great, now that we got through that there is another factor here that I would like you to understand about specifically the Sterling Tri Pacer model and no other tri pacer model that I have ever seen. Take a look at this video and take note of the horizontal stab/elevator:

https://www.youtube.com/watch?v=P5sCp7s33y

The stab/elevator on the Sterling Tripacer if built according to plan is simply humongous, almost 50% greater in size to what would be considered scale. It almost turns the model into a tandem wing aircraft. Most never notice this and I once was asked to test a new ship for a gentleman who has just completed his. Sure enough it had the giant tail feathers and when I pointed this out he did not believe me. So since I still kept a hanger over at the airport we popped over there and pulled his model out next to a full scale tripacer. Wow what a contrast ! That particular model designed in the fifties for old escapement rudder only RC was looking for the most pitch stability possible of course.

Now of course I tested this model in this form and it was OK perhaps not great and remember this model was intended for engines starting at about .19ci displacement and these days folks are hanging .40 and up on them. Later on after I pointed out the giant stab/elev to him he rebuilt it with a much more scale stab outline and I also suggested some down thrust. This he did and I also tested it in this form and all I can remember is it was a better flyer.

So my best suggestion is most definitely add four or five degrees of down trust. Yes 0-0-0 is an ideal but the Stirling will greatly benefit from down thrust. This is especially so if the original decaledge and stab/elev outline is used.

Actually it would be fun flying one these days with the giant stab and watch the guys reaction when you point out that fact to them. Please do restore your ship and enjoy

John