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I'm interested to know your thoughts on engine ground break-in vs in-flight. Curious who's doing it on the ground (and solving the cooling and FOD problems), and who's doing it in the air during Phase I Testing (new aircraft at mostly high-power settings). And what your rationale was either way.
Quite frankly, people make too much of a big deal out of break-in procedures. I prepare for the first flight the same as I would with if the plane had an engine that was already broken in. If you feel the need to do some taxi testing before flying, then do so. Three of the planes I built had fresh overhauled engines on them. In each case, I did taxi testing as needed to assure proper ground handling, braking, control, and pilot training. In each case, then engines broke in just fine and have been performing optimally for hundreds of hours with no issues. The only real difference in the first flight is that I might make it a bit longer first flight if all goes well in the flight so I can put more time on the engine before landing, and I will do a minimal amount of slow flight to explore the approach to landing regime of the flight just enough to know what to expect for approach and landing. Safety of the pilot always outranks the engine break-in, and unless you are doing some really crazy stuff to the engine, it's going to break-in just fine anyway. If something happens during the first flight that would prompt you to want to return to the airport and land, then do so. The engine is going to break-in anyway.
In my experience in building multiple airplanes as well as building a lot of aircraft engines, break-in issues are grossly overblown and many issues blamed on "improper break-in" are actually issues caused by less than stellar work on the part of the engine builder. On the engines I build, I use typically use 20-50 (Phillips XC) AD oil rather than running straight mineral oil. You can use straight mineral oil if you choose, but I haven't seen any advantage to it. Just avoid synthetics and additives for the first 25 hours. Most of the break-in is accomplished in the first couple of hours, so ideally you run high power settings if practical to keep the cylinder pressures up to seat the rings, then operate normally there-after. Otherwise, test flying and pilot safety take presedence and the engine is going to break-in anyway.
The best answer to post #1 can be found in the engine manufacturers guidance, not what "some guy" at the airport or on the internet offers as an opinion. The manufacturer has spent some $$ on engineering and gets to hear lots of feedback from the field.
There is indeed a conflict between what a new builder needs to do in phase 1 to check the aircraft flight characteristics and identify issues vs what an engine needs to break in properly. I agree that performing a safe first flight is most important. But that does not conflict with the engine manufacturer's recommendation to minimize ground running of a new engine. Taxing around at low power serves more to build pilot confidence than to confirm that the ship steers correctly. That functional check can be done on the taxi to the first flight. And a quick control check immediately after liftoff from the runway should confirm that the controls are rigged correctly, or not, in which case shutting down and landing straight ahead is the best option. If good, go fly.
The actual result of the engine's cylinders and rings not breaking in is high oil consumption and then pulling the cylinders and rehoning them. Frustrating but not fatal. Not worth overthinking it before flight #1.
All of the manufacturers have their recommended engine breakin procedures on their websites.
Best of luck,
Wes