The problem with some books, not all, is that they quote other books and so on and so forth. What was the original source? Just because 10 books all same the same thing doesn't mean much if they all used the same book as their source.

And even if you find some original source sometimes they leave off critical information like exactly what engine was used was it X or the improved X.

Again my point is that there is a lot of variability in numbers. Why? Again because testing aircraft was as young as the aviation industry itself. Add in the expediency of war and it is so hard to actually tell exactly what was what.

Tell me how do you do a speed test at 3000m in a WW1 crate? Read the report. A NACA report the ones that some of us quote and take carte blanche. Look at all the early reports. These are the experts. Look at the basic things we all take for granted now being discussed and researched.

And no we can't just accept what some book says. There needs to be some knowledgeable critical thinking to determine if the source is decent or not. Face it. We know a lot more about airplanes and how they perform now than then. And there is absolutely no reason what so ever that we shouldn't try to apply critical thought and updated knowledge.

So again get it about right. Use some benchmark airplanes and get them close and make the other planes relative to them. Don't worry if the sim plane is slightly slower or faster. Use a late Spad, a Camel, an Albatros III, and a Fokker DVII. And probably a Nieuport, too. Do your best reseach and modeling on those then make the others performance relative to those in terms of speed, climb, and turn.

And we all need to accept that just because you have a source that a Alb DVa could go 120 mph and the one in the sim goes 115 mph then don't get your panties in a bunch because there was so much variability then that they both may be right. What matters is not only the absolute sim performance but also the relative performance of that Alb DVa to its counterparts, predecessors, and descendants.