question

@FlyingRaccoon

I wanted to utilise the modern prop system for our own turboprops, but I hit the same wall and some limitations with it. I didn't spend as much time as @Simbol with it as I had a deadline to meet, but I found myself very restricted.

To clarify, I am a big advocate of legacy tables because they allow me to tune the propeller exactly how I need it and the data and formulae used for them match the academic and industry standards and I know it first-hand.

1. Documentation: a table showing what scalars and parametres work with the modern prop system would help a lot. I.e. does prop_power_absorbed work or not?
2. Undocumented parametres: prop_mod_aoa_lift_delta_align_beta_deg
3. Better explanations: advance_ratio_on_effective_beta, for example, makes no sense -even if unused. By reading it, I assume that as the advance ratio increases (moving right on the J/β table), the propeller beta also does. So, a value of 0.2 means what? That for every 0.2 of change in advance ratio the propeller β will change by 1 degree? Things like that would help a lot if they were explained or formulated.
4. Discrete control: the modern flight model should be able to simulate propeller operation throughout all advance ratios and beta angles as it uses very basic properties of the fin. The issue with that (and the previous modern propeller model) is that it is generic and you cannot predict behaviours in particular ranges. I.e. the prop may work well when flying, but when idle its RPM may be too high or too low. If I tweak power_absorbed to get my idles right, I need to re-work my flight RPM, then thrust etc.
5. I also had another situation: my prop works well for idle and takeoff, but on climb it will over-rev beyond the real RPM. Since I couldn't tune power absorbed, the solution I found was to set beta_max to 90 degrees, so the propeller has more playroom. This way if it was about to over-rev it would go from 40 degrees to 45, 50, 60 or whatever it wants to maintain RPM. The problem is that per the aircraft manual the real propeller doesn't go to above 40 degrees. This shows that the absorbed propeller power is either wrong or not-tunable in the modern propeller simulation. If our option is to move the propeller to unrealistic pitches to get it to work, so be it, but we need to know why. And here comes my next point.
6. In the previous situation I also noticed that my feathered RPM was half what it should be, which makes the problem even more pronounced. So I have a propeller that can fly from 12 to 80 degrees, but feathers at 83 and produces no thrust? That doesn't make sense from a physics standpoint.
7. Propeller discing: there is a beta angle for every prop where it will produce zero thrust and is used when taxying to keep the plane in place. Using legacy tables this can be specified, but with the modern FM it will be guessed. Again, I'm (kind of) OK with using unrealistic figures if the prop performs, but it turns propeller developement into guesswork, while there are perfectly fine physics model to predict it accurately. On to my next point.
8. Mathematical formulae: It would help me a lot if the formulae for the modern propeller model were listed next to the respective parametres and in a way that they are complete if put together -i.e. formulas posted for other things like ITT are not complete and will not give a proper output if put in excel.

TLDR, the modern prop model makes sense as a concept, but its implementation has errors that show even in moderate circumstances. The lack of tuning options for edge cases such as feathering, discing or reverse make the problem worse, as we have to rely on trial-and-error instead of predictable formulae and real data. If, at least, we have the formulae for the modern prop model, we can shape it to what we need in less time.