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Simbol avatar image
Simbol asked SWS-AlexVletsas answered

PROP Modern, Power Abosorb formulas??

Dear Asobo,

I have a propeller configured correctly with power absorb scalar to make the prop spin at idle at 1220 RPMs with a shaft torque force of 141 ft-lbs.. (idle), then at full thrust under ISA conditions it spins at 2000 RPM just as POH and gives the correct trust and power to take off, maintain speed, etc., all matches all works.. important to note, this is NOT using legacy tables.

However as soon as I enable PROP MODERN, the configurations goes high-wired:

Problem 1:

prop_mod_use_absorbed_torque = 0;

This results in wrong RPMs at idle again.. it drops from 1220 RPM to almost half, is like the power absorb scalars get totally ignored in favour of something else.. nothing I could do to influence this unless you spool the engine to the wrong N1 at idle or wrong torque at idle.

So I set it as prop_mod_use_absorbed_torque = 0.. which takes me to problem 2.

Problem 2:

prop_mod_use_absorbed_torque = 1

This keeps the correct idle torque of 141 ft-lbs, correct N1 and correct 1220 RPMs on prop, however it seems at the same time the power absorb formulas goes wrong because I end up with a propeller creating a very heavy thrust on idle (450 lbs NEW vs 180 - 200 lbs old).. causing the airplane to over taxi fairly quick!. and then at full power drops, the thrust falls too much. (losing about 300lbs of force vs non modern) making the aircraft unable to take off using the POH runway length, so clearly, the thrust goes wrong under this circumstances.

I tried altering all the PROP_MOD values to correct this issue to no availability, the only thing that helped a bit was was changing the prop_mod_aspect_ratio from 6 to 4.8 to generate more torque at the end of the curve and decrease the the prop_mod_lift_slope_cf to reduce a bit the idle thrust speed, however I could not generate more thrust at full power (969 ft-lbs as per POH), So it seems I am unable to balance the curve out with the old curve.. not matter what I tried, it seems the power absorb is calculated automatically somehow making it impossible to have the correct idle RPM vs thrust at high power.

Can we have more detail about how power absorb is calculated with the prop modern formulas and what can be used to influence it? the only solution I have so far is to reset power_propeller_absorbed_cf back to 1.0 (from 0.42 as I need it), which then returns the full power of the propeller under modern flight model, but I lose the required 1220 idle RPM speed with the required idle torque and idle N1.

Problem 3:

Yes more, if you enable CFD + Re-inject rotors.. then the propeller lose even more thrust.. up to 200 to 250 lbs more.. why is this? how Re-inject rotors affects turbo prop performance?


I want to use Prop modern, but quiet frankly after 2 weeks of no sleep trying to tweak it, I am about to give it and leave it all legacy.. there must be something I am doing wrong, we should be able to tweak these values much easier and to control results better for sure.

I am able to tweak piston engine +prop modern fairly quick, but with TurboProps, the absorved torque is causing misbehaviour that I cannot understand or control.

Thanks in advance for the time to read this question and any insight that could be provided.

Kind regards,

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DA40CGDFQ avatar image DA40CGDFQ commented ·

To add to this, IS the SDK statement wrong??


This section is for defining the properties of the propellers on a piston aircraft. The parameters available are:

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Simbol avatar image Simbol DA40CGDFQ commented ·
Totally, because for TurboProps you need to configure that section for sure..


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FlyingRaccoon avatar image FlyingRaccoon ♦♦ DA40CGDFQ commented ·
Our tech writer will fix this.
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FlyingRaccoon avatar image FlyingRaccoon ♦♦ commented ·
Hello @Simbol

Can you give me access to your package so I can have a look at what's happening in your specific case?


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MrTommymxr avatar image MrTommymxr commented ·
I had some really off performance when i first tried the modern prop model. high static thrust and low speed.

I use a negative twist value and that fixed it. I think u have -10 on my diamonds.

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Simbol avatar image Simbol MrTommymxr commented ·
where? can you elaborate more?


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MrTommymxr avatar image MrTommymxr Simbol commented ·

ok my mistake, as the values are reversed i thought i had a negative value entered into the cfg.

prop_mod_aoa_twist_delta_deg = 7.5 is what I have

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1 Answer

SWS-AlexVletsas avatar image
SWS-AlexVletsas answered


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.

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