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SWS-AlexVletsas avatar image
SWS-AlexVletsas asked SWS-AlexVletsas edited

[BUG] Propeller properties hardcoded within certain ranges

Reversible propellers have a discing/zero thrust pitch which is different for every model. Pilots use beta range to adjust their taxi speed. The RPM should begin from idle and increase up to a certain limit (95% for the PT6 engines).

What I see in MSFS is that initially my RPM decreases and then starts going up to the limit I set in the prop tables. As an example, for the Kodiak, we have the following reference values:

Low idle RPM: 1080 +/-20

Prohibited Prop Operation RPM: 450-1050

Max reverse prop RPM: 2090

What I'm getting in the sim:

Low idle RPM: 1080 +/-20

Minimum RPM during beta cycle: 790

Max reverse prop RPM: 2090

In the MSFS debug tools I observed that when the propeller beta is between beta_min and reverse, power absorbed and efficiency revert to a hardcoded 0.1 which leads to unrealistic RPM (and thrust). The propeller decreases speed and almost always enters underspeed mode. I am using legacy tables and even though I can explicitly set the Cp and n, both are completely ignored and the sim reverts to these default values.

So it would be great if this hardcoded behaviour is removed in one of the next sim updates, which will allow us to flesh out our propellers and turboprops fully.

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Simbol avatar image
Simbol answered

Sylvain, Eric, @FlyingRaccoon @EPellissier

I am experiencing the same issue as Alex.. more over because I do not use legacy tables the behaviour is even worse.

The engine and prop thrust continues to spool up forever causing Prop RPMs above 4000 despite the maximum RPM should be around 1900 on reverse. The behaviour varies with different blade pitches, for an instance on my engine between 5.9 to 6.0 triggers the behaviour, spooling the engine RPMs up to 82,000 RPMs when the maximum configured is 37468 RPMs. This causes the RPMS to go too high (4427) and the thrust totally out of scale.

There is nothing I can do to stop this behaviour, overriding the Prop:RPMs and Engine RPMs have no effect on the thrust output, so the airplane keeps accelerating pretty much to take-off speed and the engine debug continues to display the same figures as above.

The problem repeats again at -1 Degrees pitch.. but everything works fine between 2 and 3? which makes no sense. The picture below shows the results when you move the blade 1.07 degrees out of the 6 degrees zone, is too weird:

I also tried limiting the fuel to the engine via TURB ENG CORRECTED FF, but it doesn't work either since the airplane continues to spool up.

What Alex and I are trying to do here is to simulate the correct behaviour of PT6 engines, where during beta range the prop blades pitch changes to a very fine pitch slowly until the pilot continues to push the thrust level to full reverse, at which point full reverse is engaged. During this stage (Beta Ground operations) the pilot is basically adjusting the pitch blades manually to adjust the taxi speed. The engine stops using the governor and instead uses fuel flow to control the propeller RPMs so they don't go over the max RPM or under RPMs.

Ideally, we also need the ability to control fuel flow properly to code the behaviour correctly. If we could have something like we have with forced prop beta, to implement forced fuel flow, perhaps we could prevent the engine from going nuts like this?.

I am leaving here attached the description of these operations from the PT6 engines. fdocuments.net_pt6a-engine-explanation.pdf

We would really appreciate your help with this subject, both me an Alex have several new projects using PT6 engines which are affected by these behaviours, and we would like to find the best way to be able to move such projects forward.

Thanks in advance for taking the time to look into this.

Best Regards,

Raul Morales
CEO Flight Sim Technologies

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SWS-AlexVletsas avatar image
SWS-AlexVletsas answered SWS-AlexVletsas edited

Looking at Raul's post, at 5.89 degrees of pitch his Cp (power_absorbed) is very small, which causes the RPM to go crazy high. With beta less than 5 degrees (other photo) his Cp and efficiency are both 0.1 which is the hardcoded behaviour I also noticed.

Given that I'm using legacy and he's using modern, there is one common problem in both cases: the propeller has hardcoded Cp and n for certain ranges, which seem to be between -3 and +5 degrees of pitch.

The other problem has to do with the "modern" modern propeller (not the one from v1.0) which completely miscalculates the performance when it comes to Turboprops.

Additionally, the two modern propeller models seem to have weird behaviours for reverse pitch, leading to propeller overspeed when you go into full reverse. Without getting too technical, the PT6 engine in particular has two behaviours worth observing, per the maintenance manual:

  • Initially, the propeller RPM increases because it goes into finer pitch (lower Cp) and of course, N2 increases. The fuel control unit adjust the fuel flow to keep the Ng at idle during that time.
  • As you pull the power lever further back, the propeller pitch starts going below ground fine and towards reverse the more you pull it. Np increases up to 95% of the max governed value. At the same time Ng begins to increase, but the FCU controls Ng so that Np will not increase past 95% Np

What is happening in MSFS is odd:

  • Using legacy tables, one can limit the propeller to 95% Np. Ng will be dragged along by the propeller, so retaining idle Ng during the initial beta stage is impossible without some custom coding. Ng will be limited by propeller speed. When beta reaches -3 to +5 degrees, hardcoded behaviours cause the propeller to underspeed and not produce correct thrust, ignoring the tables completely.
  • Using the modern flight model and no forced beta, any throttle setting below 0% will immediately take the propeller to full reverse pitch. Ng is then used to scale the thrust produced by the propeller. The issue is that the propeller will overspeed well before maximum Ng is achieved.
  • Using the modern flight model and forced beta will yield both problems at once. You will experience propeller underspeed between -3/+5 degrees and overspeed when you go into full reverse.
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