Cannot properly increase propeller pitch and engine throttle

[S&J]

Correct it's resistance to air going in, so it's half the picture. Whereas VE is the efficiency of the engine to accept the available mixture into it's cylinders and exhaust them.

So with a normally aspirated engine at atmospheric pressure before the air filter the engine sees less after it. The airfilter lowers the manifold pressure.

Now in a turbo charged engine the waste gate is set to ensure the engine sees it's rated manifold pressure. So even if there was any impedance between the output of the supercharger and the inlet manifold the waste gate would counter this and ensure the rated pressure in the manifold.

And this is why it should be set to zero on a turbocharged engine in jsbsim.

[TheEagle]

The thing is now flying at the correct speed (well, actually, even too slow) but the propeller is still going way too fast (well, it stays within limits, but I have no possibility to reach a lower RPM (blade angle is at 28.5 all the time) even by reducing the MP to 25. What can I do ?

[S&J]

Well you have it set up for auto prop governor.

If you notice when stationary the prop pitch is a lot less than 28.5.... but as you increase speed the pitch changes to maintain max available thrust at 2600rpm

It's working correctly.

What you're looking for is to adjust the rpm. So instead of having a 'constant speed prop' having a 'variable speed prop'

For that as jsbsim isn't set up for such a thing control system code would need to be written. ( No harder than what you've done already)

A new prop would need to be written to produce thrust at a range of 1800 to 2600 rpm.

I don't know if it's been done already but check out late WWII British fighters or bombers. See if they're jsbsim and been done correctly. I wouldn't hold my breath though.

[TheEagle]

No, I can adjust the RPM - but when the aircraft is at speed, the blades are constantly at the highest pitch with an RPM of > 2600. I fixed this by setting both ct_facot and cp_factor to 1.5 (maybe 2).

[S&J]

Why would you want to reduce rpm ?

[TheEagle]

Why ? Because for example the cruise data with the highest speed in the POH is with an MP of 31 at 2500 RPM. I'm getting the correct speed and MP, but the RPM is at 2650 instead of 2500. According to the POH one should be able to do 2200 RPM with an MP of 31 - but as I said, with an MP of 31, the blades are constantly at their highest pitch setting of 28.5, but still the RPM stays at 2650. So I need to increase the Cp (with <cp_factor>), but as the speed was correct, I also need to increase thrust (<ct_factor>) to maintain the same amount of thrust.

[S&J]

Ok well in that case set constant speed to 0 in your prop file.

You can also adjust this at run time in the engine properties.

But what you really need is an adjustable auto governor. We have to do something crafty to get the engine to run at 2700rpm for the takeoff setting

So when I'm next at my pc I'll look and see the property name you'll need to be adjusting to control an auto governor.

[TheEagle]

Well, with the higher Cp and Ct everything is behaving as I want it and as described in the POH (including 2700 RPM for takeoff), so I think there is no more to do.

[TheEagle]

@S&J: do you think it would be possible for you to export the data for our propeller from the geometry tab in JavaProp as a .txt file and post that in this thread ? We'd need this for some FDM tweaking. Thanks in advance !

[S&J]

Code: Select all

12.4 degs                                          
v/(nD)   v/(R)   Ct   Cp   Cs   Tc   Pc   n   n   stalled   v   rpm   Power   Thrust   Torque
[-]   [-]   [-]   [-]   [-]   [-]   [-]   [%]   [%]   [%]   [m/s]   [1/min]   [kW]   [kN]   [Nm]
0   0   0.059169   0.023747   0.000066   9.999999   9.999999   0.01   0.02   20.00 !   0   2600   81.735   2.313   300.2
0.1   0.032   0.057872   0.023881   0.211052   9.999999   9.999999   24.23   39.97   20.00 !   8.81   2600   82.197   2.262   301.9
0.2   0.064   0.053236   0.023797   0.422403   3.389107   7.574671   44.74   64.18   15   17.61   2600   81.906   2.081   300.8
0.3   0.095   0.046486   0.022897   0.638504   1.315283   2.159548   60.91   78.84   8   26.42   2600   78.811   1.817   289.5
0.4   0.127   0.03723   0.02079   0.867939   0.592539   0.827205   71.63   87.99   0   35.22   2600   71.557   1.455   262.8
0.5   0.159   0.025331   0.016573   1.135239   0.258017   0.337632   76.42   93.96   0   44.03   2600   57.045   0.99   209.5
0.6   0.191   0.011572   0.009758   1.514521   0.081852   0.115045   71.15   97.89   0   52.83   2600   33.588   0.452   123.4
0.7   0.223   -0.00298   0.000482   3.224597   -0.015485   0.003579   0   99.99   0   61.64   2600   1.659   -0.116   6.1
0.8      -0.018   -0.01                                 
0.9      -0.035   -0.022                                 
1      -0.055   -0.035                                 
1.1      -0.075   -0.048                                 
1.2      -0.095   -0.062                                 
                                          
                                          
                                          
28.5degs                                          
v/(nD)   v/(R)   Ct   Cp   Cs   Tc   Pc   n   n   stalled   v   rpm   Power   Thrust   Torque
[-]   [-]   [-]   [-]   [-]   [-]   [-]   [%]   [%]   [%]   [m/s]   [1/min]   [kW]   [kN]   [Nm]
0   0   0.070471   0.070429   0.000053   9.999999   9.999999   0   0.01   63.00 !   0   2600   242.41   2.755   890.3
0.1   0.032   0.075647   0.077167   0.166922   9.999999   9.999999   9.8   36   98.00 !   8.81   2600   265.603   2.957   975.5
0.2   0.064   0.075084   0.075751   0.335083   4.780022   9.999999   19.82   58.18   89   17.61   2600   260.729   2.935   957.6
0.3   0.095   0.073785   0.073886   0.505137   2.087681   6.968456   29.96   71.85   89   26.42   2600   254.308   2.884   934
0.4   0.127   0.071892   0.071894   0.677208   1.144203   2.860555   40   80.41   89   35.22   2600   247.452   2.81   908.8
0.5   0.159   0.070132   0.069845   0.851417   0.714359   1.42288   50.21   85.82   63   44.03   2600   240.402   2.741   883
0.6   0.191   0.069065   0.069028   1.024109   0.488534   0.813787   60.03   89.29   41   52.83   2600   237.588   2.7   872.6
0.7   0.223   0.068736   0.068737   1.195803   0.357216   0.510314   70   91.56   14   61.64   2600   236.587   2.687   868.9
0.8   0.255   0.067296   0.06909   1.365235   0.267764   0.343623   77.92   93.26   0   70.44   2600   237.801   2.631   873.4
0.9   0.286   0.063196   0.068348   1.539208   0.198677   0.238747   83.22   94.69   0   79.25   2600   235.248   2.47   864
1   0.318   0.054459   0.063237   1.737024   0.138679   0.161032   86.12   96.07   0   88.05   2600   217.657   2.129   799.4
1.1   0.35   0.041856   0.052714   1.981564   0.088088   0.100852   87.34   97.36   0   96.86   2600   181.436   1.636   666.4
1.2   0.382   0.028984   0.039982   2.284593   0.051255   0.058919   86.99   98.38   0   105.66   2600   137.614   1.133   505.4

[TheEagle]

Thank you very much S&J !

[S&J]

No problems.....

I have to say this though.

Fudging the ct and cp values is not the way to go. It may look like at first glance it's solved the problem, but it hasn't and you'll just end up chasing your tail.

[S&J]

<ixx> 4.38 </ixx>

Could you explain the working and logic of how this value was worked out ?

Does it align with know props ?

try 8.11

[TheEagle]

If you want to know the origin of that value you have to ask dany93.

[S&J]

It's a method, a flight model derived from the prop performance. I'm open for a change, so why not

This is the first time I ever created a prop that isn't matched to the planes drag at top speed.

And I've just flown this plane.... there's something very wrong with the engine/prop

Which I will look into.