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New Aircraft - Promecc_Freccia

Questions and discussion about creating aircraft. Flight dynamics, 3d models, cockpits, systems, animation, textures.

Re: New Aircraft - Promecc_Freccia

Postby S&J » Thu Sep 29, 2022 3:28 pm

Confusion can arise because you have to understand the limitations your tools are putting on you and how that carries over into JSBsim and understand the limitations JSBsim then puts on you.

The real world is a fine thing, but you're not mimicking the real work, you're simulating it, so some flexibility has to occur, let me explain.

Using a Clark Y airfoil, designed in 1922 for a wooden prop to model a 2022 composite prop by rotating the blade to match those values found in 'the book' will deliver a prop that wont do the job.

Frankly javaprop will never model the propeller you're after correct, it simply doesn't have the airfoils. So you're going to have to make a prop using it that delivers the correct thrust for power but won't have the correct blade angle.

My advise, learn how to use javaprop correctly and understand the data it's outputting and it's limitations.
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Re: New Aircraft - Promecc_Freccia

Postby dany93 » Thu Sep 29, 2022 4:52 pm

Tweaking the parameters to get the values is a risky way, one has to be very careful at doing like this.

In an ideal world, if you have the thrust vs V/ND and power vs V/ND from the prop manufacturer, you do the job.
But you generally do not have them.
Moreover, even if you have them (ideal), your aircraft will behave like in "The Books" only if, by elsewhere (its FDM), it has the correct drag vs airspeed and weight. This last one is of first importance.

Inversely, if you tweak the prop parameters to get the aircraft behave like "in the books", say at every power between 0 and max power (at horizontal flight? At climbing? ), you have to ensure (FDM) that the aircraft drag vs airspeed is correct. Otherwise, the tuning work on propeller is useless. For example, the aircraft will not behave correctly with the propeller stopped or at idle. Or, more generally, in other circumstances.
And if, later, you realize that you have to change the aircraft drag, you have to redo the work on the propeller.

This is a general issue in simulation: you change one thing, you get a particular (correct) result, but if the change was not by the right way, you degrade something else. Like a can of worms.
Tweaking is often unavoidable. But by trying to remain as close as possible to physics and aerodynamics rules, you minimize these drawbacks.
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Re: New Aircraft - Promecc_Freccia

Postby S&J » Thu Sep 29, 2022 5:07 pm

And there lies the challenge.

And I know you probably meant to include the piston engine within the correct working of the FDM. But it should be understood that getting lift vs drag and prop thrust to power and mass Ballance working correctly will be all for naught. If the piston engine doesn't produce the correct power Vs alt for the required fuel usage.

And I can tell you that JSBsim piston engine (out of the box) is incorrect in doing so without some small bsfc and ve corrections.
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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Thu Sep 29, 2022 10:32 pm

dany93 wrote in Thu Sep 29, 2022 4:52 pm:Tweaking the parameters to get the values is a risky way, one has to be very careful at doing like this.

In an ideal world, if you have the thrust vs V/ND and power vs V/ND from the prop manufacturer, you do the job.
But you generally do not have them.
Moreover, even if you have them (ideal), your aircraft will behave like in "The Books" only if, by elsewhere (its FDM), it has the correct drag vs airspeed and weight. This last one is of first importance.

Inversely, if you tweak the prop parameters to get the aircraft behave like "in the books", say at every power between 0 and max power (at horizontal flight? At climbing? ), you have to ensure (FDM) that the aircraft drag vs airspeed is correct. Otherwise, the tuning work on propeller is useless. For example, the aircraft will not behave correctly with the propeller stopped or at idle. Or, more generally, in other circumstances.
And if, later, you realize that you have to change the aircraft drag, you have to redo the work on the propeller.

This is a general issue in simulation: you change one thing, you get a particular (correct) result, but if the change was not by the right way, you degrade something else. Like a can of worms.
Tweaking is often unavoidable. But by trying to remain as close as possible to physics and aerodynamics rules, you minimize these drawbacks.


I agree on this approach and is indeed the one I pursue. I think you first have to go this way on the first stage for each component. Only ate the end you might need to do some tweaking as a final tuning.
That being said I would like to model this propeller as best we can with the tools and knowledge we have, then move to the engine and finally to the airframe and aerodynamic of fuselage and control surfaces. Once all these are done and put together perhaps the time for the tweaking will come.

For the chat rooms let's try both and we will see which one will be the best!
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Re: New Aircraft - Promecc_Freccia

Postby TheEagle » Fri Sep 30, 2022 12:15 am

SDeAstis wrote in Thu Sep 29, 2022 10:32 pm:For the chat rooms let's try both and we will see which one will be the best!

Here you go for Discord: https://discord.gg/5A2WDZPRvk
After joining I'll give you the Pro.Mecc Freccia dev role, and then you'll be able to talk in Pro.Mecc Freccia -> #general ! :)
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Re: New Aircraft - Promecc_Freccia

Postby merspieler » Fri Sep 30, 2022 11:14 am

Very well... you're in the Mattermost Team, and already are Admin there. Just login with your mattermost credentials you've had for the hackathon and the team will show up in the bar on the left.
I've written you the invite link in the DMs there, as I'm leaving it up to you to decide if you want the team to be available publicly or just for your selected team of contributors.
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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Sat Oct 08, 2022 2:16 pm

Updated the first post with Mattermost and Discord channels! Feel free to join, support, report problems and any comment. Every sort of contribution is welcome! Thanks again so far!
Now I wish to have a session with experienced people where by sharing my desktop I could be guided for a better model production od the propeller... I was thinking after this activity to write a wiki based on this use case.
What do you think about?
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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Tue Oct 11, 2022 11:04 pm

Propeller has been modelled with the use of JavaProp and the support of S&J and TheEagle.

Now let's have a look at the Engine Rotax 912ULS. I think I already have a good file for it. But I want to review the values I put and check that all is fine. Here my current status:

Code: Select all
<piston_engine name="ROTAX912ULS">
  <minmp unit="INHG">         10.0 </minmp>
  <maxmp unit="INHG">         28.5 </maxmp>
    <displacement unit="IN3"> 187.271 </displacement>
  <maxhp>        100.0 </maxhp>
  <cycles>         4.0 </cycles>
  <idlerpm>      700.0 </idlerpm>
  <maxrpm>      5800.0 </maxrpm>
  <sparkfaildrop>  0.1 </sparkfaildrop>
  <volumetric-efficiency> 0.85 </volumetric-efficiency>
  <man-press-lag> 0.1 </man-press-lag>
  <static-friction  unit="HP"> 0.492817 </static-friction>
  <starter-torque> 78.8508 </starter-torque>
  <starter-rpm> 1400 </starter-rpm>
 <!-- Defining <bsfc> over-rides the built-in horsepower calculations -->
 <!--<bsfc>           0.45 </bsfc>-->
  <stroke unit="IN">  4.375 </stroke>
  <bore unit="IN">    5.125 </bore>
  <cylinders>         2  </cylinders>
  <compression-ratio>  8.0 </compression-ratio>
</piston_engine>
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Re: New Aircraft - Promecc_Freccia

Postby TheEagle » Tue Oct 11, 2022 11:29 pm

Good thing you posted this, the bore, stroke displacement and number of cylinders are all four wrong, here are the right ones: http://www.flyrotax.com/assets/files/pr ... 912ULS.pdf
I don't know exactly these three are used in JSBSim, but better put the right values in ! ;)
And for the static friction, one decimal place is enough ! ;)

Additionally, make sure you got the gear ratio from the linked document in your propeller's <gearratio> tag, else you'll end up with wrong propeller RPM.
Last edited by TheEagle on Wed Oct 12, 2022 11:55 am, edited 1 time in total.
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Re: New Aircraft - Promecc_Freccia

Postby Ysop » Wed Oct 12, 2022 8:17 am

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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Wed Oct 12, 2022 10:01 am



Maintenace Manual: https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/0685c00000BM5VqAAL?asPdf=false
Information on Manifold Pressure: https://rotax-docs.secure.force.com/DocumentsSearch/sfc/servlet.shepherd/version/download/0681H00000FTeDYQA1?asPdf=false

Based on these documents I digged from https://www.flyrotax.com/, this is my current state of the engine file, with still something "unknown"

Code: Select all
<?xml version="1.0"?>

<!--
  File:     ROTAX912ULS.xml
  Author:   Salvatore De Astis (SDeAstis)

  See: http://wiki.flightgear.org/JSBSim_Engines#FGPiston

  Inputs:
    name:           ROTAX912ULS
    type:           Piston Engine
    power:          100 hp
-->

<piston_engine name="ROTAX912ULS">
  <minmp unit="INHG">         18.0   </minmp>            <!--SHOULD BE OK-->
  <maxmp unit="INHG">         28.0   </maxmp>            <!--OK-->
  <displacement unit="IN3">      82.6   </displacement>         <!--OK-->
  <maxhp>                  100.0   </maxhp>            <!--OK-->
  <cycles>                   4.0   </cycles>            <!--OK-->
  <idlerpm>                 1400.0   </idlerpm>            <!--OK-->
  <maxrpm>                 5800.0   </maxrpm>            <!--OK-->
  <sparkfaildrop>             0.1   </sparkfaildrop>      <!--UNKNOWN???-->
  <volumetric-efficiency>       0.85   </volumetric-efficiency><!--UNKNOWN???-->
  <man-press-lag>             0.1   </man-press-lag>      <!--UNKNOWN???-->
  <static-friction  unit="HP">    0.493   </static-friction>      <!--UNKNOWN???-->
  <starter-torque>            78.8508   </starter-torque>      <!--UNKNOWN???-->
  <starter-rpm>              1400      </starter-rpm>         <!--UNKNOWN???-->
  <stroke unit="IN">          2.40   </stroke>            <!--OK-->
  <bore unit="IN">             3.13   </bore>               <!--OK-->
  <cylinders>                4      </cylinders>         <!--OK-->
  <compression-ratio>         10.8   </compression-ratio>   <!--OK-->
</piston_engine>
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Re: New Aircraft - Promecc_Freccia

Postby TheEagle » Wed Oct 12, 2022 12:39 pm

<sparkfaildrop> sets how much the power of the engine will drop when the engine is running on one magneto only instead of both - you'll normally have a mag check in the POH of the aircraft where you have a phrase like "Check magnetos (100 RPM maximum permissible drop)". You will have to do a little tuning of the value to achieve the correct RPM drop. As for volumetric-efficiency, this is used to adjust your fuel consumption - there should be a table for cruise fuel flow in the POH, pick a configuration (like 5000 ft, 75% power) then create a standard atmosphere condition in the sim and tune /fdm/jsbsim/propulsion/engine/volumetric-efficiency until you get the desired fuel flow, then put the value you found into the <volumetric-efficiency> tag in the engine file.

<man-press-lag> basically sets how fast the engine will accelerate, I think - this, again, has to be found by tuning, and you'd need a video including a view of the throttle and tachometer / MP gauge to make this realistic. Next, <starter-rpm> has to be set to a higher value than the <idlerpm> to get the engine to start (I think). If you got the part number of the starter you should be able to find the <starter-rpm> and <starter-torque> somewhere.
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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Wed Oct 12, 2022 12:59 pm

Great!
I believe I can find the P/N of the starter and find the datasheet.
Only information found about the starter is this: Electric starter (12 V 0,6 kW / optional: 12 V 0.9 kW )
For the rest, I now have better understanding of the parameters and makes sense to retrieve those by reverse engineering.
I thing it comes the time to script the model in JSBSIM and record some data to plot!
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Re: New Aircraft - Promecc_Freccia

Postby S&J » Wed Oct 12, 2022 8:57 pm

are you sure it's not 3.31inch for the bore ?

also you need to have a 'standard day' as reference
so
<maxmp unit="INHG"> 29.9 </maxmp>
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Re: New Aircraft - Promecc_Freccia

Postby SDeAstis » Sat Oct 15, 2022 3:28 pm

With the data I put in in the previous post I get inthe sim:
1) 2500 rpm at idle instead of 1400
2) 5300 rpm instead of 5800 rpm at max

Why?
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