Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[bomber]

This is how I do the engine, it's pretty straight forward at this point.

So with data from wiki and the operators manual
http://www.kalwishky.com/files/O-300%20Operators%20Manual.pdf

http://target4today.com/_posted_files_/Cesna_172/Continental_O-300.xml

[bomber]

ok here's the results of the Static Engine Test on the original engine...



Not Good, as there's insufficient power at sea level, but not to worry as this is because of the Volumetric Efficiency being only 0.756...

So we'll raise it up to 1.0 and retest.

[bomber]

So I ran the engine at 2450rpm (recommended cruise RPM) at 9000ft with a wind velocity of 125mph (cruise speed) and this helped me determine the cooling factor for the engine cowling to ensure that the engine cylinder head temp doesn't go above 525F..
I adjusted the BSFC value whilst cruising to ensure that the fuel consumption was within spec 12.5gallons per hour
At sea level I adjusted the Volumetric efficiency so as the engine produced 145HP at 2700RPM as per spec.

And then performed the Static Engine Test at all alts.

[bomber]

As always here's the spreadsheet that goes with it...

http://target4today.com/_posted_files_/Cesna_172/Static_Test_Results.xlsx

3 years ago I brought the subject up over on the JSBsim dev mailing list that something might very well be wrong with the piston engine code, produced data like this for the merlins. But was met with denials and assertions that this was an FG issue.. Interestingly when I brought the subject up at FG, they said it was a JSBsim issue.

I then spent the intervening years on aerodynamics of a glider, as I couldn't trust the piston engine to give the correct values in the hope that someone would come along and help rectify the problem within the JSBsim C++ code.... that's not happened.

The bottom line here is that even with the best aerodynamic model, if the power/trust unit (engine/prop) produces the wrong figures, then your aircraft performance just wont be right...

Now I can create a JSBsim 'system' using xml to correct the error in the engine....I was just hoping that I wouldn't have too...

Regards

Simon

[Revolver]

Hello Simon,

very well that you trouble in this direction. I have not argued yet with JSBSim, but I see graphics in your tests that there everything is not perfect. I mean with the fact, that the engine JSBSim as such permits to us not everybody original data simply purely press and with it get the same tests graphics which agree with original Work-Tests-Graphics with each other.
For me personally it becomes interesting when the test graphics are same with each other, because then it will show us to all that Motor-(X) is programmed so immediately, how in real world.
In my opinion it is also important because afterwards not only everything will correctly function, but one will have all originating questions (why we have differences etc.) with a blow away.
And another one I would like to say. For the easy user it would have been better and easier a lot if he writes the original-data purely and gets back the correct values as they should be.


BR,
Stefan

p.s. At the moment I understand so, what inscribing in the program original dates we won't receive the copy on an output which it is possible to call identical with the original?

[Ron Jensen]

3 years ago I brought the subject up over on the JSBsim dev mailing list that something might very well be wrong with the piston engine code, produced data like this for the merlins. But was met with denials and assertions that this was an FG issue.. Interestingly when I brought the subject up at FG, they said it was a JSBsim issue.

I think your statement is an unfair assessment. I have personally spent many, many hours responding to your posts and e-mails, and tried my best to explain how and why the code works the way it does, both on the JSBSim side and the FG side. As I have stated before, I am unable to reproduce your results using valid test methods and feel your methodology of adjusting atmospheric properties while your models stay at sea-level. is the source of your error. The simulations are not designed to allow that.

For example, as I look at your spreadsheet I see you have a MAP of 15.7 (inHg?) at "24000 ft" while you have a "Barometer" of 11.6 inHg. This should not happen as MAP should always be less than Barometric pressure in a normally aspirated engine. Going back to your reference book, 15.7 inHg yields 80+ hp at 16,000 ft, so the actual result of 90 hp at 24,000ft seems reasonable since horsepower increases with altitude when MAP is held constant.

Using my methods give me the attached graphs. The red dashed line is the expected horsepower

(Edit: Add the image as an inline link instead of an attachment)

[bomber]

I think your statement is an unfair assessment. I have personally spent many, many hours responding to your posts and e-mails, and tried my best to explain how and why the code works the way it does, both on the JSBSim side and the FG side.

As I have stated before, I am unable to reproduce your results using valid test methods and feel your methodology of adjusting atmospheric properties while your models stay at sea-level. is the source of your error. The simulations are not designed to allow that.

I'm sorry you feel that way Ron, I've always appreciated the time and effort you've put in helping me understand how the engine and thruster file contents work...

but that's a separate issue to this topic and I think it's unfair to attempt to mix the two together...

Your only response to the original post I made 3 years ago, was that and to para-phrase "you can't do it that way, I can't reproduce it, and anyway FG uses a different atmospheric model which makes it their fault"..
There was never hours spent on it, you've posted more on this topic yesterday than ever you did before and this is the first time I've ever seen any supporting data...

It wasn't my intention to be unfair but to resolve this issue... If it turns out that testing the piston engine the way I've been doing it is wrong and the piston engine is correct, then I'll be over-joyed and the issue will be resolved...

Now I will go have a look at your comments, and post my findings as soon as possible....

For example, as I look at your spreadsheet I see you have a MAP of 15.7 (inHg?) at "24000 ft" while you have a "Barometer" of 11.6 inHg. This should not happen as MAP should always be less than Barometric pressure in a normally aspirated engine. Going back to your reference book, 15.7 inHg yields 80+ hp at 16,000 ft, so the actual result of 90 hp at 24,000ft seems reasonable since horsepower increases with altitude when MAP is held constant.

[bomber]

This is the piston.cp file...

https://gitorious.org/jsbsim/jsbsim/source/eef1ac615c3d804ceaa947e839e921950e93b486:src/models/propulsion/FGPiston.cpp#LNaN-NaN

line 448

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
void FGPiston::ResetToIC(void)
{
  FGEngine::ResetToIC();
 
  ManifoldPressure_inHg = in.Pressure * psftoinhg; // psf to in Hg
  MAP = in.Pressure * psftopa;
  TMAP = MAP;
  double airTemperature_degK = RankineToKelvin(in.Temperature);
  OilTemp_degK = airTemperature_degK;
  CylinderHeadTemp_degK = airTemperature_degK;
  ExhaustGasTemp_degK = airTemperature_degK;
  EGT_degC = ExhaustGasTemp_degK - 273;
  Thruster->SetRPM(0.0);
  RPM = 0.0;
  OilPressure_psi = 0.0;
  BoostLossHP = 0.;
}
 
//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

line 358

  property_name = base_property_name + "/map-inhg";
  PropertyManager->Tie(property_name, &ManifoldPressure_inHg);

So the next search is to find in.Pressure to see how this is derived...

Any idea Ron where it might be ?

[Ron Jensen]

So the next search is to find in.Pressure to see how this is derived...

Any idea Ron where it might be ?

This is set by a call to Atmosphere->GetPressure(); in FGFDMExec.cpp line 454.

[bomber]

forgot to post this earlier, had family round..

[bomber]

Ok as it would now seem that testing the engine the 'Rolls Royce' way of adjusting the pressure and temp within a testbed doesn't work I've created a floating plane that hovers to the required altitude.... in effect allowing me to do a static engine test.

So I'm now looking forward to some more engine testing and hopefully getting values that agree with those of Ron.



There is one little niggle I have however, and that's this....... Will JSBsim be able to cope with differing planetary values for pressure and temp with alt ?

[Uriah]

Nice work Simon!!!

[bomber]

OK this is how the engine tests out using the new method....

[bomber]

Ok, it's time for some good reading...

http://www.epi-eng.com/piston_engine_technology/engine_technology_contents.htm

[HiFlyer]

Ok, it's time for some good reading...

http://www.epi-eng.com/piston_engine_technology/engine_technology_contents.htm

Little technical. I'm waiting for the action comic addition........