Thrust and moments

[fernandafenelon]

Hello everyone,

it looks like JSBSim does not consider pitch, roll and yaw moments due to thrust automatically.
I will have to add it manually. Right?

[seanmcleod]

What are you basing your statement on? Have you tried a model where the engines are above or below the cg and checked to see what happens when thrust is increased or decreased? Or what about a twin engine aircraft, what happens if the thrust is changed on the 1 engine? Do you not see a yawing moment?

For example take a look at the JSBSim 737 model and the pitch rate when the throttles are retarded, see graph - 'Control Inputs - Flare'

https://seanmcleod.github.io/2018/02/landing-ground-effect-flare-jsbsim/

[fernandafenelon]

Yes, I made this test and nothing changed (the elevator deflection didn't change at all when I changed the engine position in z). However, I am not using 737 model. I am using another aircraft model.

[seanmcleod]

We need more details. Why are you expecting the elevator deflection to change? Do you have some automatic control system controlling the elevator which you expect to make changes with changes in thrust moment?

[fernandafenelon]

I am trimming the aircraft. I expect change in elevator deflection if I change my engine position in z coordinate due to change in pitch moment. However, I don't see this change.

[seanmcleod]

Okay, so you're trimming. So show us the trim inputs and the trim results for the 2 different z positions for the engine. Are you sure the trim solution isn't coming up with a different thrust setting which counters the change in thrust arm?

[fernandafenelon]

well, I trimmed in a common condition h=10000ft, V=170knts... I just ran another test where I changed the engine position in 700 inches and I see NO difference in elevator nor thurst of equilibrium.. I am trimming using the case 0 (longitudinal).

[seanmcleod]

In terms of moments due to propulsion JSBSim exposes the following properties which you can view to see how they change as the thrust is changed etc.

  PropertyManager->Tie("moments/l-prop-lbsft", this, eX, &FGPropulsion::GetMoments);
  PropertyManager->Tie("moments/m-prop-lbsft", this, eY, &FGPropulsion::GetMoments);
  PropertyManager->Tie("moments/n-prop-lbsft", this, eZ, &FGPropulsion::GetMoments);

[fernandafenelon]

When I check these variables, they all equal 0. I will try to investigate these functions....

[seanmcleod]

Please don't be shy to show us some of the output like the trim results, how you changed the z location of the thruster etc. We may spot something, otherwise it's a lot of guessing back and forwards.

So take a look at my test and results, I've included the code so you can try it out yourself and confirm you see the same results.

So first off here is the code I use to test.

import jsbsim

fdm = jsbsim.FGFDMExec('..\\') # The path supplied to FGFDMExec is the location of the folders "aircraft", "engines", "systems"

fdm.load_model('737') # Load the 737 aircraft

# Set engines running
fdm['propulsion/engine[0]/set-running'] = 1
fdm['propulsion/engine[1]/set-running'] = 1

fdm['ic/h-sl-ft'] = 1000
fdm['ic/vc-kts'] = 200

fdm.run_ic() # Initialize the aircraft with initial conditions

# Trim
try:
    fdm['simulation/do_simple_trim'] = 1
except RuntimeError as e:
    # The trim cannot succeed. Just make sure that the raised exception
    # is due to the trim failure otherwise rethrow.
    if e.args[0] != 'Trim Failed':
        raise

print(f"moments/m-prop-lbsft - {fdm['moments/m-prop-lbsft']}")

I then ran that code with the default 737.xml in terms of the thruster location, i.e.

            <thruster file="direct">
                <location unit="IN">
                    <x>  540 </x>
                    <y> -193 </y>
                    <z>  -40 </z>
                </location>

Then I ran the code again after changing the z coordinate to -60 for both.

Results for z = -40.

  Trim successful
  Trim Results:
       Angle of Attack:   6.46  wdot:  8.95e-05 Tolerance: 1.000000e-03  Passed
              Throttle:   0.56  udot: -8.97e-05 Tolerance: 1.000000e-03  Passed
            Pitch Trim:  -0.41  qdot: -1.24e-10 Tolerance: 1.000000e-04  Passed
            Roll Angle:   0.00  vdot: -3.04e-16 Tolerance: 1.000000e-03  Passed
              Ailerons:   0.00  pdot:  2.50e-18 Tolerance: 1.000000e-04  Passed
                Rudder:   0.00  rdot: -1.91e-33 Tolerance: 1.000000e-04  Passed
moments/m-prop-lbsft - 4683.108872773994

Results for z = -60.

  Trim successful
  Trim Results:
       Angle of Attack:   6.43  wdot:  3.60e-05 Tolerance: 1.000000e-03  Passed
              Throttle:   0.55  udot: -5.36e-05 Tolerance: 1.000000e-03  Passed
            Pitch Trim:  -0.38  qdot: -5.00e-11 Tolerance: 1.000000e-04  Passed
            Roll Angle:   0.00  vdot: -2.28e-15 Tolerance: 1.000000e-03  Passed
              Ailerons:   0.00  pdot:  1.20e-16 Tolerance: 1.000000e-04  Passed
                Rudder:   0.00  rdot: -8.94e-32 Tolerance: 1.000000e-04  Passed
moments/m-prop-lbsft - 23474.563308847126

So show us what you're doing and maybe we'll spot the issue.

[fernandafenelon]

The codes are indeed helpuful. I think I found out my mistake.
I was updating the location below "Engine" and not below "Thruster".
After updating the Thruster location I got more realistic results.

Could you clarify which each of these locations mean?

	<engine file="engine">
		<location unit="IN">
			<x> 0 </x>
			<y>  0 </y>
			<z> 0 </z>
		</location>
		<feed>1</feed>
		<feed>2</feed>
		<thruster file="direct">
			<location unit="IN">
				<x>   0 </x>
				<y>   0 </y>
				<z>   700 </z>  --> I UPDATED THIS VALUE AND GOT BETTER RESULTS
			</location>
			<orient unit="DEG">
				<roll>  0 </roll>
				<pitch> 0 </pitch>
				<yaw>   0 </yaw>
			</orient>
		</thruster>
	</engine>

[seanmcleod]

The location and orient element for the thruster specify exactly where and at what angle the force generated by the engine will be applied.

A thruster definition is a requirement.

      // Locate the thruster definition
      Element* thruster_element = engine_element->FindElement("thruster");
      if (!thruster_element || !ModelLoader.Open(thruster_element))
        throw("No thruster definition supplied with engine definition.");

Whereas the location element for the engine appears optional.

// Find and set engine location

  local_element = parent_element->FindElement("location");
  if (local_element)  location = local_element->FindElementTripletConvertTo("IN");
//  else      cerr << "No engine location found for this engine." << endl;
// Jon: The engine location is not important - the nozzle location is.

My guess is this is a historical artifact probably required and used before the concept of a thruster was introduced, but once the thruster was introduced it's no longer required or used, and probably just causes confusion. @jonsberndt, @bcoconni, @agodemar can any of you confirm?

As a quick test I commented out the following members in FGEngine.h.

  //double X, Y, Z;
  //double EnginePitch;
  //double EngineYaw;

And commented out their only use in FGEngine.cpp, initialization to 0 in the constructor and being set in the SetPlacement() method without any compile issues.

So unless there is a good reason to keep the location and orientation for the engine itself I'll submit a pull request to remove the unused code, and a separate pull request to remove it from the aircraft model files.

[fernandafenelon]

Thank you for the info.
Only one more question about the thruster orient.
For a conventional aircraft, is the interpretation below correct?

orient unit="DEG"
ROLL ---> I DON'T SEE HOW TO USE THIS
PITCH ---> ENGINE IS TILTED UP OR DOWN
YAW--> ENGINE ID TILTED OUTBOARD OR INBOARD
orient

I am conducting some tests as well.

Thank you again.

[bcoconni]

I concur with @seanmcleod analysis: neither the <location> item of engines is used, nor is <orientation>. Members X, Y, Z, EnginePitch and EngineYaw can be safely removed AFAICT.

[bcoconni]

Thank you for the info. Only one more question about the thruster orient. For a conventional aircraft, is the interpretation below correct?

orient unit="DEG" ROLL ---> I DON'T SEE HOW TO USE THIS PITCH ---> ENGINE IS TILTED UP OR DOWN YAW--> ENGINE ID TILTED OUTBOARD OR INBOARD orient

@fernandafenelon your interpretation is correct.

[jonsberndt]

I seem to recall there was a reason for this code. Can you hold off for a few days? I'll see if I can find an example, or concur with it's removal. 

[seanmcleod]

Yep, no urgency.

[bcoconni]

No rush indeed.

However I can hardly see how X, Y, Z, EnginePitch and EngineYaw could be of any use since they are protected members, they are not used anywhere in JSBSim code (as @seanmcleod showed) and there are no getters through which they could be accessed.

[seanmcleod]

...protected members..., there are no getters through which they could be accessed...

Correct, I thought there may be a derived class that exposed them via a getter etc. so I did a test compile to confirm there weren't as opposed to inspecting all the derived classes.

[seanmcleod]

@jonsberndt have you been able to find an example where the position and orientation of the engine as opposed to the thruster might be useful?