Cannot modify angle 'alpha' from 0.349066 to 9.54453e-11

[gabrielcte]

Hi,

I wrote a simple Python code that adjusts various flight conditions to scan the aircraft's flight envelope.
My example used f104, f16 and 737 aircrafts;

However, when using big bank angles like 60 and 75 degrees, the following error message appears repeatedly:
Cannot modify angle 'alpha' from 0.349066 to 9.54453e-11

Sometimes, the operation is terminated.

Kind Regards,
Gabriel

[bcoconni]

Hi,

This error is most likely due to the system being overconstrained. JSBSim is computing that given the constraints that are applied to the aircraft attitude, the angle of attack cannot be set ouside the range specified (0.349066 to 9.54453e-11).

The comment in the code explains just that:

jsbsim/src/initialization/FGInitialCondition.cpp

Lines 355 to 366 in 0ac6194

	// There are situations where the desired alpha angle cannot be obtained. This
	// is not a limitation of the algorithm but is due to the mathematical problem
	// not having a solution. This can only be cured by limiting the alpha angle
	// or by modifying an additional angle (psi ?). Since this is anticipated to
	// be a pathological case (mainly when a high roll angle is required) this
	// situation is not addressed below. However if there are complaints about the
	// following error being raised too often, we might need to reconsider this
	// position.
	if (DotProduct(v0, v0) < DotProduct(u, u)) {
	cerr << "Cannot modify angle 'alpha' from " << alpha << " to " << alfa << endl;
	return;
	}

Would you mind providing the angles at which you are trying to set the attitude of your aircraft ?

[gabrielcte]

Sure :D

The angles are: 15º to 75º

ic_phi = np.linspace(np.pi/12,np.pi/2.4,10)

The angle is a input of an optimization algorith for trim

[Warning39]

@gabrielcte can you show a bit more of your code in terms of which initial conditions you're specifically setting?

And I assume this is part of your own trim algorithm as opposed to setting some initial conditions and then making use of JSBSim's trim algorithm?

[gabrielcte]

# Setup model & autopilot & log
aircraft_model='f104'
aircraft_path=(Path('.')).resolve()

fdm = jsbsim.FGFDMExec(str(aircraft_path))
fdm.set_debug_level(0)
fdm.load_model(aircraft_model)

fdm['ic/lat-geod-rad'] = np.deg2rad(-23.432)     # Latitude (rad)
fdm['ic/long-gc-rad'] = np.deg2rad(-46.470)      # Longitude (rad)
fdm['ic/terrain-altitude-ft'] = uc.m2ft(748) 


ic_h_sl_ft = np.linspace(300,25000,4)
ic_mach = np.linspace(0.1,3,10)
ic_phi =  np.linspace(np.pi/12,np.pi/2.4,10)

color = ['pb', '*r', '.g', '+k']