VTOL Fixed-wing model

[jonyMarino]

Hi,

I want to use a fixed-wing model with VTOL capabilities (through more vertical rotors, not a tilt-rotor). Does anyone know of one that can be shared?

[jonyMarino]

I am trying to create a model by myself. I am using the base c310 model and added four pointing-up rotors.
This is the branch I'm working on
This is the link to the modified c310 model
And this is the test script.
The issue is that the altitude stays the same over time, even increasing the hp.
What can be happening?

[seanmcleod]

Are you sure a pitch attitude of -90 is correct for the 4 vertical motors as opposed to +90?

https://github.com/jonyMarino/jsbsim/blob/096bfb5902f024222919cc37b780ed5168bdaadd/aircraft/c310/c310.xml#L320-L324

[seanmcleod]

Plus, log the propulsion forces, which will immediately tell you first of all whether the propulsion magnitude is greater than the weight of the aircraft, and also confirm the orientation of the propulsion forces.

jsbsim/src/models/FGPropulsion.cpp

Lines 819 to 821 in 10c6e52

	PropertyManager->Tie("forces/fbx-prop-lbs", this, eX, &FGPropulsion::GetForces);
	PropertyManager->Tie("forces/fby-prop-lbs", this, eY, &FGPropulsion::GetForces);
	PropertyManager->Tie("forces/fbz-prop-lbs", this, eZ, &FGPropulsion::GetForces);

[jonyMarino]

@seanmcleod Thanks for getting back to me so quickly.
I have changed the rotors' pitch attitude to 90, and I print the aircraft's attitude, and what I see it´s happening is that the plane rotates, which makes it crash into the ground instead of going up without changing the orientation.
If you want to run it by yourself, I've added these script, engine and aircraft files

[eferreirafilho]

Hi @jonyMarino , have you ever got the VTOL working nicely? Thanks!

[jonyMarino]

Hi @eferreirafilho . Unfortunately no. I haven't invested so much also, but I'm really interested in a model for vtol.

[seanmcleod]

Given that the aircraft is sitting on the ground and trying to take-off, during this initial take-off period there are no aerodynamic forces in play yet given the extremely low dynamic pressure.

So any movement and rotation is going to come down to the forces and moments from your 6 engines. So what did you see when you logged the propulsion forces I mentioned above?

Also take a look at the propulsion moments.

jsbsim/src/models/FGPropulsion.cpp

Lines 822 to 824 in 10c6e52

	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);

And lastly log the thrust of all 6 engines individually. Given that and the cg and moments of inertia of the aircraft you should be able to figure out what is going wrong.

[seanmcleod]

In addition to double-checking the forces from the engines and their location relative to the cg, and remember the cg isn't the same as the cg as specified in the FDM, since that is for the empty weight. Look at the mass report that is printed when the model is loaded to figure out the current cg at take-off with fuel and any other masses specified. Also take into account sense and p-factor when using a propellor.

jsbsim/src/models/propulsion/FGPropeller.cpp

Lines 258 to 273 in 39c204a

	// P-factor is simulated by a shift of the acting location of the thrust.
	// The shift is a multiple of the angle between the propeller shaft axis
	// and the relative wind that goes through the propeller disk.
	if (P_Factor > 0.0001) {
	double tangentialVel = localAeroVel.Magnitude(eV, eW);
	if (tangentialVel > 0.0001) {
	// The angle made locally by the air flow with respect to the propeller
	// axis is influenced by the induced velocity. This attenuates the
	// influence of a string cross wind and gives a more realistic behavior.
	double angle = atan2(tangentialVel, Vel+Vinduced);
	double factor = Sense * P_Factor * angle / tangentialVel;
	SetActingLocationY( GetLocationY() + factor * localAeroVel(eW));
	SetActingLocationZ( GetLocationZ() + factor * localAeroVel(eV));
	}
	}

For example take a look at quadcopter example in the JSBSim repo.

https://github.com/JSBSim-Team/jsbsim/blob/master/aircraft/F450/Propulsion.xml