Parameter missing

Gorgious · September 30, 2019, 12:16pm

Hello !
I am working on a custom tiltrotor airframe and I want to use airbrakes during back transition but the parameter VT_B_REV_OUT is missing. QGC does not give any error and the parameter does not either appear in the logs. Is there something to do to enable this parameter ?
Thanks in advance !

JulianOes · October 1, 2019, 2:53pm

Hi!

I can see that the param is only used for “standard” VTOLs:

github.com

PX4/Firmware/blob/07d656e971a72d1202651dfd3b4642736fb078d7/src/modules/vtol_att_control/standard.cpp#L68


	_mc_roll_weight = 1.0f;
	_mc_pitch_weight = 1.0f;
	_mc_yaw_weight = 1.0f;
	_mc_throttle_weight = 1.0f;


	_params_handles_standard.pusher_ramp_dt = param_find("VT_PSHER_RMP_DT");
	_params_handles_standard.back_trans_ramp = param_find("VT_B_TRANS_RAMP");
	_params_handles_standard.down_pitch_max = param_find("VT_DWN_PITCH_MAX");
	_params_handles_standard.forward_thrust_scale = param_find("VT_FWD_THRUST_SC");
	_params_handles_standard.pitch_setpoint_offset = param_find("FW_PSP_OFF");
	_params_handles_standard.reverse_output = param_find("VT_B_REV_OUT");
	_params_handles_standard.reverse_delay = param_find("VT_B_REV_DEL");
}


void
Standard::parameters_update()
{
	float v;


	/* duration of a forwards transition to fw mode */
	param_get(_params_handles_standard.pusher_ramp_dt, &v);

I assume tiltrotor does not use this param and you would have to add and implement that functionality for tiltrotors. @sfuhrer can you confirm?

Gorgious · October 2, 2019, 8:43am

Thank you for your answer.
The parameter VT_B_REV_DEL seems to be implemented (reverse_delay) only for standard VTOL but is available for tiltrotor VTOL as well.
Is it because of this line ?

github.com

PX4/Firmware/blob/07d656e971a72d1202651dfd3b4642736fb078d7/src/modules/navigator/navigator_main.cpp#L107


	_navigation_mode_array[3] = &_dataLinkLoss;
	_navigation_mode_array[4] = &_engineFailure;
	_navigation_mode_array[5] = &_gpsFailure;
	_navigation_mode_array[6] = &_rcLoss;
	_navigation_mode_array[7] = &_takeoff;
	_navigation_mode_array[8] = &_land;
	_navigation_mode_array[9] = &_precland;
	_navigation_mode_array[10] = &_follow_target;


	_handle_back_trans_dec_mss = param_find("VT_B_DEC_MSS");
	_handle_reverse_delay = param_find("VT_B_REV_DEL");


	_local_pos_sub = orb_subscribe(ORB_ID(vehicle_local_position));


	reset_triplets();
}


Navigator::~Navigator()
{
	orb_unsubscribe(_local_pos_sub);
}

rollys · October 2, 2019, 5:28pm

Besides, it wouldn’t make sense to have reverse in a tiltrotor setup. During the back transition, the motors will be in motion from vertical to horizontal. Furthermore, even if you implemented the reverse before the transition, it’ll just jerk the aircraft to a halt. I would think it would be more efficient if you allowed the aircraft to slow down on its own then the motors go vertical to hover just before stall.

Good luck.

Gorgious · October 4, 2019, 7:49am

I don’t want want to reverse the motors (it wouldn’t be allowed in the ESCs anyway). I want to output an airbrake order on the ailerons and flaps during back transition because the drone has a high lift-to-drag ratio and takes a long time to slow down on its own.

sfuhrer · October 9, 2019, 8:51am

@Gorgious as far as I know we only support braking with reversed thrust for VTOL yet, and this of course only makes sense for standard VTOL. Using the flaps for the braking sounds interesting, though probably only few VTOLs have them (as you normally only need them for runway take-off and landing).

I wouldn’t recommend using the ailerons for aerodynamic braking, as they are needed for attitude control during the transition plus usually only have a rather limited contribution to the overall drag.

The best actuator for braking would probably be an airbrake, such as gliders usually have.

I can help you with the implementation of a “braking actuator” on the px4 side (again, nothing there yet).

Have you also considered tilting the motors to the back to get some braking from them? (if it’s mechanically possible)