Plane: Document setup without GPS or airspeed sensor

[Ryanf55]

Document how to set up ArduPlane to fly in FBWA without GPS or airspeed sensor. Document some known limitations.
This is for a minimal plane build to just use AP as a stabilizer.

[Hwurzburg]

an RC failsafe results in crash or fly-away

put a warning immediately after the header

[Ryanf55]

an RC failsafe results in crash or fly-away

put a warning immediately after the header

What do you think about recommending CIRCLE as a failsafe action. That won't fly away, and over flat area, the vehicle will likely land just fine.

[Hwurzburg]

CIRCLE is not an option...only GLIDE (FBWA no throttle) is useable with no GPS

[Ryanf55]

an RC failsafe results in crash or fly-away

CIRCLE is not an option...only GLIDE (FBWA no throttle) is useable with no GPS

From the wiki: Circle mode is deliberately a very conservative mode, and doesn’t rely on GPS positioning as it is used when GPS fails

And, the code:

#include "mode.h"
#include "Plane.h"

bool ModeCircle::_enter()
{
    // the altitude to circle at is taken from the current altitude
    plane.next_WP_loc.alt = plane.current_loc.alt;

    return true;
}

void ModeCircle::update()
{
    // we have no GPS installed and have lost radio contact
    // or we just want to fly around in a gentle circle w/o GPS,
    // holding altitude at the altitude we set when we
    // switched into the mode
    plane.nav_roll_cd  = plane.roll_limit_cd / 3;
    plane.update_load_factor();
    plane.calc_nav_pitch();
    plane.calc_throttle();
}

The default for FS_SHORT_ACTN is FS_ACTION_SHORT_BESTGUESS, which means circle if I was in FBWA. You can't be in Auto , Guided, or Loiter if you don't have a GPS or airspeed sensor. Users can also select CIRCLE as their FS_SHORT_ACTN.

For FS_LONG_ACTION, we could recommend FS_ACTION_LONG_GLIDE as that is the most likely to not total the aircraft if it doesn't know where it is.

[Hwurzburg]

we should sitl the non gps case with defaults for FS actions (CIRCLE/RTL) with RC failure to be definitive on warning as to what will happen

[Ryanf55]

./Tools/autotest/sim_vehicle.py -v Plane --console --map -w
param set STALL_PREVENTION 0
param set GPS1_TYPE 0
param set ARSPD_USE 0
param set ARSPD_TYPE 0
param set AHRS_EKF_TYPE 0
param set ARMING_CHECK 0
map set showsimpos 1
# Move the map to follow the sim position.
mode fbwa
rc 2 2000
rc 3 1500
arm throttle

Wait till it gets to ~100m altitude, then trigger a failsafe.

rc 3 900

It performs the short failsafe CIRCLE, then goes to RTL and does a flyaway. Now, repeat the test with param set FS_LONG_ACTN 2 (glide). Likely on a real vehicle it will do super gentle throttle-off circles anyways because FBWA doesn't fly exactly straight.

With graph SERVO_OUTPUT_RAW.servo3_raw you can see that it's commanding no throttle. Eventually it will land.

Do you agree that GLIDE is better behavior than a flyaway?

[Hwurzburg]

yes....better

[Ryanf55]

Alright, I added that and an obvious warning about flyaway risk.

Before this wiki goes in, we either need to merge this:
ArduPilot/ardupilot#29012

OR, tell people to use the Set Home in the GCS.