Fixed wing Auto Speed Mode

[breadoven]

Adds Auto Speed mode for fixed wing similar to #10928.

Operation is pretty simple. It is enabled as an aux mode so can be activated in the usual way. Minimum and maximum allowed speeds are set using 2 settings with a range of 5 to 50 m/s, defaults of 11m/s and 22m/s respectively.. The desired speed is set between these limits using the throttle stick which acts as a speed controller only. An OSD element has been added which displays the current set speed, this is only visible when Auto speed mode is active. Speeds are 3D GPS based so a valid velocity estimated is required. There is no option to use pitot airspeed at the moment, virtual or otherwise, but it could be added.

Throttle is automatically controlled using PID control with P, I and D inputs with filtering to damp out rapid throttle changes. In Nav modes the normal fixed wing pitch to throttle control is bypassed as are the other speed related functions. There are no minimum and maximum throttle limits set at the moment, throttle is controlled over the full range from idle to maximum. This could be changed if need be, perhaps defaulting to using the current nav_fw_min_thr and nav_fw_max_thr settings.

The mode can be activated in all cases except during during Failsafe, Launch, FW Auto landing and Emergency Landing. So it can be using in Manual and Acro as well as the other navigation modes.

Currently there is no way to rapidly shut off the mode other than via the aux activation. This may need to be looked further if it's considered a problem together with other changes to improve functionality such as dealing with strong head winds etc.

HITL tested only at the moment where it works well with the default PID tune. Throttle control is smooth and responsive.

Configurator support iNavFlight/inav-configurator#2641.

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[github-actions]

Test firmware build ready — commit 83f894f

Download firmware for PR #11595

237 targets built. Find your board's .hex file by name on that page (e.g. MATEKF405SE.hex). Files are individually downloadable — no GitHub login required.

Development build for testing only. Use Full Chip Erase when flashing.

[Jetrell]

@breadoven Firstly I want to start by saying how much we all appreciate your work here on INAV. ❤️
And what follows is not ingratitude. But rather, just putting the facts out there.

When I setup the fixedwing auto speed mode with the IPF. I also tried a simplified version as you have here. But found it was just too lacking in many area's of safety of operation, and efficiency.
I noticed you mentioned many of these other issues. But I thought a rundown of why I added these extras, might help.

Example:

• The Min and Max throttle value the throttle controller can use. Plays a big part in how efficient the system can work. So their addition is practically a must.
  Plus if you do add those setpoints. It isn't a good idea to share them with nav_fw_min_thr and nav_fw_max_thr. Because altering them will significantly effect the way Pitch2Thr works when not using auto speed mode.

• I also tried using the throttle stick to set the speed target.
  It only worked very average. Because whether you fly with stick mode 1 or 2. Most of us move the throttle stick little by little as we make an aileron or rudder input. Causing the target speed to shift, and constantly require resetting.
  Even with a throttle deadband. It still occurred when flying in Acro.
  And using the throttle stick to set the speed target also rules out a safety shut off of auto speed mode. As you would have noticed. I used a delta (mildly faster) movement of the throttle stick, to exit auto speed mode if an emergency happens. Which it did once. And by pulling the throttle lower or even higher, it switches back to manual throttle control.
  I know the addition of a pot for speed target control, requires the use of an extra channel, But it does make it worth while. However I also tried just having the target setpoint being hard set as well, to a cruise speed that suits the pilots requirements. That too work well if channels are lacking.

• I also found that 3Dspeed required the "tailwind" safety airspeed throttle-up addition, to stay ahead of a low airspeed stall, if the plane was commanded to climb manually or by nav control when in RTH or a mission.
  Which would also require a setting, but I found it was absolutely worthwhile from a safety stall perspective.

• And Airspeed target was something that I noticed many people asked about. And i did find it advantageous for flight efficiency. The main time 3Dspeed was better, is if you wanted to make good headway into a strong headwind. And make it to your destination faster.

• I also found having the auto speed Throttle element on the OSD helped to determine how well the system was working. It provided a good visual reference when tuning and when commanding a steep dive or climb, so you could see how efficiently throttle was being feed-in, to hold the target speed.

I no this was long winded. And I'm not trying to make more work for you. But I thought it might be helpful for you to know what I also tried. So the feature can be made safer and more user friendly.
Plus if this feature is implemented to a higher standard, it leaves the door open for so many better fixedwing improvements.
e.g.
Fixed airspeed when running autotune. Which provides a more accurate FF and Rate tune.
I also tried using in with AutoLaunch. It also improved climb out efficiency, without risking a stall.
And it also help slow the plane, without risking a stall, when I tried it with fixedwing auto landing mode.