Fixedwing with auto speed mode

[Jetrell]

I'd often heard people on the forums talking about the benefits of INAV fixedwing having auto speed control, like the old Eagletree Vector had.
I thought the idea might have some merit. But I really wasn't sure how well it would work in practice. So I wrote the logic in the programming framework to give it a try.. The key to get it working, was to use IPF PID controller 3. Because it has the fastest looptime.

As can be seen from the video link. It works far better than I anticipated, in all flight modes. With practically no issues at all. With its climb/dive performance working as good as pitch2thr does.

The following information explains its functionality. With the zip file including all the information required to set it up in your own models.

• General flight efficiency is greater. However there are times when normal throttle control can be more efficient. It all depends on the flight conditions.

• 3Dspeed target works best for general cruising around. Being that it gets you to your location in a more timely manor than Airspeed, if you have a strong headwind.
  3Dspeed also has a minimum airspeed override. Which will work to prevent a stall in a tail wind condition, if the airplane is commanded to climb or turn.

  Airspeed target works better for the highest power management.
  However it is better to use a real Pitot sensor in this case, because virtual airspeed requires a course correction at least every 500mtrs to maintain its accuracy, as the wind shifts during the course of the flight.

• Speed mode can be used with all flight modes - MANUAL, ACRO, ANGLE, HORIZON, ANGLEHOLD, CRUISE, COURSEHOLD, LOITER, WP and RTH.
  When using with MANUAL, ACRO or HORIZON modes. It will automatically increase or decrease the throttle when you command the airplane to climb/descend or Loop.
  The same will apply to a climb or descent in ANGLE, ANGLEHOLD or COURSEHOLD modes.

  When used with navigation modes. Pitch2throttle is replaced with a speed based climb/descent target. Which changes the throttle accordingly.
  It can be more efficient IF the rate of climb and speed targets aren't set unrealistically high.

• The maximum speed target setting should not be set higher than 110km/h. Calculation will not work correctly.

• The maximum throttle that can be used to drive the speed target can be taken up to 100%. But it will reduce efficiency when tuned tightly, if it applies more throttle for time, than is required.

  • Between 1750uS to 1850uS is best, depending on the airplanes thrust and drag.

• The minimum throttle that can be used to drive the speed target, can be reduced to 0%. But is NOT recommended, due to no propulsive thrust being available. This could lead to an ESC startup de-sync if higher auto throttle is quickly called for.

  • Between 1150uS to 1250uS is best, depending on the prop size or lack of thrust available at lower throttle on some builds.

• The minimum SAFE air speed value should be set 1/3 greater than the airplane Actual Stall speed.
  e.g. the airplane stalls at 35km/h. Set the SAFE minimum airspeed to approximately 50km/h.

• It require 2 spare RC channel for operation. One Pot or Slider to adjust the speed target. And the other on a 3 position switch to enable 3D and Air based speed targeting.

• I haven't tested this. But if Miles/h is required in place of km/h. LC 20 and LC 37 should have the value of 28 replaced with 45. And LC 36 should have 110km/h replaced with 68Mi/h.

PID-FF tuning

P-term - sets how fast the throttle will increase or decrease to 'reach' the speed target. If set too higher, the throttle can oscillate.

I-term - tries to 'hold' and prevent drift once the actual speed is close to the speed target. Increasing it will hold the target speed tighter, up to a point.

D-term - will try to damping the throttle when near the target speed. * Only use in SMALL amounts.

FF-term - will attempt to drive the throttle harder to reach the target, based on instantaneous speed change. But pushing it past 480 - 500, will cause target drift without well filtered data from the speed source.

Functions

• If the throttle stick is moved greater than a set rate, above or below the its position from when speed mode was activated. It will cancel and exit speed mode for safety.
  Be sure to cycle the speed mode switch to re-enable it again.

• Speed mode has to reach approximately 35km/h GNSS ground speed, before the virtual pitot airspeed is trusted.

• Speed mode will be disabled if a GNSS fix is lost. Which still allows fixedwing dead reckoning to work as normal. It's also disabled when fixedwing Auto Landing or Auto Launch are active.. Not because it can't add great benefit to these functions, as I have found. But because it requires too many extra programming lines.

• It does not exit speed mode when a Failsafe occurs in the present programming. Which hasn't proven to be an issue. As long as its RC control switch stays active in an RX FS condition.

If anyone has any questions, comments or doubts. Ask away.

IPF speed mode setup.zip

[dcan999]

I appreciate your work on this and ill try it out! Are there any programming lines we don't absolutely need for this to work? I have 5 or 6 that I don't want to fly without.

[Jetrell]

Are there any programming lines we don't absolutely need for this to work?

Not really. Whats there is needed for it to work optimally. But it has enough room left for 6 or so extra's.

[dcan999]

Are there any programming lines we don't absolutely need for this to work?

Not really. Whats there is needed for it to work optimally. But it has enough room left for 6 or so extra's.

Ah OK thanks, I read the "we ran out of programming lines" and assumed that meant all available were used, I didn't look at all the pics in the files yet.

[KJ458]

It works perfectly, I hope it will be officially added to Inav. great job

[Jetrell]

The files have been updated to answer some peoples questions. It now explains which logic conditions you should alter, so you can run bench testing.
Without this, the OSD and programming logic outputs will not be displayed until you have a GNSS 3D fix and the minimum GNSS ground speed is obtained. Even if the speed mode switch is active,

Make sure you change the conditions back after testing. Or speed mode won't work in flight.

[dcan999]

My first flight went well and it worked as described, reminded me of my old Vector. If more people confirm this works and they are using it, can be eventually worked into the source as a part of the Advanced Tuning section (checkbox, speed box etc) so we can have all the programming lines back?

[Sibre3]

I thought I remember reading on iNAV github this wouldn't work. However it works flawlessly!
I just ran the most efficient flight I've ever done with an INAV fixedwing.
My friend and I found it so much easier to fly in formation when our aircrafts are both holding the same target speed.

It would be excellent if this makes it into iNAV 9.0 @sensei-hacker @breadoven @MrD-RC?

[sensei-hacker]

It would be excellent if this makes it into iNAV 9.0 @sensei-hacker @breadoven @MrD-RC?

To me, it certainly looks interesting. I probably won't have time to code it myself.

We would want to about how best to make defaults and such safe for users of varying levels of knowledge. It would be easy to cause a stall on the downwind if settings aren't quite right.

[Sibre3]

Thanks.

It would be easy to cause a stall on the downwind if settings aren't quite right.

The minimum tailwind air speed setting of 50km/h worked well for us. We had a stiff 27km/h tailwind in the 3D speed mode. I seen it push the throttle up a few times to stay ahead of the minimum safe stall speed. Both when the airplane was flying level and when it started to climb.

[Pundituk]

Hi, is there any chance this or similar will be included with Inav 9? Being able to set an airspeed target instead of a throttle level for cruise would be massively helpful in maintaining long flight times for a cruiser build I have. It's the only feature that arduplane is tempting me with over Inav!

[MrD-RC]

They are basically the same thing. The throttle percentage will give a set amount of thrust so a set airspeed.

However, you could set up airspeed based throttle using the programming framework.

INAV 9 is imminent. So it is extremely unlikely that anything like that will be added.

[trailx]

The benefit of having an airspeed target is that it also manages pitch changes automatically, even when not in automatic flight modes (which rely on P2T). So even flying in Acro, you can just set an airspeed, like cruise control in a car, which is really helpful when doing things like tuning PIDs at ref airspeed, or flying lower airspeeds closer to stall because it will react instantly to pitch changes, etc. I find it really useful if diving (to prevent speed from growing too fast in the dive) or when flying at low speeds through trees or other obstacles. It offers really good protection in acro mode against errant low speed stall due to pilot overload.

I've been flying a different version of this program code for a couple years and have found the feature useful at times.

Totally understand not getting this into 9, but if there's a roadmap of sorts, this would be nice to incorporate at some point as a flight mode and not get forgotten. The easiest way I have used my program is to use "current speed" as the target speed, much like hitting set on your car's cruise control. But I can see how it could be useful to also be able to adjust it while in flight, but it does require a lot more setup and thought.

[sensei-hacker]

Totally understand not getting this into 9, but if there's a roadmap of sorts, this would be nice to incorporate at some point as a flight mode and not get forgotten. The easiest way I have used my program is to use "current speed" as the target speed, much like hitting set on your car's cruise control.

A pull request would certainly be welcomed!