Add 'nav_fw_cruise_speed_source' to allow selection of airspeed or ground speed

[ZaksterBlue]

For fixed wing, stalling in low airspeed is a problem if nav mode throttle and speed settings are not well set up with a decent margin for error, eg. ground speed could be showing 60kph but with a strong 25kph tailwind, you are only at 35 kph airspeed and may stall.

At the moment the 'nav_fw_cruise_speed' setting does not allow you to base target speed on the measured or estimated airspeed. Having the option to select airspeed would reduce risk of stall. To extend on this setting, you could even have another setting to choose the balance between ground speed and airspeed, such as 50% to split the difference for the target speed if preferred.

[sensei-hacker]

This may be a good idea. In the meantime, you can use the Programming tab to accomplish the same.
Alternatively, perhaps something like:

1. Less than, airspeed, X
   2 plus, throttle, 1
   3 override throttle, condition 2, (conditioned on 1)

[Jetrell]

At the moment the 'nav_fw_cruise_speed' setting does not allow you to base target speed on the measured or estimated airspeed

@ZaksterBlue Estimated virtual airspeed is rather accurate when turns are being made at regular intervals, to update the estimator.
But if the pilot just keeps flying in a straight line for an extended distance, the virtual airspeed will become inaccurate.
So if you then fly through an area that the wind direction has shifted. That update will ikely be missed, and the virtual airspeed can become far less trusted.
A pitot is always going to be the better option.

[MrD-RC]

The problem is that people are setting the cruise throttle incorrectly. It should never be set using ground speed as a reference. Unless they are doing wind compensation when they calculate the speed. It should be set based on a physical airspeed sensor or virtual airspeed.

There are 2 things that really need to be pointed out here.

1. Stalls are not caused by speed alone. They are caused by the airflow detaching from the wing. Angle of attack plays a big part in this. It is completely possible to have a stall at full throttle if the angle of the airflow is deviating above acceptable levels for the airfoil. This is why aircraft have dedicated sensors to detect stalls.

2. Assuming that thr_comp_weight has been left enabled. Basic, level flight low speed stalls can already be prevented using the existing cruise throttle mechanism. In level flight, the same cruise throttle will always give you the same air speed. Therefore cruise throttle defines the cruise airspeed. Just set the cruise throttle above the stall airspeed (allowing a margin for gusts) and it will be fine. For example, say you know straight and level, you know your aircraft stalls at 25km/h. On a calm day, fly straight and level at 30km/h airspeed. If you don't have a pitot. Virtual airspeed can be used. Just fly a few loiter circles before flying straight to measure the speed. Check the throttle. That is your minimum cruise throttle. If the wind is more gusty, you will want to increase the margin based on the wind conditions, if you fly close to stall.

To be honest, I don't ever get why people choose such low cruise throttles anyway. There is no need to fly on the edge of stalling.

You are misunderstanding what nav_fw_cruise_speed is actually used for. It is not there to set a target cruise speed. That cruise speed is defined by nav_fw_cruise_thr. nav_fw_cruise_speed is for you to tell INAV the speed achieved at cruise throttle. So in the above example where we set cruise throttle to the percentage that we needed for 30km/h. nav_fw_cruise_speed would be set to 834cm/s. nav_fw_cruise_speed is used for remaining battery calculations. Not controlling the aircraft. Check out the docs for details.

• https://github.com/iNavFlight/inav/blob/master/docs/Settings.md#nav_fw_cruise_speed
• https://github.com/iNavFlight/inav/blob/master/docs/Settings.md#nav_fw_cruise_thr

One thing with the minimum ground speed. That has nothing to do with stall. The purpose of this is to allow the aircraft to maintain forward ground speed with headwinds at or above its cruise airspeed. Raising the minimum ground speed will make more progress against the wind. Of course, minimum ground speed being used will increase power consumption as it bumps up the throttle to achieve the desired ground speed. But, it could be the difference between the aircraft getting home (battery dependent) or being blown away.

[dcan999]

Airspeed based control would nice for people in windy areas, and also people looking to maximize efficiency. I regularly have flights where the wind changes more than 40MPH based on altitude, and I always have to manually override throttle with the stick to keep it at a constant airspeed. If I just set the cruise throttle to something high enough to work at all times, it would be way too much for half the flight and I'd lose tons of efficiency.

[sensei-hacker]

See also #10584

It was noted that while a particular airspeed is correlated with stall when flying straight and level in a particular configuration, that's no longer true if any of those words is changed. As soon as you turn, that throttle to airspeed correlation no longer holds. Swap cameras - it no longer holds. If you descend, it's no longer true.

AND it may be worth noting that's also the case with the correlation between a particular throttle value and airspeed.

A particular throttle implies a particular airspeed ONLY when flying straight and level in a particular configuration. As soon as you make a turn (or the wind is shifting), that relationship no longer holds.

The airspeed indicator (gauge) is one of the six basic instruments any full-sized plane has, for a reason. The throttle gauge is not.

Stall happens when AoA exceeds 16-18 degrees, which can be at any airspeed.

[dcan999]

Kind of what I was touching on in my reply above, I know it's probably a very small use case as are many things in this hobby it seems, but I like to do distance goal flights which are long slow climbs out and slow descents back in, at the least, I need to adjust the throttle for one leg of the trip, control by airspeed would sort of fix that. If I had it my way I would have a minimum airspeed setting for the tailwind leg, along with a minimum groundspeed setting so it would increase throttle for the headwind leg if required. I's also like to control the throttle within those two speed settings, using mah/km, to always have the most efficient throttle setting, which is sort of what do now manually to find the best cruise throttle setting for the outward leg, I think full size pilots sort of do the same?

Having those speed settings would also negate the other feature I requested awhile ago, to have separate degree settings for pitch to throttle for climb and descend, as that would let me at least not have to adjust throttle as much on different legs of the flight.

[Jetrell]

@ZaksterBlue @dcan999 You may want to have a look at this.