Fixed Wing "Global" Trimmed Pitch Angle

[VortexLattice]

Current Behavior

1. The fixed wing Autotrim (or continuous servo trim), takes a "snapshot" of the control surfaces' (elevator or elevons) current position and sets this as the new servo midpoint. This, in essence, adjusts a certain pitch angle at a certain throttle setting (airspeed).
2. The board Level Trim (or Autolevel) adjusts the angle which the controller considers as level in the Angle or other automatic modes. This, again, dictates that the plane will fly at a certain pitch angle at a certain throttle setting (airspeed), because the controller will want to keep that angle (I gain).

For the same throttle setting, if the pitch angle obtained at (2) is different than the pitch angle obtained in (1), then the plane will either dive or climb (unwanted behavior) when we change from Angle mode to Manual/ Acro modes and vice versa.

Desired Behavior

At the same throttle setting (i.e. airspeed), the plane should keep the same pitch, when changing from Angle to Acro/Manual or vice versa.

Suggested Solution

Establish a "global" trimming value through the Board level trim/ Autolevel procedure, that will have effect in ALL modes. In other words, make the Autotrim obsolete: the new servo midpoints will be adjusted based on the deflection they obtain in order to achieve the new Board level trim/Autolevel pitch value.

Who does this impact? Who is this for?

This is for fixed wing users. In essence it's the simplest way to achieve easy and precise trimming in a fixed wing, regardless of the flight mode.

Additional context

[daijoubu]

Those are two different parameters that interact in a way.

1)Board level trim is the angle of attack required to maintain altitude at a given airspeed. You will usually adjust this once at your cruise/navigation airspeed.

2. servo trim - this adjusts the servos mid point to maintain straight and level flight. It moves the I term value to the servo midpoint after the plane has spent 2 seconds in straight and level flight. You can adjust this all you want. It's intended to minimize the stick movement need when you transition to manual.

[VortexLattice]

Those are two different parameters that interact in a way.

1)Board level trim is the angle of attack required to maintain altitude at a given airspeed. You will usually adjust this once at your cruise/navigation airspeed.

2. servo trim - this adjusts the servos mid point to maintain straight and level flight. It moves the I term value to the servo midpoint after the plane has spent 2 seconds in straight and level flight. You can adjust this all you want. It's intended to minimize the stick movement need when you transition to manual.

Exactly. It does NOT need to be two different parameters, because they accomplish exactly the same thing:

->Board level trim adjusts pitch angle by feeding the desired angle value to the control surfaces and this is translated to their deflection ! (indirect user command, guided through the control board)
-> Servo trim adjusts pitch angle by taking a snapshot of the deflected control surfaces, commanded directly by the user.

To simplify things, we need to have only ONE parameter for trimming and this parameter should adjust the board level trim (BUT it should have effect also in Manual/ Acro modes as well!)

[VortexLattice]

In order to practically achieve what I mean, I propose for the servos, when in Angle or other Navigation mode, to obtain a new I value - the one caused by deflecting control surfaces to achieve board level trim (or Autolevel) - and transfer this value to the other modes as well (Manual and Acro). The term for this procedure in the configurator could be named "Obtain global pitch trim servo values", or "Trim fixed wing pitch for all modes".

[daijoubu]

Acro (and angle) don't care about the servo mid position. The PID loop directly calculates a servo position needed to achieve the desired response. Only manual cares about servo mid position. The first few iterations of the PID loop care, but it iterates fast so the effect is small enough to be not relevant

Think of fw level pitch trim as redefining the angle held when the pitch stick is centered. By default, Inav will hold 0 degrees when pitch is 1500. For quads, this is great, but most planes will descend if held at 0 degrees and pilots don't want to hold pressure on the stick to fly level. Adjusting fw level pitch trim means Inav will hold 4 degrees at 1500 input from the radio.

The board's orientation in space means nothing in manual. The channel that used to command pitch angle now commands elevator deflection with 1500 meaning no deflection from midpoint. We want the aircraft to continue flying straight and level immediately after switching to manual so the midpoint should be "whatever was required to maintain board level trim in angle"

Fw level pitch trim is how you trim Inav as angle mode is the basis for everything. Servo trim is a convenience for pilots that fly manual. You can achieve what you want by enabling continuously trim servos as it will update the midpoint everytime you fly straight and level.

[VortexLattice]

Acro (and angle) don't care about the servo mid position. The PID loop directly calculates a servo position needed to achieve the desired response. Only manual cares about servo mid position. The first few iterations of the PID loop care, but it iterates fast so the effect is small enough to be not relevant

Think of fw level pitch trim as redefining the angle held when the pitch stick is centered. By default, Inav will hold 0 degrees when pitch is 1500. For quads, this is great, but most planes will descend if held at 0 degrees and pilots don't want to hold pressure on the stick to fly level. Adjusting fw level pitch trim means Inav will hold 4 degrees at 1500 input from the radio.

The board's orientation in space means nothing in manual. The channel that used to command pitch angle now commands elevator deflection with 1500 meaning no deflection from midpoint. We want the aircraft to continue flying straight and level immediately after switching to manual so the midpoint should be "whatever was required to maintain board level trim in angle"

Fw level pitch trim is how you trim Inav as angle mode is the basis for everything. Servo trim is a convenience for pilots that fly manual. You can achieve what you want by enabling continuously trim servos as it will update the midpoint everytime you fly straight and level.

You say that "The board's orientation in space means nothing in manual. The channel that used to command pitch angle now commands elevator deflection with 1500 meaning no deflection from midpoint"...

Exactly this is the fact I would like to emphasize: the channel that commands pitch angle by fw_level_pitch_trim achieves this pitch angle by adjusting the elevator deflection!!! It is not magic.
This is the SAME as the servo trim in Manual! You want the same angle of deflection. Why try to achieve the same angles by trial and error, when the software can easily transfer these deflections when changing over to Manual mode????

[daijoubu]

I agree, it's not magic. We do set the servo midpoint based on the servo deflection required to hold the fw_level_trim value in angle. You can choose to do this on demand with the Autolevel servos "mode" or you can have Inav do this automatically every time you fly straight and level by enabling the continuously trim servos feature on the configuration page. There is no new configuration value required to achieve this, the feature just needs to be turned on.

Be careful though, continuously trim servos could set your trims to some funky value if the last time you flew straight and level was at a very, very low airspeed.

[VortexLattice]

I agree, it's not magic. We do set the servo midpoint based on the servo deflection required to hold the fw_level_trim value in angle. You can choose to do this on demand with the Autolevel servos "mode" or you can have Inav do this automatically every time you fly straight and level by enabling the continuously trim servos feature on the configuration page. There is no new configuration value required to achieve this, the feature just needs to be turned on.

Be careful though, continuously trim servos could set your trims to some funky value if the last time you flew straight and level was at a very, very low airspeed.

So, why not make the "Autolevel mode new servo position" have effect even in Manual mode?
In other words, take the same value when changing in Manual - making any Autotrim unessecary.

[MrD-RC]

Auto-level trim adjusts the FC level offset, not the servos. Only the self levelling flight modes care about the level trim. All flight modes are affected by the servo trim.

Ideally, set the servos trim at cruise speed. Then when it's flying level hands off in manual. Go into Angle and set the level trim.

[b14ckyy]

Exactly. It does NOT need to be two different parameters, because they accomplish exactly the same thing:

You have a fundamental misunderstanding here.
Servo trim affects ROTATION rate of the plane at a given airspeed to keep it at zero. Flying straight is not the same thing as flying level.
Level trim adjusts an offset to the sensor based reading where the ground and horizon is. The Level or Attitude controller is constantly moving the servos to keep the plane at a specific attitude that is level with the horizon and the Pitch level trim just adjusts how much the nose should be held above or below the horizon to maintain a somewhat consistent altitude at a specific speed.

Both are 2 fundamentally different things. Your servo trim can be entirely off and the plane would still fly level as the angle controller will force the servos to hold the plane level. It would just fly like crap in manual and badly in Acro.

[VortexLattice]

b14ckyy, imagine that you are flying line of sight. You have NO flight controller. You have trimmed an aeroplane to fly straight & level, with the throttle stick midpoints (50%). Straight & level means no roll angle and the most important, at the same attitude. You have the experience to do so. How do you achieve that? By deflecting the elevator using the elevator servo trim. The crucial to remember is that the plane flies with a pitch angle θ at this situation.

Now, you install a flight controller that has only ANGLE mode (i.e. it tends to keep the calibrated angles of the plane).
Unfortunately, you calibrated this flight controller on a perfectly flat table and because you are a good craftsman, you installed it perfectly flat WITH RESPECT TO THE LONGITUDINAL AXIS on your aeroplane, i.e ZERO degrees. No other fiddling on the aeroplane, neither on your transmitter trims

Now, you are going to fly your aeroplane. Initially, you launch WITHOUT the flight controller engaged. All good, you observe that the plane flies with the throttle stick midpoints (50%), straight & level as it was before, at pitch angle θ.

Now you engage the flight controller. Ooops! The controller remembers that the level he must achieve is exactly in line with the longitudinal aircraft axis. So, the controller ignores the servo trim and puts the servos in such position, as for the aeroplane to obtain ZERO pitch angle! Now, because the lift at zero pitch angle is less (or even zero), the plane cannot support its weight, and falls, unless you increase the throttle (and you are lucky).
If you revert to manual mode, i.e. disengage the controller, the servos will revert to their previously trimmed position and the plane will continue to fly straight and level (but at a reduced altitude).

But, if you had calibrated the pitch controller at a pitch angle θ, then when you engaged the controller, it would recognise that the angle θ is already there and the servos would not receive any signal to move. Thus, engaging and disengaging the controller would NOT cause any descent. THAT IS what a pilot wants from his autopilot.

Do you agree with my above thoughts?

So, what I actually propose, is to achieve this balanced behaviour when engaging and disengaging the inav autopilot without proceeding into multiple procedures, like autotrim AND autolevel. It would be easier to program INAV autolevel only, so as to give the same pitch trim in manual mode.

[b14ckyy]

You have trimmed an aeroplane to fly straight & level, with the throttle stick midpoints (50%). Straight & level means no roll angle and the most important, at the same attitude.

There is your mistake. An RC pilot does not trim servos to fly straight and LEVEL. Servos are trimmed to make the plane NOT rotate around roll or pitch when in level flight. Has nothing to do with actual self leveling (aside from dihedral effects but we are not talking about that right now).

Now you engage the flight controller. Ooops! The controller remembers that the level he must achieve is exactly in line with the longitudinal aircraft axis. So, the controller ignores the servo trim and puts the servos in such position, as for the aeroplane to obtain ZERO pitch angle!

Incorrect. The Angle controller pushes the servos to rotate the plane in the target attitude. In your described case with a neutral AOA so slightly nose down. Then the servos go back to mid point (that is correctly trimmed neutral) and the plane will by itself keep that attitude and if it diverts, the Angle controller will correct it.
the target of servo trim is, to get work of any PID loop if the plane is not supposed to rotate at its current attitude.

Again, here is your point of misunderstanding: The attitude of the aircraft and the trim center of the Servos have NOTHING to do with each other. The servos only deflect while the airplane is supposed to rotate around any of its 3 axis. AS long as the rotation rate is supposed to be 0, the servos will be centered. That's what servo trim achieves.

Yo can be at a 60° bank with a 40° nose down attitude in a banked high speed dive and the servos will be centered until you want to rotate the plane back to level flight.

This is why servo trim has still NOTHING to do with Pitch Level Trim.

So, what I actually propose, is to achieve this balanced behaviour when engaging and disengaging the inav autopilot without proceeding into multiple procedures, like autotrim AND autolevel. It would be easier to program INAV autolevel only, so as to give the same pitch trim in manual mode.

technically you can already do that without problem. Fly angle mode, enable Pitch Level trim until altitude stays somewhat constant. Then disable pitch level trim and engage Autotrim mode. While in angle. It will trim the servos neutral for level flight that the ANGLE controller ensures here. Then lend with autotrim enabled and disarm. Done.

or you just use continuous servo trim all the time. as this will adapt to airspeed changes dynamically based on your I-Term.

[VortexLattice]

You have trimmed an aeroplane to fly straight & level, with the throttle stick midpoints (50%). Straight & level means no roll angle and the most important, at the same attitude.

There is your mistake. An RC pilot does not trim servos to fly straight and LEVEL. Servos are trimmed to make the plane NOT rotate around roll or pitch when in level flight. Has nothing to do with actual self leveling (aside from dihedral effects but we are not talking about that right now).

Strongly disagree, you CAN trim the servos to fly level. This is done by adjusting the plane's angle of incidence (through the elevator) and also by adjusting the throttle to balance drag. And, yes, the rates, at this situation are zero, i.e. the plane does not rotate.

Now you engage the flight controller. Ooops! The controller remembers that the level he must achieve is exactly in line with the longitudinal aircraft axis. So, the controller ignores the servo trim and puts the servos in such position, as for the aeroplane to obtain ZERO pitch angle!

Incorrect. The Angle controller pushes the servos to rotate the plane in the target attitude. In your described case with a neutral AOA so slightly nose down. Then the servos go back to mid point (that is correctly trimmed neutral) and the plane will by itself keep that attitude and if it diverts, the Angle controller will correct it. the target of servo trim is, to get work of any PID loop if the plane is not supposed to rotate at its current attitude.

What you miss here: if the target attitude is achieved and servos go back to neutral, then the plane will start rotating again to miss that target attitude. And the controller will again try to obtain the target attitude, etc... The elevator's position has a direct effect in the target attitude and the controller will keep that position in angle mode.

Again, here is your point of misunderstanding: The attitude of the aircraft and the trim center of the Servos have NOTHING to do with each other. The servos only deflect while the airplane is supposed to rotate around any of its 3 axis. AS long as the rotation rate is supposed to be 0, the servos will be centered. That's what servo trim achieves.

What you miss here is that you can trim the servos position so as to obtain different airplane attitudes. The more the servos are deflected up, the higher the pitch angle, etc.

Yo can be at a 60° bank with a 40° nose down attitude in a banked high speed dive and the servos will be centered until you want to rotate the plane back to level flight.

This is why servo trim has still NOTHING to do with Pitch Level Trim.

So, what I actually propose, is to achieve this balanced behaviour when engaging and disengaging the inav autopilot without proceeding into multiple procedures, like autotrim AND autolevel. It would be easier to program INAV autolevel only, so as to give the same pitch trim in manual mode.

technically you can already do that without problem. Fly angle mode, enable Pitch Level trim until altitude stays somewhat constant. Then disable pitch level trim and engage Autotrim mode. While in angle. It will trim the servos neutral for level flight that the ANGLE controller ensures here. Then lend with autotrim enabled and disarm. Done.

or you just use continuous servo trim all the time. as this will adapt to airspeed changes dynamically based on your I-Term.

Thank you for the tip above.
I do not know if I explain correctly, or not due to the language gap. However, I am flying fixed wing RC models for 20 years now. I have also studied aeronautical engineering and flight mechanics. And I know how and when a plane, especially an RC model is trimmed. And what I want to achieve. I do not mean that you guys not know, I just hope you are not explaining trimming and levelling a fixed wing in terms of multicopter flight mechanics. Because they are completely different.

[breadoven]

The flight controller could be aligned with the longitudinal axis of the plane OR the chord line of the wing at the root, i.e. angle of incidence of the wing. The setting for the FC pitch alignment would depend on how the wing is aligned with the fuselage in this case (and also how the FC mounting is aligned with the fuselage but we can assume it's aligned with the longitudinal axis of the plane ). If the wing is aligned with the fuselage with a defined angle of incidence (the normal case) then the FC pitch alignment should be set to 0 with the longitudinal axis of the plane level. If on the other hand the wing chord is aligned with the longitudinal axis of the plane (unusual surely ?) then the FC pitch alignment should be adjusted to give a pitch reading of 0 when the longitudinal axis of the plane of pitched up 3 to 4 degrees. How you do this depends on how you decide to use align_board_pitch and fw_level_pitch_trim (I don't bother with fw_level_pitch_trim).

This is why servo trim has still NOTHING to do with Pitch Level Trim.

Not sure this isn't strictly true. Pitch Level Trim (Autolevel) is achieved by deflecting the pitch servo which will require a fairly consistent servo offset to maintain level flight IF the elevator is not perfectly mechanically adjusted at that speed to maintain level flight. In fact Autolevel mode doesn't maintain level flight it maintains constant altitude flight just to confuse things (it will get confused when there is lift/sink about). Yes it will be actively adjusting the servo to maintain level but it will have an offset in one direction or the other and this offset will be similar on average to the offset you get when using Servo Autotrim. If the elevator is perfectly mechanically trimmed for the given speed then the FC pitch trim should be 0, the servo output should be centred (1500). However, this doesn't mean the elevator is perfectly aligned with the tailplane it may have a mechanical trim offset.

I tend to just look at elevator output in the flight log during Alt hold flight to understand how well mechanically trimmed the elevator is and mechanically adjust accordingly.

At the end of the day the plane doesn't care what's adjusting the elevator (Autolevel or Pitch trim) it just knows that at a certain speed a certain elevator position will maintain level flight or zero pitch rate flight if you want to think of it in rate terms. As mentioned above though this is complicated by the weather, wind, lift/sink etc.

Having said all that maybe I've got it wrong which wouldn't be surprising because I do think the whole pitch/level situation in INAV has become a bit confusing with too many settings and modes which seem to do similar things in a slightly different way.

[daijoubu]

To do what you want, set the fw_level_trim in angle mode and enable continuously trim servos on the configuration page. You will never have to manually engage servo trim, Inav will set the servo midpoints based on what it determined the servo deflection was the last time if flew straight and level for 2 seconds. The only thing you will need to think about is "Do I want to reset my neutral AoA because I plan to fly at a different airspeed?", so the complication of having separate trims for Angle mode vs manual mode is gone.

Turn on stats and Inav will save these new midpoints every time you disarm. Just make sure you let it sit for a few seconds after disarming so you don't lose power (unplug) while writing to flash as that will corrupt the config and Inav will reset to defaults next time it powers up. Not a problem if you have to take your goggles off and walk out past the flight line and back after disarming.