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# Trim function (pitch) of planes, strange behavior?

## Question

Hey guys,

I think the trim function when flying planes is very confusing!   (I am talking about jet planes in atmosphere, < 10,000 m).

(I am talking about KSP 1.6.0, I think this behavior exists until the TRIM function exists...)

Let me explain the "strange" behavior in KSP:

A pilot assumes that if you trim pitch elevator DOWN (with ALT + W) that the nose will go DOWN immediately a bit,

and vice versa, if you trim pitch elevator UP (with ALT + S) that the nose will go UP a bit.

Instead, KSP will keep pitching UP when you tip or hold ALT + W (instead of going DOWN immediately) until you reach the "upper reversal point",

then it will pitch DOWN, regardless if you press (tip or hold) ALT+W or ALT+S.

After reaching the "bottom reversal point" it will keep pitching UP, regardless if you press (tip or hold) ALT+W or ALT+S.

This behaviour is absolutely unrealistic and very confusing while flying planes!

This should be fixed very soon...

Where can I report such a "bug"?

Sorry for my English (I am German), I hope you understand what I mean...

Answers are welcome, maybe you can check and confirm this behavior...

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1 hour ago, KSPJoe said:

Hey guys,

I think the trim function when flying planes is very confusing!   (I am talking about jet planes in atmosphere, < 10,000 m).

(I am talking about KSP 1.6.0, I think this behavior exists until the TRIM function exists...)

Let me explain the "strange" behavior in KSP:

A pilot assumes that if you trim pitch elevator DOWN (with ALT + W) that the nose will go DOWN immediately a bit,

and vice versa, if you trim pitch elevator UP (with ALT + S) that the nose will go UP a bit.

Instead, KSP will keep pitching UP when you tip or hold ALT + W (instead of going DOWN immediately) until you reach the "upper reversal point",

then it will pitch DOWN, regardless if you press (tip or hold) ALT+W or ALT+S.

After reaching the "bottom reversal point" it will keep pitching UP, regardless if you press (tip or hold) ALT+W or ALT+S.

This behaviour is absolutely unrealistic and very confusing while flying planes!

This should be fixed very soon...

Where can I report such a "bug"?

Sorry for my English (I am German), I hope you understand what I mean...

Answers are welcome, maybe you can check and confirm this behavior...

Are you flying with SAS on ?

SAS conflicts with trim inputs,  preventing the desired effect.

It sounds like you already know how to fly an airplane with pitch trim,  but i made a tutorial video last year showing the method.    I don't turn SAS on till over 41km - and two seconds later the engines are shut down as we coast to Apoapsis.         Make sure CC / Subtitles are enabled on Youtube so you can see the comments,  I don't actually speak :-)

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Yes, you are right, I usually fly with SAS ON...(some plane designs are almost not to handle with SAS OFF...)

Okay, then this "strange" behavior should be fixed if SAS is ON...!

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21 minutes ago, KSPJoe said:

Yes, you are right, I usually fly with SAS ON...(some plane designs are almost not to handle with SAS OFF...)

Okay, then this "strange" behavior should be fixed if SAS is ON...!

I think it is easier to design a plane to be naturally stable,  than it is to fight SAS for control.

Use RCS build aid to make sure your engines are in line with centre of mass,  so that the plane does not pitch up or down as engine power changes.

Also use RCS build aid to make sure your centre of mass is the same with empty and full tanks.

Be generous with vertical stabilizer area/and try to put as far behind centre of mass as possible - this gives it more "leverage".

Dihedral/Roll stability is difficult.     I  only trim for pitch,  and make manual roll inputs to keep the heading at 90 degrees East.   In theory Dihedral should give roll stability,  but i've never been able to make an airplane that flies to orbit without some manual corrections to level the wings.      The problem with Dihedral on a conventional airplane layout is that it makes the wing sections act more like vertical stabilizer,  but as the main wing is ahead of centre of mass, this can end up reducing your yaw stability.      However,   if you have a canard  / delta configuration,  perhaps you can put dihedral on the outboard and rearmost wing sections.   This will cause the dihedral sections to be aft of CG and not reducing yaw stability.

However,  most unwanted rolling appears to come from bugs in the physics engine not aerodynamic reasons.     For example,  if you attach engine nacelles either side of the main fuselage and then attach wings to the nacelles,  this can cause problems in some KSP versions, as can importing craft built  on earlier versions of the game.     Attaching the wings directly to the fuselage,  or simply reattaching parts again without changing anything, or adding struts,  can fix this.

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@KSPJoe, the SAS function is like an autopilot.  Usually you do not change trim while autopilot is in control. Players are also frustrated that normal control inputs do not interact smoothly with SAS; and there is discussion about what players want in this thread

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13 hours ago, OHara said:

@KSPJoe, the SAS function is like an autopilot…

It’s important to point out that while SAS may be like an autopilot, it is not one, and attempting to use it as one for planes is a kluge at best. If you’re on PC, there are mods available that have much better autopilot-like functions.

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I know the difference between SAS and an autopilot (there are mods like "MechJeb" with a built-in autopilot or "Kramax Autopilot", etc.)

But sometimes I want to fly with SAS ON (without an autopilot mod) and I want to keep a defined height (e.g. 8,000 m or whatever) constantly over a long-travel distance.

To do this it is necessary to trim pitch up or down a bit because the speed of the plane changes due to the fact that the air becomes thinner with increasing height etc.

It is impossible to trim with SAS ON to keep a defined height because of the strange behavior mentioned above.

If this issue is reported in the bug-tracker already then it is okay for me...

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On 1/8/2019 at 2:50 AM, KSPJoe said:

Yes, you are right, I usually fly with SAS ON...(some plane designs are almost not to handle with SAS OFF...)

Okay, then this "strange" behavior should be fixed if SAS is ON...!

Actually, should be SAS OFF. You are fixing the problem when it is OFF, and causing the problem when it is ON.

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On 1/8/2019 at 10:48 PM, KSPJoe said:

But sometimes I want to fly with SAS ON (without an autopilot mod) and I want to keep a defined height (e.g. 8,000 m or whatever) constantly over a long-travel distance.

To do this it is necessary to trim pitch up or down a bit because the speed of the plane changes due to the fact that the air becomes thinner with increasing height etc.

It is impossible to trim with SAS ON to keep a defined height because of the strange behavior mentioned above.

If this issue is reported in the bug-tracker already then it is okay for me...

It's not a bug because it is by definition what SAS is and does.

SAS is like using a perfect gyroscope  - it always points exactly the same direction, no matter whether you're deep in a planet's gravity well or out in open space.
And the trim functions simply don't do anything when SAS is on, because trim alters the position of aeodynamic surfaces, while SAS aims at a given direction. The two functions are simply impossible to reconcile. SAS moves the aerodynamic surfaces for you, and doesn't care much at all where their neutral point is.

And since you're travelling around a sphere, SAS can never keep you flying at a given height or on a given heading. Because again, a height or a heading are impossible to reconcile with the "perfect gyroscope fixed direction" that SAS maintains.

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Mechjeb's version of SAS could be useful for this ... that allows you to set heading and pitch relative to the horizon, so it would be fairly close anyway.  There's also supposed to be a Spaceplane Guidance...

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I use prograde lock with SAS on planes, that seems to make a bad plane flyable.

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