What is Trim and what do I need it for?

[davidpsummers]

I read someone mentioning "trim".  What is it?  When/Why do I need it?

[Snark]

20 minutes ago, davidpsummers said:

I read someone mentioning "trim".  What is it?  When/Why do I need it?

Trim is a very rarely-used feature.  I've been playing KSP for a couple of years and have never used it even once (don't even know what the key mapping is).  My impression is that most players don't use it, either.

Every reference to trim that I've seen in the forums over the past year has been "you may have accidentally set trim, press alt-X to re-zero it" as an answer to people who are having trouble controlling their craft.  That's it.  So if that's the only context in which it comes up, I think it's safe to say it's not a feature you can't live without. 

That said:  perhaps someone who actually uses the feature could give a better description than I could, but basically it's re-setting the "zero torque" point in some fashion for your controls-- I think the idea is that you can use it if you have an uneven mass distribution, or off-axis thrust, or something like that.  Maybe it's useful for shuttles?  I almost never fly spaceplanes, and never fly shuttles, so that may be why I never need the trim feature. 

Certainly it's a feature that the game is very coy about.  Hunt around on the KSP wiki, or the forums, and there's practically nothing about it.

[ForScience6686]

Trim is only worthwhile if you aren't using SAS in a plane.  Mainly used for pitch to set level flight at cruising speeds.  Alt+directional input will set trim, alt+x to reset.  It basically holds the control surface at the set trim level.

[Vegetal]

I use it all the time. You adjust trim by using alt-wasd, and reset it by alt-X.

What is it? You readjust your center point for your controls. It's very useful for planes because you have to build them with the CoP behind CoM, which gives them a constant nose down tendency that you have to neutralize. With a keyboard, that is impossible, with a joystick, it's boring, and with SAS, it's messy. So you adjust trim to neutralize that, it's the best solution, and used in pretty much all real aircraft.

[Plusck]

1 hour ago, Snark said:

That said:  perhaps someone who actually uses the feature could give a better description than I could, but basically it's re-setting the "zero torque" point in some fashion for your controls-- I think the idea is that you can use it if you have an uneven mass distribution, or off-axis thrust, or something like that.  Maybe it's useful for shuttles?  I almost never fly spaceplanes, and never fly shuttles, so that may be why I never need the trim feature. 

This is actually a very good description. In real life, trim controls on an aircraft will move a tiny mini-aileron (or mini-rudder) at the trailing edge of the aileron (or rudder). This causes the airflow to push the main body of the control surface the other way, so if you let go of the stick (or rudder) it will settle naturally in a different position. Pilots should constantly re-trim the controls so that the plane maintains level flight with zero effort on the controls - and this changes with airspeed and (for monoprops) engine speed.

KSP approximates this, with much the same aim. I don't do planes much in KSP but whenever I do I use trim, mostly because SAS is so odd.

[AeroGav]

If you trim an airplane it will naturally tend to settle at a particular angle of attack - difference between prograde and where the nose is pointing.   Eg.  you set trim for 5 degrees angle of attack in level flight at part throttle.  If you then shut the throttle, it will settle into a glide at 5 degrees angle of attack.  If you firewall the throttle, it will eventually settle into a nose angle that =  your climb angle based upon your excess power + 5 degrees that was trimmed for (after a few oscillations).

Eventually, it will cimb to an altitude where the air is so thin, the engines no longer have any excess power and you climb no higher.  The thin air means less drag but also less lift, and you will be in level flight at much higher speed than before.   But you'll still have about the same angle of attack.

I nearly always use pitch trim to fly my low thrust/weight ratio spaceplanes to orbit,  because optimum lift drag ratio occurs at a certain angle of attack and that's the easiest way to keep it constant while thrust , lift , drag and air density fluctuate all over the shop.         

Yaw or Roll trim?    Never use these.   I design my planes to be stable and fly straight,  if the plane won't fly straight by itself that means my joystick isn't centering properly/is generating phantom inputs or the physics have glitched, in which case it's best to restart.  Never had any success trimming out phantom forces ...   always under or over corrected.

[Geschosskopf]

6 hours ago, davidpsummers said:

I read someone mentioning "trim".  What is it?  When/Why do I need it?

Good question.  I've been playing since 0.20 and haven't ever used it.

[Temstar]

It's also used for cruise control on long range rover trips. Switch to docking mode, trim "pitch axis" all the way forwards and the rover will drive itself. You just have to point it in the right direction.

[Snark]

2 minutes ago, Temstar said:

It's also used for cruise control on long range rover trips. Switch to docking mode, trim "pitch axis" all the way forwards and the rover will drive itself. You just have to point it in the right direction.

Really?  Are you sure it's "pitch axis" and not "accelerator"?

Reason I ask:  the fact that KSP maps the rover controls (accelerate, decelerate, turn left, turn right) to the same WASD keys that are used for pitch and yaw is a major pain in the butt that makes rovers very hard to drive (and extremely confusing for many folks).  Lots of people (myself included) remap the rover controls to some other set of keys (I use the numpad) to eliminate the conflict.  So the question is, does "rover cruise control" really go with pitch, or does it go with "accelerate"?

I'm guessing it goes with "accelerate".  I should go try this out. If it works, cool!  Learned something new today. 

[Temstar]

13 minutes ago, Snark said:

Really?  Are you sure it's "pitch axis" and not "accelerator"?

That's why I said switch to docking mode first and put "pitch axis" in quotes. I'm not actually sure which axis (port-starboard, fore-aft, zenith-nadir) W and A correspond to in docking mode actually because I always dock in staging mode. I only ever use docking mode to drive rovers.

[GoSlash27]

I use trim on long- distance atmospheric flights, but that's about it. I find getting it set is more of a hassle than just hand- flying with SAS on short trips, and I have no use for it in space or on rovers.

Best,

-Slashy

[davidpsummers]

Thanks everyone.  It actually good to know as I use spaceplanes and have this same issue.