Trim limits

[Spurts]

Hello,

I just wanted to highlight some observations I've made about trim.  Full after trim results in incontrollable pitch-up at any speed (full stick forward with full aft trim results in pitch up to stall) and full forward trim still requires forward stick at high speed (~1.9IMN) to avoid climbing.

Is this intentional?  It seems like there would be benefit from moving the trim limits "forward" in both cases.

These are far from game-breaking issues, I'm just curious if they are correct implementation.

[Golo]

You would need non linear stick/trim implementation, such is in F-5. In current implementation if you trim aft/fwd to certain value you will not have full range of stick motion available in opposite direction. At least last time I checked. That is what may cause your problems. With that said I have not noticed anything unusual at high speeds, I can control the aircraft/trim it out just fine. 

[Spurts]

I am fine with not having full range of motion (and you are correct), I do it intentionally for that purpose.  I go full forward trim before a dogfight to reduce my chance of overcontrolling in pitch.  

I did once, as a test, go full aft stick trim on the ground.  Once airborne I was full forward stick cleaning up and kept pitching up.  THAT problem is due, as you said, to not having full range of motion available but it is also not a realistic scenario.

At 1.9M+ 36,000ft I can full forward trim but I still have to push forward to avoid climbing, so I am at a loss there.  At low alt and 1.2M full forward trim I need to pull back a bit.  What speeds and alts are you doing that you can trim?

I guess in short I am saying that full aft trim is uncrontrollable with wings forward (can be controlled with manual sweep aft at low speed).  Full forward trim has been insufficient to stabilize at top speed around 36,000ft.

[Golo]

You must be doing something wrong, I just tested it at high speed and its working just fine. F-14B from take off (0 trim) full mil 10-15° nose up, M0,7ish to 25k ft, then burner and nose down about 10° to accelerate to M1.5ish then climb 5-10° nose up to 40k ft at that point I was at M1.8-1.9 and I pitched little down to accelerate to M2 that was roughly at 38k ft. At no point in that whole exercise I had problems with controlability or trim issues, My trim ranged from 0 to 6ish nose up.

If you test it still have problems link me your track Ill take a look at it.

[Spurts]

will do

 

[Spurts]

file size is too big.  Here's what I did.  Hot start on runway, F-14A clean (no Phoenix rails) full internal fuel.  no stick movement, full aft trim, watch h-stab indicator for cessation of movement.  Full AB.  Let plane take off due to trim.  Full forward stick.  Observe plane remains in pitch up and increasing.   Neutral stick input, full forward trim.  Watch h-stab indicator for cessation of movement.  climb using aft stick pressure at 350 KIAS/0.75IMN until 36,000ft.  Do best to maintain 36,000ft using only stick pressure (currently full forward trim) untill ~1.9M when zero forward pressure results in climb.  Observe 2.0IMN @49,000ft and 6,000+ft/m climb on the instruments.  Observe zero stick input h-stab position reads ~+4.  Push forward to h-stab position 0, press forward trim.  observe trim moves again, hold until cessation of movement.  observe 2.2IMN at 36,000ft with aft stick pressure needed to maintain altitude! 

Tentative Conclusion?  you cannot set a trim setting to max forward when subsonic and have it apply to full forward when supersonic?  

I can definitely see how if you trim as you go you have no problems, but if you try and set trim ahead of time you will climb like crazy.

[Golo]

How is your track file too big? Trk is usually in kB, must be hell of a mission. And your write up is perfectly consistent with how trim works if I understand it correctly.

If you trim full nose up on take off, it will result in uncontrollable pitch up (due to limited fwd stick authority, that is just the stick/trim implementation now). Next you went full nose down trim, that will stop the pitch up, but will need aft stick to control which is what you did (and is fine since controlability is not symmetrical, its biased to for nose up, you have more stick authority there). 

Then you climbed to 36k ft and leveled off still with an aft stick, and accelerated. Acceleration will result in speed increase and higher force on h-stabs resulting in a need to ease aft stick you were holding to maintain level fight, or you would climb (track would be nice here to know what your stick position was). Im not exactly sure what you mean by zero fwd pressure here, if just easing aft stick from what you were holding thats fine.

Next you say you were at 49k ft M2 hands of stick and 4 nose up trim with 6k fpm climb, sounds about right. At M2 slightest pitch deviation will result in thousands of FMP vertical speeds.

Then you said you trimmed full nose down at M2.2, well your nose is gonna go down here, thats not surprising is it? Of course you need aft stick to hold level flight. 

Im not seeing anything out of ordinary really, maybe except that part that you said zero forward pressure (with full nose down trim!) at M1.9 36k ft will result in climb, I dont exactly know what you mean by that (track would be nice for that one).

Edited December 11, 2021 by Golo

[Spurts]

That last part is my point.  I went full nose down trim until the stab indicator stopped moving shortly after take off, so full forward trim and controlling the nose with aft stick (as expected).  Level acceleration at 36,000ft from 0.8M to 1.9M resulted in reducing aft pressure, as expected, until there was zero aft pressure.  So at this point, I am hands off the stick, having previously gone full forward trim (trim held until stabs stopped moving while not holding the stick).  I can still push full forward trim and the stabs wont move from 4up.  I will however begin climbing rapidly while hands off stick.

 

Only when I both push the stick forward from neutral AND push forward trim again will the trim change from 4up to 2up.  In this case I pushed to 0 on the stabs, pushed nose down trim again, saw them move, released the stick, and saw they stabilised at 2 up instead of 4 up.

 

As for the track file, I don't know what to tell you.  Empty PG map with just me flying around for a few minutes testing trim resulted in a 6MB file.  My inability to add the .trk is why I detailed my procedure to see if you wanted to test it yourself.  After you leave a mission and you have the debrief you just hit the "Save Track" button right?

 

Not saying it's a bug (I didn't see that I was in the bug sub-forum when I made this) just that it seemed like odd behaviour (especially the uncontrollable full aft trim).  I appreciate you taking the time to work through this with me @Golo

[Golo]

@Spurts Not sure you will be able to see this since I run some mods but you can try

Trim test.trk

Edit: Actually scratch that, you can not watch it, because it goes stupid right after takeoff even if its just 2MB of data (fkng EDs tracks)

Suffice to say that I did test it like you full nose up trim on take off then full nose down and climb to 36k ft and level acceleration past M2, and I did not experience any weird nose up tendencies (less back stick required but that is normal).

Edited December 11, 2021 by Golo

[Spurts]

I don't have time to watch all of it right now, but I watched some of it.  You gave me an idea of how to troubleshoot though, that control input thing.  Between that and screenshots I should be good.

[Spurts]

Did not get that control input overlay to come up, where do you find that.  Did more detailed situational recordings though.

• Sitting on Runway
  • default trim: -1 
    • stick range: -13 - +27
  • full forward trim: -4
    • Stick range: -13 - +24
  • full aft trim: +21
    • Stick range: +9 - +35

per prior experience, full aft trim, hands off stick take off, full forward trim as recovery

• 3,350ft 240KIAS
  • Full forward trim: -4

per prior experience, use stick pressure to climb to 36,000ft leaving full forward trim

• 36,800ft 261KIAS 0.84IMN
  • hands off trim: -2

use stick pressure to try a level acceleration towards 2.0M

• 36,950ft 683KIAS 2.19IMN
  • hands off trim: +2
    • 3,500fpm climb
  • push trim forward result?: +2 not even pushing forward on the stick and trim at the same time changed it, not sure what happened before now

Notable difference, I realized in previous attempts I was trying to set autopilot once I got to 36,000ft.  Autopilot uses trim.  This is 99% likely the source of my previous experience of "Why is my trim +4?".  Also of note, full forward trim DOES change.

[Golo]

That control overlay is toggable with RCtrl + Enter iirc.