Simviation Forums

I am wondering about aircraft trim. It seems if I get the aircraft all trimmed out, and then do a few manuveurs, the trim is all wrong again like I never trimmed it at all. If I set the trim and then do a turn or climb or dive, if I get back to the same FL and attitude and spped, that trim should still be correct, shouldn't it? It's like once I get the trim set, if I move the stick at all, the trim goes right back to where I started. Why is this and what can i do about it?

What aircraft are we talking about here?
All I can say at this point is to check the trim indicator gauge that is hopefully on the panel, and see if it is actually resetting itself for some reason.
The only other thing I can think of is that your controller needs to be re-calibrated. It may be not centering precisely and is consequently holding some of the movement values just executed.


Good Luck,
-SeanTK

It happens. Elevator trim is pretty weird in most MSFS models compared to the real thing. Drives me batty sometimes.
But... you can minimize this by using trim very sparingly, and being sure to wait a moment after changing trim to see what the airplane wants to do. Without real force feedback, it's almost impossible to properly trim anything in the sim in the normal real-life way ("trimming the pressure off").

If I set the trim and then do a turn or climb or dive, if I get back to the same FL and attitude and spped, that trim should still be correct, shouldn't it?

Depends, brettt777.

Yo guys!

In the FS9 Student Pilot Lessons, ole Rod Machado starts you off with the ever-present and alway 'doing' .....attitude, power, and trim.

Well, I did an experiment yesterday...I was using CFS2 instead of FS9 but pretty much the same thing. I was flying an L-39 in Combat Jet Trainer and tried exactly what you talked about. Trim the a/c straight and level at a specific altitutde, do a few manuevers and get back to that same altitude and attitude and the trim should be the same. Only it isn't. It's almost as if the trim resets itself back to zero every time I touch the stick.

Well, I did an experiment yesterday...I was using CFS2 instead of FS9 but pretty much the same thing. I was flying an L-39 in Combat Jet Trainer and tried exactly what you talked about. Trim the a/c straight and level at a specific altitutde, do a few manuevers and get back to that same altitude and attitude and the trim should be the same. Only it isn't. It's almost as if the trim resets itself back to zero every time I touch the stick.

Seems mysterious... until you consider the throttle.
"Trimmed for XX airspeed at XX altitude" implies a power setting. The slightest change in throttle will necessitate a trim change, especially if you maneuver while moving the throttle. No matter what the plane, "hands-off" trim means hands off everything, including the throttle. To fly level at a given airspeed and power setting, the plane wants to assume a particular A of A. Change the power and it will need to change that A of A in order to stay at that airspeed and altitude. It will "ask" for trim.

Maybe you didn't move the throttle, in which case I can only shrug... I've never performed that experiment consciously, because I was taught to re-trim often.

To see if it's really returning to neutral, just look at the trim gauge, if the plane has one. Most have a mark on the wheel or the gauge to show "zero", which is nothing more than the center of travel of the whole trim mechanism (which is set up, I guess, to yield level flight within part of the average cruise performance curve).

brettt777,

I sense some reluctance not to take the wonderful free flying lessons provided by FS9.

This is going to sound too simple but are you allowing the A/C to "pfugoid out"?

Bear in mind...an airplane has no brains.  Once it is airborne it doesn't know which way the wind is blowing, how much throttle is being applied, the angle of attack or anything else.  It follows the laws of physics.  In further response to your trim question I tried a little experiment, even though I knew from real piloting experience what would happen.  I took my L-19 Simulator A/C to 10,000 feet. trimmed it for level flight with a constant airspeed of 117 knots.  Without touching the trim I reduced the throttle to idle.  Initially the rate of descent went to -2500 fpm.  The airspeed climbed to about 125 and the nose started to lift and the descent rate fell off to -700 fpm.  after three oscillations the airspeed stabilized at 117 knots and a descent rate of -1100fpm.  I had lost 3500 feet in altitude by the time everything had balanced out.  Then I applied full throttle.  The rate of climb went to 1200 fpm and dropped to 400fpm.  it finally setteled into a 600 fpm climb for a few minutes.  The airspeed steadied at 117 knots and then, slowly, the rate of climb started to decay.  The rate of climb bled off rather quickly until it reached 300 fpm.  The airspeed stayed pegged at 117 knots.  The airplane continued to climb while the rate of climb retarded to zero.  It took 47 minutes for the airplane to regain the 3500 feet of altitude initally lost but the airplane finally came back to 10,020 feet by the time the rate of climb reached zero.  During this entire time the only thing I changed was the throttle setting.  I used full throttle in level trimmed flight, reduced it to idle and went back to full throttle to assure there were no veriables in the throttle settings during the experiment.  I held the wings level to assure full lifting capacity of the wings but I never put in any forward or aft stick.  The trim was never changed.  I did this experiment four different times each in an ordinal direction.  The results were the same every time.  At no time did the air speed change more that two knots and the altitudes never varied more than 10 feet. In answer to the laws of physics the lift was equal to the drag and the lift was balanced out by gravity.What ever anyone else tells you, try it for yourself.  I know of no better proof of an A/C responding to the laws of physics without regard to the wind or anything else.  

What YellowroseVI describes is actually the way it works out in real life. The aircraft will "hunt" for its trim speed overshooting first to the slow side and then to the fast side, (or thew other way around)  but eventually settling in.

What Rottydaddy says is also true, that this aspect of FS is not as it is in real life. The simulator is generally not as well  damped as a real aircraft when it comes to settling in at a trim speed.

I have taken aircraft in the sim, trimmed them up all nice and straight and level. I then press ever so gently in the stick, not enought  to even see the airspeed change, but enough for the VSI to start to indicate the slightest decent, and then I let the stick go. In real aircraft the nose would immediately return to a point just barely high of the aformentioned straight and level, and then dip a bit, and then rise a bit again. Repeating about two or three times until things settle out. BUT in the sim the nose continues to drop and drop, airspeed builds, significant altitude is lost, then the nose rises way above straight and level....etc. You get the picture. It is very annoying since in a real aircraft, once you have it trimmed it pretty much flies itself, but in FS any slight manover, such as a course corection, and your off to messing about withthe trim for several minutes.

The good news is that if you "hand fly" it back to the trim speed in straight and level using just the very, very lightest of touch, you can usually get things to setle down quickly without playing around with the throttle or trim settings. It takes a feather touch though. Much more so than in real flying.

JLCGull

Brett777, RottyDaddy, et al;

I was messing about with some fun new aircraft and noticed that some are better W.R.T. trim response than others. A little peek into the Aircraft.CFG file, which is found in the indivdiual aircraft folders revealed a simple difference.

I'll bet some of you already know this, but it was news to me;

I just discovered a keyboard command to hold constant airspeed:

ctrl+R
toggles this function on and off.