Explain rudder trim to me

[Grarsch]

Hi.

I'm struggling to understand how rudder trim works in this game. I'm no pilot but I know the purpose of that 'side slip' indicator in the cockpit so I can tell when it's not aligned. Can also understand the need to trim rudder for different speeds and altitudes, it's probably similar to the reaons behind aileron and elevator trim.

What I fail to understand is how, if Combat Trim is disabled, the plane instantly goes out of trim rudder-wise.

Am flying straight as an arrow, 80% power, 5000 feet, CT engaged. The 'ball' is centered. I disable CT - the ball goes to either side (left it seems). Why is this and what's the reason behind it? I've tried manually trimming the rudder but mostly can't seem to get it properly aligned.

If I somehow manage to get it right, I then do some turns and dives and climbs, then get back to 5000 feet, straight as an arrow. Haven't touched trim or anything. The flight parameters are the same, yet now the rudder is out of trim. What gives?

I want to fly without Combat Trim because trim is a fun thing to micro-manage. However, if I am chasing someone, barely gaining ground, then all the side slipping will slow me down and I'll have no choice but to engage CT.

Explanations and tips would be welcome

[Wizkid]

@  Grarsch
 
 
If the rudder (the ball indicator shows it) is off center during a shooting attempt, you are flying sideways and the whole ballistics are screwed.
You will not hit where the gunsight is pointed at.

It is vitally important to have the nose in the line of flight (the rudder centered), to get all the ballistical computations right.

[Grarsch]

Right, this is something I am aware of and one of the reasons why I asked 'why does it work like I described'.

Got an idea why it gets out of trim under the circumstances I described?

Thanks.

[Wizkid]

There is a different rudder trim setting for every speed. The reason is the propwhash (it turns around the aircraft and hits the rudder sideways) and the slightly inclined (offset by 1-3 degrees) rudder fin to counter that force.

Therefore you will have to retrim the rudder for every little speed change.

BTW a rudder out of trim does not only affect the gunnery but the acceleration and top speed as well.

[Grarsch]

Right, so if you keep speed, trim and altitude the same, the aircraft should be in trim.

So say this is set so the aircraft is in trim, 5000 feet AGL, 250mph. I then do a dive, followed by a turn. I then climb back to 5000 feet AGL, get to speed 250mph. During all of this, I have not touched trim at all, or used CT or something like that.

According to my limited understanding of aerodynamics, since the important variables are the same, the aircraft should now still be in trim.

But it isn't in AH. What am I missing?

Thanks again.

My hit % is 5% so I'll blame the rudders for that

[Wizkid]

With all parameterts staying the same, the rudder trim should stay as it was before.

But, are really all the parameters the same?
You have changed the grossweight of the aircraft by burning fuel, thereby flying with different angle of attack at the same speed as before. That is a change isnīt it? A small one, but definitely noticable, at least in RL.

If you really want to blame it on AH or the game tuning in general, I would try a twinengined aircraft for a change and see if the issue still persits. I would try a Me 110 or a P38 for instance.

[indy007]

Make sure you do not have it assigned to any sort of rotary if your joystick has one.

For example, if you have elevator trim on a rotary, and turn off combat trim, the trim will go to wherever the rotary is set for. If it's set for rudder, and the rotary is twisted all the way left, and combat trim is then turned off, you will immediately yaw left.

Happened to me once or twice.

[stantond]

The explanation of trim and how it is used on a RL aircraft and how it's use in AH is not the same. Trim (RL) is used to remove some of the stick load from the pilot. Trim tab settings keep the physical position of the stick (or rudder) fixed for that axis, however the stick travel is the same as if someone was holding the stick (or pressing on a rudder pedal).  

Trim in AH works by moving a control surface some % of its maximum value for attitude adjustments, with the remaining portion scaled to the maximum positional movement in the joystick.  For example, rudder trim full left does not move the rudder 100% of its full distance but 30%.  The remaining 70% of rudder movement is from moving the rudder pedal left though 100% of the pedal's available deflection.  In an actual aircraft, the stick (or rudder pedal) is physically moved and 'trimmed' so that only 70% of the stick (or rudder) movement remains in that axis direction.

Control surface maximum movement is completely independent of trim position, but who flys using maximum inputs?  Maybe maximum inputs are used momentarily for a roll, loop, or a snap roll, but other wise not. So, the stick deflection is rescaled and 'exaggerated' for maximum trim conditions for that trim axis.  For example, this can be seen during takeoff when elevator trim is full up and only a slight pull back on the stick is needed. Taking off with that trim full down needs almost full stick deflection to take off.

I typically hit angle trim in a dive or going straight to get the plane automatically 'trimed up' then disable auto trim.  I really haven't experienced what you are describing regarding the manual trim being off much after a few maneuvers. Do you calibrate your stick regularly in AH?  It almost sounds like your joystick is drifting.  


Regards,

Malta

[bozon]

Any use of ailerons will cause a yaw secondary effect toward the rising wing (the lowered aileron). Roll right and it yaws left.

Any sharp pitch movements will cause yaw due to gyro-effect of the prop disc. I don't know if and how AH models this, but it's terrible in some planes (try a mosquito).

[Grarsch]

Thanks gents, this has helped me understand what's going on. As I said, I'm not a pilot so these things aren't precisely second nature to me.

Thanks again.

[Clifra Jones]

I pretty much never use combat trim. I usually don't fart around with the rudder trim while in a fight, I use the rudder pedals, but my thumb is constantly on my elevator trim. In level flight or in a dive what Malta said is good advice, hit trim for angles, let it settle and then turn it off. If you need to do minor trim adjustment.

Even while in auto-trim for angles mode, elevator and rudder trim are still adjustable. I use this to set specific dive to climb angles.

One exercise you can do to learn trim is to put the plane into a shallow bank turn and trim the plane to say in that turn with your hands off the stick. While I don't recommend this for actual flying and fighting it will give you the feel of how much trim to add and how you can relax the stick while applying trim. You can do this with different degrees of turns, climbing turns and diving turns.

Your doing the right thing in learning trim. CT is a crutch that will hinder you in certain planes. ki-84 and p-38 are notoriously bad with CT on.

[Krusty]

Check your stick. If you've got a twisty-stick rudder you're probably spiking one way or the other. Check the stick mapping, the "advanced" button, and see about putting a little deadband in, and/or scaling it. When I use combat trim then turn it off it usually stays nice and level until I change something. I have CT mapped on my stick so I can toggle it on then off.

If you're slipping the second you take it off I'd guess that you have a stick input/calibration issue.

[Grarsch]

Thanks Krusty, seems the issue was with the 'mini-stick' on my CH Pro Throttle. The trim was as the mini-stick was - if it was centered, so was the rudder trim. I had thought it would just keep the current trim after I released it and it didn't.

Mea culpa.

Unfortunately, my gunnery hasn't improved since I discovered this which leads me to believe that AH incorrectly models...umm...'thermal layers' or maybe 'heat induction' or something like that.

[hubsonfire]

Originally posted by Pyro
Combat trim is simply a lookup table of trim settings.  For example, if you are doing 200 mph and have combat trim enabled, it will go to a table and lookup your speed and set the three trims to whatever corresponds to the 200 mph entry.  Combat trim is meant to keep you trimmed condition for just one condition- military power at a standard fuel and ordnance load.  Change your throttle settings, drop your flaps, change your loadout, etc. and combat trim may no longer hold you in a trimmed out condition.  For those conditions, you need to use manual trim to get your plane trimmed out.  If you use combat trim all the time, you'll notice that when you're on final approach with power reduced and gear and flaps down, you're out of trim by a good deal and are having to put in a lot of joystick and rudder input to hold your plane at the correct attitude.  If you want to get your plane trimmed out in these other conditions, you'll need to use the manual trim system when you're in those conditions.

As soon as you start dialing in some manual trim, your combat trim will be disabled.  To reenable it, you either need to press the combat trim key or there is an option in the flight setup that automatically enables combat trim whenever you leave autopilot mode.  

So you're on final with your power reduced and flaps and gear down and combat trim has you out of trim.  You're having to push forward on the stick to keep your nose from coming up.  You start repeatedly hitting the down trim key to alleviate this.  As you dial in more elevator trim, you can start relaxing the amount of joystick input you are giving until the stick is centered.  You would then rinse and repeat for the aileron and rudder trims and that should put you in a trimmed out condition.  

What the new feature does is move all three of those trims for you when you make a single button press.  Going back to the above example, how I would trim out my plane on final approach would be to hold the correct amount of elevator, aileron, and rudder input to keep the plane flying like I want it to.  Then I just hit the set trim button and it looks at how much input I'm giving to each axis and begins dialing in trim to get each of those axes back to a neutral position.  As it does this I just begin relaxing the amount of deflection I'm feeding into the joystick and rudder until they are neutral.  

This is just an addition to the manual trim system.  It doesn't replace anything.  If you don't use it, you wouldn't know that anything has changed.  

Although the outcome is essentially the same, trim is different in a PC sim versus a real aircraft because PC's have self-centering spring loaded sticks.

[Traveler]

Originally posted by Grarsch
Right, so if you keep speed, trim and altitude the same, the aircraft should be in trim.

So say this is set so the aircraft is in trim, 5000 feet AGL, 250mph. I then do a dive, followed by a turn. I then climb back to 5000 feet AGL, get to speed 250mph. During all of this, I have not touched trim at all, or used CT or something like that.

According to my limited understanding of aerodynamics, since the important variables are the same, the aircraft should now still be in trim.

But it isn't in AH. What am I missing?

Thanks again.

My hit % is 5% so I'll blame the rudders for that

I am not sure if center of lift and center of gravity are modeled into the aircraft, if they are,  the part you are missing is the changing moment of the aircraft as it burns fuel.  The weight change as fuel is burned and the resulting change in the center of lift which effects the need for trim.