Trim issue on the Viper

[Mirage2425]

So I've posted a couple of times on this with trim issues relating to the TGP.  ED has come back with it being as designed.  That's because they are correct, it is as designed.  However, I've done a lot of research and talked to a real life Viper pilot and I believe the real issue is that there is a minor issue with the turn and slip indicator (the Viper pilot told me about it that instrument).  The turn and slip indicator detects if an aircraft is skidding.  If the ball isn't centered then the aircraft's center of gravity is off and the aircraft's yaw is off.  Below is an article from Wikipedia on the turn and slip indicator:

 

https://en.m.wikipedia.org/wiki/Turn_and_slip_indicator#:~:text=The turn indicator is a,is mounted in a gimbal.&text=As the aircraft yaws%2C a,precession around the roll axis.

 

Every aircraft is a little off due to bent airframes, regardless of whether the TGP is on the aircraft and requires trimming.  When you takeoff you set a takeoff trim to get the aircraft safely off the ground.  The aircraft is not perfectly trimmed at this point.  Then, when you are at altitude and you have a few seconds you perfectly trim the aircraft.  To do this you first set a constant speed and adjust yaw trim so the ball is centered and then use roll trim to perfectly trim the aircraft.  Then, when you drop ordinance you can compensate simply with roll trim.  Here's where the problem comes in.  When you start trimming the ball is perfectly centered.  It shouldn't be.  It should be off to the left or right to indicate the aircraft is slipping.  As a result I have to move the ball all the way to the line on the left or right using yaw trim and then use roll trim to trim the aircraft perfectly.  This works but for realism it would be better if the ball is offset if the aircraft is slipping.  I can completely trim the aircraft at this point.  It took that Viper pilot to get me there.  I would suggest that Wags do a video on this very important topic as a lot of people struggle with it.

[TobiasA]

Planes with small wings like fighters are don't really have a tendency to slip like gliders to with their large wings. The viper's fly by wire compensates for it when you roll the aircraft, and we don't have real life issues like a slightly bent airframe (which are caused by production tolerances back then). That is why you almost never see the ball offset to any side. Kick the rudder, and it will move as it should.

If you have an asymmetric loadout, you can compensate that with a chart. I usually don't do those, because if you have too much asymmetric loadout, it becomes real nasty and increases the risk of a stall. Also, the flight model the viper currently has, is having not enough lift on slow speeds so landing speeds and such are higher. With an asymmetric loadout, this is even more a problem.

A HARM on one wing and two GBU-12 on the other won't hurt tho.

You can actually see that if you release ordnance and there is for example one maverick less on one side, the ball on the yaw indicator touches the line on this side because drag on the other side is a little bit higher. You can simulate the "bent airframe" if you load two racks with 3 bombs each on one side and leave the other side empty, the plane will skid to a noticable amount. Then, you trim yaw, then roll and it'll be cool. As long as you fly straight of course, because the weight will be a pain in the a** when turning.
If the ball is centered, the plane isn't slipping. And in our perfect world, we have perfect airframes without tolerances, no bents, no uneven weight distribution- just nice and sweet. It would be cool to have an option to place some random "trim offset" but then most people would leave that option unchecked.

Once you have the yaw centered (which is the case for us), you can stick with roll trim.

I developed the habit of as clicking the trim hat once or twice when I pull the finger off the trigger, depending on what I released. AAM's are one click (if any, you can live without), GBU-10's might be 3 or 4, or hold the button longer.  However, you often release bombs in pairs, so...

Since roll and pitch trim is on the HOTAS next to the trigger, it becomes second nature.

The reason you trim yaw first is that yaw affects roll, but roll doesn't really impact yaw.

Edited January 31, 2021 by TobiasA

[Mirage2425]

So with the TGP loaded the ball should be dead center?  How do you trim with the TGP loaded except for what I'm doing now which is moving the ball to the left line then roll trimming.  Is that how they do it IRL?

[kengou]

Worth noting there is a rudder trimmer on the left side on the trim panel. If you're out of yaw trim for whatever reason try using that?

[TobiasA]

vor 11 Stunden schrieb Mirage2425:

So with the TGP loaded the ball should be dead center?  How do you trim with the TGP loaded except for what I'm doing now which is moving the ball to the left line then roll trimming.  Is that how they do it IRL?

I do not have RL experience on the Viper, but know of one manual that mentions that the yaw trim is always centered on takeoff (HAF F-16, you can google this and download it for free), and I know of one fatal accident caused by accidently turning the yaw trim all the way to one side by storing the checklist there. 

If the ball is between the two lines in the middle of the ADI, then you are flying straight and you do not slip. Middle means flying straight. 

Having the TGP attached might lead to a little slip, but I usually ignore it as it is very small. You may however click the yaw trim to have it exactly centered. 

The stick controls do the same as turning the knob on the left side panel, except we don't have the yaw trim on the stick. Only pitch and roll. Because you usually only need roll and probably a little pitch sometimes, but rarely yaw trim. 

[randomTOTEN]

On 1/30/2021 at 2:05 PM, Mirage2425 said:

When you start trimming the ball is perfectly centered.  It shouldn't be.

It isn't in my experience.

 

When I start my trimming demonstrations with the TGP mounted, the ball is deflected to the side. The first step I perform is yaw trim to center the ball, then roll. This is to avoid trying to correct a yaw mistrim with adverse yaw from ailerons/stabs.

You probably need to zoom in some to see the deflection of the inclinometer in finer detail.

In the previous discussion, a user pointed out that this will actually result in a small amount of sideslip due to combined aerodynamic effects of the asymmetrical drag and trimming force. Uncorrected it's skidding turn. The ideal would probably be about half the correction, known as a "zero sideslip" condition. But really, it's such a small amount of slipping or skidding to really care about such minute details. You can just adjust the controls until you're satisfied despite being less than aerodynamically perfect.

[Mirage2425]

That's usually what I do.  Let me see if I can get some screenshots amd a track file.  I'm using VR super zoom.  I'll post them once I have them.  It's good to know that you don't experience this issue and it's probably something I'm doing wrong.  That I can correct

 

 

It's not that I'm out of yaw trim.  It's just that it's not working as I'd expect, meaning center the ball with yaw trim and then correct with roll.  I'm only moving the knob about 1/8 of the way to the right, which moves the ball left.  Then I can use roll trim to correct and perfectly trim the aircraft.

[randomTOTEN]

1 hour ago, Mirage2425 said:

I'm only moving the knob about 1/8 of the way to the right, which moves the ball left.

that's a lot in my opinion (assuming the aircraft is otherwise symmetrical)

I have my mouse wheel set to "clicks" instead of continuous rotation, and it's two clicks of the wheel for me... nowhere near 1/8th deflection. It's an incredibly tiny amount. If you were sitting in the aircraft with your hand on the knob you would probably apply enough pressure just to barely feel it moving under your finger tips. As soon as you felt it move you would probably release pressure, then check the change.

1/8th sounds like way too much.

Edited February 1, 2021 by randomTOTEN

[Mirage2425]

How do you set the mouse wheel to clicks?

[randomTOTEN]

I have a button below my mouse wheel. The point is it's a very very very small amount of movement.

[TobiasA]

You guys notice anything when mounting the TGP besides a small roll tendency to the right due to the weight? I can't see the ball offset to any side on my system, maybe my monitor is too small or something. I have a typical loadout (2 bags, 4 slammer, 2 heater) and the TGP and it is hard to tell if there is any offset in yaw trim. I just do one short click on "trim left wing down" (coolie hat left) and that's it. Straight and level.

It's true that due to tolerances, the real thing requires some trim depending on speed and such, but our viper is perfect so it doesn't require that.

 

Are you sure you don't have a wrong middle position on your pedal axis by accident?

[ruddy122]

In the KC10 I put my feet flat and don’t touch the rudder pedals in flight

The only time I put my feet on rudder pedals is taking off, taxiing and landing for crosswind control

I use ailerons only to turn the plane in flight


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[ruddy122]

AR is tricky you have to be stable and patient

If you are stable in the precontact position chances are you’ll be stable in the contact position

The 10 was weird that you need to make small movements in the opposite direction we call that action chopping wood if you put controls in the direction of roll drift you’ll become a giant slingshot and the IP (me) will freak out and get scared you are about to swap paint with another airplane


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[Mirage2425]

On 2/1/2021 at 9:35 AM, randomTOTEN said:

that's a lot in my opinion (assuming the aircraft is otherwise symmetrical)

I have my mouse wheel set to "clicks" instead of continuous rotation, and it's two clicks of the wheel for me... nowhere near 1/8th deflection. It's an incredibly tiny amount. If you were sitting in the aircraft with your hand on the knob you would probably apply enough pressure just to barely feel it moving under your finger tips. As soon as you felt it move you would probably release pressure, then check the change.

1/8th sounds like way too much.

 

You are correct, I was compensating for too much roll trim.  It's just a little now.  The ball is still touching the left line.  Not sure if that's supposed to be that way.  However, given the small amount of deflection I could see the ball being centered before I start trimming.