Trim indicator needle doesn't move

[Oiz0]

Hi,

I'm starting with this module, and during the F-4 startup procedure, I see it's recommended to set trim nose down to 3 degrees.

I try to do so, and I see well my stick move forward, but the pitch trim indicator needle doesn't move !

I have watched startup videos and needle moves there.

So, I suppose I don't understand something and/or I have not done something, but could someone explain me whhat happens and what I have to do ?

Thanks

[Oiz0]

Well, searching info in this forum, in pinned topic "Issues you may encounter when trying the F-4E for the first time, and how to avoid them.", I read this :

Issue: My trim is not working on the ground, the stick is either full forward or full back and when I trim it does not change position.

Fix: This is correct as is on the ground. The bellows system pulls at the stick with increasing airspeed, while the bobweight system pushes the stick forward with increasing G. In flight and when you are trimmed out well, these forces roughly balance each other out naturally.

On ground however, the bellows measure airspeed 0 and hence do not add any force that would counter the bobweight system pushing the stick forward. You also will not see trim doing anything in that situation, since the trimming merely changes the length of the lever to which the bellows can apply force to the stick and do not move the stick around directly.

I'm not sure to understand well (English is not my native language) and if it's normal, why it's working in some youtube videos. has that changed since 1 year ?

And if it's "normal" that trim indicator doesn't work, how can we know how to trim to 3 degrees nose down during pre-takeoff setup ?!

 

Edited 3 hours ago by Oiz0

[Ivandrov]

The stick on the ground at zero airspeed will move very fast and hit stick traverse limits while adjusting the trim. But, that's not an indication of the actual trim adjustment limit. Don't look at the stick, just look at the gauge and continue holding the trim until you get to the desired setting.

Edited 2 hours ago by Ivandrov

[Zabuzard]

Yeah, it sounds to me like you are not trimming enough yet and let go the trim hat after you saw your stick in the cockpit moving around a lot already.

The explanation in the FAQ is correct and that is why your stick will, on the ground, immediately bump into the limit after just touching the trim hat. That is correct behavior.

In the F4, trimming does not move the stick directly. It changes the arm length of the lever used by the bellows to pull the stick aft with increasing speed. As opposing force to the bobweights who try to push the stick forward with increasing G.

You know, imagine two children fighting over a toy. One left, one right. And both pull on the toy in their direction at the same time. If they both pull with the same strength, the toy doesnt move and no one gets it. The force is balanced out.
Trimming in the F4 changes the strength of one of the two children, making them stronger or weaker, shifting the balance.
And bc one kid pulls stronger with increasing airspeed, while the other kid pulls stronger with increasing G, you will get a highly dynamic experience in practice.

And at ground, where you have nearly 0 airspeed, one of the two forces is almost disabled, so the other force dominates the play. As soon as you start your takeoff and gain speed, the other force will rise and start to move your stick naturally.

[Zabuzard]

Think of it like that:


(and ofc i drew the arrows the wrong way around, lmao. Increasing airspeed pulls aft. Increasing G pushes forward)

The idea is that it self-balances the aircraft, more or less. When you dive, your airspeed increases. So the bellows start to pull the stick aft to lift you back up.
High G typically means high AoA, typically means in order to get back to level flight the stick needs to be pushed forward. So the bobweights get heavier which causes the stick to push forward.

[Zabuzard]

Worth noting that the bellows measure "airspeed" at the bellows intake sitting on your tail, next to the anti-collision light.

If that probe gets damaged (e.g. combat damage) or clogged (e.g. ice/frozen when flying without pitot-heat turned on), it cant do its job and the bellows will go absolutely bonkers on your stick. And since the trim-system connects to the bellows, the trimming wont do anything useful anymore at all either.

And since the engineers didnt think of adding a way to disable the bellows/bobweight linkage, you will have a fun time flying back home.

The procedure is to disable all assistance systems by pulling the ARI CB (left subpanel), disabling the STAB AUG system (left console aft) and pulling the CBs for AIL, STAB and TRIM (right wall).
Set trim to 0 so when the system kicks back in you wont be caught by surprise.
Then hard grip the stick, not let it go, and descent to hotter air to hopefully remove any bellow intake blockage.
Until the problem is resolved you have to actively fight the bellows wanting to throw your stick around like crazy.