What is the F-4E's overbalance weight system?

[zarusoba10]

While reading the F-4E's flight manual, I read about a device called overbalance weight in the section for artificial force feedback on the control stick.
The FFB for the F-4E's stabilizer seems to include a classic bob weight (a device that pushes the stick forward as the vertical G of the aircraft increases) and bellows (also called Q-feel). (Hydraulic resistance device that operates in proportion to IAS)

It seems that this bellows is equipped with an overbalance weight, but I am not sure how this weight works.

Reading some materials, it seems that this overbalance weight prevents the stick from falling toward the pilot when the pitch angle of the aircraft is large, but I don't know the details.

English is not my native language, so could someone please explain it to me concisely?

 

I hope HB recreates these behaviors in the game's FFB.

 

 

 

[Yannick Pancake]

As far as I know, the Phantom has a ram air System, wich increases the forces on the stick with an increasing airspeed.

I'm not entirely sure how it works, but basically you have a ram air duct in the root of the vertical stabilizer (those 2 openings left and right on the front of the stabilizer) and with an increase in airspeed you get more air in wich increases the forces on the stick you need to move the stick. so basically a force feedback. I heard it was done to prevent too large inputs while flying fast which could damage the plane. 

[zarusoba10]

4 hours ago, Yannick Pancake said:

As far as I know, the Phantom has a ram air System, wich increases the forces on the stick with an increasing airspeed.

I'm not entirely sure how it works, but basically you have a ram air duct in the root of the vertical stabilizer (those 2 openings left and right on the front of the stabilizer) and with an increase in airspeed you get more air in wich increases the forces on the stick you need to move the stick. so basically a force feedback. I heard it was done to prevent too large inputs while flying fast which could damage the plane. 

The air scoop at the base of the F-4E's vertical stabilizer is for engine bay cooling.
The pressure used to generate the control stick force is detected by a pitot tube in the vertical tail.

 

 

 

 

[zarusoba10]

The force generated by airspeed pulls the control column toward the pilot. (nose up)
The force generated by the weights pulls the control column toward the nose of the aircraft. (nose down)

As the aircraft increases speed, the stick will fall towards the pilot; as the aircraft increases vertical acceleration, the stick will fall towards the nose.

On the ground, when you let go of the control stick, the stick will fall forward. In the air, the stick remains in a neutral position as the speed begins to increase. However, as the speed increases, the stick begins to fall back.

The place where these front and rear forces are balanced and neutral is said to be the place where the trims meet.

Most likely, the F-4E's hydraulic control system does not have any feedback from the control surfaces, so it cannot determine the neutral point of the trim by default. You can use a spring to create a neutral point for the stick, but that point is fixed and you don't know the aerodynamic trim center point.
For this reason, the above system may be used to inform the pilot of the neutral point of trim.

By the way, the bob weight generates a stronger force the higher the G turn, so it provides feedback to the pilot to stop the pilot from making a high G turn. (This feature also has the ability to force the control stick forward if the pilot blacks out.)

[zarusoba10]

There is an explanation of overbalance weights on page 22 of this document.

https://www.acc.af.mil/Portals/92/Docs/ACC SAFETY/COMBAT EDGE/TAC70_02.pdf

 

When the F-4 was flying at a high AoA, the pulling force from the bob weight decreased and the control stick tended to become lighter, which threatened flight stability.
Therefore, this characteristic has been improved by installing a "balance weight" within the trim system.

I just don't understand why the bob weight force decreases when flying at high AOA. Is it due to the movement of the wing's center of lift?

[Smyth]

I think it is just based on the geometry of the linkages. Note that article says the problem was identified through ground testing.

Just imaging tipping the aircraft nose-up while it sits on the ground. If the control stick falls backwards (or forwards), then there is an imbalance in the control system. If it falls backwards, that will reduce the centering force of the stick at high AoA.

It probably doesn't help that at high AoA the direction of G-force is no longer aligned with the direction of the bobweight motion (only up/down), so the component of G-force acting on the bobweight itself to generate feel/resistance is reduced.

These are the kinds of issues that are not such a big deal at 20deg AoA, but get rapidly more severe with higher angles thanks to trigonometry. I'm pretty sure that's why the overbalance adjustment was implemented on the F-4 production line at the same time as leading edge slats.

Note that McDonnell's next fighter (F15) along with most 4th generation designs eliminated the bobweight feel system entirely. However they still added weights as necessary to counterbalance the forward-backward motion of the stick.

[zarusoba10]

 

In a high AoA posture, the force of the red arrow in the image decreases and the force of the blue arrow increases, so is it correct to say that the "weight" on the stick decreases?

And does the overbalance weight try to bridge this difference in power between blue and red?