# What are the "Aft Bobweights" for on a full power hydraulic aircraft?

I like military aircraft, and recently I've been researching the feel system for the control stick.

It seems that there is something called bob weights to improve the feel of an aircraft. It's a logical system to provide resistance to the stick according to vertical acceleration.

My question is that some hydraulically controlled aircraft have two bob weights.

Why do I need two bob weights? Looking at the diagram, it appears that the rear bob weight pulls the stick toward you. In other words, does it mean that the mass balance is achieved by the front bob weight and the aft bob weight?

The bobweights provide two functions:

1. A artificial stick force gradient that rises with increasing G.
2. Inertial damping to the control circuit by adding mass to the linkage to dampen out sudden or jerky pilot inputs independent from G load. With hydraulic controls, the only feedback the pilot feels is whatever resistance is in the linkage and any springs. You could say the bobweights add "heft" to the control system that would otherwise be too easy to move. Viscous dampers are often included to further fine tune the feedback of the system. The objective is to try to replicate the feel of a normal manually operated control surface where the mass of the surface itself influences control feedback.

To achieve both objectives at the same time, you use two bobweights that oppose each other. The two weights combined provide the desired intertial effects the designer is looking for. For stick force-per-G, you make the nose-down bobweight heavier, or with a longer arm, than the nose-up one, giving you a net nose-down bias that provides the required stick force per G.

• Wouldn't the mass of the pushrods give enough inertial damping? It seems silly to make them as light as possible and then add masses for feel. My version of a "bobweight" is the upgoing pushrod into a T-tail - you only need to make sure that the gearing works the right way. Dec 10, 2023 at 17:20
• Using dedicated weights allows you to find tune the effect you want by adjusting the mass and leave the control linkage alone. Modern transports with cable operated hydraulic controls do away with bobweights and use a pitch feel bungee spring (or hydraulic) devices with variable spring rates and damping. The Challenger bizjet just connected the feel units to the moveable horiz stab to vary the stick force by trim speed. On the CRJ line, an outgrowth of Challenger, that was replaced with an electronic controller with linear input actuators. Dec 10, 2023 at 18:09
• Opposing bob weights apply forward and backward pulling force in response to vertical acceleration, so the back and forth movement of the stick becomes heavy''. understood. And since the front bob weight is heavier, you feel a stronger force pulling you forward. understood. It seems that stabilization by inertial mass can be achieved with just the front bob weight, but is a rear bob weight necessary for balance? The rear bob weight is basically built into the tail of the aircraft. Is this also relevant? Dec 12, 2023 at 8:05