I have been wondering why the ailerons on the A320 (and other Airbus aircraft) droop when there is no power from the hydraulic pump. From my understanding of hydraulic actuators, the position where it was before the pump was shut off, is where it is gonna stay (if that makes sense) because you are closing the fluid intake and outake valves. I think I understand how Boeing actuators work, and why they stay in place with no hydraulic power coming from the pump. Just why do these Airbus ailerons droop with no hydraulic power?
The A320 aileron actuators have servo valves of course, which when closed indeed allow no flow through them from pressure to return. They additionally have mode valves, which do provide a direct (damped and narrow) connection, and it is this connection that allows for a slow and smooth aileron droop when unpowered.
Pic above is from this site, which contains a thorough explanation and simulation of the actuator. The right hand side is the valve in stand-by mode,
In active mode, the mode valve establishes connection between the servo valve control ports and the cylinder ports enabling nominal functioning of the system. In stand-by mode, the mode valve isolates the control ports of the servo valve and connects both cylinder chambers through a damping orifice and anti-cavitation valves – refilling the chambers in case the pressure goes below return pressure.
@JohnK added a valuable comment: the main reason to do this is, when the hydraulic system is unpressurised there is passive flutter damping...
...meaning fewer or no dedicated passive flutter dampers are required for that surface than if you have actuators that hydraulically lock with no pressure present and no spool valve displacement from a control input.