ChristianJ wrote:
The transfer function for a electro-hydraulic actuator is a bit more complicated than a simple low-pass filter, but the suggestion of oscillations in the system is somewhat too far-fetched... and would indeed imply a fundamental design flaw.... The technology has been around too long, and is too well understood, for this to be a plausible cause, IMO.
I would agree. Like any feedback servo, hydraulics are naturally rate limited in update terms by the inherent phase lags in the system. That is, the lag or delay between demand input and controlled entity reaching demand position. In the case of, for example, an aircraft rudder, this might be in the seconds range for full scale input. Because this is a rate of change limit, it means that increasing oscillatory demand input above the slew rate capability will result in ever decreasing amplitude at the control surface. I would think it unbelievable that any modern aircraft servo could have corners in the response plot that would permit oscillation at all. It's such fundamental part of servo design.
With a hydraulic failure, say a valve stuck so that the surface slews to one or other limit, all bets are off as the system is then running open loop. Istr, this sort of failure has been implicated a/c losses in the past, but forget the details. As for hydraulic capacity, I would expect any modern aircraft system to have a flow rate at least double that of worse case expected, but may be wrong there.
Have an old book on servo design that has a section on hydraulic servos:
Ahrendt, Servomechanism Practice, McGraw Hill, 1955. P 191. Old, but worth a read. The basic theory doesn't change...
Regards,
Chris