Paul C, I'm no Nick Lappos but I hope my simple QHI's explanation below helps you to understand what happens:
The main rotor hydraulic control system is there to reduce the feedback forces to the pilot's controls, essentially similar to power steering on a car. The feedback forces originate from effort required to drive the pitch angles of the blades to where the pilot wants them to be and hold them there against variable aerodynamic forces.
A small helicopter is controllable by the pilot without hydraulic assistance due to the small feedback forces involved. A larger aircraft less so, in some cases there is no manual reversion due to the design; instead there are two or more hydraulic systems, giving a failure backup.
As feedback forces get higher, at extremes of speed / and or manoeuvre, the hydraulic system comes under greater strain. This could, subject to design, overstress hydraulic components such as the pump drive, or the rubber seals in the servos, causing complete failure.
To ensure the system cannot be damaged by an overstress, the hydraulic system of a Gazelle has a Pressure Regulating Valve set at 40 bar. The AS 355 has a PRV set at 35 bar. If the system pressure reaches these figures, the valve will open, allowing hydraulic fluid to bleed off back to the reservoir. Whilst this is happening, the jack will effectively stop moving because it cannot overcome the feedback force working against it.
This is what the British military have traditionally called "jack stall". The French recently prefer the term "servo transparency".
As far as the effect on the aircraft is concerned, it will go wherever the aerodynamic forces take it. The pilot can reduce the feedback forces by reducing the severity of the manoeuvre, usually by reducing collective pitch.
If he has allowed the aircraft into a tight corner (literally and figuratively speaking), he may not have that option.
Last edited by ShyTorque; 20th February 2008 at 20:32.
Reason: A typo/spelling mistake.