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Old 6th Dec 2001, 02:50
  #27 (permalink)  
Lu Zuckerman

Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132

To: Grey Area

“What you invisage happening is not what I described and is, frankly, rubbish. The feedback forces exert a force equal to the maximum available from the jack so it can no longer move. It has nothing to do with bypass valves”.

The very purpose of a hydraulic servo is to not only resist feedback forces from the rotor but to be able to overcome any and all feedback forces. To do this it must be properly designed and be supported by an adequate supply system. If you have what you call jack stall then the system is not designed properly. In order to do this; the hydraulic system must be able to support the maximum demands placed on the system by manipulation of the controls. That is why hydraulic systems incorporate an accumulator large enough to take care of these maximum demands while the pump catches up. Most new hydraulic systems do not incorporate an accumulator but use a constant speed variable displacement pump. In a few cases the system may incorporate both an accumulator and a constant speed variable displacement pump.

Again, I state that if you get jack stall then the system is not designed correctly and I don’t give a damn what Aerospatial thinks.

I have seen helicopter flight control systems that incorporate constant speed variable displacement pumps and in monitoring the pressure gage the system pressure can drop by one third or more when the cyclic stick is moved in a circle or if the collective is raised and lowered. When the movement stopped the pressure immediately went up to the green line. At two thirds rated pressure the system could still resist the maximum design feedback forces. If this type of pump can’t keep up with the heavy demand then think about what a constant displacement pump does relative to system pressure when a high demand is placed on it.

On a Sikorsky servo on the H-34, H-37 and H-19 the bypass valve would open at pressures lower than 750 pounds PSI. Please tell me what the hydraulic pressure is on the AS-350 and tell me at what pressure the bypass valves on the servo open up?
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