WO1985001326A1

WO1985001326A1 - Ram air turbine hydraulic power system

Info

Publication number: WO1985001326A1
Application number: PCT/US1984/001368
Authority: WO
Inventors: Albert L. Markunas
Original assignee: Sundstrand Corporation
Priority date: 1983-09-16
Filing date: 1984-08-27
Publication date: 1985-03-28
Also published as: GB8512204D0; GB2157461A; DE3490419T1; JPS60502221A; GB2157461B; EP0157794A4; EP0157794A1

Abstract

A ram air turbine hydraulic power system and pressure compensator valve (24) usable therein for controlling the displacement of a variable displacement pump (12) driven by a ram air turbine (10). The pressure compensator valve has a valve member (42) with a pilot responsive to discharge pressure of the pump and a compensator spring (50) acting on the valve member in opposition to the discharge pressure, a movable control piston (54) engaging the compensator spring, a pair of control springs (70, 72) engaging said control piston and acting in opposition to each other, and a control pressure which is proportional to the square of pump speed is applied to the control piston whereby the force of the compensator spring is varied depending upon the position of the control piston.

Description

RAM AIR TURBINE HYDRAULIC POWER SYSTEM

Description

Technical Field This invention pertains to a ram air turbine driven hydraulic power system wherein a variable displacement pump is driven by a ram air turbine and a pressure compensator valve is used for controlling displacement of the pump and for reducing pump displacement to prevent stall of the turbine upon decrease in turbine speed.

Background Art

A conventional ram air turbine for driving a pump to provide emergency hydraulic power for an aircraft is designed to provide a specified hydraulic power at some minimum specified airspeed. The ram air turbine has a plurality of variable pitch blades mounted in a hub. The governor for the ram air turbine controls the pitch of the turbine blades to control rotational speed of the turbine under normal conditions of varying hydraulic load demand by the system and airspeed. Because of size and weight considerations, there is very little adjustment of the blade pitch available beyond the design point to allow the governor to control rotational speed of the turbine under reduced airspeed conditions. Once the blade pitch limit is reached, the ram air turbine operates as a fixed pitch unit. For any airspeed of the aircraft, there is a definite rotational speed limit of the ram air turbine below which there is a stall of the turbine and the hydraulic power available is critically reduced. A conventional pressure compensated variable displacement hydraulic pump used in the hydraulic power system for an aircraft has a maximum power absorption characteristic which is essentially proportional to rotational speed. Operation of such a pump for airspeeds more than approximately 5% below the design air speed will stall the ram air turbine driving the pump rotational speed to zero. Unless the air speed is increased to the design point, any attempt to bring the ram air turbine back into operation will only result in another stall, with the resultant loss of hydraulic power. This problem has led aircraft manufacturers to overspecify the requirements for the ram air turbine driven hydraulic power system, resulting in a size and weight penalty.

Disclosure of the Invention

A primary feature of the invention is to provide a ram air turbine driven hydraulic power system for an aircraft utilizing a variable displacement pump wherein the displacement of the pump is controlled to provide a maximum power absorption characteristic for the pump that is approximately linear with rotational speed of the pump above a design set point and follows a speed cubed relationship below the design point to provide a power absorption characteristic for the pump below the design point, that essentially follows the stall line of the ram air turbine as the airspeed decreases with a constant stall margin to result in a no-stall ram air turbine hydraulic power system.

Another feature of the invention is to provide, in combination, a variable displacement pump and a ram air turbine for driving the pump with means for controlling the displacement of the pump to prevent stall of the ram air turbine including a pilot valve having a valve member for hydraulically controlling the displacement of the pump and being positionable by a compensator spring acting on the valve member in opposition to pump discharge pressure, and means responsive to the speed of the pump for varying the effective force of the compensator spring.

An additional feature of the invention is to provide a pressure compensator valve for a pump, said valve comprising a pilot valve with a movable valve member having a pilot responsive to discharge pressure of the pump and a compensator spring acting on the valve member in opposition to the pump discharge pressure and the improvement in the pressure compensator valve comprising a movable control piston engaging the compensator spring, a pair of control springs acting in opposition on said control piston, and means for applying a control pressure proportional to the square of pump speed to said control piston to control the position thereof and establish a force which results in a pressure set point for the pump which is a function of the compensator valve geometry and spring rates and increases linearly with the control pressure.

Still another feature of the invention is to provide a pressure compensator valve for controlling the displacement of a variable displacement pump driven by a ram air turbine to have the pump operate below a design point at progressively decreasing power absorption levels which stay within the progressively decreasing power generating levels of the ram air turbine comprising, a displacement actuator for the pump, a pilot valve member positionable for controlling the displacement, actuator, a compensator spring urging the pilot valve member in a first direction and the pilot valve member subject to pump discharge pressure acting in opposition to the compensator spring, and means for varying the set point at which the pressure compensator valve will operate to destroke the pump comprising, a control piston having a range of movement between two limit positions and movably supporting an end of the compensator spring, a pair of control springs acting in opposition on the control piston, and means for applying a control pressure proportional to the square of the pump speed to the control piston which acts to set the position of the control piston and control the force of the compensator spring. Brief Description of the Drawings

Fig. 1 is a schematic view of the ram air turbine hydraulic power system; Fig. 2 is a central sectional view of the pressure compensator valve; and

Fig. 3 is a graph plotting ram air turbine power against ram air turbine speed and showing stall characteristics of the ram air turbine and power absorption characteristics of the variable displacement pump at various flight speeds.

Best Mode for Carrying Out the Invention

The ram air turbine hydraulic power system is shown generally in Fig. 1 wherein a ram air turbine, indicated generally at 10, drives a variable displacement pump 12 through suitable drive connections including a drive shaft 14. One specific use of the ram air turbine hydraulic power system is to provide emergency hydraulic power for an aircraft. When operated, the pump 12 pumps oil from a reservoir 16 to a discharge line 18. The variable displacement pump 12 may be of the type having a series of pistons whose stroke is controlled by a hanger with a face cam. A displacement control for the hanger includes a cylinder 20 having an actuator piston 22 connected to the hanger for setting the position thereof.

A pressure compensator valve, indicated generally at 24, is connected by a line 26 to the pump discharge line 18 to sense pump discharge pressure and controls the communication of the actuator piston 22 with either pump discharge through a line 28 or case drain, indicated at 30. The pressure compensated valve 24 also receives a control pressure through a line 32 which connects to a unit 34 which senses the speed of the shaft 14 and, therefore, the speed of the pump 12 and provides an output control pressure which is proportional to the square of pump speed. The pressure compensator valve is shown particularly in Fig. 2 and has a body 40 having a pilot valve with a pilot valve member 42 having a central land 44 which controls communication of the actuator piston 22 through line 28 with either pump discharge existing at the upper end of the pressure compensator valve body or to case drain 30. As shown in Fig. 2, the pilot valve member 42 is at a pressure set point wherein the land 44 is positioned to block communication of the actuator piston line 28 with both pump discharge and case drain whereby the actuator piston is neither stroked nor destroked.

The pilot valve member 42 has a land 46 at the upper end thereof providing a pilot section responsive to pump discharge pressure. The pump discharge pressure urges the pilot, valve member in a direction to connect pump discharge with the actuator piston 22. This force is opposed by a compensator spring 50 which, at one end, engages an end 52 of the pilot valve member. The structure of the pressure compensator valve, as so far described, is conventional. The pressure compensator valve embodying the invention has a control piston 54 mounted within a chamber 56 of the valve body 40 for movement between a pair of stops defined by end walls 58 and 60 of the chamber 56, and which provide limit positions for movement of the control piston 54 from the position shown in Fig. 2. The control piston 54 movably supports the compensator spring 50 by a transverse wall 62 of the control piston engaging an end of the compensator spring 50. A pair of control springs 70 and 72 are positioned within the body chamber 56 and act in opposition on the control piston, with the control spring 70 positioned between the end wall 58 of the body chamber and the transverse wall 62 of the control piston. The control spring 72 is positioned between the transverse wall 62 of the control piston and the end wall 60.

The control pressure which is responsive to the speed of the pump and, more particularly, which is proportional to the square of pump speed, enters the body chamber 56 from the control pressure line 32 and acts on the annular end of the control piston wall 76 and the transverse wall 62 of the control piston.

The pressure compensator valve provides a pump maximum power absorption characteristic that is approximately linear with rotational speed above the design point and follows a speed cubed relationship below the design point. Referring particularly to Fig. 3, the relationship between ram air turbine power and ram air turbine speed at various flight speeds of the aircraft is shown by the curves 80, 82, 84, 86 and 88 representing increasing flight speeds, and with the curve 86 illustrating the relation at the approximate normal cruise speed of the aircraft. The stall line for the ram air turbine is shown at 90 which indicates the speed at which the ram air turbine will stall at a particular power requirement on the ram air turbine. As the ram air turbine speed decreases, the available power without stall decreases.

The power absorption curve for the pump 12 is illustrated by a straight line 92 above a design point 94 for the operation of the pump and by a curve 96 below the design point showing a power absorption characteristic below the design point that is a cubic function of the pump speed.

The pilot valve 24 controls the flow of oil to and from the actuator piston 22 to either destroke or stroke the pump 12 which results in less or more pump delivery to either lower or raise the pump discharge pressure.

The control is achieved by a force balance and the pump discharge pressure which will neither stroke nor destroke the actuator piston 22 is the pressure set point and is determined by the diameter of the land 46 providing the pilot section and by the force of the compensator spring 50. A primary feature is the control of the compensator spring force which is determined by the position of the control piston 54 relative to the pilot valve member 42. The position of the control piston 54 is dependent on the forces in the control springs 70 and 72, the force in the compensator spring 50, and the. value of the control pressure representing pump speed. Therefore, the pressure set point is a function of the compensator valve geometry and the spring rates and increases linearly with the control pressure representative of pump speed. For low values of the control pressure, the control piston is on the stop defined by end wall 60 at one limit position and the pressure set point is constant at some low value. For high values of the control pressure, the control piston 54 is on the stop defined by end wall 58 at the other limit position and the pressure set point is constant at the nominal design pressure of the pump. For the control piston at either stop, the pressure compensator valve 24 acts like a conventional compensator valve. When the control piston 54 is between the stops defined by end walls 58 and 60, the pressure compensator valve is a variable pressure set point valve, with the pressure set point being linearly dependent on the control pressure. During operation as the flight speed drops below the design point, the speed of the ram air turbine would begin to drop causing the control pressure delivered to the pressure compensator valve to drop. As the control pressure is reduced, the pump discharge pressure required to overcome the compensator spring 50 drops, thus allowing oil to flow to the actuator piston 22. This destrokes the pump, resulting in a lower system pressure corresponding to the lower airspeed and reducing the pump horsepower demand to match the horsepower available from the ram air turbine. As seen in Fig. 3, the curve 96 representing the power absorption of the pump 12 below the design point is offset from the stall line 90 for the ram air turbine, to provide a constant margin to assure that the ram air turbine will not stall as flight speed reduces. in order to control the maximum power absorption of the pump to be a cubic function of pump speed, the control pressure applied to the pressure compensator valve must be made proportional to the square of pump speed. This is shown in Fig. 1 by use of the unit 34. This can be a commercially available unit which would electronically sense the speed of the pump and control an electro-hydraulic servo valve to set the control pressure. There are a number of alternatives to the unit 34. These alternatives include the use of a small fixed displacement gear pump driven by the ram air turbine 10 with the discharge from this pump passing through an orifice provided in the transverse wall 62 of the control piston 54. A second alternative would be adding a small centrifugal stage pump driven by the ram air turbine 10 with its outlet applied against the control piston. A third alternative would be the use of a small weight arranged to generate a force varying with the square of pump speed to be used in a pressure servo to generate the control pressure.

In view of the foregoing description, it will be evident that the ram air turbine hydraulic power system provides for reduced power demand by a variable displacement pump as the air speed causing operation of the ram air turbine reduces thus avoiding the sudden loss of hydraulic power during some unforeseen flight condition of the aircraft.

In operation at the design point indicated at 94 in the graph of Fig. 3, the valve land 44 of the pressure compensator valve is positioned as shown in

Fig. 2 and the pump 12 is operating to deliver oil under pressure to the discharge line 18. If the aircraft flight speed should decrease from the design point, the control pressure applied through line 32 to the control piston 54 will be reduced, with the result that the force of the compensator spring 50 is reduced and pump discharge pressure acting on the pilot land 46 lowers the valve member 42 from the position shown in Fig. 2 whereby pump discharge oil can flow through line 28 to the actuator piston 22. This will reduce the pump displacement along the curve 96 shown in Fig. 3 whereby the power requirement imposed on the ram air turbine will be reduced.

Once there is a force balance within the pressure compensator valve resulting from the decreased pump discharge pressure, the valve land 44 will move to the position to block communication with line 28. Subsequent increase in aircraft speed back to the design point will increase the control pressure acting on the control piston 54 to raise the valve land above the position shown in Fig. 2 whereby oil can flow from the actuator piston 22 to the case drain 30 and when a force balanced condition is achieved, the land 44 has returned to the position of Fig. 2.

If there is an extreme reduction in airspeed, such as represented by curve 82 in Fig. 3, it is obvious there will be a substantially greater reduction in the control pressure. This results in a greater delivery of oil to the actuator piston 22 resulting in a greater reduction in pump displacement and flow of oil from the pump and, therefore, a greater reduction in pump discharge pressure until a force balance is achieved.

Claims

ClaimsI CLAIM:

1. A pressure compensator, valve for controlling the displacement of a variable displacement pump driven by a ram air turbine to have the pump operate below a design point at progressively decreasing power absorption levels which stay within the progressively decreasing power generating levels of the ram air turbine comprising, a displacement actuator for the pump, a pilot, valve member positionable for controlling the displacement actuator, a compensator, spring urging, the pilot valve member in a first direction and the pilot valve member subject to pump discharge pressure acting in opposition to the compensator spring, and means for varying the set point at which the pressure compensator valve will operate to destroke the pump comprising, a control piston having a range of movement between two limit positions and movably supporting an end of the compensator spring, a pair of control springs acting in opposition on the control piston, and means for applying a control pressure proportional to the square of the pump speed to the control piston which acts to set the position of the control piston and control the force of the compensator spring.

2. A pressure compensator valve for a pump and having a pilot valve with a movable valve member having a pilot responsive to discharge pressure of the pump. and a compensator spring acting on the valve member in opposition to the pump discharge pressure the improvement comprising, a movable control piston engaging the compensator spring, a pair of control springs engaging said control piston and acting in opposition to each other, and means for applying a control pressure related to pump speed to said control piston to control the position thereof.

3. A pressure compensator valve as defined in claim 2 wherein said control piston has at least two positions which establish two different effective forces for the compensator spring, and a decrease in control pressure below a certain valve results in movement of the control piston in a direction to reduce the effective force of the compensator spring.

4. A pressure compensator valve as defined in claim 3 wherein said control pressure is proportional to the square of pump speed.

5. In combination, a variable displacement pump, a ram air turbine for driving the pump, and means for controlling the displacement of the pump to prevent stall of the ram air turbine comprising, a pilot valve having a valve member for hydraulically controlling the displacement of the pump, said valve member being positionable by a compensator spring acting on the valve member in opposition to pump discharge pressure acting on a pilot section of the valve member, and fluid pressure responsive means for varying the effective force of the compensator spring dependent upon the speed of the pump.

6. In the combination of claim 5, said means for varying the effective force of the compensator spring comprising a movable control piston which movably supports the compensator spring, and means responsive to the speed of the pump for setting the position of the control piston.

7. In the combination of claim 5, said means for varying the effective force of the compensator spring comprising a control piston movable between two limit positions and supporting an end of the compensator spring, a pair of control springs positioned one to each side of the control piston and acting on the control piston in opposition to each other, and means for applying a control pressure proportional to the square of pump speed to the control piston whereby an increase in control pressure above a certain value shifts the control piston against the force of one control spring and the compensator spring.

8. A pressure compensator valve for controlling displacement of a variable displacement pump driven by a ram air turbine to prevent stall of the ram air turbine comprising, means responsive to the pump discharge pressure for controlling the fluid pressure applied to a displacement control for the pump including a compensator spring acting on a pilot valve member, and means operable by fluid pressure and responsive to the speed of the pump for controlling the force with which the compensator spring acts on the pilot valve member.

9. A pressure compensator valve as defined in claim 8. wherein said means responsive to the speed of the pump for controlling the force of the compensator spring comprises a movable control piston supporting one end of the compensator spring, a pair of control springs acting in opposition on said control piston, and means for applying a pump speed indicating control pressure to said control piston.

10. A pressure compensator valve as defined in claim 9 wherein said means for applying a pump speed indicating control pressure applies a control pressure which is proportional to the square of the pump speed.

11. A pressure compensator valve for controlling the displacement of a variable displacement pump driven by a ram air turbine to have the pump operate below a design point at progressively decreasing power absorption levels which stay within the progressively decreasing power generating levels of the ram air turbine comprising, a displacement actuator for the pump, a pilot valve member positionable for controlling the connection of the displacement actuator to pump discharge and case drain, a compensator spring urging the pilot valve member in a direction to connect the displacement actuator to case drain, a pilot section of the pilot valve member subject to pump discharge pressure acting in opposition to the compensator spring, and means sensitive to pump speed for varying the set point at which the pressure compensator valve will operate to destroke the pump comprising, a control piston movably supporting an end of the compensator spring and having a range of movement between two limit positions, a pair of control springs acting in opposition on the control piston, and means for applying a control pressure proportional to the square of the pump speed to the control piston which urges the control piston toward one of said limit positions to increase the force of the compensator spring whereby the force of the compensator spring and thus the control of the pump displacement varies with the control pressure established by the speed of the pump.