GB2095342A

GB2095342A - Variable pitch propeller mechanism

Info

Publication number: GB2095342A
Application number: GB8207508A
Authority: GB
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1981-03-16
Filing date: 1982-03-15
Publication date: 1982-09-29
Also published as: GB2095342B

Abstract

The mechanism comprises a hollow shaft 1 movable in a first direction A to move the propeller blades towards coarse pitch and feather and in a second direction B to move the blades towards fine pitch. An internal axially fixed shaft 2 carries engagement elements 3 interconnected by links 8 which are pivotted to provide knuckle joint 9. The latter is acted upon by a biassing member 11 and a hydraulic piston 14 and when the pitch control means are operative, the piston 14 overcomes the bias of the member 11 thereby to displace the knuckle joint in the second direction B and to retract the engagement elements 3. When the pitch control means are inoperative e.g. due to hydraulic failure, the bias of member 11 overcomes the urging of piston 14 to extend the engagement elements 3 to a position in which their serrations 4 engage corresponding serrations on hollow shaft 1. The engagement elements 3 may be carried by the fixed shaft 2 via constraining means 6, 7 which allow movement of the hollow shaft 1 in the first direction A but prevent movement thereof in the second direction B when the engagement elements are extended. <IMAGE>

Description

SPECIFICATION Variable pitch propeller mechanism This invention relates to variable pitch blade propellers, that is to say, propellers in which the blades can be moved between fine and coarse pitch positions by a blade pitch control system. In some such propellers it is found that a failure in the blade pitch control system allows the blade to drift towards a fine pitch position under the influence of aerodynamic and centrifugal forces. Accordingly, it is an object of the present invention to provide a mechanism which will automatically prevent such drift on failure of the control system. Moreover, it is a further object of the invention to provide such a mechanism which cannot prevent the blades being moved to a coarse pitch position.

According to the present invention, there is provided a variable pitch blade propeller assembly which includes pitch control means having hollow shaft means and inner shaft means protruding into the hollow shaft means, one of the inner shaft means and the hollow shaft means being fixed and the other being axially movable in a first direction to effect blade movement towards coarse pitch and feather and in a second opposite direction to affect blade movement towards fine pitch, engagement elements carried by ghe inner shaft means for radial movement between an extended position in which the hollow shaft means is engaged and a retracted position in which the hollow shaft means is disengaged, interconnecting means having an intermediate knuckle joint interconnecting said elements, biassing means permanently biassing the knuckle joint in such a direction to urge the elements towards the extended position, urging means urging the knuckle joint in an opposite direction thereby tending to draw the elements towards the retracted position, the arrangement being such that when the pitch control means are operative the urging means overcomes the bias of the biassing means and retracts the engagement elements but that when the pitch control means are inoperative the bias of the biassing means overcomes that of the urging means and extends the engagement elements to the extended position, thereby to prevent relative movement between the inner shaft means and the hollow shaft means.

Preferably, the inner shaft means carries a pair of oppositely directed engagement elements pivotally mounted on each end of a knuckle joint, the knuckle joint comprising a pair of links pivoted together.

The assembly is preferably arranged so that the knuckle joint is fully opened to provide a geometric lock when the engagement elements are in their extended position.

Advantageously, the engagement means are carried on the inner shaft means by constraining means such that axial movement of the engagement means with respect to the inner shaft means in one sense is prevented but that limited axial movement in an opposed sense is allowed whereby, when the engagement elements are extended, operation of the pitch control means to effect blade movement towards coarse pitch and feather is allowed but that blade movement towards fine pitch is prevented.

Where the blade pitch control means is hydraulically operated, preferably the urging means is a hydraulic piston sharing the same hydraulic supply.

Preferably, the inner shaft means is axially fixed.

Preferably, the mating surfaces of the engagement elements and the hollow shaft means are provided with mating serrations.

Alternatively, the mating surfaces of the engagement elements and the hollow shaft means are provided with relatively smooth friction surfaces.

Conveniently, the constraining means include compression spring means to allow said limited movement in said opposite sense.

One embodiment of the invention is described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional side view of part of a variable pitch propeller assembly, the cross-section being upon the rotational axis X - X of the propeller, Figure 2 is a view upon arrow II of Figure 1, and Figure 3 is a view upon Section Ill - Ill of Figure 1.

In the Figures, a variable pitch blade propeller assembly includes a hydraulically pov'ered pitch controller of which only a hollow shaft 1 is illustrated. This shaft- is longitudinally movable to the right as drawn (the first direction A) to effect blade movement toward coarse pitch and eventually to a fathered position. In the feathered position, the blades are edged on to the free airstream to provide low drag when an engine driving the propeller assembly is shut down.

The shaft 1 is longitudinally movable to the left as drawn (the second direction B) to effect blade movement toward fine pitch.

Protruding into the hollow shaft and lying coaxially within it is a fixed shaft 2. This carries twin oppositely directed engagement elements 3 which are capable of radial movement between an extended position (illustrated) in which a serrated outer face 4 on each engages a mating serrated region 5 on the interior surface of the hollow shaft 1, and a retracted position in which the faces 4 disengage the region 5.

The elements 3 are prevented from lateral move menttothe left (the second direction B) by reaction pads 6 but are allowed limited lateral movement to the right (the first direction A) against the bias of compression springs 7.

Interconnecting the elements 3 are twin generally radially directed links 8, each pivoted at an outer end to a respective element 3, and at an inner end to one another to form a knuckle joint 9. The pivot axes of the links 8 are parallel to one another and lie transverse to the axis)(-X. Twin spaced rollers 10 are carried on the axis of the knuckle joint 9. The links 8 are of a length chosen such that when the elements 3 are in the extended position, engaging the hollow shaft 1, the pivot axes are aligned with one another to provide a geometric lock.

Displacement of the knuckle joint 9 in the first and second directions along the axis X - X effects extension and retraction of the engagement elements. The knuckle joint 9 is urged to the right by a biassing member 11, a concave end face 12 of which contacts the rollers 10. A powerful coil spring 13 provides the biassing force.

The knuckle joint 9 is urged to the left by an urging member in the form of a hydraulic piston 14, an end face 15 of which also contacts the rollers 10. The piston 14 is provided with residual bias to the left by a coil spring 16. In effect, the knuckle joint 9 is trapped between the member 11 and the piston 14. The rate of the spring 13 is chosen to be much greater than that of the spring 16.

The assembly comprising the engagement elements 3, the links 8, and hence the knuckle joint 9, is free to move across the diameter of the hollow shaft 1 to provide a self-centring, that is to say a load equalising effect. When the engagement elements 3 are other than in their extended position, the concave end face 12 of the biassing member ensures that the knuckle joint 9 is maintained on the axis X X. In addition, the provision of a spaced pair of rollers 10 which contact concave face 12 ensure correct radial alignment of the assembly comprising the engagement elements 3 and the links 8.

Should the hydraulic pressure biassing the piston 14 to the left fall below a chosen level, either through a propeller rotation overspeed signal or a hydraulic pressure failure, the spring 13 urges the knuckle joint 9 to the right (in the first direction A) until piston 14 reaches the end of its travel thus tending to bring the axes of the links 8 into alignment thereby urging the engagement elements to the extended position in which their serrated faces 4 mate with the serrated region 5 on the shaft. The hollow shaft 1 cannot then move toward the left, that is to say toward fine pitch.

Nevertheless, it is still possible to drive the blades to feather, that isto say, in the coarse pitch direction, since the engaged elements move to the right (in the first direction A) with the hollow shaft 1 against the bias of the coil springs 7. The knuckle joint 9 is restrained by the residual bias of the spring 16 and accordingly the engagement members 3 are retracted from engagement with the shaft 1. The blades can thus be moved to coarse pitch.

Claims

1. Avariable pitch blade propeller assembly which includes pitch control means having hollow shaft means and inner shaft means protruding into the hollow shaft means, one of the inner shaft means and the hollow shaft means being fixed and the other being axially movable in a first direction to effect blade movement towards coarse pitch and feather and in a second opposite direction to effect blade movement towards fine pitch, engagement elements carried by the inner shaft means for radial movement between an extended position in which the hollow shaft means is engaged and a retracted position in which the hollow shaft means is disengaged, interconnecting means having an intermediate knuckle joint interconnecting said elements, biassing means permanently biassing the knuckle joint in such a direction to urge the elements towards the extended position, urging means urging the knuckle joint in an opposite direction thereby tending to draw the elements towards the retracted position, the arrangement being such that when the pitch control means are operative the urging means overcomes the bias of the biassing means and retracts the engagement elements but that when the pitch control means are inoperative the bias of the biassing means overcomes that of the urging means and extends the engagement elements to the extended position, thereby to prevent relative movement between the inner shaft means and the hollow shaft means.

2. A variable pitch blade assembly according to Claim 1, wherein the inner shaft means carries a pair of oppositely directed engagement elements pivotally mounted on each end of a knuckle joint, the knuckle joint comprising a pair of links pivotted together.

3. A variable pitch blade assembly according to Claim 1 or Claim 2, wherein the knuckle joint is fully opened to provide a geometric lock when the engagement elements are in their extended position.

4. Avariable pitch blade assembly according to any of the preceding Claims, wherein the engagement means are carried on the inner shaft means by constraining means such that axial movement of the engagement means with respect to the inner shaft means in one sense is prevented but that limited axial movement in an opposed sense is allowed whereby, when the engagement elements are extended, operation of the pitch control means to effect blade movement towards coarse pitch and feather is allowed but that blade movement towards fine pitch is prevented.

5. Avariable pitch blade assembly according to any of the preceding Claims, wherein the pitch control means are hydraulically powered and the urging means is a hydraulic piston sharing the same hydraulic supply.

6. A variable pitch blade assembly according to any of the preceding Claims, wherein the inner shaft means is axially fixed.

7. A variable pitch blade assembly according to any of the preceding Claims, wherein the mating surfaces of the engagement elements and the hollow shaft means are provided with mating serrations.

8. A variable pitch blade assembly according to any of Claims 1 to 6, wherein the mating surfaces of the engagemerit elements and the hollow shaft means are provided with relatively smooth friction surfaces.

9. A variable pitch blade assembly according to Claim 4, wherein the constraining means include compression spring means to allow said limited movement in said opposite sense.

10. A variable pitch blade assembly substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.