GB2151712A

GB2151712A - Ram air turbine

Info

Publication number: GB2151712A
Application number: GB08333723A
Authority: GB
Inventors: John A Chilman
Original assignee: Sundstrand Corp
Current assignee: Sundstrand Corp
Priority date: 1983-12-19
Filing date: 1983-12-19
Publication date: 1985-07-24
Also published as: GB2151712B; GB8333723D0

Abstract

A ram air turbine having a rotatable hub assembly 10 mounting a plurality of bifurcated blades 20 having pairs of spaced-apart airfoils 50,51 offset from the pitch axis of rotation of the blade relative to the hub assembly. Centrifugal twisting moments associated with the blade airfoils act in a direction to urge the variable pitch blade toward a coarse pitch with resulting reduction in speed of rotation of the hub assembly in the event there is a failure in the control mechanism for controlling the pitch of the blade. <IMAGE>

Description

SPECIFICATION Ram air turbine Technical field This invention relates to a ram air turbine and, more particularly, to a ram air turbine having a rotatable hub assembly with a plurality of variable pitch bifurcated blades extending therefrom. In operation, the spaced-apart airfoils of each bifurcated blade have centrifugal twisting moments which act in opposition to governor spring means whereby the pitch of the blades can be controlled during operation to obtain a constant speed of rotation of the hub assembly. The bifurcated blades result in reduction in the size or omission of normally used governor weights which act in opposition to the governor spring means, with a reduction in the weight of the ram air turbine.

Background art For many years, aircraft have had a ram air turbine as standard equipment for supplying backup power when the normal power source is inoperative. The ram air turbine is normally stored within the fuselage of the aircraft and, when needed for operation, is lowered into the airstream whereby relative speed of the aircraft through the ambient air causes rotation of blades mounted to a hub assembly connected to a drive shaft whereby the drive shaft can drive either electrical generating gear or a source of hydraulic power. The ram air turbine conventionally includes governor mechanisms that vary the pitch of the blades in order to provide a constant speed for the drive shaft even though the relative speed of the aircraft to the ambient air may vary.

The prior art includes the Blackburn Patents Nos.

2,876,847 and 3,013,613 which illustrate typical constructions wherein a plurality of variable pitch blades have their blade roots rotatably mounted in a rotatable hub assembly. The pitch of the blades is controlled by governor means including a governor spring which urges the blades toward a fine or zero pitch limit position and weights associated with the blades responsive to rotation of the blades and hub assembly urge the blades to a coarse pitch limit position. Any tendency of the ram air turbine hub assembly to rotate at a speed in excess of the desired constant speed results in the governor weights exerting a force greater than that exerted by the governor spring to move the blades toward the coarse pitch limit position.

In the Blackburn Patent No. 3,013,613, not only are there governor counterweights, but there are also auxiliary counterweights.

It is a desired objective in a ram air turbine to minimize the weight because of the use on aircraft and, therefore, the weight of the counterweights should be minimized.

The prior art also includes a ram air turbine as shown in Patent No. 4,411,596, owned by the assignee of this application. This patent shows a ram air turbine having a governor system for controlling the pitch of the blades with a governor weight acting against governor spring means. The ram air turbine embodying the invention disclosed in this application distinguishes therefrgm in the use of blades constructed to either avoid the use of a governor counterweight or minimize the size thereof with resulting reduction in the weight of the ram air turbine.

The Reissner Patent No. 2,344,266 discloses a fixed bifurcated blade and discusses the advantages resulting from such a blade including a reduction in weight and the modification in the direction of action of the centrifugal twisting moment. Weight is added by counterweights used to act in opposition to the centrifugal twisting moment. This patent does not disclose the use of such a blade in a ram air turbine wherein the blades are of variable pitch and provide improved results in coaction with a governor control system.

Disclosure of the invention A primary feature of the invention is to provide a ram air turbine utilizing variable pitch bifurcated blades for constant speed drive of a drive shaft.

Each blade has a pair of airfoils positioned whereby, during operation, a centrifugal twisting moment on the blade acts in opposition to a governor spring means to urge the blade toward a coarse pitch limit and thus either avoid the need for a governor counterweight or minimize the size thereof with resulting reduction in the weight of the ram air turbine. An additional weight reduction is achieved by the use of the bifurcated blade which weighs less than a single blade having the same power extraction capability. This enables reduction in weight of bearings and other structures required for support of the blades.

The pair of airfoils of a bifurcated blade are displaced relative to the pitch adjustment axis of the blade, with the result that a bifurcated blade has a centrifugal twisting moment urging the blade toward coarse pitch. This moment acts in the same direction as the previously known governor weight and in opposition to the governor spring to provide the appropriate governing action. Because of this, the governor weight can either be omitted or the weight thereof minimized. The bifurcated blade inherently attempts to adjust its pitch towards a coarse pitch limit position and, in the case of a ram air turbine, this is in a direction to reduce the rpm of the ram air turbine whereby an inherently safe arrangement results since, should the blade become free from its control mechanism, it will move to a low-speed generally feathered position.

For a given diameter of a ram air turbine, the power extraction capability thereof can be defined by the chord of the airfoils of the blades for a given rotational speed and airspeed. A blade having a plurality of airfoils will have substantially the same aerodynamic performance as a single blade airfoil whose chord is the sum of the chords of the blade having plural airfoils. This will result in the airfoils being proportionally thinner for the same thickness/chord ratio and the net result will be a lighter blade. This results in lower centrifugal loading on the bearings which rotatably mount the blade at the root thereof leading to lower bearing requirements and potentially smaller bearings as well as a lighter hub assembly which mounts the blades.This reduction in the weight of the blades along with the reduced requirements for a governor counterweight leads to further lowering the bearing requirements and the required strength of the hub assembly.

Another feature of the invention is to provide a ram air turbine having a rotatable hub assembly connected to a drive shaft and mounting a plurality of blades which are rotatably adjustable relative to the hub assembly for varying the pitch thereof between fine and coarse limit positions and with the blades being bifurcated to provide spaced-apart airfoils positioned at either side of the pitch adjustment axis of the blade. The centrifugal twisting moments on the blade airfoils act in opposition to a governor spring to assist in achieving a balance and provide an inherently safe arrangement, since, in the event the governor control of the blade is lost, the blade will move toward a coarse pitch position resulting in low-speed operation of the ram air turbine.

Brief description of the drawings Figure 1 is a perspective view of a ram air turbine embodying the invention; Figure 2 is a plan section of a conventional ram air turbine blade and associated counterweight for purpose of illustrating the centrifugal twisting moments associated therewith; Figure 3 is a view similar to Fig. 2 illustrating the bifurcated blade and the centrifugal twisting moments associated therewith; and Figure 4 is a generally central vertical section of the ram air turbine as shown in Fig. 1.

Best mode for carrying out the invention The ram air turbine is indicated generally at 10 in Fig. 1. A frame 11, shown in broken line, has a support 12 which rotatably mounts the ram air turbine. The frame 11 is pivotally mounted to the aircraft whereby it may have a stowed position within the aircraft fuselage or be moved outwardly from the fuselage to an operative position, as shown in Fig. 1.

The ram air turbine has a rotatable hub assembly 15 with a cover 16 and which mounts a pair of bifurcated blades, indicated generally at 20 and 21.

Before further describing the invention, reference is made to Fig. 2 illustrating a conventional blade as used in the prior art ram air turbines previously referred to. This blade has an airfoil 25 carried by a hub assembly and rotatable relative thereto about an axis of rotation 26 whereby the pitch of the blade can vary between fine and coarse pitch limit positions. The direction of flight of an aircraft having the ram air turbine is shown by the arrow 27 and the hub assembly rotates in the direction of the arrow 28. Considering an element 29 of the airfoil, the centrifugal force indicated by the arrow 30 imparts a rotational moment in the clockwise direction about the axis 26, as indicated by the arrow 31.This action occurs in a quadrant identified as 32 and similarly an element 33 of the airfoil in a quadrant 34 has a centrifugal force, indicated by the arrow 35, acting in the clockwise direction, as indicated by the arrow 36. Considering the elements 29 and 33 of the airfoil, it will be evident that any part of the airfoil 25 in the quadrants 32 and 34 has a centrifugal twisting moment in the clockwise direction, as viewed in the drawing. The prior art also includes a counterweight, shown in broken line at 40, having its mass located in quadrants 41 and 42, whereby the centrifugal twisting moments on the counterweight are in a counterclockwise direction, as indicated by the arrow 43.

The centrifugal twisting moments operating in the clockwise direction, as indicated by the arrows 31 and 36, act to urge the airfoil toward a zero or fine pitch position with resulting increase in speed of rotation of the hub assembly 15. The centrifugal twisting moments associated with the counterweight 40 urge the airfoil 25 toward a coarse pitch limit position, with movement of the airfoil toward that position resulting in a lower speed of rotation of the hub assembly.

With the variable pitch airfoil 25 shown in Fig. 2, the centrifugal twisting moment about the pitchadjusting axis 26 of the airfoil causes the airfoil to move towards a zero or fine pitch position with the latter position having the plane of the airfoil parallel to the plane of rotation identified by the arrow 28. In such variable pitch device, particularly with regard to ram air turbines, this moment towards zero pitch is a potential hazard as any failure in the control mechanism allowing the airfoil to rotate on its axis would cause excessive overspeed, particularly at high forward airspeeds of the aircraft. With a mechanically governed variable pitch ram air turbine, the governor spring urges the airfoil to turn towards a fine pitch and the governor weights urge the airfoil towards a coarse pitch, with the forces equating at the design speed.Normally, the airfoil centrifugal moment is balanced by placing the counterweight 40 in a plane perpendicular to that of the airfoil, as illustrated in Fig. 2. The centrifugal twisting moment of this counter weight can be used to either balance or to provide a net centrifugal twisting moment of the airfoil and counter weight assembly towards the coarse pitch position, thus reducing the governor weight requirements and provide a degree of overspeed protection in the event of the control mechanism failure.

A bifurcated blade with airfoils occupying the quadrants normally occupied by the counterweight 40 would have a centrifugal twisting moment towards coarse pitch, thus resulting in a blade having an inherently fail-safe mode toward coarse pitch and assisting the function of the governor weight.

The bifurcated blades 20 and 21 used in the ram air turbine embodying the invention each have a pair of spaced-apart airfoils, with the blade 20 having the airfoils 50 and 51 and the blade 21 having the airfoils 52 and 53. The action of the bifurcated blade and the centrifugal twisting moments associated therewith are illustrated in Fig. 3 in a manner similar to the illustration of the prior art in Fig. 2.

With the direction of flight indicated by the arrow 55 and the direction of rotation of the hub assembly indicated by the arrow 56, it will be noted that the airfoils 50 and 51 are displaced relative to an axis of rotation 55 about which the bifurcated blade rotates in adjusting its pitch. Identifying the quadrants 32, 34, 41 and 42, similarly to the quadrants as identified in Fig. 2, it will be noted that the airfoils 50 and 51 are in the quadrants 41 and 42 whereby the centrifugal twisting moments act in a counterclockwise direction, as indicated by the arrow 57. The spaced-apart airfoils 50 and 51 thus are inherently urged, during operation, toward a coarse pitch low-speed position and provide forces similar to those provided by the counterweight 40 as used in the prior art and described in Fig. 2. Because of this, the prior art counterweight either may be omitted or reduced in size.A reduced size counterweight is shown in broken line at 60 in Fig.

2.

The structure of the ram air turbine utilizing the bifurcated blades is shown more particularly in Fig.

4. The support 12 rotatably mounts by means of bearings, one of which is shown at 65, a drive shaft 66 which connects to a constant speed device, such as electrical generating gear or a hydraulic pump mounted within the frame 11. The drive shaft 66 is integral with an end plate 70 of the hub assembly 15. The hub assembly has a generally cylindrical member 71 secured to the end plate 70 as by machine bolts 72 and mounts the cover 16.

The cylindrical member 71 of the hub assembly has a support member 75 with a series of radial arms having a peripheral flange connected to the cylindrical member 71 and at its center has a tubular mounting member 76 which movably receives a governor rod 77 movable lengthwise of the hub assembly. The governor rod 77 has a spring seat member 78 at one end thereof which seats an end of a governor spring 79 and with the other end of the governor spring engaging a washer 80 abutting the mounting member 76.

The bifurcated blades are diametrically opposite each other and are mounted similarly to the hub assembly, with the mounting of the bifurcated blade 20 being shown particularly in Fig. 4. The section of Fig. 4 has been taken through the ram air turbine along a line to illustrate the mounting of the bifurcated blade 20 and not pass through the mounting of the bifurcated blade 21.

The bifurcated blade 20 has a root 80 which, at its inner end, is secured to a plate 81. A bearing 82 rotatably mounts the blade root 80 and plate 81 relative to the cylindrical member 71 of the hub assembly, whereby the bifurcated blade can rotate about the axis of rotation 55 shown in Fig. 3.

A pin 85 extending downwardly from the plate 81 coacts with a yoke, indicated generally at 86, defined by a tubular member having a flange 87 and a disc 88 mounted thereon and spaced from the flange 87. The yoke is held in position by a threaded member 89 threaded on an end of the governor rod 77 and urging the flange 87 against a shoulder on the governor rod 77.

In operation of the ram air turbine, the governor spring 79 urges the governor rod 77 toward the left whereby, through the yoke 86 and the pin 85, the bifurcated blade 20 is urged in a direction to move the airfoils 50 and 51 toward a zero or fine pitch limit position. This force is opposed by the centrifugal twisting moments associated with the airfoils which, as described in connection with Fig. 3, are urging the bifurcated blade in a counterclockwise direction with a force acting in a direction to move the blade toward a coarse pitch limit position.

The design of the bifurcated blade and the governor spring 79 can be selected whereby an appropriate governing action can be obtained to vary the pitch of the bifurcated blade as required to maintain a constant speed of rotation of the hub assembly 15 and the drive shaft 66. This avoids the need for a counterweight having the same centrifugal twisting moment as the bifurcated blade; however, if it is found desirable to use such a counterweight, the size thereof can be minimized and a small counterweight 69, as previously described, can be used and is shown associated with the plate 81 in Fig. 4.

The reduction in the size of the counterweight or omission thereof results in a reduced weight for the ram air turbine which is important in connection with the use thereof with aircraft. As stated previously, the bifurcated blade with the spacedapart airfoils can, with less weight, have the same aerodynamic performance as a blade with a single airfoil whose chord is the sum of the chords of the bifurcated blade airfoils. This leads to further reduction in the weight of the ram air turbine because of lower centrifugal loading on the airfoil bearings 82, with resulting lower requirements and less weight for the bearing 65 and other bearing not shown. Over-all, the entire system can be of lighter weight.

The principles of the invention have been disclosed as utilized in a ram air turbine. It will be recognized that the invention would be equally applicable to other power extraction devices having variable pitch blades for driving a device at a constant output speed, such as a windmill.

The invention is also applicable to a device having more than two sets of blades, i.e., three or four or more.

Further, the invention is applicable to a plurality of airfoils rotating about the same blade rotational axis in excess of two. Three, four, or more airfoils would give varying degrees of weight saving and reduction in counterweight size.

Claims

1. In a ram air turbine having a rotatable hub assembly connected to a drive shaft for driving a constant speed device, a plurality of blades extending from the hub assembly and rotatable about axes of rotation whereby the pitch of the blades may be varied to maintain a constant speed of rotation of the hub assembly, and governor means including a yieldable member for urging the blades toward a fine pitch position characterized by said blades each having a pair of spaced-apart airfoils offset from the blade axis of rotation.

2. In a ram air turbine as defined in claim 1 wherein said airfoils have centrifugal twisting moments acting in opposition to said yieldable member 4 of said governor means.

3. In a ram air turbine as defined in claim 2 wherein said yieldable member is a spring.

4. A ram air turbine having a rotatable hub assembly comprising, a bifurcated blade having a pair of spaced-apart airfoils, means rotatably mounting said blade in said hub assembly for rotation about a pitch axis to assume a position between coarse and fine 6 pitch limits, means in the hub assembly urging said blade toward said fine pitch limit, and said pair of spaced-apart airfoils positioned whereby a centrifugal twisting moment on said blade resulting from rotation of the hub assembly acts in opposition to said urging means to urge the blade toward the coarse pitch limit.

5. A ram air turbine as defined in claim 4 wherein said airfoils are offset from said pitch axis whereby centrifugal forces occurring during rotation of the hub assembly will result in centrifugal twisting moments acting to urge the blade toward the coarse pitch limit.

6. A ram air turbine as defined in claim 4 including a minimal size counterweight secured to said blade and positioned to provide a centrifugal twisting moment which acts in an aiding direction to the centrifugal twisting moment on the blade.

7. A ram air turbine having a support, a hub assembly rotatably mounted on said support, a drive shaft connected to said hub assembly for driving a device by rotation of the hub assembly, a pair of blades extending radially from said hub assembly and each having a blade root, means associated with each blade root mounting the blades for rotation relative to the hub assembly whereby the blades are of variable pitch, a governor spring, means interconnecting the governor spring and the blades whereby the force of the governor spring urges the blades toward a fine pitch position, and said blades being of a construction whereby the centrifugal twisting moments of said blades when the hub assembly is rotated act against the governor spring to urge the blades toward a coarse pitch position.

8. A ram air turbine having a support, a hub assembly rotatably mounted on said support, a drive shaft connected to said hub assembly for driving a device by rotation of the hub assembly, a pair of blades extending radially from said hub assembly and each having a blade root, means associated with each blade root mounting the blades for rotation relative to the hub assembly to enable varying the pitch of the blades, a governor spring, means interconnecting the governor spring and the blades whereby the force of the governor spring urges the blades toward a fine pitch position, and said blades each having a pair of spaced-apart airfoils offset from the axis of blade rotation whereby the centrifugal twisting moments of said blades when the hub assembly is rotated act against the governor spring to urge the blades toward a coarse pitch position.

9. In a power extraction device having a rota table hub assembly connected to a drive shaft, a plurality of blades extending from the hub assembly and rotatable about axes of rotation whereby the pitch of the blades may be varied to maintain a substantially constant speed of rotation of the hub assembly, and governor means including a yieldable member for urging the blades toward a fine pitch position characterized by said blades each having a pair of spaced-apart airfoils offset from the blade axis of rotation.

10. A power extraction device having a rotatable hub assembly comprising, a bifurcated blade having a pair of spaced-apart airfoils, means rotatably mounting said blade in said hub assembly for rotation about a pitch axis to assume a position between coarse and fine pitch limits, means in the hub assembly urging said blade toward said fine pitch limit, and said pair of spaced-apart airfoils positioned whereby a centrifugal twisting moment on said blade resulting from rotation of the hub assembly acts in opposition to said urging means to urge the blade toward the coarse pitch limit.