WO2002016900A2

WO2002016900A2 - Flutter exciter

Info

Publication number: WO2002016900A2
Application number: PCT/IB2001/001497
Authority: WO
Inventors: Louwrens Hermias Van Zyl
Original assignee: Csir
Priority date: 2000-08-21
Filing date: 2001-08-20
Publication date: 2002-02-28
Also published as: WO2002016900A3; AU2001277652A1

Abstract

A flutter exciter includes first and second impellers rotatably connected together by coupling means and rotatable about a common axis in opposite directions, each impeller having at least one vane so that fluid flowing over the vanes causes the impellers to rotate and each to generate a rotating force, the resultant of the rotating forces being an oscillating substantially rectilinear excitation force.

Description

FLUTTER EXCITER

FIELD OF THE INVENTION

THIS invention relates to a flutter exciter.

BACKGROUND TO THE INVENTION

A flutter exciter induces vibration in an aircraft either during actual flight- testing or during wind tunnel model testing. Typically the flutter exciter is used to excite the vibration modes of an aircraft during testing as part of the aircraft certification procedure.

Generally the flutter exciter must generate a force on the aircraft that is controllable and measurable; the frequency of the force must cover the frequency range of interest .for a particular aircraft; the amplitude of the force must be adequate over the frequency range of interest; and changes to structural and aerodynamic characteristics of the aircraft with the flutter exciter connected thereto, must be small.

Various flutter excitation techniques are known, for instance pilot input via the normal aircraft . controls ("stick rap"); rotating mass exciters - unbalanced rotating masses spun by electric motors; explosive devices ("Bonkers"); a rotating vane; electrical inputs to hydraulic control systems to oscillate normal control surfaces; and flutter vanes - fixed vanes with electrically driven, rotating slotted cylinders mounted directly behind the fixed vanes. US 4,809,553 describes the rotating slotted cylinders and US 3,552,192 describes the rotating vane. The rotating vane is mounted adjacent to the surface of the outer tip of an aircraft wing or horizontal stabilizer at right angles to the direction of flight. The vane is rotated at a constant rate about its mid-chord axis.

The fixed vane to which the rotating slotted cylinders are attached constitutes a large non-representative aerodynamic surface. In addition, the fixed vane and the rotating cylinder comprise a non-representative mass added to the aircraft. The rotating vane of US 3,552,192 requires a large amount of power because the chordwise center of pressure of the rotating vane changes with the angular position of the vane during each rotation.

SUMMARY OF THE INVENTION

According to the invention a flutter exciter includes first and second impellers rotatably connected together by coupling means and rotatable about a common axis in opposite directions, each impeller having at least one vane so that fluid flowing over the vanes causes the impellers to rotate and each to generate a rotating force, the resultant of the rotating forces being an oscillating substantially rectilinear excitation force.

Each impeller preferably includes at least two aerodynamically unbalanced vanes.

The first impeller is preferably connected to a first shaft and the second impeller is preferably connected to a second shaft and the first and second shafts are preferably connected together by the coupling means.

The first and second shafts may be co-axial.

The coupling means may be a contra-rotation device. The flutter exciter may include pitch control means for controlling the pitch of the vanes of at least one of the impellers.

The flutter exciter may include drive means for driving the impellers. The drive means may be a motor. The drive means is preferably coupled to either the first or the second shaft.

The flutter exciter may include a cylindrical housing within which the flutter exciter is mounted and from which the vanes project. A nose cone for the cylindrical housing may be provided with the impellers being interposed between the nose cone and the cylindrical housing.

The flutter exciter may include measuring means for measuring the excitation force generated.

According to another aspect of the invention a method of generating an excitation force for flutter testing includes the steps of generating first and second rotating forces which rotate at the same speed in opposite directions and maintaining a phase relationship between the two rotating forces so that an excitation force resulting from the two rotating forces is oscillating and substantially rectilinear.

The rotating forces may be generated by rotating two impellers in opposite directions at the same rotational speeds, each impeller having at least one vane and preferably two aerodynamically unbalanced vanes.

The method may include the step of varying the frequency of the excitation force. The frequency of the excitation force may be varied by varying the rotational speed of the impellers.

The rotational speed of the impellers may be varied by varying the pitch of the vanes of at least one of the impellers. The rotational speed of the impellers may also be varied by driving the impellers. The torque required to drive the impellers may be varied by varying the pitch of the vanes of at least one of the impellers.

The method may include the step of measuring the resultant excitation force generated.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a partial diagrammatic perspective view of a flutter exciter according to the invention with its missile body removed;

Figure 2 is a cross-sectional side view on line ll-ll of figure 3 of the flutter exciter;

Figure 3 is a cross-sectional plan view on line Ill-Ill of figure 2;

Figure 4 is a cross-sectional view on line IV-IV of figure 2;

Figure 5 is a cross-sectional view on line V-V of figure 2;

Figure 6 is a cross-sectional view on line VI-VI of figure 2;

Figure 7 is a cross-sectional view on line VII-VII of figure 2;

Figure 8 is a cross-sectional view on line VIIl-VIII of figure 2;

Figure 9 is a cross-sectional view on line IX-IX of figure 2;

Figure 10 shows the forces generated by the flutter exciter, the force in the top row being the resultant force generated by the front impeller, the force in the middle row being the resultant force generated by the rear impeller, and that in the bottom row being the resultant force of the two resultant forces generated by the front and rear impellers; and

Figure 11 shows the actual forces and the resultant forces generated by the impellers.

DETAILED DESCRIPTION OF THE DRAWINGS

A flutter exciter 10 is designed to be fitted within a cylindrical housing in the form of missile body 12. The missile body 12 can then be connected to an aircraft (not shown), particularly to a wing of the aircraft. A nose cone 13 for the missile body is provided.

The flutter exciter 10 includes a support in the form of a floor plate 14 made from six mm thick aluminium. The flutter exciter 10 further includes a front impeller 16 rotatable in a first direction indicated by the arrow A. The front impeller 16 comprises a front impeller hub 18 made from aluminium and mounted on a twelve mm diameter stainless steel front drive shaft 20 as can be seen more clearly in figure 2. The front drive shaft is rotatably mounted on bearings 22 and 24. The front impeller 16 includes a first front vane 26 made from aluminium or steel, and a second front vane 28 also made from aluminium or steel. As can be seen, the vanes are bolted to the hub 18. The first front vane 26 is positioned for maximum incidence at an angle of between 10° and 20°, in this embodiment 15°, with respect to the axis of rotation of the front drive shaft 20. The second front vane 28 is positioned for zero or minimum incidence at an angle of between -5° and 5°, in this embodiment 0°, with respect to the axis of rotation of the front drive shaft 20. A non-zero minimum incidence could be used to increase the generated force or to increase the maximum rotational speed, as described in more detail later in the specification. The orientation of the vanes can be altered using the bolts which retain the vanes on the hub. It should be understood that the angle of incidence of the vanes of the front impeller 16 could differ from the angles indicated in this embodiment. The flutter exciter 10 also includes a rear impeller 30 rotatable in a second direction, indicated by the arrow B, opposite to the first direction of rotation. The rear impeller 30 comprises a rear impeller hub 32 made from aluminium and mounted on a twenty mm outer diameter rear drive shaft 34. Figure 2 shows how the rear drive shaft 34 is rotatably mounted on bearings 36 and 38. The rear drive shaft 34 is hollow and is co-axial with the front drive shaft 20.

The rear impeller 30 includes a first rear vane 40 made from aluminium or steel, and a second rear vane 42 also made from aluminium or steel. The first rear vane 40 is positioned at an angle of maximum incidence, namely 15°, with respect to the axis of rotation of the rear drive shaft 34. However, the first rear vane 40 is aligned so that the angle of incidence is opposite to that of the angle of incidence of the first front vane 26. The second rear vane 42 is positioned at an angle of zero incidence, namely 0°, with respect to the axis of rotation of the rear drive shaft 34. The vanes of the rear impeller 30 are movably connected to the rear impeller hub 32 so that the pitch of the vanes, and hence the effective angle of incidence, can be altered. This process is described in more detail later in this specification. It should be understood that the angle of incidence of the vanes of the rear impeller 30 could differ from the angles indicated in this embodiment. As described above, the first rear vane could be positioned for maximum incidence at an angle of between 10° and 20°, and the second rear vane could be positioned for zero or minimum incidence at an angle of between -5° and 5°.

The alignment of the first vanes 26 and 40 on the front and rear impellers 16 and 30 is such that when air is forced over these vanes, the impellers rotate in opposite directions. In addition, because the front and rear impellers are aerodynamically unbalanced (this is described in more detail below), the contra-rotation generates an oscillating rectilinear force. The flutter exciter 10 includes a contra-rotation device, indicated generally by the reference numeral 44, connected to the front and rear drive shafts. The contra-rotation device 44 can be seen more clearly in figures 1 , 2 and 8. The contra-rotation device 44 ensures that both drive shafts rotate at the same speed. The front drive shaft 20 is connected to a driving gear 46. The rear drive shaft 34 is connected to a driven gear 50. The contra- rotation device 44 includes two transfer cogs 54 and 56 which are engaged with the driving and driven gears and ensure that these gears rotate at the same speed.

The second vanes 28 and 42 of each impeller are set at zero incidence, namely 0°, to the axis of rotation, and the first vanes 26 and 40 at an angle of incidence providing maximum force, namely 15° to the axis of rotation. The force on the first vanes 26 and 40 provides both a resultant excitation force that is the ultimate objective of the flutter exciter, as well as a torque that tends to turn the impellers. The impellers do not generate thrust. The force generated by each impeller rotates rather than oscillating back and forth as can be seen from figure 10. However, the resultant of the two rotating forces is a radially extending oscillating rectilinear excitation force. This arrangement provides maximum rotational speed range without stalling a vane, and a fairly constant force amplitude.

Referring now to figure 11 , the diagrams in the top row relate to the front impeller, and those in the bottom row to the rear impeller. The diagrams in the first column relate to the impellers when stationary, in the second column to the impellers when rotating freely with balanced torques and in the third column to the impellers driven at their maximum speed without stalling a blade. It will be appreciated that in figure 11 the impellers are shown at the 0° degree position.

The forces on the vanes are indicated by reference numeral 94, the resultant force by reference numeral 96, the resultant moment by reference numeral 98 and the direction of rotation by reference numeral 100. When the system is stationary with the vanes set as in figure 1 , the torque on the first and second impellers will be in opposite directions and the impellers will tend to rotate.

If the two impellers are allowed to rotate freely, the effective incidence (and hence the force) on the first vanes 26 and 40 decreases, whereas the effective incidence (and hence the force) on the second vanes 28 and 42 increases. The effect is that the resultant excitation force remains constant, but the overall torque reduces. Each impeller will rotate up to a speed where the positive torque on one of the vanes balances the negative torque on the other of the vanes. For controlled rotation below this speed, braking is required and for controlled rotation above this speed, additional power is required.

Braking and power are provided by means of an electrical main drive motor 58 shown more clearly in figures 2 and 9. Power to the electrical motor is via a 110 / 115 VAC or a 28 VDC power supply of the aircraft (not shown). As shown in figure 8, the main drive motor 58 is attached to the floor plate 14. The main drive motor is connected via a flexible coupling 60 to the front drive shaft 20 and, by controlling the speed of rotation of the front drive shaft, effectively provides power or a braking force to both impellers via the contra-rotation device 44.

Typically an electrical motor which meets the space constraints does not have sufficient torque to cover the full operational range of impeller speeds. In order to meet the torque requirement, a pitch control mechanism is provided to change the pitch of the vanes of one or both impellers. The pitch control mechanism a pitch control motor 64 which drives a threaded shaft 70 of a ball screw arrangement 62 via a flexible coupling 66. The threaded shaft 70 is supported by a bearing 72. The pitch control motor 64 is driven in parallel with the main drive motor 58. The pitch control mechanism could be provided by other means which are known in the art. The pitch control mechanism includes a linear bearing 74 which is bolted to the floor plate 14. The linear bearing includes a body 76 which can be moved laterally along the axis of the rotating shafts by the ball screw arrangement 62 upon rotation of a threaded shaft 70 of the ball screw arrangement 62. Four pitch control rods 78 extend from the movable body 76 towards the rear impeller 30. The pitch control rods 78 pass through holes in a balance block 80 which is bolted to the floor plate 14. The body 76 moves the pitch control rods 78.

As can be seen in figure 2, the pitch control rods are connected to a body which moves laterally under the influence of the pitch control rods. The body includes a flanged inner bearing formation 82 and an outer bearing formation 84. The pitch control rods are connected to the flanged inner bearing formation. As can be seen in figure 5, the outer bearing formation 84 is connected by means of two universal joints 86 to the ends of each of two pitch control arms 88. The other end of each pitch control arm includes a collar 90 which is connected to a vane of the rear impeller. The outer bearing formation is separated from the inner bearing formation by means of a bearing 92. Accordingly, although the bearing as a whole moves laterally under the influence of the pitch control rods, the outer bearing formation can rotate relative to the inner bearing formation. As the outer bearing formation 84 moves laterally, the pitch control arms 88 are caused to pivot via the action of the universal joints 86. This pivoting action of the pitch control arms translates into a rotation in the vanes of the rear impeller.

In order to minimize the torque required to keep the system stationary in the position shown in figure 1 , the pitch of the vanes of the rear impeller would be changed so that the first rear vane 40 is at zero incidence and the second rear vane 42 is at a maximum incidence producing a force to the left when viewed from the rear. The torque on the two impellers would then be in the same direction and rotation would be restrained by the contra- rotation device. It should be understood that in this embodiment the pitch control mechanism only alters the pitch of the vanes on the rear impeller arrangement. In another embodiment, the adjustment mechanism also alters the pitch of the vanes on the front impeller, and in a further embodiment, any combination of the vanes on the front and rear impeller arrangements can be altered.

Although not shown in the drawings, the flutter exciter according to the embodiment includes two accelerometers for measuring horizontal and vertical acceleration, two strain gauge bridges for measuring horizontal and vertical force, instrumentation for measuring rotational speed of the impellers, pitch setting of the vanes and current consumption of the main drive and pitch control motors. The strain gauge bridges are located on the balance block 80. Data will be transmitted but the flutter exciter may include a data recorder.

It will be appreciated that because the flutter exciter has a missile body 12 and a nose cone 13, the changes to the structural and aerodynamic characteristics of the aircraft are very small because the exciter may replace part of a missile or other external store that forms part of the aircraft configuration to be tested.

It will be appreciated that many modifications or variations of the invention are possible without departing from the spirit or scope of the invention.

Claims

1. A flutter exciter including first and second impellers rotatably connected together by coupling means and rotatable about a common axis in opposite directions, each impeller having at least one vane so that fluid flowing over the vanes causes the impellers to rotate and each to generate a rotating force, the resultant of the rotating forces being an oscillating substantially rectilinear excitation force.

2. The flutter exciter of claim 1 wherein each impeller includes at least two aerodynamically unbalanced vanes.

3. The flutter exciter of claim 1 or claim 2 wherein the first impeller is connected to a first shaft and the second impeller is connected to a second shaft and the first and second shafts are connected together by the coupling means.

4. The flutter exciter of claim 3 wherein the first and second shafts are co-axial.

5. The flutter exciter of any one of claims 1 to 4 wherein the coupling means is a contra-rotation device.

6. The flutter exciter of any one of the above claims including pitch control means for controlling the pitch of the vanes of at least one of the impellers.

7. The flutter exciter of any one of the above claims including drive means for driving the impellers.

8. The flutter exciter of claim 7 wherein the drive means is a motor.

9. The flutter exciter of claim 7, insofar as it is dependent on claim 3, wherein the drive means is coupled to either the first or the second shaft.

10. The flutter exciter of any one of the above claims including a cylindrical housing within which the flutter exciter is mounted and from which the vanes project.

11. The flutter exciter of claim 10 including a nose cone for the cylindrical housing with the impellers being interposed between the nose cone and the cylindrical housing.

12. The flutter exciter of any one of the above claims including measuring means for measuring the excitation force generated.

13. A flutter exciter substantially as herein described and illustrated with reference to the accompanying drawings.

14. A method of generating an excitation force for flutter testing including the steps of generating first and second rotating forces, which rotate at the same speed in opposite directions and maintaining a phase relationship between the two rotating forces so that an excitation force resulting from the two rotating forces is oscillating and substantially rectilinear.

15. The method of claim 14 wherein the rotating forces are generated by rotating two impellers in opposite directions at the same rotational speeds, each impeller having at least one vane.

16. The method of claim 15 including the step of varying the frequency of the excitation force.

17. The method of claim 16 including the step of varying the frequency of the excitation force by varying the rotational speed of the impellers.

18. The method of claim 17 including the step of varying the rotational speed of the impellers by varying the pitch of the vane of at least one of the impellers.

19. The method of claim 18 including the step of varying the rotational speed of the impellers by driving the impellers.

20. The method of claim 18 including the step of varying the torque required to drive the impellers by varying the pitch of the vane of at least one of the impellers.

21. The method of any one of claims 14 to 20 including the step of measuring the resultant excitation force generated.

22. A method of generating an excitation force substantially as herein described and illustrated with reference to the accompanying drawings.