Propeller blade arrangement

Abstract

A propeller blade arrangement comprises a propeller blade attached to a hub via a retaining bearing and rotatable with the hub, the blade being rotatable about a centerline of the blade, the retaining bearing being configured to tilt the blade such that the centerline of the blade is tilted relative to the hub.

Description

Technical Field

[0001] The present disclosure relates to propeller design for an aircraft and optimizing blade centrifugal restoring moments. Background Art

[0002] The propeller has a number of blades mounted around a hub. The hub and blade assembly rotate around the hub axis. In addition, each blade is mounted to the hub by a retaining bearing that allows the blade itself to rotate around an axis of rotation perpendicular to the hub axis. This allows the pitch of the blades to be varied to suit different flight conditions and output power requirements.

[0003] Depending on the flight conditions, the propellers rotate at different speeds and in forward or reverse operating mode. During the climb, greater power and forward thrust are required. During the cruise phase, the forward thrust moves the aircraft forward. During landing, the propellers operate in reverse thrust mode to slow the aircraft down.

[0004] The engine torque applied to the blades via the propeller hub causes the blades to rotate and generate thrust. The airflow circulating around the different blade airfoil stations generates lift and drag at each station. The sum of the lift and drag generated along the blades produces shear loads and high bending moments at the root of the blade where the retaining bearing is located. The retaining bearing must be designed to have sufficient size and strength to withstand such bending forces. Modern propeller blades are designed to be as light as possible while maintaining strength. Composite materials allow for the manufacture of strong and lightweight blades, but the retaining bearing still needs to be designed to withstand the bending forces that may act on the root of the blade, as well as smaller G forces and shear forces. This places a limit on the extent to which the size and weight of the entire propeller assembly can be reduced.

[0005] One way to reduce the effect of aerodynamic forces on the blades is to balance the aerodynamic forces by creating an opposing centrifugal bending moment - the restoring bending moment - from the G forces created by the rotation of the propeller. The restoring bending moment created in the same plane as the aerodynamic bending moment but in a substantially opposite direction means that when these moments are added together, a reduced bending moment will result on the blades.

[0006] The way to achieve this centrifugal restoring bending moment is by designing the stack of blades - that is, the structure of their multiple airfoil sections - so that the blade's center of gravity is offset relative to its centerline and therefore its pitch changes the axis of rotation. The direction and magnitude of the offset relative to the centerline (CG offset) are chosen to optimize the bending moment for given operating conditions. The restoring bending moment varies with the rotation rate, the weight of the blade, and the CG offset.

[0007] The restoring bending moment is usually equal to:

[0008] G 载荷 x CG offset

[0009] in:

[0010] G 载荷 = W x (RPM x π x ) 2 x R

[0011] Where W is the weight of the blade, RPM is the rotation rate of the blade, and R is the radial distance between the center of gravity of the blade and the centerline (axis of rotation) of the propeller.

[0012] Normally, the blade will be designed with a center of gravity to produce a restoring moment that is optimal in magnitude and opposite to the aerodynamic bending moment in initial climb conditions - the centrifugal bending moment. These are the conditions where power and airflow forces are typically at a maximum. Stacking shifts the center of gravity in such conditions so that the total bending moment is very significantly reduced.

[0013] While this works very well in operating conditions where the blades are stacked to minimize the overall bending moment, the design can be detrimental to the blades in other flight conditions.

[0014] In particular, when the propeller is operated in reverse thrust mode, for example during landing, the pitch angle of the blades is different and the aerodynamic forces act on the other side of the blades, and the offset center of gravity causes a different centrifugal bending moment. The total bending moment resulting from the addition of the aerodynamic bending moment to the centrifugal bending moment will no longer be minimized in reverse thrust mode - instead, the total bending moment may actually be doubled, because the moments act in essentially the same direction, and the restoring bending moment therefore has an additive effect on the aerodynamic bending moment rather than a subtractive effect.

[0015] It is desirable to provide a propeller design that optimizes the restoring torque under different operating conditions. Summary of the invention

[0016] Therefore, a propeller blade arrangement is provided, comprising a propeller blade attached to a hub via a retaining bearing and rotatable with the hub, the blade being rotatable about a centerline of the blade defined by an axis of rotation of its retaining bearing, the retaining bearing being configured to tilt the blade so that the centerline of the blade is tilted relative to the hub (i.e., the blade centerline does not intersect and / or is not orthogonal to the hub centerline).

[0017] The blade may be configured such that the centre of gravity of the blade is offset relative to its centre line. Alternatively, the blade may be configured such that the centre of gravity of the blade is aligned with the centre line.

[0018] A propeller is also provided, the propeller comprising a hub and a plurality of blades, each of the plurality of blades being mounted around the hub by a respective retaining bearing, whereby the retaining bearing is configured to tilt the blades so that the centerline of the blades is tilted relative to the hub. The centerline of the blades should not intersect and / or be perpendicular to the centerline of the hub.

[0019] A method for operating a propeller is also provided, the method comprising: rotating the propeller in a first direction and tilting one or more blades of the propeller relative to a hub by a certain amount to position the center of gravity of the blade at a certain position, thereby generating a centrifugal bending moment to offset the aerodynamic bending moment acting on the blade. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1A and Figure 1B The forces acting on the blades in forward thrust mode as in a known arrangement are shown.

[0021] Figure 2A and Figure 2B The forces acting on the blades in reverse thrust mode are shown as in a known arrangement.

[0022] Figure 3A and Figure 3B The forces acting on the blades in reverse thrust mode are shown for an arrangement according to the present disclosure.

[0023] Figure 4 is a schematic diagram of a blade arrangement of the present disclosure. DETAILED DESCRIPTION

[0024] The described embodiments are by way of example only. The scope of the present disclosure is limited only by the claims that follow.

[0025] First reference Figure 1A and Figure 1B , the existing arrangement for generating the blade centrifugal restoring moment will be briefly explained.

[0026] Blade 1 is shown with a centre line C about which the blade rotates. In the example shown, it has been calculated that in forward thrust operation in 'cruise mode', the engine torque load E and the external thrust load A will combine to produce a resulting aerodynamic bending moment R on the blade hold which is oriented in the fourth quadrant. In order to produce a restoring centrifugal bending moment which will counteract the load on the blade, the blade will be constructed or stacked so that its centre of gravity CG is offset relative to its centre line C. The azimuthal position of the centre of gravity will be determined so that Figure 1BAs shown, the centrifugal bending moment will be in the same plane as the aerodynamic bending moment R (quadrant 2) or as close as possible to the aerodynamic bending moment, but in the opposite direction to the aerodynamic bending moment. Therefore, under these flight conditions, the sum of these moments will produce an optimally small total bending moment. CL indicates the centerline of the hub.

[0027] However, Figure 2A and Figure 2B The force on this blade 1 when the pitch of the blade is rotated to reverse the thrust mode angle is shown. The direction of the thrust A acting on the blade is now the same as Figure 1A and Figure 1B The directions shown are opposite, while the direction of the torque load remains the same. The resulting bending moment R in the blade holding direction is therefore in the first quadrant. The position of the center of gravity, which rotates with the blade, now produces a centrifugal bending moment that is no longer in the opposite direction to the aerodynamic bending moment, but essentially in the same direction (also in the first quadrant). Therefore, when added, these moments produce a larger total bending moment - that is, a total bending moment that is actually greater than the aerodynamic bending moment itself. As mentioned above, the holding bearing needs to be designed to withstand this bending moment.

[0028] In the arrangement of the present disclosure, the situation under forward thrust is similar to Figure 1A and Figure 1B Same as shown.

[0029] According to the present disclosure, the blade 10 is attached to the propeller hub via a retaining bearing 20 that allows the blade to rotate about the bearing centerline to adjust and change the blade pitch. The hub arm incorporating the retaining bearing 20 is designed so that the blade can be tilted in the hub 30. By tilting the blade retaining bearing 20, the blade centerline CL1 is tilted so that the position of the blade center of gravity CG1 is offset compared to an axis passing through the center of the retaining bearing and intersecting / perpendicular to the hub centerline to achieve a restoring moment, which is subtracted from the aerodynamic bending moment under flight conditions such as takeoff, as detailed in the background description. In reverse thrust mode, as Figure 3A As shown, the offset in the position of CG1 caused by the pitch does not rotate with the blade and remains constant in magnitude and orientation (still in the second quadrant) compared to the hub reference axis. Therefore, the resulting restoring bending moment does not add significantly to the aerodynamic bending moment.

[0030] The force acting on the blade 10 in the reverse thrust mode will be the same as that relative to Figure 2A The forces described are the same, but since the CG1 offset effect from the tilt does not rotate with the blade and maintains at least part of the relative position of the center of gravity CG1, the centrifugal bending moment is Figure 2B The aerodynamic bending moments shown in will be at or near a 90° angle and will not increase the aerodynamic bending moments too much or may even cancel out some of the aerodynamic bending moments.

[0031] The sum of the aerodynamic bending moment and the centrifugal bending moment will produce a total bending moment that is almost equal to or even smaller than the aerodynamic bending moment.

[0032] The blades and hub can be designed by varying the position of CG1 and / or the tilt angle α to optimize the total bending moment under different flight conditions.

[0033] Figure 4 It is schematically shown how the blade can be tilted at the bearing relative to the hub, so that the centre line CL1 of the blade is tilted relative to the centre line CL defined by the hub 30 .

[0034] Since the arrangement avoids loads much greater than the aerodynamic bending moment, the worst case bearing design can be smaller and lighter. This also provides the ability to blend reverse thrust at higher airspeeds without overstressing the retaining bearings.

[0035] It is contemplated that the concepts described and claimed herein may be applied to any propeller or fan, not just aircraft propellers.

Claims

1. A propeller blade arrangement, comprising a propeller blade attached to a hub via a retaining bearing and rotatable with the hub, the blade being rotatable about a centerline of the blade defined by an axis of rotation of the retaining bearing, the retaining bearing being configured to tilt the blade relative to the hub such that the centerline of the blade is tilted relative to the hub such that the centerline of the blade does not intersect and is not orthogonal to the centerline of the hub, wherein in the forward thrust operation mode, the blades are arranged so that the center of gravity of the blades is offset so that a restoring bending moment is generated, thereby reducing the total bending moment to offset the aerodynamic bending moment acting on the blades; and Wherein in the reverse thrust operating mode, the tilt angle is selected so that the centerline of the blade is tilted so that the position of the center of gravity of the blade relative to the hub is substantially the same as in the forward thrust operating mode.

2. A propeller blade arrangement as claimed in claim 1, wherein the blade is configured such that the centre of gravity of the blade is offset relative to the centre line of the blade.

3. The propeller blade arrangement of claim 1, wherein the blade is configured such that the center of gravity of the blade is aligned with the centerline.

4. A propeller blade arrangement, comprising a hub and a plurality of blades, each of the plurality of blades being mounted about the hub by a respective retaining bearing, whereby the retaining bearing is configured to tilt the blades such that a centerline of the plurality of blades is tilted relative to the hub such that the centerline of the plurality of blades does not intersect and is not orthogonal to a centerline of the hub, wherein in the forward thrust operation mode, the blades are arranged so that the center of gravity of the blades is offset so that a restoring bending moment is generated, thereby reducing the total bending moment to offset the aerodynamic bending moment acting on the blades; and Wherein in the reverse thrust operation mode, the tilt angle is selected to tilt the center lines of the plurality of blades so that the position of the center of gravity of the blades relative to the hub is substantially the same as in the forward thrust operation mode.

5. A method of operating a propeller, the method comprising: rotating the propeller in a first direction and tilting one or more blades of the propeller relative to a hub so that a centerline of the one or more blades is tilted relative to the hub so that the centerline of the one or more blades does not intersect and is not orthogonal to a centerline of the hub to position the center of gravity of the blades so that a centrifugal bending moment is generated to counteract an aerodynamic bending moment acting on the blades, wherein in the forward thrust operation mode, the blade is arranged so that the center of gravity of the blade is offset so that a restoring bending moment is generated, thereby reducing the total bending moment to offset the aerodynamic bending moment acting on the blade; and Wherein in the reverse thrust operating mode, the tilt angle is selected to tilt the centerline of the one or more blades so that the position of the center of gravity of the blade relative to the hub is substantially the same as in the forward thrust operating mode.

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