WO2023063816A1 - Method for determining a position of the center of gravity (cog) of an elongated body - Google Patents
Method for determining a position of the center of gravity (cog) of an elongated body Download PDFInfo
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- WO2023063816A1 WO2023063816A1 PCT/NL2022/050559 NL2022050559W WO2023063816A1 WO 2023063816 A1 WO2023063816 A1 WO 2023063816A1 NL 2022050559 W NL2022050559 W NL 2022050559W WO 2023063816 A1 WO2023063816 A1 WO 2023063816A1
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- Prior art keywords
- elongated body
- rotation axis
- plane
- offset
- geometrical center
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- 238000000034 method Methods 0.000 title claims abstract description 59
- 230000005484 gravity Effects 0.000 title claims abstract description 16
- 239000000725 suspension Substances 0.000 claims description 7
- 238000003780 insertion Methods 0.000 claims description 6
- 230000037431 insertion Effects 0.000 claims description 6
- 230000000284 resting effect Effects 0.000 claims description 6
- 239000003733 fiber-reinforced composite Substances 0.000 claims description 5
- 230000001154 acute effect Effects 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 3
- 239000002131 composite material Substances 0.000 description 2
- 238000005303 weighing Methods 0.000 description 2
- 241000220317 Rosa Species 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000003908 quality control method Methods 0.000 description 1
- 239000012783 reinforcing fiber Substances 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 229920001187 thermosetting polymer Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
- 239000002023 wood Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M1/00—Testing static or dynamic balance of machines or structures
- G01M1/12—Static balancing; Determining position of centre of gravity
- G01M1/122—Determining position of centre of gravity
Definitions
- the invention relates to a method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body.
- the invention in particular relates to a method for determining an offset of the center of gravity (CoG) of an elongated body made of fiber reinforced composites with respect to a geometrical center in a plane of the elongated body.
- the invention further relates to a method for determining an offset of the center of gravity (CoG) of a rotor or propeller blade with respect to a geometrical center in a plane of the rotor of propeller blade.
- An exemplary body involves a rotor blade or propeller blade, or a rotor comprising at least two of such rotor blades or propeller blades.
- a known method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body is disclosed in ‘A Body for Accurately Determining the Mass and
- the traditional methods usually include two parts with a certain location, i.e., a fixed platform and a mobile platform.
- a certain location i.e., a fixed platform and a mobile platform.
- One of the problems of the known method is that its accuracy is not sufficient, or it requires a complicated set-up.
- the invention in particular aims at providing a method for determining an offset of the CoG of an elongated body made of fiber reinforced composites with respect to a geometrical center in a plane of the elongated body.
- the invention further aims at providing a method for determining an offset of the CoG of a rotor or propeller blade with respect to a geometrical center in a plane of a rotor blade, or propeller blade, or a rotor that comprises at least two of such rotor blades or propeller blades.
- the method is used for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body, and comprises the following steps of: providing a first rotation axis and a second rotation axis at a distance to the first rotation axis and both perpendicular to the plane and at offset positions to the geometrical center of the elongated body; balancing the elongated body on the first rotation axis into an equilibrated first natural balanced position and subsequently balancing the elongated body on the second rotation axis into an equilibrated second natural balanced position; measuring a first and second rotation angle with respect to a reference direction in the plane of the elongated body in the equilibrated natural first and second balanced positions respectively; and determining the offset of the CoG in the plane by geometrical triangulation using the measured rotation angles, which implies that the location of
- the method of the invention provides a reliable and reproducible method for determining the position of the CoG of an elongated body.
- the inventors have found that the method is able to measure an unbalance defined by the product of the mass of the elongated body and the offset of the CoG with respect to the geometrical center of the elongated body (M*E) as low as 50 g*mm, more preferably as low as 40 g*mm, even more preferably as low as 30 g*mm, and most preferably as low as 20 g*mm.
- the accuracy of the method allows measuring deviations in the unbalance of less than 5 g*mm, more preferably of less than 4 g*mm, even more preferably of less than 3 g*mm, and most preferably of less than 2 g*mm. Such accuracies are difficult if not impossible to reach with the known method.
- measuring the first and second rotation angle with respect to the reference direction in the plane of the elongated body in the equilibrated natural first and second balanced positions may involve measuring two angles with respect to the vertical direction. Indeed, an object hanging at an angle with the vertical direction makes an acute angle with this vertical direction and an obtuse angle which is larger than the acute angle.
- the first and second rotation angle should both be acute or obtuse, and the acute angle is preferably used in the triangulation.
- the reference direction is preferably the vertical direction. This embodiment simplifies the geometrical triangulation.
- the method may in principle be used on any elongated body.
- a body has a length to largest width ratio of at least 1.5, more preferably of at least 2.0, even more preferably of at least 2.5 and most preferably of at least 3.0.
- a particular embodiment provides a method wherein the offset positions are located left and right of a vertical line passing through the geometrical center when the object is in a vertical resting position.
- a method is provided wherein the offset positions are both located above, or both located below a horizontal line passing through the geometrical center when the object is in a vertical resting position.
- the offset positions are located above and below a horizontal line passing through the geometrical center when the object is in a vertical resting position.
- a method is provided wherein the offset positions are both located left or right of a vertical line passing through the geometrical center when the object is in a vertical resting position.
- Another embodiment provides a method wherein the offset positions are equidistant to said vertical and/or horizontal line through the geometrical center.
- Yet another embodiment provides a method wherein the offset positions are located on a horizontal line parallel to said horizontal line passing through the geometrical center and/or are located on a vertical line parallel to said vertical line passing through the geometrical center.
- Yet another embodiment of the invention provides a method wherein the distance between the first and second rotation axes is A, the first rotation angle is al, the second rotation angle is a2, and the geometrical triangulation results in the following formula:
- E [tan(a2)/(tan (al) + tan (a2))] * A - A/2
- E is the offset of the CoG with respect to the geometrical center.
- the offset may be defined in any direction but will preferably be defined in a direction that is parallel to the direction of a line that connects the positions of the first and second rotation axes.
- the elongated body may not show any axis or plane of symmetry, or may have a plane of symmetry.
- a rotor or propeller blade may have a middle plane that provides a plane of symmetry between an underside of the blade and an upper side. The middle plane runs through a leading edge of the rotor or propeller blade.
- Another preferred embodiment provides a method wherein the plane of the elongated body is a plane of symmetry of the elongated body.
- a method wherein providing the first rotation axis and/or the second rotation axis is carried out by inserting a rod in a hole of the elongated body, the hole being provided at at least one of the offset positions. The at least one rod then extends perpendicular to the plane of the elongated body.
- the first rotation axis and the second rotation axis are both provided by inserting a rod in a hole of the elongated body, the hole being provided at the offset positions.
- the insertion of the rod in the hole of the elongated body is a rigid insertion and the suspension of the rod in the support comprises a bearing, preferably an air bearing.
- This arrangement effectively reduces friction between the balanced elongated body and the support.
- the bearing is also provided at an offset position to the geometrical center of the elongated body.
- an embodiment provides the rod in a bearing in provided on the first rotation axis and on the second rotation axis.
- Another embodiment provides the bearing on the first (or on the second) rotation axis and uses a counterweight to balance the elongated body such that the center of gravity is located in a middle position of the bearing.
- a rod is used as suspension where the rod has a smaller diameter than the hole of the elongated body in which it is inserted.
- the rod may be supported at both sides of the elongated body by a suitable support, such as one horizontal support at each side of the elongated body.
- the elongated body will automatically seek an equilibrium position.
- This embodiment is advantageous in that friction is virtually non-existent, thereby substantially eliminating any measuring mistakes.
- the set-up is substantially friction-free since the elongated body is allowed to ‘walk’ onto the rod. Measuring the angle of equilibrium may be carried out by measuring the horizontal distance between both tips of the elongated body in the equilibrium position.
- a practical method involves providing two plumb lines that may be translated in a horizontal direction until they are both in line with the tips of the elongated body. The angle may then be determined by calculating arccos (horizontal distance/diameter) where diameter is the tip-to-tip distance along the elongated body.
- This embodiment is very accurate and robust.
- the method according to a practical embodiment of the invention comprises providing the first rotation axis and the second rotation axis by a mounting hole of the elongated body.
- Such mounting holes are used for mounting the elongated body to a another part of a complete body to which it is connected in use.
- the mounting holes are used to connect the rotor to a hub of a helicopter.
- the mounting holes are usually positioned at t distance of each other in order to accommodate force moments that occur because of rotation of the rotor around the hub.
- the method is characterized in that the elongated body is substantially made of a fiber-reinforced composite.
- the composite material may be based on a thermosetting and/or a thermoplastic matrix material, and may use any type of reinforcing fiber used in the art today.
- the method according to another embodiment may be used advantageously for determining the position of the CoG of a rotor blade or a propeller blade, or of a rotor comprising at least two of the rotor blades or propeller blades.
- Figure 1 schematically shows a front view of method steps according to an embodiment of the invention:
- Figure 2 schematically shows a front view of mounting holes of a rotor and important parameters used in determining the position of the CoG of the rotor in accordance with an embodiment of the invention
- Figure 3A schematically shows a perspective view of a suspension of a rotor using an air bearing in accordance with an embodiment of the invention.
- Figure 3B schematically shows a front view of a dial plate used in accordance with an embodiment of the invention.
- FIG 1 a front view of two method steps according to an embodiment of the invention is shown.
- the embodiment shown aims at determining an offset E of the center of gravity (CoG) of an elongated body which is a rotor 1, comprising two rotor blades (11, 12) that are connected to a centrally disposed mounting plate 10.
- the offset E of the center of gravity (CoG) is determined with respect to a geometrical center 13 in a plane 14 of the rotor 1.
- the plane 14 is defined as a middle plane of the rotor 1 (which corresponds to the plane of the figure), and represents a plane of symmetry of the rotor 1.
- Each rotor blade (11, 12) has a leading edge (Ila, 12a) and a trailing edged (11b, 12b).
- the geometrical center 13 of the rotor is located in the center of a central hole 15, which is used for attachment to a rotating shaft (not shown) of a helicopter for instance.
- the mounting plate 10 is thereto provided with a number of holes (10a,..., lOd) through which a screw connection may be provided for attachment to a corresponding mounting plate of the helicopter.
- the embodiment of the method uses the holes (10a, ..., lOd) in providing a first rotation axis and a second rotation axis for a free-hanging rotor 1.
- a first step of the method involves providing a first rotation axis by inserting a rod 16 (see figure 3 A) in one of the holes (10a, ..., lOd) of the mounting plates, for instance hole 10a, and balancing the rotor 1 on the first rotation axis into an equilibrated first balanced position, as shown on the left of figure 1.
- a first rotation angle al with respect to a vertical direction is then measured in the plane 14 of the rotor 1 in the equilibrated first position.
- a second rotation axis is provided by inserting the rod (or another rod) in another one of the holes (10a, .. lOd) of the mounting plates, for instance hole 10b, and balancing the rotor 1 on the second rotation axis into an equilibrated second balanced position, as shown on the right of figure 1.
- a second rotation angle a2 is then measured in the plane 14 of the rotor 1 in the equilibrated second position.
- first and second rotation axes (in the example defined around the holes 10a and 10b respectively) are provided at a distance A to each other and also both perpendicular to the plane 14.
- the first and second rotation axes (around holes 10a and 10b in the example) are provided at offset positions to the geometrical center 13 of the rotor 1.
- the rod 16 may be suspended on a support plate 17 for instance to hold the rotor 1.
- a useful embodiment comprises inserting the rod 16, for instance a machined shaft, in one of the mounting holes (10a, ..., lOd) and suspend the rod 16 in the support plate 17 using an air bearing 18. Once the air bearing 18 is supplied with sufficient pressured gas, the rotor 1 may easily find its equilibrated balance.
- One skilled in the art is able to correctly adjust the pressure of the gas of the bearing by routine experimentation. Due to the nature of the setup according to this embodiment, friction or hysteresis is eliminated from the measurement, such that the exact location of the CoG may be measured even more accurately. It should be noted that the use of other bearings is also possible.
- the suspension of the rod 16 in the support 17 may be a rigid suspension and the insertion of the rod 16 in one of the holes (10a, ..., lOd) of the rotor 1 may comprise a bearing, preferably an air bearing. It is also possible to carry out the method such that the insertion of the rod 16 in one of the holes (10a, ..., lOd) of the rotor 1 comprises a bearing, preferably an air bearing, and the suspension of the rod 16 in the support 17 also comprises a bearing, preferably the air bearing 18.
- the angles of rotation (al, a2) of the rotor 1 may be measured accurately from a dial plate or compass rose 19 that is hung vertically behind the rotor 1.
- the vertical direction corresponds to an angle of rotation (al, a2) of zero.
- the angular markings on the dial plate 19 may for instance be provided every 0.1°, in particular in the expected rotation angle ranges (19a, 19b), but other accuracy readings also possible.
- the angles of rotation (al, a2) of the rotor 1 may be measured by measuring a horizontal distance between the two tips of the rotor 1, for instance by using plumb lines at each tip.
- the position of the CoG may be determined by using trigonometry, i.e. by geometrical triangulation using the measured rotation angles (al, a2). It can easily be shown (see figure 2) that the offset E of the CoG with respect to the geometrical center 15 of the rotor 1 may be calculated as follows:
- the center of gravity (CoG) of an elongated body may be determined by using two off-center hinge points and finding the two angles on which the elongated body will balance itself.
- the CoG will be located directly below the hinge point. In this balanced situation the elongated body will be suspended at an angle ((al, a2) with the vertical direction 20.
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Abstract
A method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body is described. The method comprises providing a first rotation axis and a second rotation axis at a distance to the first rotation axis and both perpendicular to the plane and at offset positions to the geometrical center of the elongated body. A subsequent step involves balancing the elongated body on the first rotation axis into an equilibrated first balanced position and subsequently balancing the elongated body on the second rotation axis into an equilibrated second balanced position. A first and a second rotation angle in the plane of the elongated body in the equilibrated first and second balanced positions respectively is then measured. The offset of the CoG in the plane is then determined by geometrical triangulation using the measured rotation angles.
Description
METHOD FOR DETERMINING A POSITION OF THE CENTER OF GRAVITY (COG) OF AN ELONGATED BODY
TECHNICAL FIELD OF THE INVENTION
The invention relates to a method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body. The invention in particular relates to a method for determining an offset of the center of gravity (CoG) of an elongated body made of fiber reinforced composites with respect to a geometrical center in a plane of the elongated body. The invention further relates to a method for determining an offset of the center of gravity (CoG) of a rotor or propeller blade with respect to a geometrical center in a plane of the rotor of propeller blade.
BACKGROUND OF THE INVENTION
In many applications, it is important to be able to accurately determine the position of a center of gravity of the body after manufacture. Any manufacturing method may entail some unbalances in the body and knowing the extent of the unbalance may be seen as an important quality control measure. The above is true for bodies made by more traditional materials, such as wood or metal for instance. The introduction of fiber reinforced composites in modern bodies has led to more flexibility in design shapes and a decrease in body mass, but manufacturing of composite bodies may also entail unbalances that need to be verified. This is particularly of concern for bodies or parts thereof that are moved or rotated in use at relatively large speeds. An exemplary body involves a rotor blade or propeller blade, or a rotor comprising at least two of such rotor blades or propeller blades.
A known method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body is disclosed in ‘A Body for Accurately Determining the Mass and
Center of Gravity of Rigid Bodies’, Meibao Wang, Xiaolin Zhang*, Wenyan Tang and Jun Wang, Appl. Sci. 2019, 9, 2532; doi:10.3390/app9122532. The known method involves measuring the mass and the CoG of rigid bodies with a multi-point weighing
method. A similar method is disclosed in US 5,081,865 and in US 2016/0305842 Al. CN 106813833 A discloses a weighing method for determining the center of gravity of a propeller blade specifically.
The traditional methods usually include two parts with a certain location, i.e., a fixed platform and a mobile platform. One of the problems of the known method is that its accuracy is not sufficient, or it requires a complicated set-up.
SUMMARY OF THE INVENTION
It is an aim of the present invention to provide an improved method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body. The invention in particular aims at providing a method for determining an offset of the CoG of an elongated body made of fiber reinforced composites with respect to a geometrical center in a plane of the elongated body. The invention further aims at providing a method for determining an offset of the CoG of a rotor or propeller blade with respect to a geometrical center in a plane of a rotor blade, or propeller blade, or a rotor that comprises at least two of such rotor blades or propeller blades.
According to the invention, this and other aims are fulfilled by a method in accordance with claim 1. The method is used for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body, and comprises the following steps of: providing a first rotation axis and a second rotation axis at a distance to the first rotation axis and both perpendicular to the plane and at offset positions to the geometrical center of the elongated body; balancing the elongated body on the first rotation axis into an equilibrated first natural balanced position and subsequently balancing the elongated body on the second rotation axis into an equilibrated second natural balanced position; measuring a first and second rotation angle with respect to a reference direction in the plane of the elongated body in the equilibrated natural first and second balanced positions respectively; and
determining the offset of the CoG in the plane by geometrical triangulation using the measured rotation angles, which implies that the location of the offset positions is selected such that the geometrical triangulation is possible.
DETAILED DESCRIPTION OF THE INVENTION
It has turned out that the method of the invention provides a reliable and reproducible method for determining the position of the CoG of an elongated body. The inventors have found that the method is able to measure an unbalance defined by the product of the mass of the elongated body and the offset of the CoG with respect to the geometrical center of the elongated body (M*E) as low as 50 g*mm, more preferably as low as 40 g*mm, even more preferably as low as 30 g*mm, and most preferably as low as 20 g*mm. This means that the accuracy of the method allows measuring deviations in the unbalance of less than 5 g*mm, more preferably of less than 4 g*mm, even more preferably of less than 3 g*mm, and most preferably of less than 2 g*mm. Such accuracies are difficult if not impossible to reach with the known method.
It should be noted that measuring the first and second rotation angle with respect to the reference direction in the plane of the elongated body in the equilibrated natural first and second balanced positions may involve measuring two angles with respect to the vertical direction. Indeed, an object hanging at an angle with the vertical direction makes an acute angle with this vertical direction and an obtuse angle which is larger than the acute angle. In the method, the first and second rotation angle should both be acute or obtuse, and the acute angle is preferably used in the triangulation.
The reference direction is preferably the vertical direction. This embodiment simplifies the geometrical triangulation.
The method may in principle be used on any elongated body. Such a body has a length to largest width ratio of at least 1.5, more preferably of at least 2.0, even more preferably of at least 2.5 and most preferably of at least 3.0.
A particular embodiment provides a method wherein the offset positions are located left and right of a vertical line passing through the geometrical center when the object is in a vertical resting position. In a further preferred embodiment of the invention, a method is
provided wherein the offset positions are both located above, or both located below a horizontal line passing through the geometrical center when the object is in a vertical resting position. These embodiments allow determining the offset in a direction that is perpendicular to the vertical direction, i.e. in a horizontal direction, as exemplified in figure 2, discussed further below.
In another embodiment, the offset positions are located above and below a horizontal line passing through the geometrical center when the object is in a vertical resting position. In a further preferred embodiment of the invention, a method is provided wherein the offset positions are both located left or right of a vertical line passing through the geometrical center when the object is in a vertical resting position. These embodiments allow determining the offset in a direction that is perpendicular to the horizontal direction, i.e. in the vertical direction.
Another embodiment provides a method wherein the offset positions are equidistant to said vertical and/or horizontal line through the geometrical center.
Yet another embodiment provides a method wherein the offset positions are located on a horizontal line parallel to said horizontal line passing through the geometrical center and/or are located on a vertical line parallel to said vertical line passing through the geometrical center.
Yet another embodiment of the invention provides a method wherein the distance between the first and second rotation axes is A, the first rotation angle is al, the second rotation angle is a2, and the geometrical triangulation results in the following formula:
E = [tan(a2)/(tan (al) + tan (a2))] * A - A/2 wherein E is the offset of the CoG with respect to the geometrical center. The offset may be defined in any direction but will preferably be defined in a direction that is parallel to the direction of a line that connects the positions of the first and second rotation axes.
The elongated body may not show any axis or plane of symmetry, or may have a plane of symmetry. For instance, a rotor or propeller blade may have a middle plane that
provides a plane of symmetry between an underside of the blade and an upper side. The middle plane runs through a leading edge of the rotor or propeller blade.
Another preferred embodiment provides a method wherein the plane of the elongated body is a plane of symmetry of the elongated body.
In another embodiment, a method is provided wherein providing the first rotation axis and/or the second rotation axis is carried out by inserting a rod in a hole of the elongated body, the hole being provided at at least one of the offset positions. The at least one rod then extends perpendicular to the plane of the elongated body.
In one suitable embodiment, the first rotation axis and the second rotation axis are both provided by inserting a rod in a hole of the elongated body, the hole being provided at the offset positions.
In an embodiment, the insertion of the rod in the hole of the elongated body is a rigid insertion and the suspension of the rod in the support comprises a bearing, preferably an air bearing. This arrangement effectively reduces friction between the balanced elongated body and the support. This means that the bearing is also provided at an offset position to the geometrical center of the elongated body. In order to increase accuracy, an embodiment provides the rod in a bearing in provided on the first rotation axis and on the second rotation axis. Another embodiment provides the bearing on the first (or on the second) rotation axis and uses a counterweight to balance the elongated body such that the center of gravity is located in a middle position of the bearing.
In yet another embodiment, a rod is used as suspension where the rod has a smaller diameter than the hole of the elongated body in which it is inserted. The rod may be supported at both sides of the elongated body by a suitable support, such as one horizontal support at each side of the elongated body. In the suspended state, the elongated body will automatically seek an equilibrium position. This embodiment is advantageous in that friction is virtually non-existent, thereby substantially eliminating any measuring mistakes. The set-up is substantially friction-free since the elongated body is allowed to ‘walk’ onto the rod.
Measuring the angle of equilibrium may be carried out by measuring the horizontal distance between both tips of the elongated body in the equilibrium position. A practical method involves providing two plumb lines that may be translated in a horizontal direction until they are both in line with the tips of the elongated body. The angle may then be determined by calculating arccos (horizontal distance/diameter) where diameter is the tip-to-tip distance along the elongated body.
This embodiment is very accurate and robust.
The method according to a practical embodiment of the invention comprises providing the first rotation axis and the second rotation axis by a mounting hole of the elongated body. Such mounting holes are used for mounting the elongated body to a another part of a complete body to which it is connected in use. For instance, in case of a rotor, the mounting holes are used to connect the rotor to a hub of a helicopter. The mounting holes are usually positioned at t distance of each other in order to accommodate force moments that occur because of rotation of the rotor around the hub.
In another embodiment, the method is characterized in that the elongated body is substantially made of a fiber-reinforced composite. The composite material may be based on a thermosetting and/or a thermoplastic matrix material, and may use any type of reinforcing fiber used in the art today.
The method according to another embodiment may be used advantageously for determining the position of the CoG of a rotor blade or a propeller blade, or of a rotor comprising at least two of the rotor blades or propeller blades.
The embodiments of the invention described in this patent application can be combined in any possible combination of these embodiments, and each embodiment can individually form the subject-matter of a divisional patent application.
BRIEF DESCRIPTION OF THE FIGURES
The invention will now be elucidated with reference to the following figures, without however being limited thereto. In the figures:
Figure 1 schematically shows a front view of method steps according to an embodiment of the invention:
Figure 2 schematically shows a front view of mounting holes of a rotor and important parameters used in determining the position of the CoG of the rotor in accordance with an embodiment of the invention;
Figure 3A schematically shows a perspective view of a suspension of a rotor using an air bearing in accordance with an embodiment of the invention; and
Figure 3B schematically shows a front view of a dial plate used in accordance with an embodiment of the invention.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
Referring to figure 1 , a front view of two method steps according to an embodiment of the invention is shown. The embodiment shown aims at determining an offset E of the center of gravity (CoG) of an elongated body which is a rotor 1, comprising two rotor blades (11, 12) that are connected to a centrally disposed mounting plate 10. As shown in more detail in figure 2, the offset E of the center of gravity (CoG) is determined with respect to a geometrical center 13 in a plane 14 of the rotor 1. The plane 14 is defined as a middle plane of the rotor 1 (which corresponds to the plane of the figure), and represents a plane of symmetry of the rotor 1. Each rotor blade (11, 12) has a leading edge (Ila, 12a) and a trailing edged (11b, 12b). The geometrical center 13 of the rotor is located in the center of a central hole 15, which is used for attachment to a rotating shaft (not shown) of a helicopter for instance. The mounting plate 10 is thereto provided with a number of holes (10a,..., lOd) through which a screw connection may be provided for attachment to a corresponding mounting plate of the helicopter.
The embodiment of the method uses the holes (10a, ..., lOd) in providing a first rotation axis and a second rotation axis for a free-hanging rotor 1. Indeed, a first step of the method involves providing a first rotation axis by inserting a rod 16 (see figure 3 A) in one of the holes (10a, ..., lOd) of the mounting plates, for instance hole 10a, and balancing the rotor 1 on the first rotation axis into an equilibrated first balanced position, as shown on the left of figure 1. A first rotation angle al with respect to a vertical direction is then measured in the plane 14 of the rotor 1 in the equilibrated first position. In a second step of the method, a second rotation axis is provided by inserting
the rod (or another rod) in another one of the holes (10a, .. lOd) of the mounting plates, for instance hole 10b, and balancing the rotor 1 on the second rotation axis into an equilibrated second balanced position, as shown on the right of figure 1. A second rotation angle a2 is then measured in the plane 14 of the rotor 1 in the equilibrated second position.
It should be noted that the first and second rotation axes (in the example defined around the holes 10a and 10b respectively) are provided at a distance A to each other and also both perpendicular to the plane 14. The first and second rotation axes (around holes 10a and 10b in the example) are provided at offset positions to the geometrical center 13 of the rotor 1.
The rod 16 may be suspended on a support plate 17 for instance to hold the rotor 1. A useful embodiment comprises inserting the rod 16, for instance a machined shaft, in one of the mounting holes (10a, ..., lOd) and suspend the rod 16 in the support plate 17 using an air bearing 18. Once the air bearing 18 is supplied with sufficient pressured gas, the rotor 1 may easily find its equilibrated balance. One skilled in the art is able to correctly adjust the pressure of the gas of the bearing by routine experimentation. Due to the nature of the setup according to this embodiment, friction or hysteresis is eliminated from the measurement, such that the exact location of the CoG may be measured even more accurately. It should be noted that the use of other bearings is also possible. Also, the suspension of the rod 16 in the support 17 may be a rigid suspension and the insertion of the rod 16 in one of the holes (10a, ..., lOd) of the rotor 1 may comprise a bearing, preferably an air bearing. It is also possible to carry out the method such that the insertion of the rod 16 in one of the holes (10a, ..., lOd) of the rotor 1 comprises a bearing, preferably an air bearing, and the suspension of the rod 16 in the support 17 also comprises a bearing, preferably the air bearing 18.
The angles of rotation (al, a2) of the rotor 1 may be measured accurately from a dial plate or compass rose 19 that is hung vertically behind the rotor 1. The vertical direction corresponds to an angle of rotation (al, a2) of zero. The angular markings on the dial plate 19 may for instance be provided every 0.1°, in particular in the expected rotation angle ranges (19a, 19b), but other accuracy readings also possible.
Alternatively, the angles of rotation (al, a2) of the rotor 1 may be measured by measuring a horizontal distance between the two tips of the rotor 1, for instance by using plumb lines at each tip.
The position of the CoG may be determined by using trigonometry, i.e. by geometrical triangulation using the measured rotation angles (al, a2). It can easily be shown (see figure 2) that the offset E of the CoG with respect to the geometrical center 15 of the rotor 1 may be calculated as follows:
E = [tan(a2)/(tan (al) + tan (a2))] * A - A/2 wherein the distance between the first and second rotation axes is A, the measured first rotation angle is al , and the measured second rotation angle is a2. The rotation angles (al, a2) are both positive numbers, i.e. the absolute value of the measured rotation angles (al , a2) is used in the formula.
According to the invention, the center of gravity (CoG) of an elongated body may be determined by using two off-center hinge points and finding the two angles on which the elongated body will balance itself. When the elongated body finds a natural balance, the CoG will be located directly below the hinge point. In this balanced situation the elongated body will be suspended at an angle ((al, a2) with the vertical direction 20.
The invention is not limited to the above given examples and variations thereto may be envisaged within the scope of the appended claims.
Claims
1. Method for determining an offset of the center of gravity (CoG) of an elongated body with respect to a geometrical center in a plane of the elongated body, the method comprising: providing a first rotation axis and a second rotation axis at a distance to the first rotation axis and both perpendicular to the plane and at offset positions to the geometrical center of the elongated body; balancing the elongated body on the first rotation axis into an equilibrated first natural balanced position and subsequently balancing the elongated body on the second rotation axis into an equilibrated second natural balanced position; measuring a first and second rotation angle with respect to a reference direction, preferably the vertical direction, in the plane of the elongated body in the equilibrated first and second natural balanced positions respectively; and determining the offset of the CoG in the plane by geometrical triangulation using the measured rotation angles, which implies that the location of the offset positions is selected such that the geometrical triangulation is possible.
2. Method according to claim 1, wherein the offset positions are located left and right of a vertical line passing through the geometrical center and/or are both located above, or both located below a horizontal line passing through the geometrical center.
3. Method according to claim 1 or claim 2, wherein the offset positions are located above and below a horizontal line passing through the geometrical center when the object is in a vertical resting position and/or are both located left or right of a vertical line passing through the geometrical center when the object is in a vertical resting position.
4. Method according to claim 2 or 3, wherein the offset positions are equidistant to said vertical and/or horizontal line through the geometrical center.
5. Method according to any one of claims 2-4, wherein the offset positions are located on a horizontal line parallel to said horizontal line passing through the
geometrical center and/or are located on a vertical line parallel to said vertical line passing through the geometrical center.
6. Method according to any one of the preceding claims, wherein the distance between the first and second rotation axes is A, the first rotation angle is al, the second rotation angle is a2, and the geometrical triangulation results in the following formula:
E = [tan(a2)/(tan (al) + tan (a2))] * A - A/2 wherein E is the offset of the CoG with respect to the geometrical center.
7. Method according to any one of the preceding claim s, wherein the plane of the elongated body is a plane of symmetry.
8. Method according to any one of the preceding claims, wherein providing the first rotation axis and/or the second rotation axis is carried out by inserting a rod in a hole of the elongated body, the hole being provided at at least one of the offset positions.
9. Method according to claim 8, wherein the first rotation axis and the second rotation axis are both provided by inserting a rod in a hole of the elongated body, the hole being provided at the offset positions.
10. Method according to claim 8 or 9, wherein the at least one rod is suspended in a support.
11. Method according to claim 10, wherein the insertion of the rod in the hole of the elongated body is a rigid insertion and the suspension of the rod in the support comprises a bearing, preferably an air bearing.
12. Method according to any one of the preceding claims, wherein the first rotation axis and the second rotation axis are provided by a mounting hole of the elongated body.
13. Method according to any one of the preceding claims, wherein the elongated body is substantially made of a fiber-reinforced composite.
14. Method according to any one of the preceding claims, wherein the elongated body comprises a rotor blade or a propeller blade, or a rotor comprising at least two of the rotor blades or propeller blades.
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NL2029427A NL2029427B1 (en) | 2021-10-15 | 2021-10-15 | Method for determining a position of the center of gravity (cog) of an elongated body |
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Citations (3)
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US5081865A (en) | 1990-06-29 | 1992-01-21 | The United States Of America As Represented By The Secretary Of The Air Force | Center of gravity locating method |
US20160305842A1 (en) | 2015-04-17 | 2016-10-20 | Raytheon Company | Automated work piece center of mass identification system and method for same |
CN106813833A (en) | 2017-03-27 | 2017-06-09 | 江苏科技大学 | A kind of tuning for Controllable Pitch Propeller blade center of gravity measurement method |
-
2021
- 2021-10-15 NL NL2029427A patent/NL2029427B1/en active
-
2022
- 2022-10-06 WO PCT/NL2022/050559 patent/WO2023063816A1/en unknown
Patent Citations (3)
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US5081865A (en) | 1990-06-29 | 1992-01-21 | The United States Of America As Represented By The Secretary Of The Air Force | Center of gravity locating method |
US20160305842A1 (en) | 2015-04-17 | 2016-10-20 | Raytheon Company | Automated work piece center of mass identification system and method for same |
CN106813833A (en) | 2017-03-27 | 2017-06-09 | 江苏科技大学 | A kind of tuning for Controllable Pitch Propeller blade center of gravity measurement method |
Non-Patent Citations (1)
Title |
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MEIBAO WANGXIAOLIN ZHANGWENYAN TANG AND JUN WANG: "A Body for Accurately Determining the Mass and Center of Gravity of Rigid Bodies", APPL. SCI, vol. 9, 2019, pages 2532 |
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