Design method and system of coaxial counter-rotating propellers based on fast aerodynamic calculation method

Abstract

The present application discloses a coaxial counter-rotating propeller design method and system based on a rapid aerodynamic calculation method, which relates to the field of aircraft design technology. The method includes obtaining the layout and parameter design of the coaxial counter-rotating propeller, determining the blade load distribution in the cruise state using the Goldstein theory; calculating the thickness distribution and chord length distribution of the blade according to the dimensionless spanwise position of the blade and the diameter of the rotor disk; and determining the airfoil lift coefficient and airfoil distribution based on these data. The torsion distribution of the blade is calculated by the blade element theory, and the forward / backward sweep distribution is determined according to the aerodynamic and structural requirements. The rapid momentum source aerodynamic calculation method is used to evaluate whether the aerodynamic performance meets the cruise and take-off conditions. If it meets the requirements, the coaxial counter-rotating propeller design is completed; if it does not meet the requirements, the chord length distribution, airfoil distribution and torsion distribution of the blade are updated until the design requirements for the cruise and take-off conditions are met. The present application can realize the rapid design of coaxial counter-rotating propellers.

Description

Technical Field

[0001] The present application relates to the technical field of aircraft design, and in particular to a design method and system for coaxial counter-rotating propellers based on a rapid aerodynamic calculation method. Background Art

[0002] Propeller efficiency is a primary factor in determining the cruising speed, range, and endurance of aircraft using propellers as their primary propulsion system. Coaxial counter-rotating propellers help control propeller geometry and improve propulsion efficiency. This configuration consists of two identical or similar propellers, rotating coaxially and counter-rotating. Compared to conventional propellers, the presence of the rear propeller in the propulsion system can recover some of the circulation in the front propeller's slipstream, resulting in higher propulsion efficiency. For a given size, a counter-rotating propeller can provide more power and lower fuel consumption than a single propeller.

[0003] Due to the advantages of coaxial counter-rotating propellers in aerodynamics and other aspects, researchers at home and abroad have conducted many studies on them, such as the study of the interference flow field characteristics of the front and rear propellers, and the study of aerodynamic and noise characteristics. However, in terms of propeller design, most research focuses on single-row propellers, and there is less research on the design process of coaxial counter-rotating propellers. During the preliminary design of the propeller, it is necessary to use engineering methods to quickly evaluate its aerodynamic performance in order to iteratively design. The aerodynamic theory used for single-row propellers is difficult to consider the mutual influence of the two propellers. Some scholars have developed some aerodynamic theories suitable for coaxial counter-rotating propellers based on the aerodynamic theory of single-row propellers, such as the strip theory, based on the airflow model obtained through experiments or experience. However, the value of the empirical coefficient is related to parameters such as aerodynamic layout and incoming flow conditions.

[0004] The momentum source method can be used to simulate the interference effect of propellers on the flow field. This method has the advantages of requiring less computational resources and having a short computational cycle while meeting engineering accuracy. Since its introduction by Rajagopalan and Chaffin in the 20th century, it has been widely used in engineering design. The basic idea of ​​the momentum source method is to replace the propeller with an action disk and add a momentum source term to the governing equation within the area of ​​the action disk. This causes the airflow momentum around the disk to change, replacing the effect of the blades on the surrounding air. Because this method can consider the mutual influence of two adjacent propellers from the perspective of flow field momentum, and the unsteady aerodynamic characteristics of a coaxial counter-rotating propeller in an axial flow state have strong regularity within a rotation cycle, it is suitable to a certain extent for the aerodynamic characteristic analysis and downwash flow field simulation of coaxial counter-rotating propellers in an axial flow state.

[0005] Although some research has been carried out on the aerodynamic, acoustic and structural characteristics of coaxial counter-rotating propellers at home and abroad, the existing coaxial counter-rotating propeller design process is not clear and systematic, and there are few calculation methods suitable for quickly evaluating its aerodynamic performance, which is affected by its aerodynamic layout and changes in incoming flow parameters. Therefore, the present invention proposes a set of reference coaxial counter-rotating propeller design processes, and uses the momentum source method as a rapid aerodynamic evaluation tool. Through this design process and aerodynamic evaluation tool, the purpose of rapid design of coaxial counter-rotating propellers is achieved. Summary of the Invention

[0006] The purpose of this application is to provide a coaxial counter-rotating propeller design method and system based on a rapid aerodynamic calculation method, which can realize the rapid design of coaxial counter-rotating propellers.

[0007] To achieve the above objectives, this application provides the following solutions:

[0008] In a first aspect, the present application provides a design method for coaxial counter-rotating propellers based on a rapid aerodynamic calculation method, comprising:

[0009] Step 1: Obtain the layout design and operating parameter design of the coaxial counter-rotating propeller; the layout includes propeller diameter, number of blades, pitch, rear propeller diameter cutting ratio, rotational speed and load distribution ratio; the operating parameters include the incoming flow velocity of the spanwise position airfoil section and the cruising state; the cruising state is the state of an aircraft equipped with a coaxial counter-rotating propeller.

[0010] Step 2: Based on the layout design and operating parameter design, the Goldstein theory is used to determine the spanwise load distribution of the blades in the cruise state.

[0011] Step 3: Calculate the blade thickness distribution and chord length distribution based on the dimensionless spanwise position of the blade and the disc diameter.

[0012] Step 4: Determine the blade spanwise airfoil lift coefficient based on the blade spanwise load distribution, blade thickness distribution, and blade chord length distribution, and determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient.

[0013] Step 5: Determine the torsion distribution of the blade based on the blade element theory according to the incoming flow velocity of the airfoil section at the spanwise position.

[0014] Step 6: Determine the forward / backward sweep distribution of the blade based on the aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord line of the blade does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to the moment of the blade around the pitch rotation axis under stress state is not greater than the upper limit of the structural strength.

[0015] Step 7: Based on the rapid momentum source aerodynamic calculation method, determine whether the aerodynamic performance of the blades meets the design requirements for cruise and takeoff states.

[0016] When the design requirements for cruise and take-off states are met, the coaxial counter-rotating propeller design is completed.

[0017] When the cruise and takeoff state design requirements are not met, the chord length distribution, airfoil distribution, and twist distribution of the blade are updated, and based on the updated chord length distribution, airfoil distribution, and twist distribution, steps 5 to 7 are executed until the cruise and takeoff state design requirements are met.

[0018] In a second aspect, the present application provides a coaxial counter-rotating propeller design system based on a rapid aerodynamic calculation method, comprising:

[0019] The design parameter acquisition module is used to obtain the layout design and working parameter design of the coaxial counter-rotating propeller; the layout includes the propeller diameter, the number of blades, the spacing, the rear propeller diameter cutting ratio, the rotation speed and the load distribution ratio; the working parameters include the incoming flow velocity of the spanwise position airfoil section and the cruising state; the cruising state is the state of the aircraft equipped with the coaxial counter-rotating propeller.

[0020] The blade spanwise load distribution determination module is used to determine the blade spanwise load distribution under cruise conditions based on layout design and operating parameter design using the Goldstein theory.

[0021] The thickness distribution and chord length distribution calculation module is used to calculate the thickness distribution and chord length distribution of the blade based on the dimensionless spanwise position of the blade and the propeller disk diameter.

[0022] The airfoil distribution determination module is used to determine the blade spanwise airfoil lift coefficient according to the blade spanwise load distribution, blade thickness distribution and blade chord length distribution, and to determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient.

[0023] The twist distribution calculation module is used to determine the twist distribution of the blade based on the blade element theory according to the incoming flow velocity of the airfoil section in the span direction.

[0024] The forward / backward sweep distribution determination module is used to determine the forward / backward sweep distribution of the blade based on the aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section of the blade in the direction perpendicular to the quarter chord line does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to that the moment of the blade around the pitch rotation axis under the load state is not greater than the upper limit of the structural strength.

[0025] The evaluation module is used to determine whether the aerodynamic performance of the blades meets the design requirements for cruise and takeoff states based on the rapid momentum source aerodynamic calculation method.

[0026] When the design requirements for cruise and take-off states are met, the coaxial counter-rotating propeller design is completed.

[0027] When the cruise and take-off state design requirements are not met, the chord length distribution, airfoil distribution and twist distribution of the blade are updated, and calculations are performed based on the updated chord length distribution, airfoil distribution and twist distribution until the cruise and take-off state design requirements are met.

[0028] According to the specific embodiments provided in this application, this application discloses the following technical effects:

[0029] The present application provides a coaxial counter-rotating propeller design method and system based on a rapid aerodynamic calculation method, step one: obtain the layout design and working parameter design information of the coaxial counter-rotating propeller. The layout design covers key elements such as propeller diameter, number of blades, pitch, rear propeller diameter cutting ratio, speed and load distribution ratio; the working parameters include the incoming flow velocity of the airfoil section at the spanwise position and the state of the aircraft carrying the propeller in the cruise state. Step two: Based on the layout design and working parameter design, use the Goldstein theory to determine the spanwise load distribution of the blade in the cruise state. Step three: Combine the dimensionless spanwise position and disc diameter data of the blade to accurately calculate the thickness distribution and chord length distribution of the blade. Step four: Based on the spanwise load distribution, thickness distribution and chord length distribution of the blade, determine the spanwise airfoil lift coefficient of the blade, and further determine the airfoil distribution of the blade accordingly. Step five: Based on the incoming flow velocity of the airfoil section at the spanwise position, combined with the blade element theory, accurately determine the torsion distribution of the blade. Step 6: Determine the forward / backward sweep distribution of the blades, taking into account both aerodynamic performance and structural strength requirements. Aerodynamic performance requires ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter-chord line does not exceed the drag divergence Mach number corresponding to the cruise design point. Structural strength requirements require that the moment generated about the rotation axis by the blade under load exceeds the structural standard, and the moment about the pitch rotation axis does not exceed the upper limit of the structural strength. Step 7: Use a rapid momentum source aerodynamic calculation method to evaluate whether the blade's aerodynamic performance meets the cruise and takeoff design requirements. If so, the design of the coaxial counter-rotating propeller is complete. If not, the blade's chord length distribution, airfoil distribution, and twist distribution are updated. Based on the updated data, steps 5 through 7 are repeated until the cruise and takeoff design requirements are met. The method described in this application is applicable to evaluating the aerodynamic performance of coaxial counter-rotating propellers under any aerodynamic configuration. This method can account for the interaction between the two propellers and eliminates the need to rely on experimental data or empirical coefficients to adjust the inflow model. In order to further improve the computational efficiency of the momentum source method, the calculation speed is significantly improved by applying periodic boundary conditions, which greatly shortens the calculation time to 1 / n of the original time, thereby promoting the rapid design process of coaxial counter-rotating propellers. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 Schematic diagram of a flow chart of a coaxial counter-rotating propeller design method based on a fast aerodynamic calculation method in one embodiment of the present application.

[0032] Figure 2 A flow chart of a coaxial counter-rotating propeller design provided in one embodiment of the present application.

[0033] Figure 3 A flow chart of a momentum source method suitable for coaxial counter-rotating propellers provided in one embodiment of the present application.

[0034] Figure 4 A grid diagram of a momentum source method for an isolated coaxial counter-rotating propeller provided in one embodiment of the present application; the left side of the diagram is a side view, and the right side is a longitudinal cross-sectional view.

[0035] Figure 5 An embodiment of the present application provides a momentum source method grid for an isolated coaxial counter-rotating propeller after applying periodic boundary conditions.

[0036] Figure 6 A schematic diagram of an area for adding a momentum source term to a coaxial counter-rotating propeller provided in one embodiment of the present application.

[0037] Figure 7 A flow chart for solving a momentum source term is provided in one embodiment of the present application.

[0038] Figure 8 A schematic diagram of a periodic boundary condition provided in one embodiment of the present application.

[0039] Figure 9 This is a comparison chart of calculation results and test results when the incoming flow Mach number Ma=0.35 provided in one embodiment of the present application; wherein (a) is a curve chart showing the change of total thrust coefficient and total power coefficient with the forward ratio; (b) is a curve chart showing the change of comprehensive efficiency with the forward ratio.

[0040] Figure 10 Schematic diagram of the structure of a coaxial counter-rotating propeller design system based on a fast aerodynamic calculation method in one embodiment of the present application. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0042] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0043] Example 1

[0044] like Figure 1 As shown, this embodiment provides a design method for coaxial counter-rotating propellers based on a fast aerodynamic calculation method, including:

[0045] Step 1: Obtain the layout design and operating parameter design of the coaxial counter-rotating propeller; the layout includes propeller diameter, number of blades, pitch, rear propeller diameter cutting ratio, rotational speed and load distribution ratio; the operating parameters include the incoming flow velocity of the spanwise position airfoil section and the cruising state; the cruising state is the state of an aircraft equipped with a coaxial counter-rotating propeller.

[0046] Step 2: Based on the layout design and operating parameter design, the Goldstein theory is used to determine the spanwise load distribution of the blades in the cruise state.

[0047] Step 3: Calculate the blade thickness distribution and chord length distribution based on the dimensionless spanwise position of the blade and the disc diameter.

[0048] Step 4: Determine the blade spanwise airfoil lift coefficient based on the blade spanwise load distribution, blade thickness distribution, and blade chord length distribution, and determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient.

[0049] Step 5: Determine the torsion distribution of the blade based on the blade element theory according to the incoming flow velocity of the airfoil section at the spanwise position.

[0050] Step 6: Determine the forward / backward sweep distribution of the blade based on the aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord line of the blade does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to the moment of the blade around the pitch rotation axis under stress state is not greater than the upper limit of the structural strength.

[0051] Step 7: Based on the rapid momentum source aerodynamic calculation method, determine whether the aerodynamic performance of the blades meets the design requirements for cruise and takeoff states.

[0052] When the design requirements for cruise and take-off states are met, the coaxial counter-rotating propeller design is completed.

[0053] When the cruise and takeoff state design requirements are not met, the chord length distribution, airfoil distribution, and twist distribution of the blade are updated, and based on the updated chord length distribution, airfoil distribution, and twist distribution, steps 5 to 7 are executed until the cruise and takeoff state design requirements are met.

[0054] In this embodiment, the design steps for a coaxial counter-rotating propeller can be divided into the following: the first step is layout and operating parameter design; the second step is blade shape design. Both steps aim to achieve a coaxial counter-rotating propeller with better overall performance; and the third step is aerodynamic performance evaluation of the coaxial counter-rotating propeller. During the second design step, the layout and operating parameters determined in the first step are used as a basis for the aerodynamic performance evaluation and propeller blade design.

[0055] Among them, Figure 2 As shown, in this embodiment, when executing step 101, the specific steps may be as follows:

[0056] Determine the layout and operating parameter design of the coaxial counter-rotating propeller, such as propeller diameter, number of blades, pitch, rear propeller diameter cutting ratio, speed, load distribution ratio and other parameters.

[0057] Specifically, when selecting the propeller diameter, factors such as its structure, weight and spatial layout must be comprehensively considered. The selected diameter should ensure that the cruise performance and maximum thrust requirements at takeoff are met, while ensuring that the noise level meets the specified standards. The cruise efficiency of the propeller is closely related to the disc load; reducing the disc load (i.e., increasing the propeller diameter) can improve efficiency because it reduces the induced speed and thus reduces the induced loss. Under the same disc load conditions, the counter-rotating propeller has higher efficiency than the single propeller. However, the efficiency of the counter-rotating propeller is less sensitive to changes in the disc load, and it can operate under higher disc loads without significantly reducing efficiency. However, a larger propeller diameter will have a negative impact on the weight of the propulsion system because larger propellers are heavier and require a heavier gearbox to reduce their rotational speed to ensure that the tip speed remains within an appropriate range. Therefore, it is crucial to reasonably select the propeller diameter.

[0058] For example, when designing a small drone, engineers need to select an appropriate propeller diameter to meet its performance requirements. Considering the compact structure and weight restrictions of the drone, engineers may choose a propeller with a smaller diameter to reduce the overall weight. For example, if the drone's takeoff weight is 5 kg, engineers may choose a carbon fiber propeller with a diameter of 30 cm to ensure sufficient thrust and a low noise level. Although a smaller propeller diameter may result in a higher rotor disk load, thereby reducing cruising efficiency, this is acceptable considering the size of the drone and mission requirements. At the same time, in order to keep the propeller speed within a safe range, engineers will design a lightweight gearbox to ensure that the propeller operates efficiently without significantly affecting the drone's endurance.

[0059] Specifically, the number of propeller blades has a significant impact on aerodynamics, acoustic characteristics and structural strength. In terms of aerodynamics, in order to maintain constant thrust, increasing the number of blades to increase the aspect ratio of the blades helps reduce tip losses, thereby improving aerodynamic efficiency. From an acoustic point of view, increasing the total number of blades and ensuring that the number of front and rear blades does not have a common multiple can help reduce noise levels. In terms of structural design, in order to minimize blade stress and reduce the risk of flutter, it is better to use fewer blades and a larger chord length. It is recommended that the total number of blades of a coaxial counter-rotating propeller be between 14 and 20, with a difference of 2 to 3 blades between the front and rear blades. Taking into account the impact of load distribution on aerodynamic performance and the maximum acceptable stress of the blades in the structure, if the difference in the number of blades exceeds 3, it may have an adverse effect on the aerodynamic efficiency of the propeller.

[0060] Specifically, the propeller spacing parameter has a relatively limited impact on aerodynamic efficiency, but it has a more significant impact on noise levels and structural weight. In this case, setting the propeller spacing to 0.2 to 0.3 times the front propeller diameter is a more reasonable range.

[0061] Specifically, the diameter of the rear propeller of the rotating propeller is trimmed: by reducing the diameter of the rear propeller, the interference of the front propeller tip vortex on the rear propeller can be reduced, thereby reducing the noise level generated by the rotating propeller. However, excessive trimming of the rear propeller diameter will have a significant negative impact on cruise efficiency. Generally, a 10-15% trimming ratio of the rear propeller diameter may cause a 0.5%-2% decrease in cruise efficiency. Therefore, when trimming the rear propeller diameter, a balance must be found between meeting aerodynamic performance and reducing noise.

[0062] Propeller speed: This parameter has a significant impact on aerodynamic performance and noise characteristics. Under the premise of keeping the propeller thrust unchanged, the aerodynamic performance will improve with the increase of the speed. This is because the increase in speed causes the thrust component of the blade perpendicular to the propeller disc plane to increase, while the drag component within the propeller disc plane decreases, thereby improving the aerodynamic efficiency of the propeller. However, when the blade tip Mach number approaches 1, the aerodynamic and noise performance of the propeller will drop sharply due to the compressibility of the air and the generation of shock waves. Modern high-speed propeller design uses thin airfoils and swept technology to keep the blade tip Mach number between 0.9-1.0 in the cruising state and the blade tip Mach number between 0.6-0.7 in the take-off state. This is a relatively suitable parameter selection.

[0063] Load distribution between the two propellers of a counter-rotating propeller: This parameter mainly affects the aerodynamic performance of the counter-rotating propeller. Compared with a single propeller, the rear propeller of a coaxial counter-rotating propeller can recover the energy consumed by the front propeller to distort the air, so that under the same thrust and size conditions, the overall efficiency of the counter-rotating propeller is higher. Therefore, the comprehensive aerodynamic efficiency of the counter-rotating propeller is affected by the degree to which the rear propeller recovers the energy of the front propeller. When the torque generated or the power consumed by the two propellers of the counter-rotating propeller is evenly distributed, the aerodynamic efficiency is the highest. At this time, the circumferential distortion of the air behind the counter-rotating propeller is minimized, and the circumferential induced loss is also minimized. Under the same thrust conditions, the aerodynamic efficiency of the counter-rotating propeller is insensitive to small changes near the average load distribution. When the diameter of the rear propeller is reduced, a slight tilt of the load distribution towards the front propeller helps to reduce the circumferential induced loss of the counter-rotating propeller.

[0064] Among them, when designing the blade shape of the second part, if Figure 2 As shown, steps 2 to 6 may be specifically as follows:

[0065] Specifically, when executing step 2, the spanwise load distribution of the blades in the cruise state is determined. From the perspective of optimal aerodynamic efficiency, the spanwise load distribution of the blades can be determined using the Goldstein theory.

[0066] Specifically, when executing step 3, the thickness distribution and chord length distribution of the blade are determined. This can be given based on the design experience of modern high-speed propellers. For example, the thickness and chord length distribution of a certain high-speed propeller are given by the following formula:

[0067]

[0068]

[0069] in, is the airfoil thickness, c is the airfoil chord length, D is the disc diameter, and x is the dimensionless spanwise position of the blade.

[0070] Specifically, when executing step 4, the specific steps may be as follows:

[0071] The blade spanwise airfoil lift coefficient distribution is determined based on the blade thickness distribution and chord length distribution as well as the incoming flow conditions and operating parameters in the cruise state; the incoming flow conditions are determined based on the incoming flow velocity.

[0072] The airfoil distribution of the blade is determined based on the spanwise airfoil lift coefficient distribution of the blade; the spanwise airfoil lift coefficient distribution of the blade is such that at the spanwise position of 0.75R of the blade, the lift coefficient ranges from 0.4 to 0.6.

[0073] Specifically, steps 4-5 need to clarify the airfoil distribution and torsion distribution of the blade. Based on the spanwise load distribution of the blade obtained in step 2, the blade thickness distribution and chord length distribution determined in step 3, and the incoming flow conditions and operating parameters in the cruise state, the spanwise airfoil lift coefficient distribution of the blade can be preliminarily determined. Subsequently, the airfoil distribution and torsion distribution of the blade are determined based on this information. When selecting an airfoil, priority should be given to airfoils with a better lift-to-drag ratio under high-speed conditions to ensure efficiency in the cruise state; at the same time, airfoils with a higher maximum lift coefficient should be selected to ensure sufficient thrust during takeoff. Taking into account aerodynamic performance and acoustic effects, the lift coefficient at the 0.75R spanwise position of the blade should be maintained between 0.4 and 0.6. The determination of the torsion distribution can be achieved by analyzing the incoming flow velocity of the airfoil section at each spanwise position and applying the blade element theory.

[0074] When executing step 6, the forward sweep and the swept distribution of the blade need to be accurately determined. By configuring the forward sweep and the swept distribution of the blade, it can be ensured that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord is lower than the drag divergence Mach number of the cruise design point, thereby optimizing the aerodynamic performance and improving the acoustic characteristics. When configuring the forward sweep and the swept distribution of the blade, its relationship with the pitch rotation axis should also be considered to ensure that when subjected to force, the torque around the rotation axis can meet the structural requirements. When performing CAD modeling of blades with forward sweep and the swept distribution, the airfoil at each spanwise position should be moved along the airfoil chord direction after the installation angle is arranged.

[0075] In this embodiment, the step of evaluating the aerodynamic performance of the coaxial counter-rotating propellers in the third part is step 7. When executing step 7, if Figure 3 As shown, the specific details can be as follows:

[0076] Based on the spacing between the two blades, a grid is generated for momentum source calculation of the coaxial counter-rotating propeller.

[0077] According to the diameters of the two blades, the mesh cells in the disk plane where the source term is applied are determined.

[0078] The size of the source term on the grid cells in the disk plane is determined based on the rotational speed of the two blades and the number of blades.

[0079] The source terms on the grid cells in the disk plane of the two blades are substituted into the RANS equation, and the flow field is solved iteratively to obtain a stable flow field considering the mutual influence of the two blades.

[0080] The aerodynamic performance of the blades is evaluated based on the steady flow field considering the interaction between the two propellers.

[0081] When generating a grid for momentum source calculation for a coaxial counter-rotating propeller based on the spacing between the two blades, the specific steps are as follows:

[0082] According to the spacing between the two blades, a cylindrical structured momentum source grid is generated for momentum source calculation of the coaxial counter-rotating propeller; the cylindrical structured momentum source grid is used to make the two propeller disk planes located on both sides of the 1 / 2 height plane of the cylindrical momentum source grid, and the distances to the 1 / 2 height plane are equal.

[0083] Specifically, when using the momentum source method to simulate the aerodynamic characteristics of an isolated coaxial counter-rotating propeller, a cylindrical structured momentum source grid is generated according to the spacing between the two propellers, so that the two propeller disk planes are located on both sides of the 1 / 2 height plane of the cylindrical momentum source grid and are equidistant from the plane. The grids near the two propeller disks are encrypted, and the generated grid is as follows: Figure 4 In addition, record the grid numbers corresponding to the height positions of the two propeller disk planes so that the source terms can be applied later.

[0084] This embodiment also introduces periodic boundary conditions into the momentum source method for coaxial counter-rotating propellers. Since the steady momentum source method allows the propeller's effect on the flow field to be expressed in the form of time averaging, the flow field information obtained within the disc plane is periodic. Therefore, the periodic flow field region can be artificially intercepted and the entire flow field can be simulated by applying periodic boundary conditions, thereby reducing the amount of grids and improving computational efficiency. The grid of the momentum source method for isolated coaxial counter-rotating propellers after applying periodic boundary conditions is as follows: Figure 5 As shown in Figure 2, the parameter settings during mesh generation are basically the same as those for the complete momentum source mesh, except that only a few meshes are retained around the propeller disk.

[0085] The determination of the grid cells in the disk plane where the source term is applied based on the diameters of the two blades can be performed as follows:

[0086] The size of the propeller disk plane is determined according to the diameters of the two blades, and the grid area in the propeller disk plane where the source term needs to be applied is determined; the source term is applied to the grid unit by a marking method; and the grid area includes a plurality of grid units.

[0087] Specifically, the basic idea of ​​the momentum source method is to replace the propeller with an action disk. A momentum source term is added to the governing equation within the action disk's region, causing the airflow momentum around the disk to change, replacing the effect of the blades on the surrounding air. The calculation of the momentum source term primarily utilizes blade element theory and Newton's third law (the law of action and reaction).

[0088] When using the momentum source method, it is first necessary to determine the propeller disk position, that is, the momentum source term addition area. The two propeller disks of the coaxial counter-rotating propeller in the axial flow state are perpendicular to the incoming flow direction. The schematic diagram of the momentum source term addition area of ​​the coaxial counter-rotating propeller is shown in Figure 6 shown.

[0089] The area where the momentum source term is added is determined by the height positions of the two disk planes and the grid cells within the disk planes. In the first step, the corresponding height positions of the two disk planes in the momentum source grid are determined. The diameters of the two propellers can be used to determine the grid cells within the disk plane. When the distance from a grid point within the disk plane to the center of the disk plane is less than the disk diameter, the grid point is considered to be within the disk plane. The grid cell formed by all the grid points within the disk plane is the grid cell within the disk plane to which the source term is applied.

[0090] Among them, when determining the upper source limit size of the grid unit in the propeller disk plane based on the rotation speed and number of blades of the two blades, the specific method can be as follows:

[0091] According to the relevant parameters such as the rotational speed of the two propellers and the number of propeller blades, the source item size on the grid unit in the propeller disk plane is determined. Among them, the momentum source item size on the grid unit in the propeller disk plane needs to be determined. Figure 7 The flowchart for solving the momentum source term is shown. Finally, the flow field information is solved through iterative advancement in conjunction with CFD methods. This section is consistent with the traditional method for solving the momentum source term, so it will not be elaborated here.

[0092] Compared with the traditional momentum source method, the momentum source method of coaxial counter-rotating propellers requires special attention to the following matters: According to the momentum source term calculation theory, source terms need to be added at the corresponding positions of the front and rear propeller disks, and the grid units in the propeller disk plane where the source terms are applied are determined based on the results of the second step; when applying the source terms, the size of the source terms on the grid units in the propeller disk plane needs to be determined based on conditions such as the rotation speed of the two propellers and the number of blades, and the size of the momentum source terms and their distribution forms in the two propeller disk planes may be different; in addition, since the front and rear propellers rotate in opposite directions, the tangential velocity calculation method of the airfoils in the two propeller disks is different, and the circumferential induction direction of the airflow is also opposite. This is a key issue in correctly applying the momentum source method to the aerodynamic calculation of coaxial counter-rotating propellers.

[0093] Among them, when substituting the source term size on the grid unit in the disk plane of the two blades into the RANS equation and solving the flow field iteratively, a stable flow field considering the interaction between the two blades is obtained. Specifically, it can be as follows:

[0094] The source term size on the grid unit in the disk plane of the two blades is substituted into the RANS equation, and the improved CFD iterative propulsion method is used to iteratively propel the flow field to obtain a stable flow field that takes into account the mutual influence of the two blades; the improved CFD iterative propulsion method is a CFD iterative propulsion method that introduces the momentum source term into the control equation in each iterative step according to the local incoming flow conditions at the position of the coaxial counter-rotating propellers.

[0095] Specifically, the determined source terms on the in-plane grid cells of the two propeller disks are substituted into the RANS equations, and the flow field is solved iteratively. This is similar to the traditional CFD iterative method and will not be described in detail here. During the calculation process, each iterative step adds a momentum source term to the governing equations based on the local flow conditions of the coaxial counter-rotating propellers, ultimately obtaining a stable flow field that takes into account the interaction between the two propellers.

[0096] In order to improve the calculation speed of the momentum source method of coaxial counter-rotating propellers, this embodiment uses periodic boundary conditions, as shown in the schematic diagram. Figure 8 As shown, it needs to be used in conjunction with the momentum source method grid of an isolated coaxial counter-rotating propeller after applying periodic boundary conditions.

[0097] Assuming the symmetry plane is in the direction I, then the symmetry boundary i=i max The physical quantities at and i=1 are as follows:

[0098]

[0099]

[0100]

[0101] Where p is the pressure of the grid cell, ρ is the density of the grid cell, and subscript i is max / 1 means i=i max or the grid cell on i=1, 2 / i max -1 means i=2 or i=i max-1 The grid cells on the disc are represented by u, v, and w, respectively, representing the velocity components of the grid cells along the three axes of the coordinate system. Data related to the thrust, power consumption, and spanwise load distribution of the two propellers were obtained. After the calculated flow field reaches a steady state, the force exerted on the air by each grid cell in the disc plane can be calculated based on the application of the momentum source term. This information can then be derived from the thrust, power consumption, and spanwise load distribution of the two propellers.

[0102] Finally, determine whether the aerodynamic performance of the coaxial counter-rotating propeller meets the requirements.

[0103] That is, analyze whether the blade load distribution and aerodynamic performance in the cruise state meet expectations; analyze whether the aerodynamic performance in the take-off state meets the requirements; if it does not meet the requirements, it is necessary to iteratively adjust the chord length distribution of the blade in step 3 and the airfoil distribution and torsion distribution of the blade in step 4.

[0104] This embodiment also provides a specific example for verification, which is as follows:

[0105] 1) In order to ensure that the established numerical simulation method can effectively analyze the aerodynamic characteristics of the coaxial counter-rotating propeller, the calculation method used in this embodiment is verified using the test data of the eight-blade coaxial counter-rotating propeller NACA3-(3)(05)-05. The diameter of the front and rear propellers of the counter-rotating propeller are both 0.91m, the distance between the two propellers is 0.1524m, both have 4 blades and the root cutting ratio is 0.36. The blades are composed of NACA-16 series airfoils and have similar geometric shapes, the same chord length distribution, different linear negative torsion slopes, and no forward or backward sweep distribution. The test measures the aerodynamic performance of the coaxial counter-rotating propeller by fixing the blade angle and changing the rotation speed of the two propellers at multiple speeds within the inflow Mach number range of 0.35-0.925.

[0106] Verification and comparison of the coaxial counter-rotating propeller's forward propeller ratio J at the incoming flow Mach number of 0.53 F The corresponding total tension coefficient C T , total power coefficient C P , comprehensive efficiency η, such as Figure 9 As shown in (a) and (b) in . When the forward propeller ratio is relatively large, the calculated results of the total thrust coefficient and the total power coefficient are basically consistent with the test results. When the forward propeller ratio is relatively small, the aerodynamic angle of attack of many blade sections of the front and rear propellers is large. At this time, these sections are in a stall state, and the momentum source method cannot make reliability predictions. Within the test range, the calculated results of the comprehensive efficiency changing with the forward propeller ratio are basically consistent with the test results. In summary, this shows that the calculation method has a certain reliability in evaluating the comprehensive aerodynamic performance of coaxial counter-rotating propellers, and can be used for preliminary engineering design or theoretical analysis of the aerodynamic aspects of coaxial counter-rotating propellers within an appropriate range.

[0107] 2) Comparison of calculation results and efficiency before and after applying periodic boundaries using the momentum source method:

[0108] In the comparison of this embodiment, the propeller disk of the complete cylindrical momentum source grid is evenly distributed with 90 grids, and the total number of grids is about 900,000. After applying the periodic boundary condition, the propeller disk of the momentum source grid only retains 3 grids, and the total number of grids is about 30,000. The calculation amount of using this grid is 1 / 30 of the calculation amount of using the complete cylindrical momentum source grid.

[0109] Table 1 Comparison of calculation results

[0110]

[0111] Among them, for the comparison of computational efficiency, the computational time consumed before applying the periodic boundary condition is 16606.3s, and the computational time consumed after applying the periodic boundary condition is 643.3s.

[0112] In summary, the application of periodic boundary conditions does not have a significant impact on the calculation results of the aerodynamic performance of the coaxial counter-rotating propeller, but the single calculation time is greatly reduced, shortening the calculation time to 1 / 26 of the original calculation time, that is, the calculation time of this method is approximately shortened to 1 / n of the original calculation time, where n is equal to "the amount of grid before applying the periodic boundary conditions / the amount of grid after applying the periodic boundary conditions".

[0113] 3) Applicability of the momentum source method:

[0114] This method can be used to calculate the aerodynamic performance of coaxial counter-rotating propellers with arbitrary aerodynamic layouts, such as arbitrary pitch, combination of two-propeller disk diameters, combination of two-propeller blade numbers, and combination of two-propeller speeds. The mutual influence of the two propellers can be taken into account and there is no need to modify the inflow models of the two propellers through experiments or empirical coefficients.

[0115] Example 2

[0116] like Figure 10 As shown, this embodiment provides a coaxial counter-rotating propeller design system based on a fast aerodynamic calculation method, including:

[0117] The design parameter acquisition module 1001 is used to obtain the layout design and working parameter design of the coaxial counter-rotating propeller; the layout includes the propeller diameter, the number of blades, the spacing, the rear propeller diameter cutting ratio, the rotation speed and the load distribution ratio; the working parameters include the incoming flow velocity of the spanwise position airfoil section and the cruising state; the cruising state is the state of the aircraft equipped with the coaxial counter-rotating propeller.

[0118] The blade spanwise load distribution determination module 1002 is configured to determine the blade spanwise load distribution in a cruising state using Goldstein theory based on the layout design and the operating parameter design.

[0119] The thickness distribution and chord length distribution calculation module 1003 is used to calculate the thickness distribution and chord length distribution of the blade according to the dimensionless spanwise position of the blade and the diameter of the propeller disk.

[0120] The airfoil distribution determination module 1004 is configured to determine the blade spanwise airfoil lift coefficient according to the blade spanwise load distribution, the blade thickness distribution, and the blade chord length distribution, and determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient.

[0121] The twist distribution calculation module 1005 is used to determine the twist distribution of the blade according to the incoming flow velocity of the airfoil section at the spanwise position based on the blade element theory.

[0122] The forward / backward sweep distribution determination module 1006 is used to determine the forward / backward sweep distribution of the blade based on aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord line of the blade does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to the moment of the blade around the pitch rotation axis under stress state is not greater than the upper limit of the structural strength.

[0123] The evaluation module 1007 is used to determine whether the aerodynamic performance of the blade meets the design requirements for cruise and take-off states based on the rapid momentum source aerodynamic calculation method.

[0124] When the design requirements for cruise and take-off states are met, the coaxial counter-rotating propeller design is completed.

[0125] When the cruise and take-off state design requirements are not met, the chord length distribution, airfoil distribution and twist distribution of the blade are updated, and calculations are performed based on the updated chord length distribution, airfoil distribution and twist distribution until the cruise and take-off state design requirements are met.

[0126] The airfoil distribution determination module 1004 specifically includes:

[0127] The blade spanwise airfoil lift coefficient distribution determination submodule is used to determine the blade spanwise airfoil lift coefficient distribution based on the blade thickness distribution and chord length distribution as well as the incoming flow conditions and operating parameters in the cruise state; the incoming flow conditions are determined based on the incoming flow velocity.

[0128] The airfoil distribution determination submodule is used to determine the airfoil distribution of the blade based on the spanwise airfoil lift coefficient distribution of the blade; the spanwise airfoil lift coefficient distribution of the blade is such that when the blade is at the spanwise position of 0.75R, the lift coefficient ranges from 0.4 to 0.6.

[0129] In summary, this application has the following technical effects:

[0130] This application comprehensively considers the aerodynamic, acoustic and structural characteristics of coaxial counter-rotating propellers, and proposes a reference coaxial counter-rotating propeller design process, which can be used to guide the design of coaxial counter-rotating propellers with better aerodynamic efficiency, lower aerodynamic noise and better structural characteristics; since the CFD high-precision aerodynamic calculation method has a large amount of calculation, in order to be able to evaluate the aerodynamic characteristics of the coaxial counter-rotating propeller and iteratively design it more quickly during the preliminary design process, this application proposes a momentum source method suitable for the aerodynamic performance evaluation of coaxial counter-rotating propellers. This method can be used to calculate the aerodynamic performance of coaxial counter-rotating propellers with any aerodynamic layout, and can consider the mutual influence of the two propellers without the need to modify and adjust the inflow model of the two propellers through experiments or empirical coefficients; in order to further improve the calculation speed of the momentum source method, the calculation speed of this method is greatly improved by applying periodic boundary conditions, so that its calculation time is approximately shortened to 1 / n of the original calculation time, where n is equal to "grid amount before applying periodic boundary conditions / grid amount after applying periodic boundary conditions".

[0131] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0132] This document uses specific examples to illustrate the principles and implementation methods of this application. The description of the above examples is only intended to help understand the method and core concept of this application. At the same time, for those skilled in the art, based on the concept of this application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A design method for coaxial counter-rotating propellers based on a fast aerodynamic calculation method, characterized in that: The coaxial counter-rotating propeller design method includes: Step 1: Obtain a layout design and operating parameter design for the coaxial counter-rotating propeller; the layout includes propeller diameter, number of blades, pitch, rear propeller diameter trim ratio, rotational speed, and load distribution ratio; the operating parameters include the incoming flow velocity of the spanwise airfoil section and the cruise state; the cruise state is the state of an aircraft equipped with the coaxial counter-rotating propeller; Step 2: Based on the layout design and operating parameter design, the Goldstein theory is used to determine the spanwise load distribution of the propeller blades in the cruise state. Step 3: Calculate the blade thickness distribution and chord length distribution based on the blade's dimensionless spanwise position and the disc diameter; Step 4: Determine the blade spanwise airfoil lift coefficient based on the blade spanwise load distribution, blade thickness distribution, and blade chord length distribution, and determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient; Step 5: Determine the blade twist distribution based on the blade element theory according to the incoming flow velocity of the airfoil section at the spanwise position; Step 6: Determine the forward / backward sweep distribution of the blade based on the aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord line of the blade does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to ensuring that the moment of the blade around the pitch rotation axis under load does not exceed the upper limit of the structural strength; Step 7: Based on the rapid momentum source aerodynamic calculation method, determine whether the aerodynamic performance of the blade meets the design requirements for cruise and takeoff states; When the cruise and take-off state design requirements are met, the coaxial counter-rotating propeller design is completed; When the cruise and takeoff state design requirements are not met, the chord length distribution, airfoil distribution, and twist distribution of the blade are updated, and based on the updated chord length distribution, airfoil distribution, and twist distribution, steps 5 to 7 are executed until the cruise and takeoff state design requirements are met.

2. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 1, characterized in that: The step 3 specifically includes: According to the formula Calculate the thickness distribution of the blade; According to the formula Calculate the chord length distribution of the blade; in, is the airfoil thickness, c is the airfoil chord, D is the disk diameter, and x is the dimensionless spanwise position of the blade.

3. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 1, characterized in that: The step 4 specifically includes: Determining the lift coefficient distribution of the blade spanwise airfoil according to the thickness distribution and chord length distribution of the blade, the incoming flow conditions and operating parameters in a cruising state; the incoming flow conditions are determined according to the incoming flow velocity; The airfoil distribution of the blade is determined based on the spanwise airfoil lift coefficient distribution of the blade; the spanwise airfoil lift coefficient distribution of the blade is such that at the spanwise position of 0.75R of the blade, the lift coefficient ranges from 0.4 to 0.

6.

4. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 1, characterized in that: The step 5 specifically includes: According to the formula Calculate the installation angle of the cross-section airfoil at different spanwise positions of the blade; where, is the profile airfoil installation angle, V y is the axial flow velocity, V yi is the induced velocity, ω is the rotational angular velocity, and α0 is the aerodynamic angle of attack corresponding to the lift coefficient of the cross-section airfoil; The installation angle of the cross-section airfoil at each spanwise position of the blade is calculated to determine the torsion distribution of the blade.

5. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 1, characterized in that: Based on the rapid momentum source aerodynamic calculation method, determine whether the aerodynamic performance of the blades meets the design requirements for cruise and takeoff states, including: Generate a grid for momentum source calculation of a coaxial counter-rotating propeller based on the spacing between the two blades; According to the diameters of the two blades, determine the grid cells in the disk plane where the source term is applied; According to the rotation speed and number of blades of the two blades, the size of the source term on the grid cell in the disk plane is determined; Substituting the source terms on the grid cells in the disk plane of the two blades into the RANS equation, the flow field is solved iteratively to obtain a stable flow field considering the mutual influence of the two blades. The aerodynamic performance of the blades is evaluated based on the steady flow field considering the interaction between the two propellers.

6. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 5, characterized in that: Based on the spacing between the two blades, a grid is generated for the momentum source calculation of the coaxial counter-rotating propeller, specifically including: According to the spacing between the two blades, a cylindrical structured momentum source grid is generated for momentum source calculation of the coaxial counter-rotating propeller; the cylindrical structured momentum source grid is used to make the two propeller disk planes located on both sides of the 1 / 2 height plane of the cylindrical momentum source grid, and the distances to the 1 / 2 height plane are equal.

7. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 5, characterized in that: Based on the diameters of the two blades, determine the grid cells in the disk plane where the source term is applied, specifically including: The size of the propeller disk plane is determined according to the diameters of the two blades, and the grid area in the propeller disk plane where the source term needs to be applied is determined; the source term is applied to the grid unit by a marking method; and the grid area includes a plurality of grid units.

8. The method for designing coaxial counter-rotating propellers based on a rapid aerodynamic calculation method according to claim 5, characterized in that: Substituting the source terms on the grid cells in the disk plane of the two blades into the RANS equation, the flow field is iteratively solved to obtain the stable flow field considering the mutual influence of the two blades. Specifically, it includes: The source term size on the grid unit in the disk plane of the two blades is substituted into the RANS equation, and the improved CFD iterative propulsion method is used to iteratively propel the flow field to obtain a stable flow field that takes into account the mutual influence of the two blades; the improved CFD iterative propulsion method is a CFD iterative propulsion method that introduces the momentum source term into the control equation in each iterative step according to the local incoming flow conditions at the position of the coaxial counter-rotating propellers.

9. A coaxial counter-rotating propeller design system based on a fast aerodynamic calculation method, characterized in that: include: A design parameter acquisition module is used to obtain the layout design and operating parameter design of the coaxial counter-rotating propeller; the layout includes propeller diameter, number of blades, pitch, rear propeller diameter cutting ratio, speed and load distribution ratio; The operating parameters include the incoming flow velocity of the airfoil section at the spanwise position and the cruise state; the cruise state is the state of the aircraft equipped with coaxial counter-rotating propellers; The blade spanwise load distribution determination module is used to determine the blade spanwise load distribution in the cruise state based on the layout design and operating parameter design using the Goldstein theory; Thickness distribution and chord length distribution calculation module, used to calculate the thickness distribution and chord length distribution of the blade based on the dimensionless spanwise position of the blade and the propeller disk diameter; an airfoil distribution determination module, configured to determine a blade spanwise airfoil lift coefficient according to the blade spanwise load distribution, the blade thickness distribution, and the blade chord length distribution, and to determine the blade airfoil distribution based on the blade spanwise airfoil lift coefficient; The twist distribution calculation module is used to determine the twist distribution of the blade based on the blade element theory according to the incoming flow velocity of the airfoil section at the spanwise position; A forward / backward sweep distribution determination module is used to determine the forward / backward sweep distribution of the blade based on aerodynamic performance and structural strength requirements; the aerodynamic performance refers to ensuring that the local Mach number of each airfoil section in the direction perpendicular to the quarter chord line of the blade does not exceed the drag divergence Mach number corresponding to the cruise design point; the structural strength requirement refers to ensuring that the moment of the blade around the pitch rotation axis under load does not exceed the upper limit of the structural strength; An evaluation module, which uses a rapid momentum source aerodynamic calculation method to determine whether the blade's aerodynamic performance meets the design requirements for cruise and takeoff conditions; When the cruise and take-off state design requirements are met, the coaxial counter-rotating propeller design is completed; When the cruise and take-off state design requirements are not met, the chord length distribution, airfoil distribution and twist distribution of the blade are updated, and calculations are performed based on the updated chord length distribution, airfoil distribution and twist distribution until the cruise and take-off state design requirements are met.

10. A coaxial counter-rotating propeller design system based on a fast aerodynamic calculation method according to claim 9, characterized in that: The airfoil distribution determination module specifically includes: A blade spanwise airfoil lift coefficient distribution determination submodule is used to determine the blade spanwise airfoil lift coefficient distribution based on the blade thickness distribution and chord length distribution, as well as the incoming flow conditions and operating parameters in the cruise state; the incoming flow conditions are determined based on the incoming flow velocity; The airfoil distribution determination submodule is used to determine the airfoil distribution of the blade based on the spanwise airfoil lift coefficient distribution of the blade; the spanwise airfoil lift coefficient distribution of the blade is such that when the blade is at the spanwise position of 0.75R, the lift coefficient ranges from 0.4 to 0.6.

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