Propeller design for unmanned aerial vehicle based on aerodynamic performance and noise optimization

Abstract

The invention discloses an unmanned aerial vehicle propeller design based on aerodynamic performance and noise optimization, and relates to the field of propeller structure design. Aiming at the problem of contradiction between performance and noise optimization design during design of an existing propeller structure, a new design and research thought of the propeller structure for the unmanned aerial vehicle is given, and the propeller structure for the unmanned aerial vehicle which meets aerodynamic performance and is low in aerodynamic noise is designed in a multi-main-microparameter cooperation mode. According to the research thought, the influence effect on the aerodynamic performance and noise of the propeller under different measures can be analyzed, meanwhile, the aerodynamic performance and noise reduction are considered, the design scheme of the propeller for the unmanned aerial vehicle with the maximum take-off thrust, the optimal cruise efficiency and the minimum operation noise is designed under the condition that the service time and the service life of the propeller are guaranteed, and the design method is suitable for popularization and application. The thinking steps are simple and short in time consumption, and basis is provided for subsequent propeller design in the same field.

Description

Technical Field:

[0001] The present invention relates to the field of the structural design of propellers for unmanned aerial vehicles, and particularly to an optimal design method and process for the installation angle - blade radius defined by different operating points. Background Art:

[0002] As the main power source of turboprop aircraft, propellers are more fuel-efficient than turbofan engines because of their advantages such as low fuel consumption rate, good economy, high safety, and short takeoff and landing distance. Therefore, turboprop aircraft play an important role in the modern regional aviation market. At present, for the design of the structural parameters of propellers for unmanned aerial vehicles, only the design of parameters such as the number of blades, chord length, outer radius, blade angle, and hub radius meets the process constraints and actual production requirements. Based on the blade element theory, the thrust and efficiency are deduced to analyze and obtain the structural parameters suitable for propellers for unmanned aerial vehicles. However, the above analysis methods and processes do not fully consider the influence brought by aerodynamic performance and noise, resulting in the failure to achieve the optimal design of the installation angle - blade radius under different operating points for the designed propeller parameters of the aircraft; on the other hand, there is also a certain deviation problem between the performance of the propeller of the aircraft designed by the existing design process for propellers for unmanned aerial vehicles and the actual operating performance of the physical object. Summary of the Invention:

[0003] In view of the above-mentioned defects in the prior art, the present invention proposes a design of a propeller for an unmanned aerial vehicle optimized based on aerodynamic performance and noise, and sets the design ranges of multiple main parameters such as the number of blades, chord length, outer radius, blade angle, and hub radius according to requirements. Ensure that the design ranges meet the rationality of process constraints and actual production, and based on the blade element theory, new calculation formulas for thrust and efficiency are deduced under the design ranges of multiple main parameters. The new calculation formulas make the target values such as the propeller thrust, efficiency, and engine input power only related to the interference angle and the lift-drag ratio angle. The lift-drag ratio angle is limited within 3° according to design experience, the interference angle is analyzed and designed, and the error of the interference angle between different operating points is corrected according to a large number of simulation data. Combining this theory, and based on different operating points, an optimal design curve of the installation angle - blade radius is defined. Using the above design method and process for the propeller for an unmanned aerial vehicle, the traditional efficiency calculation method that requires complex integral operations can be abandoned, which greatly saves time. At the same time, it is also necessary to consider that the power absorbed on the propeller shaft of the propeller should match the power provided by the engine in the aircraft, so as to meet the propeller parameter design for the unmanned aerial vehicle under different operating point performance requirements.

[0004] The purpose of the present invention is to propose a propeller design for unmanned aerial vehicle based on aerodynamic performance and noise optimization based on the engineering problem of the deviation between the operating performance and the actual value of the propeller for unmanned aerial vehicle in combination with the actual design requirements of the engineering, determine the assumed range of multiple main parameters of the propeller according to the operating performance parameter requirements of the actual operating point of the propeller for unmanned aerial vehicle, and accurately calculate the thrust and efficiency of the propeller based on the blade element theory. The method of interference angle correction can accurately calculate the pulling force, torque, input power, output power and efficiency generated by the propeller in the actual scenario that meets the engineering requirements, and then obtain the optimal curve of the installation angle limited based on different operating points; on the other hand, by analyzing the noise spectrum measured by the propeller aircraft, the propeller aerodynamic noise prediction value at different operating points is carried out, and according to the reasonable propeller noise reduction theory, the propeller multi-objective-multi-main parameter optimization based on the NSGA-Ⅱ algorithm is carried out, and the propeller thrust, speed cloud map, pressure cloud map, propeller air domain flow velocity cloud map, and aerodynamic noise sound pressure level curve numerical simulation are used to verify it.

[0005] Compared with the prior art, the present invention can achieve the following technical effects:

[0006] The proposed UAV propeller design process based on aerodynamic performance and noise optimization can realize the optimal curve calculation based on different working points to limit the installation angle, as well as the UAV propeller parameter design. The entire design process gets rid of complex integral operations, making the calculation process simpler and greatly shortening the calculation time. In this process, aerodynamic performance requirements and noise reduction can also be considered.

[0007] The concept and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, features and effects of the present invention. Description of the drawings:

[0008] Figure 1 The propeller front view and plan view;

[0009] Figure 2 Propeller blade theory blade element forces;

[0010] Figure 3 Propeller thrust simulation in real-world scenarios;

[0011] Figure 4 Propeller speed cloud map in the example scenario;

[0012] Figure 5 Propeller pressure cloud diagram in the example scenario;

[0013] Figure 6 Propeller air domain velocity cloud map in the example scenario;

[0014] Figure 7Sound pressure level curve of propeller aerodynamic noise under example scenarios;

[0015] Figure 8 Propeller design process for drones based on aerodynamic performance and noise optimization; Specific implementation method:

[0016] To make the objectives, technical solutions, and advantages of this invention patent clearer, the following will, in combination with specific examples, clearly and completely describe the technology in this invention. However, it should be understood that these descriptions are exemplary and do not limit the scope of this invention patent.

[0017] Step 1: Select 6 parameters that can characterize and determine the range of propeller size and shape parameters. As Figure 1 shown, where X1 is the number of blades. Generally, there is no strict regulation, but the number of blades has a significant impact on aspects such as aerodynamic performance, noise characteristics, structural stress, and manufacturing cost. For a propeller with a given power, adding one more blade will add an additional three-dimensional loss source, which means an increase in the tip loss of the propeller. At the same time, as the number of blades increases, the load on a single propeller blade will decrease accordingly. Selecting more propeller blades can absorb the power generated by the engine more fully. Therefore, the design of the number of blades should be determined first; X2 is the outer radius, which is usually pre-determined by the overall parameters of the drone and appropriately adjusted according to the actual needs of propeller performance design. The diameter of the fan section should be minimized under the premise of meeting the requirements of aerodynamic design indicators; X3 is the hub radius. The ratio of the propeller hub radius to the outer radius is defined as the hub ratio. The hub ratio has an important impact on the propeller rotation speed, efficiency, etc. In addition to considering the dimensions required by the structural design, the hub radius should be sized to improve the fan efficiency and needs to be matched and designed with the propeller radius, rotation speed, etc., and iteratively optimized; X4 is the chord length; X5 is the blade angle. The angle between the blade chord line and the rotation axis is called the blade angle; X6 is the thickness. The thickness distribution at each radius of the propeller needs to be further determined according to the actual use of the propeller. The overall structural strength and gas flow characteristics of the blade determine the thickness distribution at different radii of the blade. Generally, the thickness decreases monotonically from the root to the tip of the blade. To ensure that the blade does not fall off, the relative thickness at the root is 20%-30%, and at the tip, considering the comprehensive requirements for noise and structural strength, the thickness is generally 3%-5%;

[0018] Step 2: Now intercept the blade element at radius r and obtain the forces on the blade element according to aerodynamic theory as Figure 2 shown, where dL is the lift force acting on the blade element; dD is the drag force acting on the blade element; dR is the total aerodynamic force on the blade element; dT is the thrust force on the blade element; dF is the tangential force on the blade element; V0 is the forward speed of the propeller; V t is the circumferential speed of the propeller; V ais the axial velocity of the propeller; Φ0 is the angle between the resultant velocity and the rotational plane; θ is the installation angle of the blade element; α is the angle of attack of the airflow relative to the blade element; β is the interference angle; γ is the lift-drag angle; the angle between the resultant velocity Φ0 and the rotational plane satisfies Let the thrust obtained from the momentum equation be equal to the thrust obtained from the blade element force analysis, then we can get: In the formula, σ is the propeller solidity, which satisfies:

[0019] Step 3: The lift-drag angle γ is generally designed to be within 3 degrees. Thus, according to the derived theoretical formula, the thrust and efficiency of the propeller are mainly affected by the interference angle β. Therefore, considering the thrust and efficiency requirements under three working conditions: the propeller aircraft at the working point one serves at an altitude of 0 km, the propeller aircraft at the working point two serves at an altitude of 4.5 km, and the propeller aircraft at the working point three serves at an altitude of 7 km. Take β = θ - α - Φ0. To make the interference angle β meet the above requirements, the optimized design of the installation angle θ should be more fitted to the Φ0 curve at 7 km in the first half of the propeller and higher than the Φ0 curve at 7 km in the second half. The fitting function of the θ curve is obtained as;

[0020] Step 4: Calculate the sound power of the propeller. In the formula: E is the sound power generated by all the blades of the propeller, B is the number of blades of the propeller, g is the acceleration due to gravity, γ and a0 are the density of still air and the airspeed respectively, D is the width of the blade trailing edge wake, W is the airspeed relative to the blade, R is the radial distance from the propeller blade tip to the axis of rotation, D R is the propeller diameter, U t is the tangential velocity at the blade tip; From Step 1, W and D are estimated in advance, and D can be appropriately defined as the sum of the blade thickness D t and the displacement thickness δ of the turbulent boundary layer at the trailing edge of a flat plate with the same chord length as the blade and an incident angle of zero, that is * The sum, that is In the formula: R ec is the Reynolds number based on the blade chord length C and the flow velocity relative to the blade; On the other hand, the relationship between the radiated sound power E measured at a distance Z along the axis of the impeller and the sound pressure level SPL is: In addition, according to the process of the propagation and attenuation of the propeller aerodynamic noise, the sound pressure level attenuation formula is given: In the formula: L p is the sound pressure level (in dB) at a distance r from the source point, L p0 is the sound pressure level at the reference distance r0, and r0 is usually 1 meter. So when the distance doubles (10lg2 = 3), the sound pressure level attenuates by 3 dB;

[0021] Step 5: After obtaining the calculation formulas for the three objectives of thrust, efficiency, and noise in Steps 2 and 3, the multi-objectives of the propeller can be abstracted as a function controlled by the main parameters of the propeller geometric model (a1 - number of blades, a2 - outer radius, a3 - hub radius, a4 - chord length, a5 - thickness), that is: Combined with the main parameter range limitations described in Step 1, a mathematical model for multi-objective optimization of the propeller main parameters is obtained. Finally, multi-objective and multi-main parameter optimization is carried out based on the NSGA-II genetic algorithm, and it is verified through numerical simulations of the propeller thrust, velocity cloud map, pressure cloud map, flow velocity cloud map in the propeller air domain, and the curve of the aerodynamic noise sound pressure level under different operating point scenarios, as Figure 3 、 4 、5, 6, and 7 show. The proposed design process of the propeller for UAVs based on aerodynamic performance and noise optimization is as Figure 8 shown.

[0022] To verify the accuracy of the engineering application of the present invention, a certain propeller design example is used to prove it. The Fluent software is used for fluid-structure interaction calculation to simulate whether the performance indicators of the propeller under the operating point of serving at an altitude of 4.5 km meet the theoretical calculation;

[0023] The propeller thrust simulation under the example scenario is as Figure 3 shown. When the propeller serves at an altitude of 4.5 km, after tending to be stable, the thrust value is 4730 N. When the propeller serves at an altitude of 7 km, after tending to be stable, the thrust value is 2399 N. For the propeller serving at 0 km, because the curve tends to be stable with more steps, it is set to 6000 here. After tending to be stable, the thrust value is 10375 N; the error between the theoretically obtained value and the simulation value meets the design requirements.

[0024] The propeller thrust simulation under the example scenario is as Figure 4 shown. When the propeller serves at an altitude of 4.5 km, the speed at the propeller tip is the maximum, with a maximum speed of 323 m / s, and it gradually decreases to the root position, with a minimum speed of 1.63 m / s. When the propeller serves at an altitude of 7 km, the speed at the propeller tip is the maximum, with a maximum speed of 294 m / s, and it gradually decreases to the root position, with a minimum speed of 1.47 m / s. When the propeller serves at 0 km, its rotational speed is the same as that when serving at an altitude of 4.5 km.

[0025] The propeller pressure cloud map under the example scenario is as Figure 5As shown in the figure, when the propeller serves at an altitude of 4.5 km, the pressure value at the tip of the propeller blade is the smallest, and it continuously increases from the tip of the blade. The connection part between the root of the blade and the hub reaches the maximum value. The maximum pressure is 22000 Pa, and the minimum pressure is -29200 Pa. When the propeller serves at an altitude of 7 km, the pressure value at the tip of the propeller blade is the smallest, and it continuously increases from the tip of the blade. The connection part between the root of the blade and the hub reaches the maximum value. The maximum pressure is 13000 Pa, and the minimum pressure is -17000 Pa. When the propeller serves at an altitude of 0 km, the pressure value at the tip of the propeller blade is the smallest, and it continuously increases from the tip of the blade. The connection part between the root of the blade and the hub reaches the maximum value. The maximum pressure is 38800 Pa, and the minimum pressure is -49200 Pa. The main reason is not only that the rotational speed of the propeller when serving at an altitude of 0 km is greater than that at an altitude of 4.5 km which is greater than that at an altitude of 7 km, but also that the air density at an altitude of 7 km is less than that at an altitude of 4.5 km which is less than that at an altitude of 0 km.

[0026] The velocity cloud diagram of the air domain of the propeller under the example scenario is as Figure 6 shown. The air flow enters from the inlet and is accelerated when encountering the propeller. The air flow above the propeller is continuously compressed and distributed in an umbrella shape. And although the indicated airspeed of the propeller when serving at an altitude of 4.5 km is less than that when serving at an altitude of 7 km, the fluid velocity of the air domain of the propeller. The maximum velocity at the working point of 4.5 km is 89.7 m / s, which is still greater than the maximum velocity of 78.4 m / s at the working point of 7 km. For the propeller when serving at an altitude of 0 km, the velocity cloud diagram of the air domain of the propeller is significantly different from that of the other two working points. The main reason is that when the propeller serves at an altitude of 0 km, its indicated airspeed is zero, and the fluid velocity of most parts in the air domain is 0 m / s. The maximum velocity is 81.3 m / s. This figure can also best reflect the aerodynamic performance generated by the designed propeller.

[0027] The curve of the aerodynamic noise sound pressure level of the propeller under the example scenario is as Figure 7 shown. The amplitude of the aerodynamic noise sound pressure level of the designed propeller is 131.5 dB. It meets the basic target value of the design requirements, further demonstrating the effectiveness of the present invention. Transplanting the results of the present invention into the calculation of the same type of propeller can also achieve the same effect, and no example demonstration will be given here.

[0028] The above examples describe the principle and technical route of the present invention for the patent, rather than limiting the present invention. What is described in the above examples and the specification is only to illustrate the logic and idea of the present patent. Without departing from the spirit, idea and scope of the invention patent, various changes and improvements will occur to the present invention patent, and these changes and improvements will fall within the scope of the invention patent to be protected.

Claims

1. Design of propeller for UAV based on aerodynamic performance and noise optimization, characterized by: The propeller design for the UAV is a parameter design process under the coordination of multiple main and micro parameters. The process can define the optimal design curve of the installation angle-blade radius according to the design range of the multiple main parameters and in combination with different working points. The specific implementation process of the method can be divided into the following steps: S1. Select 6 parameters that can characterize and determine the range of propeller size and shape parameters, among which X1 is the number of blades. There is usually no strict regulation, but the number of blades has a dramatic impact on aerodynamic performance, noise characteristics, structural stress, and manufacturing costs. For a propeller with a given power, each additional blade will add an additional three-dimensional loss source, which means that the propeller tip loss is increased. At the same time, as the number of blades increases, the load of a single propeller blade will also decrease. Selecting more propeller blades can more fully absorb the power generated by the engine. Therefore, the design of the number of blades should be determined first; X2 is the outer radius, which is usually predetermined by the overall parameters of the UAV and adjusted appropriately according to the actual needs of the propeller performance design. The fan section diameter should be minimized as much as possible while meeting the requirements of the aerodynamic design indicators; X3 is the hub radius, and the ratio of the propeller hub radius to the outer radius is defined as the hub ratio. The hub ratio has an important influence on the propeller speed, efficiency, etc. In addition to considering the dimensions required by the structural design, the hub radius should be conducive to improving the fan efficiency. It needs to be designed to match the propeller radius, speed, etc. and iteratively optimized; X4 is the chord length; X5 is the blade angle. The angle between the blade chord and the axis of rotation is called the blade angle; X6 is the thickness. The thickness distribution at each radius of the propeller should be further determined based on the actual use of the propeller. The overall structural strength of the blade and the gas flow characteristics determine the thickness distribution at different radii of the blade. In general, the thickness decreases monotonically from the root to the tip of the blade. In order to ensure that the blade will not fall off, the relative thickness at the root of the blade is 20%-30%, and at the tip of the blade, in order to consider the comprehensive requirements for noise and structural strength, the thickness is generally 3%-5%; S2. Now intercept the blade element at radius r and obtain the blade element force diagram according to aerodynamic theory, where dL is the lift acting on the blade element; dD is the drag acting on the blade element; dR is the total aerodynamic force on the blade element; dT is the pull on the blade element; dF is the tangential force on the blade element; V0 is the propeller forward speed; V t is the propeller circumferential speed; V a is the propeller axial speed; Φ0 is the angle between the composite speed and the rotating plane; θ is the installation angle of the blade element; α is the angle of attack of the airflow relative to the blade element; β is the interference angle; γ is the drag lift angle; the angle Φ0 between the composite speed and the rotating plane satisfies, Let the tension obtained by the momentum equation be equal to the tension obtained by the blade element force analysis: Where σ is the propeller solidity, which satisfies: S3. The drag lift angle γ is generally designed to be within 3 degrees. According to the derived theoretical formula, the thrust and efficiency of the propeller are mainly affected by the interference angle β. Therefore, considering the thrust and efficiency requirements under three working conditions, namely, the working point 1 propeller aircraft is in service at an altitude of 0 km, the working point 2 propeller aircraft is in service at an altitude of 4.5 km, and the working point 3 propeller aircraft is in service at an altitude of 7 km, β=θ-α-φ0 is taken. In order to make the interference angle β meet the above requirements, the optimal design of the installation angle θ should be closer to the Φ0 curve of 7 km in the front half of the propeller, and higher than the Φ0 curve of 7 km in the back half, and the fitting θ curve function is obtained; S4. Calculate the propeller sound power. Where: E is the sound power generated by all propeller blades, E is the number of propeller blades, g is the acceleration of gravity, γ and a0 are the density and airspeed of still air respectively, D is the trailing edge width of the blade, W is the air speed relative to the blade, R is the radial distance from the propeller tip to the rotation axis, and D R is the propeller diameter, U t is the tangential velocity at the blade tip; W and D are estimated in advance from the main parameters of step 1, and D can be appropriately defined as the blade thickness D t The displacement thickness of the turbulent boundary layer at the trailing edge of the flat plate with the same chord length as the blade and zero incidence angle is δ * The sum of Where: R ec is the Reynolds number based on the blade chord length C and the flow velocity relative to the blade; on the other hand, the relationship between the radiated sound power E measured at the distance Z from the impeller axis and the sound pressure level SPL is: In addition, according to the propagation and attenuation process of propeller aerodynamic noise, the sound pressure level attenuation formula is given: Where: L p is the sound pressure level (in dB) at a distance r from the source, L p0 It is the sound pressure level at the reference distance r0, which is usually 1 meter. So when the distance doubles (10lg2=3), the sound pressure level attenuates by 3dB. S5. After obtaining the calculation formulas for the three objectives of thrust, efficiency and noise by combining steps 2 and 3, the propeller multi-objective can be abstracted into a function controlled by the main parameters of the propeller geometric model (a1-number of blades, a2-outer radius, a3-hub radius, a4 chord length, a5-thickness), that is: Combined with the main parameter range restrictions described in step one, the mathematical model for multi-objective optimization of propeller main parameters is obtained, and finally multi-objective and multi-main parameter optimization is performed based on the NSGA-Ⅱ genetic algorithm.

2. According to the chord length X4 of claim 1, the chord length assumption range is selected at 75% of the outer radius of the propeller. As the propeller chord length spreads along the outer radius, the chord length value increases first and then decreases; and two assumptions are provided: Assumption 1: The chord length along the outer radius from position X3 to position X2 is a piecewise linear function relationship: Assumption 2: The chord length along the outer radius from position X3 to position X2 is a piecewise linear function relationship: In the formula: a1, a2, b1, b2, c1, c2, d1, d2, e2, f2 are all constants, and it is assumed that a1<c1 in 1.

3. The multi-objective multi-primary parameter optimization according to claim 1, characterized in that: According to the genetic algorithm, the propeller thrust formula, efficiency formula and noise formula are simulated and verified by the fluid-solid coupling method.

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