Propeller design method suitable for distributed electric propulsion aircraft

Abstract

According to the propeller design method suitable for the distributed electric propulsion aircraft, the strong coupling effect of aerodynamic force and propulsive force between the distributed propeller and wings is utilized, and the change of aerodynamic force of the wings is controlled by changing slip flow velocity distribution of the distributed propeller. The method comprises the following steps: firstly, according to an aircraft design task requirement, obtaining propeller design parameters, calculating a required axial induced velocity, obtaining geometric parameter distribution of the propeller based on a blade element momentum theory and an eddy current theory, and locally optimizing key positions such as a propeller tip and a propeller root, so as to obtain the high-efficiency propeller suitable for the distributed electric propulsion aircraft. The efficiency of the propellers is higher while the lift-drag characteristics of the whole aircraft are kept consistent.

Description

Technical Field

[0001] The invention belongs to the technical field of aviation aircraft, and in particular relates to a propeller design method suitable for distributed electric propulsion aircraft. Background Art

[0002] Distributed Electric Propulsion (DEP) is a new technology that uses electric motors to drive multiple propellers distributed on the wings or fuselage to provide power for the aircraft, making full use of the propulsion-aerodynamic coupling effect to reduce the wing area, increase the lift-to-drag ratio, and improve the economy of the aircraft. It often uses tilt propellers, which can achieve multi-state flight such as vertical takeoff and landing on the ground, hovering in the air, and high-speed cruise forward flight by tilting the propellers within a certain angle range. It is currently widely used by emerging electric vertical takeoff and landing (eVTOL) aircraft. As the type of aircraft energy changes from oil to electric, the efficiency of its power system is particularly significant.

[0003] Compared with traditional aircraft, there are complex aerodynamic interferences between propellers and other components of distributed electric propulsion aircraft, which seriously affects the efficiency of the power system. Through reasonable propeller aerodynamic design and full use of the favorable coupling between propellers and wings, the efficiency of propellers in distributed electric propulsion aircraft can be improved. Summary of the invention

[0004] For emerging electric-driven propeller aircraft such as eVTOL and eSTOL, it is necessary to propose a propeller design method that is suitable for propeller designs of different configurations and different design requirements.

[0005] The present invention is achieved through the following technical solutions.

[0006] A propeller design method suitable for a distributed electric propulsion aircraft comprises the following steps:

[0007] Step S1: according to the performance of the distributed electric propulsion aircraft, the aircraft dimensions and the propeller disk load requirements, determine the aircraft flight speed V, the radius R of the propeller, the thrust T and the number of blades B;

[0008] Step S11: determining the maximum rotation speed according to the blade tip Mach number limit, and selecting a given rotation speed N;

[0009] Step S12: selecting an airfoil whose lift-to-drag ratio meets the initial design requirements, and determining the design angle of attack α of the airfoil;

[0010] Step S13: Control the slipstream speed: According to the blade element momentum theory, the propeller slipstream axial induction factor a is obtained through the thrust

[0011] T=2πR 2 ρV 2 (1+a)a (1)

[0012] Step S14: According to the blade element momentum theory, the thrust expression has the following relationship:

[0013] V 2 (1+a)a=Ω 2 r 2 (1-b)b (2)

[0014] The tangential induction factor can be obtained:

[0015]

[0016] After the axial induction factor a is determined, the tangential induction factor b will also be determined. However, according to equation (3), the maximum value of the tangential induction factor b is 0.5, so when equation (3) fails, equation (4) needs to be used to determine the tangential induction factor b.

[0017] In formula (1-4), T is the design thrust, R is the propeller radius, ρ is the air density, a is the axial induction factor, V is the flight speed, Ω is the angular velocity of the cross-sectional blade element, r is the distance from the blade element to the hub, and b is the tangential induction factor.

[0018] Step S2: Geometric parameter distribution

[0019] After determining the distribution of the axial induction factor and the tangential induction factor, the local inflow angle at the radial position r can be calculated by equation (5): The airfoil attack angle α is given before design, and the twist angle β is determined according to equation (6):

[0020]

[0021] Due to the tip loss, the propeller blade tip will not bear much load, which will also reduce the tip speed. Increasing the tip load can be achieved by increasing the axial induction factor near the blade tip. The desired effect can be achieved by applying the modified Prandtl tip loss coefficient F to the axial induction factor, as shown in equation (9), where f can be calculated from equation (8).

[0022]

[0023] According to the blade element theory and momentum theory, there are two separate expressions of thrust T, equation (10) and equation (11), and the expression of local chord length c is obtained (12).

[0024] dT=4πrρV 2 (1+a)aFdr (10)

[0025]

[0026] In formula (5-12), V is the flight speed of the aircraft, Ω is the angular velocity of the blade element, r is the distance from the blade element to the hub, a is the axial induction factor, b is the tangential induction factor c l is the airfoil lift coefficient, c d is the airfoil drag coefficient, B is the number of blades, and R' is taken as 1.03 to 1.05 times R according to experience;

[0027] Step S3: Iteratively calculate the torsion angle β and chord length c at each radial point of the propeller, where the iterative convergence condition is the inflow angle The difference between the two calculations is less than 0.5°;

[0028] Step S31: The final propeller torsion angle β and chord length c and other geometric parameters are obtained through iteration of step S3 to complete the rapid propeller design.

[0029] Step S4: Slipstream fast calculation method

[0030] The induced velocity field of propeller slipstream is non-uniform and the calculation is complex. This model adopts the slipstream tube model and simplifies the spiral wake into two continuous vorticity distributions: axial vortex γ x and tangential vortex γ t The axial vortices are parallel to the axis of rotation and are distributed on a concentrically placed cylindrical tube; the tangential vortices are distributed on the same cylindrical surface but perpendicular to the axial vortices.

[0031] Step S41: In the slipstream tube model, the induced velocity dV of the vortex element segment dl at any point P p It can be expressed as formula (13):

[0032]

[0033] The induced velocity of the axial vortex is calculated by formula (14-16):

[0034] u x =0 (14)

[0035]

[0036] The induced velocity of the tangential vortex is calculated by formula (17-19):

[0037]

[0038] In equations (16) and (19):

[0039] e=rsinψ-y p , k = -rcosψ-zp ;

[0040] In formula (13-19), M is the vector from point P to the infinitesimal segment; Г represents the size of the infinitesimal segment circulation;

[0041] The propeller design method based on blade momentum and vortex theory itself has simplifications and assumptions, and the design results generally have certain deviations from the actual situation. After obtaining the propeller chord length distribution and torsion angle distribution, the propeller slipstream distribution is checked by the above-mentioned propeller slipstream fast calculation method, and it is iteratively corrected until the requirement of uniform axial induced velocity distribution is met.

[0042] Compared with the prior art, the advantages of the present invention are: while maintaining the lift and drag characteristics of the entire aircraft, the present invention can design a propeller with uniform slipstream by thrust or slipstream speed, and the propeller efficiency is higher. The design method of the present invention can adapt to the design of propellers with different configurations and different design requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a flow chart of the present invention;

[0044] Figure 2 is the blade element velocity triangle of the present invention;

[0045] Figure 3 The slipstream tube model of the present invention;

[0046] Figure 4 is the induced velocity of the vortex system of the slipstream tube model of the present invention at any point P, the left side is the axial vortex system, and the right side is the tangential vortex system;

[0047] Figure 5 It is the propeller chord length and torsion angle station diagram of the example of the present invention;

[0048] Figure 6 is a propeller axial velocity diagram of an example of the present invention;

[0049] Figure 7 The geometric model diagram of the propeller is quickly modeled by the geometric parameters of the present invention. DETAILED DESCRIPTION

[0050] In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in combination with implementation methods and flowcharts, but it is not intended to limit the present invention.

[0051] The key concept of the present invention is to use a rapid design method, consider the slipstream generated by the propeller, perform CFD calculation and momentum blade element theory calculation to obtain the aerodynamic characteristics of the propeller, and iteratively correct the propeller geometric parameters to quickly model the propeller geometric model.

[0052] The specific design method is:

[0053] (1) In view of the need for a large number of propellers in distributed electric propulsion aircraft configurations, the propeller cruise conditions, including cruise speed V, rotation speed N, propeller radius R, number of blades B, air density ρ, thrust T, and airfoil angle of attack, are given. Based on blade element momentum theory and vortex theory, the radial distribution of the propeller axial induction factor a is determined;

[0054] (2) Control the propeller slipstream by changing the axial induced velocity distribution and iteratively determine the propeller geometric parameters chord length and torsion angle distribution;

[0055] (3) The thrust and axial induction factor are obtained based on the blade element momentum theory, and the tangential induction factor is then calculated;

[0056] (4) Calculate the local inflow angle at the radial position. The sum of the local inflow angle and the angle of attack is the torsion angle.

[0057] (5) By increasing the axial induction factor near the blade tip, the blade tip loss is compensated and a more realistic chord length and twist angle are obtained. The twist angle needs to be iteratively calculated to obtain the value at each radial point of the propeller;

[0058] (6) After the above calculations are completed, calculate the drag coefficient, power coefficient and hovering efficiency in hovering and cruising states;

[0059] (7) After the axial induced velocity is evenly distributed, the geometric parameters are imported into the rapid modeling program to obtain the propeller geometric model.

[0060] First, according to the design requirements, the initial chord length and torsion angle distribution are quickly designed; after the initial chord length and torsion angle distribution are determined, the propeller slipstream fast calculation method can be used to calculate the distribution of the propeller slipstream axial velocity along the propeller radial direction;

[0061] By giving the propeller cruising conditions, the axial and tangential induction factors are determined.

[0062] Step S1: According to the performance of the distributed electric propulsion aircraft, the aircraft dimensions and the propeller load requirements, the flight altitude H = 500m and the air density ρ at this altitude = 1.1566kg / m 3 , flight speed V0 = 60m / s, propeller radius R = 0.48m, number of blades B = 5, design tension T = 850N;

[0063] Step S11: determining the maximum rotation speed according to the blade tip Mach number limit, and selecting a given rotation speed N=3000 rpm;

[0064] Step S12: selecting the MH114 airfoil whose lift-to-drag ratio meets the initial design requirements, and determining the design angle of attack α of the airfoil;

[0065] Step S13: Control the slipstream speed: According to the blade element momentum theory, the propeller slipstream axial induction factor a is obtained through the thrust

[0066] T=2πR 2 ρV 2 (1+a)a (1)

[0067] Step S14: According to the blade element momentum theory, the thrust expression has the following relationship:

[0068] V 2 (1+a)a=Ω 2 r 2 (1-b)b (2)

[0069] The tangential induction factor can be obtained:

[0070]

[0071] After the axial induction factor a is determined, the tangential induction factor b will also be determined. However, according to equation (3), the maximum value of the tangential induction factor b is 0.5, so when equation (3) fails, equation (4) needs to be used to determine the tangential induction factor b.

[0072] After determining the distribution of the axial induction factor a and the tangential induction factor b, the local inflow angle is calculated, and the twist angle is determined by the given airfoil angle of attack.

[0073] In formula (1-4), T is the design thrust, R is the propeller radius, ρ is the air density, a is the axial induction factor, V is the flight speed, Ω is the angular velocity of the cross-sectional blade element, r is the distance from the blade element to the hub, and b is the tangential induction factor.

[0074] Step S2: Geometric parameter distribution

[0075] The local inflow angle at the radial position r is calculated by equation (5): The airfoil attack angle α is given before design, and the twist angle β is determined according to equation (6):

[0076]

[0077] Due to the tip loss, the propeller blade tip will not bear much load, which will also reduce the tip speed. Increasing the tip load can be achieved by increasing the axial induction factor near the blade tip. The desired effect can be achieved by applying the modified Prandtl tip loss coefficient F to the axial induction factor, as shown in equation (9), where f can be calculated from equation (8).

[0078]

[0079] According to the blade element theory and momentum theory, there are two separate expressions of thrust T, equation (10) and equation (11), and the expression of local chord length c is obtained (12).

[0080] dT=4πrρV 2 (1+a)aFdr (10)

[0081]

[0082] In formula (5-12), V is the flight speed of the aircraft, Ω is the angular velocity of the blade element, r is the distance from the blade element to the hub, a is the axial induction factor, b is the tangential induction factor c l is the airfoil lift coefficient, c d is the airfoil drag coefficient, B is the number of blades, and R' is taken as 1.03 to 1.05 times R according to experience;

[0083] Step S3: Iteratively calculate the torsion angle β and chord length c at each radial point of the propeller, where the iterative convergence condition is the inflow angle The difference between the two calculations is less than 0.5°;

[0084] Step S31: The final propeller torsion angle β and chord length c and other geometric parameters are obtained through iteration of step 7 to complete the rapid propeller design. The torsion angle β and chord length c are distributed as follows: Figure 5 shown.

[0085] Step S4: Slipstream fast calculation method

[0086] The induced velocity field of propeller slipstream is non-uniform and the calculation is complex. This model adopts the slipstream tube model and simplifies the spiral wake into two continuous vorticity distributions: axial vortex γ x and tangential vortex γ t The axial vortices are parallel to the axis of rotation and are distributed on a concentrically placed cylindrical tube; the tangential vortices are distributed on the same cylindrical surface but perpendicular to the axial vortices.

[0087] Step S41: In the slipstream tube model, the induced velocity dV of the vortex element segment dl at any point P p It can be expressed as formula (13):

[0088]

[0089] M is the vector from point P to the infinitesimal segment; Г represents the size of the circulation of the infinitesimal segment.

[0090] The induced velocity of the axial vortex is calculated by formula (14-16):

[0091] u x =0 (14)

[0092]

[0093] The induced velocity of the tangential vortex is calculated by formula (17-19):

[0094]

[0095] In equations (16) and (19):

[0096] e=rsinψ-y p , k = -rcosψ-z p ;

[0097] In formula (13-19), M is the vector from point P to the infinitesimal segment; Г represents the size of the infinitesimal segment circulation;

[0098] The propeller design method based on blade momentum and vortex theory itself has simplifications and assumptions, and the design results generally have certain deviations from the actual situation. After obtaining the propeller chord length distribution and torsion angle distribution, the propeller slipstream distribution is checked by the above-mentioned propeller slipstream fast calculation method, and it is iteratively corrected until the requirement of uniform axial induced velocity distribution is met. Figure 6 Comparison of iterative corrections for propeller slipstream axial velocity.

[0099] Step S42: The geometric parameters are imported into the rapid modeling program to obtain the propeller geometric model, such as Figure 7 shown.

[0100] Step S43: At the flight altitude H = 500m, the air density at this altitude is ρ = 1.1566kg / m 3 , flight speed V0 = 60m / s, propeller radius R = 0.48m, number of blades B = 5, the propeller performance parameters calculated by numerical simulation are thrust 796N, torque 207N·M, efficiency 73.13%.

[0101] The above description is only a preferred embodiment of the present invention, and does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the description and illustrations of the present invention should be included in the protection scope of the present invention.

Claims

1. A propeller design method suitable for a distributed electric propulsion aircraft, characterized in that: A method for designing a uniform slipstream propeller is derived from blade element momentum theory, comprising the following steps: Step S1: according to the performance of the distributed electric propulsion aircraft, the aircraft dimensions and the propeller disk load requirements, determine the aircraft flight speed V, the radius R of the propeller, the thrust T and the number of blades B; Step S11: determining the maximum rotation speed according to the blade tip Mach number limit, and selecting a given rotation speed N; Step S12: selecting an airfoil whose lift-to-drag ratio meets the initial design requirements, and determining the design angle of attack α of the airfoil; Step S13: Control the slipstream speed: According to the blade element momentum theory, the propeller slipstream axial induction factor a is obtained through the thrust T=2πR 2 pV 2 (1+a)a (1) Step S14: According to the blade element momentum theory, the thrust expression has the following relationship: V 2 (1+a)a=Ω 2 r 2 (1-b)b (2) The tangential induction factor can be obtained: After the axial induction factor a is determined, the tangential induction factor b will also be determined; however, according to equation (3), the maximum value of the tangential induction factor b is 0.5, so when equation (3) fails, equation (4) needs to be used to determine the tangential induction factor b; In formula (1-4), T is the design tension, R is the propeller radius, ρ is the air density, a is the axial induction factor, V is the aircraft flight speed, Ω is the angular velocity of the cross-sectional blade element, r is the distance from the blade element to the hub, and b is the tangential induction factor; Step S2: Geometric parameter distribution After determining the distribution of the axial induction factor a and the tangential induction factor b, the local inflow angle at the radial position r can be calculated by equation (5): The airfoil attack angle α is given before design, and the twist angle β is determined according to equation (6): Due to the tip loss, the propeller blade tip will not bear too much load, which will also reduce the tip speed; increasing the tip load can be achieved by increasing the axial induction factor near the blade tip. The modified Prandtl tip loss coefficient F can be applied to the axial induction factor to achieve the desired effect, as shown in equation (9), where f can be calculated by equation (8); According to the blade element theory and momentum theory, there are two separate expressions of thrust T in equations (10) and (11), and the expression of local chord length c is obtained in equation (12); dT=4πrρV 2 (1+a)aFdr (10) In formula (5-12), V is the flight speed of the aircraft, Ω is the angular velocity of the blade element, r is the distance from the blade element to the hub, a is the axial induction factor, b is the tangential induction factor c l is the airfoil lift coefficient, c d is the airfoil drag coefficient, B is the number of blades, and R' is taken as 1.03 to 1.05 times R according to experience; Step S3: Iteratively calculate the torsion angle β and chord length c at each radial point of the propeller, where the iterative convergence condition is the inflow angle The difference between the two calculations is less than 0.5°; Step S31: obtaining the final propeller torsion angle β and chord length c and other geometric parameters through iteration of step S3, and completing the rapid propeller design; Step S4: Slipstream fast calculation method The induced velocity field of propeller slipstream is non-uniform and the calculation is complex. This model adopts the slipstream tube model to simplify the spiral wake into two continuous vorticity distributions: axial vortex γ x and tangential vortex γt ;Axial vortices are parallel to the axis of rotation and are distributed on concentrically placed cylindrical tubes; Tangential vortices are distributed on the same cylindrical surface but perpendicular to the axial vortices; Step S41: In the slipstream tube model, the induced velocity dV of the vortex element segment dl at any point P p It can be expressed as formula (13): The induced velocity of the axial vortex is calculated by formula (14-16): u x =0 (14) The induced velocity of the tangential vortex is calculated by formula (17-19): In formula (16) and formula (19): e=rsinψ-y p , k = -rcosψ-z p ; In formula (13-19), M is the vector from point P to the infinitesimal segment; Г represents the size of the infinitesimal segment circulation; The propeller design method based on blade element momentum and vortex theory itself has simplifications and assumptions, and the design results generally have certain deviations from the actual situation; after obtaining the propeller chord length distribution and torsion angle distribution, the distribution of the propeller slipstream is checked by the above-mentioned propeller slipstream fast calculation method, and it is iteratively corrected until the requirement of uniform axial induced velocity distribution is met.

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