A blunt trailing edge weakly separated low-drag airfoil with a symmetric shape design at the front and back based on uniform incoming flow conditions

Abstract

The present invention proposes a blunt trailing edge weakly separated low-drag airfoil with a symmetric front and rear shape design based on uniform incoming flow conditions. The leading edge radius of the airfoil is 0.06, the maximum thickness of the airfoil is 0.26, which is located at the 50% chord length of the airfoil, and the maximum camber is 0.0408, which is located at the 54.2% chord length of the airfoil. Compared with the classic DBLN-526 airfoil, the leading edge radius of this airfoil is increased, which is beneficial to maintaining stall starting from the trailing edge, increasing the laminar flow range during positive flow. The lower surface of the airfoil presents a double S shape, and the drag is reduced at a design Mach number of 0.2-0.4, further improving the lift-to-drag ratio.

Description

Technical Field

[0001] The present invention relates to the field of airfoils of coaxial rotor helicopter rotor blades, and specifically to a blunt trailing edge weakly separated low-drag airfoil with a front-back symmetric shape design based on uniform incoming flow conditions. Background Art

[0002] Early helicopter rotors generally directly adopted simple airplane wing airfoils. However, compared with the wings of fixed-wing aircraft, helicopter rotors face complex aerodynamic phenomena such as forward-side compressibility, rearward-side dynamic stall, and reverse flow. The biggest reason for the difficult design of helicopter blades is that the incoming flow velocity and angle of attack of the sectional airfoil are in a large range of changes. For example, when the X2 helicopter cruises at a speed of Ma = 0.5, the incoming flow velocity range of its blades can range from reverse flow of -0.2 Mach to forward flow of 0.9 Mach. The wide-speed-range incoming flow velocity results in different design requirements and focused characteristics at different stations along the span of the helicopter blade airfoil.

[0003] According to the spanwise velocity distribution of the forward and rearward blades of the X2 configuration coaxial rotor high-speed helicopter during forward flight and hovering, the minimum forward incoming flow Mach number is about 0.2 Mach, and the maximum is about 0.4 Mach. The maximum reverse incoming flow velocity of the rearward blade is 0.22 Mach. In order to improve the cruise efficiency and high-speed performance of the rotor, attention should be focused on the drag at medium angles of attack under 0.2 - 0.4 Mach, and the drag coefficient at small angles of attack under reverse flow. In addition, when the high-speed helicopter is flying forward at high speed, the rearward blades of the helicopter rotor are in a large area of reverse flow. When flying at high speed (250 kn / h), 85% of the airflow of the rearward blades flows from the trailing edge of the airfoil to the leading edge. Conventional airfoils are prone to separation in reverse flow, resulting in an increase in the drag of the rearward blades and a decrease in cruise efficiency. In order to reduce flow separation, the X-2 verification aircraft of the compound thrust high-speed helicopter using the "ABC" rotor adopts the airfoil DBLN-526 in the inner wing section (x / R < 0.3) of the rotor, achieving certain aerodynamic effects. However, as the performance requirements for coaxial rotor helicopters become higher and higher, the airfoil DBLN-526 is also difficult to meet the requirements. Summary of the Invention

[0004] In view of the situation that the incoming flow velocity of the high-speed helicopter blade is in a large range of changes and the reverse flow is serious, requirements for reducing the drag coefficient at medium angles of attack under low subsonic conditions and reducing flow separation at small angles of attack are put forward for the airfoil near the root station of the blade. For this purpose, using the front-back symmetric shape design method under uniform incoming flow conditions, the present invention designs and proposes a blunt trailing edge weakly separated low-drag airfoil for within 0.35R of the coaxial rotor helicopter rotor blade, where R is the blade span. Compared with the classic DBLN-526 airfoil with a thickness of 26%, this airfoil increases the leading edge radius, which is beneficial to maintaining stall starting from the trailing edge and increasing the laminar flow range during forward flow. The lower surface of the airfoil presents a double S, which can further improve the lift-drag ratio.

[0005] The technical solution of the present invention is as follows:

[0006] A blunt trailing edge weakly separated low-drag airfoil with a front-to-back symmetric shape design based on uniform incoming flow conditions, and its airfoil characteristics are as follows:

[0007] The leading edge radius of this airfoil is 0.06, the maximum thickness of the airfoil is 0.26, which is located at the 50% chord length of the airfoil, and the maximum camber is 0.0408, which is located at the 54.2% chord length of the airfoil. It should be noted that in the field of airfoil design, parameter descriptions are all made using dimensionless quantities. Therefore, the above leading edge radius, maximum thickness, maximum camber, and subsequent airfoil coordinate descriptions are all made using dimensionless quantities, and the dimensionless process is based on the airfoil chord length c.

[0008] Furthermore, the geometric coordinate expressions of the upper and lower surfaces of the airfoil are:

[0009]

[0010] where x represents the abscissa of the upper or lower surface of the airfoil, y represents the corresponding ordinate of the upper or lower surface of the airfoil, n represents the order of the CST parameterization method, and y tail represents the y coordinate of the step at the airfoil root;

[0011] The fitting coefficients of the upper surface of the airfoil are:

[0012]

[0013] The fitting coefficients of the lower surface of the airfoil are:

[0014]

[0015] Furthermore, the fitting coefficients of the upper and lower surfaces of the airfoil are preferably:

[0016] The fitting coefficients of the upper surface of the airfoil are:

[0017]

[0018] The fitting coefficients of the lower surface of the airfoil are:

[0019]

[0020] Furthermore, the positions of the coordinate points on the upper and lower surfaces of the blunt trailing edge weakly separated low-drag airfoil with a front-to-back symmetric shape design based on uniform incoming flow conditions are:

[0021] Upper surface coordinates:

[0022]

[0023]

[0024] The coordinates of the lower surface are as follows:

[0025]

[0026]

[0027] Beneficial effects

[0028] Compared with the prior art, the present invention has the following technical effects:

[0029] The present invention provides a blunt trailing-edge weakly separated low-drag airfoil with a front-to-back symmetric shape design based on a uniform oncoming flow condition. For large rigid coaxial double-rotor helicopters, the flow field of the blunt trailing-edge weakly separated low-drag airfoil has characteristics such as a large laminar flow range and strong flow separation at the trailing edge. Based on the front-to-back symmetric shape design method under uniform oncoming flow conditions, the present invention designs a new blunt trailing-edge weakly separated low-drag airfoil. Compared with the classical DBLN-526 airfoil, the leading-edge radius is increased, which is beneficial to maintaining stall starting from the trailing edge and increasing the laminar flow range during positive flow. The lower surface of the airfoil presents a double S shape, and the drag is reduced at a design Mach number of 0.2 - 0.4, further improving the lift-to-drag ratio.

[0030] The additional aspects and advantages of the present invention will be partially given in the following description, partially become apparent from the following description, or be understood through the practice of the present invention. Description of the drawings

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0032] Figure 1 is the geometric shape diagram of the airfoil of the present invention

[0033] Figure 2 Comparison of the geometric shape diagram of the airfoil of the present invention with the geometric shape diagram of DBLN-526

[0034] Figure 3 Comparison of the mean camber line of the airfoil of the present invention with the mean camber line of DBLN-526

[0035] Figure 4 Comparison of the lift and drag characteristic curves of the airfoil of the present invention and the DBLN-526 airfoil at the design point (Ma = 0.2, Re = 1.1×10 6 )

[0036] Figure 5 Comparison of the lift and drag characteristic curves of the airfoil of the present invention and the DBLN-526 airfoil at the design point (Ma = 0.3, Re = 1.65×10 6 )

[0037] Figure 6Comparison of lift and drag characteristics curves of the airfoil of the present invention and the DBLN-526 airfoil at the design point (Ma = 0.4, Re = 2.2×10 6 )

[0038] Figure 7 Drag variation with time-averaged angle of attack of the airfoil of the present invention and the DBLN-526 airfoil

[0039] Figure 8 Lift variation with time-averaged angle of attack of the airfoil of the present invention and the DBLN-526 airfoil

[0040] Figure 9 Comparison of unsteady variation curves of lift coefficients of the airfoil of the present invention and the DBLN-526 airfoil at the design point (α = -6°, Ma = 0.1, Re = 1.2×10 6 )

[0041] Figure 10 Comparison of unsteady variation curves of lift coefficients of the airfoil of the present invention and the DBLN-526 airfoil at the design point (α = -4°, Ma = 0.1, Re = 1.2×10 6 )

[0042] Figure 11 Comparison of unsteady variation curves of lift coefficients of the airfoil of the present invention and the DBLN-526 airfoil at the design point (α = -2°, Ma = 0.1, Re = 1.2×10 6 )

[0043] Figure 12 Comparison of unsteady variation curves of drag coefficients of the airfoil of the present invention and the DBLN-526 airfoil at the design point (α = 4°, Ma = 0.1, Re = 1.2×10 6 ) Detailed implementation manners

[0044] The embodiments of the present invention are described in detail below. The described embodiments are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0045] When the helicopter cruises at 0.3 Mach, the speeds of the rotor blade at 0.3R at the azimuth angles of 90° and 270° are 0.45 Mach and 0.15 Mach respectively, and the speed ratio is 3:1. Therefore, the importance weight ratio of the forward flow to the reverse flow is approximately 9:1. In order to improve the cruise efficiency and high-speed performance of the rotor, the drag at medium angles of attack under 0.2 - 0.4 Mach should be focused on. Considering the characteristics of the flow field such as a large laminar flow range and strong flow separation at the trailing edge, the rotor is required to have a certain ability to resist flow separation, which poses extremely high requirements on the unsteadiness of the airfoil.

[0046] To this end, according to the aerodynamic problems faced by helicopter blades, the present invention conducts multi-point and multi-objective optimization design on the rotor airfoil at 0.3R of the helicopter blade based on surrogate optimization means. First, optimization is carried out with the goal of drag reduction for the working conditions of incoming flow Mach numbers of 0.3 and 0.4 and angles of attack of 7° and 5° to ensure that the optimized airfoil has low drag characteristics at medium angles of attack. On this basis, further optimization design is carried out for the working conditions of incoming flow Mach numbers of 0.2 and 0.3 and Cl = 0.6 with the goal of improving the lift-to-drag ratio to ensure that the airfoil has low-speed high-lift characteristics. Finally, the last optimization is carried out for the working condition of incoming flow Mach number of 0.55 and Cl = 0 with the goal of minimizing drag, because the zero-lift drag determines the high-speed performance of the airfoil. Through the optimization design, a new blunt trailing edge weakly separated low-drag airfoil is obtained based on the front and rear symmetric shape design method under uniform incoming flow conditions.

[0047] (1) Symmetric shape design method based on uniform incoming flow

[0048] Based on the reference airfoil DBLN-526, using FFD parameterization, the airfoil is divided into two parts according to the axis of symmetry. Taking the left half as an example, 5 control points are selected from the upper and lower parts respectively for control, with a total of 10 variables. The control point information of the right half is the same as that of the left half.

[0049] (2) Optimization model

[0050] Table 1 Design point working conditions of the blunt trailing edge weakly separated low-drag airfoil

[0051]

[0052] Design objectives and constraints:

[0053] Table 2 Design objectives and constraints of the blunt trailing edge weakly separated low-drag airfoil

[0054]

[0055] (3) Optimized airfoil data

[0056] As Figure 2 Shown by the comparison between the airfoil geometric shape diagram of the present invention and the geometric shape diagram of the reference airfoil DBLN-526, compared with the classic DBLN-526 airfoil with a thickness of 26%, the leading edge radius of this airfoil is further increased, which is beneficial to maintaining stall starting from the trailing edge and increasing the laminar flow range during positive flow. The lower surface of the airfoil presents a double S, which can further improve the lift-to-drag ratio.

[0057] Specifically, for the blunt trailing edge weakly separated low-drag airfoil designed with a front and rear symmetric shape based on uniform incoming flow conditions in this embodiment, its airfoil characteristics are as follows:

[0058] Leading edge radius Airfoil area Maximum thickness Maximum thickness position Maximum camber Maximum camber position 0.06 0.18973 26％ 0.500 0.0408 0.542

[0059] It should be noted that in the field of airfoil design, parameter descriptions are all in dimensionless quantities. Therefore, the above leading-edge radius, maximum thickness, maximum camber, and subsequent airfoil coordinate descriptions are all in dimensionless quantities, and the dimensionless process is based on the airfoil chord length c.

[0060] Furthermore, the unified expressions obtained by fitting the upper and lower surfaces of the airfoil using 7th-order CST parameterization are as follows:

[0061]

[0062] where x represents the abscissa of the upper or lower surface of the airfoil, y represents the corresponding ordinate of the upper or lower surface of the airfoil, n represents the order of the CST parameterization method, and y tail represents the y-coordinate of the airfoil root step;

[0063] The fitting coefficients for the upper surface of the airfoil are:

[0064]

[0065] The fitting coefficients for the lower surface of the airfoil are:

[0066]

[0067] The specific positions of the coordinate points on the upper and lower surfaces are:

[0068] Coordinates of the upper surface:

[0069]

[0070]

[0071] The coordinates of the lower surface are as follows:

[0072]

[0073]

[0074] (4) Comparison of aerodynamic data at design points

[0075] Such as Figure 4 、 Figure 5 、 Figure 6The comparison of the lift and drag characteristics between the optimized airfoil and the reference airfoil DBLN-526 at different design points is given. The main design objectives of the optimized airfoil at the design point have been improved. Aiming at the optimization objectives of drag reduction at medium angles of attack and further increasing the lift-to-drag ratio based on this, the drag coefficient of the optimized airfoil at the design point has decreased, with a maximum reduction of 20.88%, and the lift-to-drag ratio has increased by up to 19.94%, which is overall better than the initial airfoil; all constraints are met, and the optimized airfoil generally meets the verification indicators well, except that the maximum lift-to-drag ratio at high speeds has slightly decreased.

[0076] (5) Unsteady characteristics of the optimized airfoil

[0077] Unsteady calculations are carried out on the designed airfoil, such as Figure 7 、 Figure 8 The time-averaged variation diagram of the lift coefficient of the airfoil with the angle of attack is given. As shown in the figure, the time-averaged drag coefficient has decreased, and the time-averaged lift coefficient has increased, and obvious optimization effects have been achieved. Such as Figure 9 、 Figure 10 、 Figure 11 、 Figure 12 As shown, in the unsteady calculation state, the mean value of the lift coefficient has increased compared with the reference airfoil, the drag coefficient has decreased, and the amplitudes of the two curves have decreased, which has changed the separation characteristics to a large extent, significantly reduced the aerodynamic noise, and the perturbation of the lift coefficient at large angles of attack has increased, but still meets the verification indicators.

[0078] In summary, the optimization process itself conforms to the law of multi-objective aerodynamic optimization, and the target aerodynamic characteristics of the optimized airfoil under various working conditions have been greatly improved.

[0079] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and purposes of the present invention.

Claims

1. A blunt trailing edge weakly separated low-drag airfoil with a symmetric shape design at the front and back based on uniform oncoming flow conditions, characterized in that: The leading-edge radius of the airfoil is 0.06, the maximum thickness of the airfoil is 0.26, which is located at the 50% chord length of the airfoil, the maximum camber is 0.0408, which is located at the 54.2% chord length of the airfoil; the above-mentioned leading-edge radius, maximum thickness, maximum camber and subsequent airfoil coordinate descriptions are all described using dimensionless quantities, and the dimensionless process is based on the airfoil chord length c; The airfoil is used for the position within 0.35R of the rotor blade of a coaxial rotor helicopter, where R is the blade span; The airfoil is designed by using a symmetric shape design method based on uniform incoming flow, with the DBLN-526 airfoil as the reference airfoil for optimization design, and the design point conditions are: At design state 1 and design state 2, the goal is to reduce drag; at design state 3 and design state 4, the goal is to increase the lift-to-drag ratio; At design state 5, the goal is to minimize drag; The geometric coordinate expressions of the upper and lower surfaces of the designed airfoil are: where x represents the abscissa of the upper or lower surface of the airfoil, y represents the corresponding ordinate of the upper or lower surface of the airfoil, n represents the order of the CST parameterization method, and y tail represents the y-coordinate of the step at the airfoil root; The fitting coefficients of the upper surface of the airfoil are: The fitting coefficients of the lower surface of the airfoil are:

2. The blunt trailing-edge weakly separated low-drag airfoil with a front-to-back symmetric shape design based on uniform incoming flow conditions according to claim 1, characterized in that: The fitting coefficients of the upper surface of the airfoil are: The fitting coefficients of the lower surface of the airfoil are:

3. The blunt trailing-edge weakly separated low-drag airfoil with a front-back symmetric shape design based on uniform oncoming flow conditions according to claim 1, wherein: The positions of the upper and lower surface coordinate points of the blunt trailing-edge weakly separated low-drag airfoil with a front-to-back symmetric shape design based on uniform incoming flow conditions are: The upper surface coordinates: The lower surface coordinates are as follows:

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