Calculation method of tiltrotor transition corridor based on aerodynamic simulation

Abstract

The present invention discloses a method for calculating a tiltrotor transition corridor based on aerodynamic simulation. The method of the present invention is as follows: first, a force analysis is performed on the rotor-wing system to obtain the aerodynamic balance equation in the transition state; the variation trend of the aerodynamic coefficient with the tilt angle is calculated using CFD software, and data fitting is performed on it. On this basis, the wing stall angle boundary curve and the zero lift angle boundary curve of the transition corridor are calculated to obtain the left and upper boundaries of the transition corridor; the rotor thrust boundary curve is calculated to obtain the right boundary of the transition corridor; the three boundaries of the transition corridor obtained are combined with the lower boundary limit of the 0° tilt angle to finally obtain the tiltrotor transition corridor. Based on the method of the present invention, a transition corridor suitable for electric tiltrotor drones that is closer to the actual situation can be obtained.

Description

Technical Field

[0001] The invention relates to the fields of simulation calculation and flight control, and in particular to a method for calculating a tiltrotor transition corridor based on aerodynamic simulation. Background Art

[0002] With advances in electric energy storage and motor technology, as well as the application of distributed electric drive systems, electric vertical take-off and landing (EVTL) aircraft have seen significant growth as greener and safer aircraft. They combine the high-speed cruising performance of fixed-wing aircraft with the vertical take-off and landing capabilities of helicopters, offering a wider flight range and enhanced mission capabilities. Compared to other power systems like turbine engines, they also offer the advantage of lower noise levels.

[0003] To switch between fixed-wing and helicopter modes, tiltrotor aircraft incorporate a transition state. The rotor and other related components must switch vertically and horizontally. This phase exhibits highly nonlinear aerodynamic characteristics, impacting the aircraft's safety and stability during flight. The tilting path for the tiltrotor's transition state cannot be arbitrarily set; it must be performed within a transition corridor to ensure a safe transition. The transition corridor is the flight envelope, expressed as variables of airspeed and rotor pitch angle. This corridor effectively maps the feasible trim region within the aircraft's performance constraints.

[0004] When a drone is operating, the propellers rotate to generate thrust. The pressure differences between the upper and lower surfaces of the wings as air flows over them generate lift. During transitional states, the propellers tilt to achieve the transition between modes. During this transition, the propellers and wings share the weight of the drone. The transition corridors reveal the flight speed ranges for tilt-rotor drones in different configurations. At each tilt angle, the speed must not exceed certain upper and lower limits. If the speed is too low, the wings may stall, providing too little lift, leading to an imbalance between the aircraft's lift and weight, and consequently, a loss of altitude. If the speed is too high, the aircraft is limited by factors such as the maximum propeller speed and dynamic stability, failing to generate sufficient thrust and resulting in a lack of stability.

[0005] Currently, calculations for transition corridor boundaries are primarily based on engine-related calculations, with one constraint being the engine's power limit. However, electric tilt-rotor UAVs use electric propulsion systems, making engine-based transition corridor boundary calculations infeasible. Furthermore, fixed values ​​for the wing's aerodynamic coefficients are used when calculating the zero-lift and maximum-lift boundaries. However, in actual tilting, the aerodynamic coefficients vary with the tilt angle, making the use of fixed values ​​for calculating transition corridor boundaries impractical. Summary of the Invention

[0006] To address the issues with the currently used transition corridor calculation methods, this paper proposes a tiltrotor transition corridor calculation method based on aerodynamic simulation. This method considers the variation of aerodynamic coefficients (lift and drag coefficients) with tilt angle when calculating the zero-lift and maximum-lift boundaries of the transition corridor. This method is applicable to electric tiltrotor UAVs and provides more accurate calculation results.

[0007] To achieve this object, the present invention adopts the following specific technical solutions.

[0008] S1. Perform force analysis on the rotor-wing system and obtain the aerodynamic equilibrium equation under the transient state;

[0009] S2. Using CFD software, calculate the variation of the aerodynamic coefficients with the pitch angle at both the stall and zero-lift angles of attack. Fit the quadratic polynomials for the variation of the aerodynamic coefficients with the pitch angle at both the stall and zero-lift angles of attack to obtain the quadratic polynomials.

[0010] S3. Based on steps S1 and S2, calculate the wing stall angle of attack boundary curve and the zero lift angle of attack boundary curve to obtain the left boundary and the upper boundary of the transition corridor;

[0011] S4. Calculate the rotor thrust boundary curve to obtain the right boundary of the transition corridor.

[0012] In the above technical solution, further, in step S1:

[0013] Due to the limitation of simulation calculation, only the longitudinal trim characteristics of the tiltrotor are considered.

[0014] The aerodynamic balance equation in the transition state can be simplified as follows:

[0015]

[0016] Where T is the thrust generated by the rotor; α is the tilt angle; G is the system weight; L is the lift generated by the wing; and D is the drag generated by the wing. The calculation formulas for L and D are as follows:

[0017]

[0018] Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient and S is the wing area.

[0019] Furthermore, in step S2, the CFD software is used to calculate the variation trend of the aerodynamic coefficient with the tilt angle when the wing is in the stall angle of attack and the zero lift angle of attack state, and the quadratic polynomial of the variation of the aerodynamic coefficient with the tilt angle when the wing is in the stall angle of attack and the quadratic polynomial of the variation of the aerodynamic coefficient with the tilt angle when the wing is in the zero lift angle of attack are obtained by fitting. The specific method is:

[0020] First, determine the stall angle of attack and zero lift angle of attack of the wing, and establish Model 1: the calculation model when the wing is at the stall angle of attack, and Model 2: the calculation model when the wing is at the zero lift angle of attack.

[0021] Then, CFD calculation software is used to simulate the above models 1 and 2. The specific steps are as follows:

[0022] S2_1: Set the computational domain and perform meshing on Model 1;

[0023] S2_2: Set the rotor motion trajectory. In the transition state, the rotor rotates at a certain speed and tilts at the same time, simulating the process of the rotor tilting from helicopter mode to fixed-wing mode.

[0024] S2_3: Use CFD software to perform simulation calculations to obtain the changes in the vertical force and the horizontal force along the chord length of the wing, that is, the changes in the lift and drag of the wing during the tilting process;

[0025] S2_5: Calculate the lift coefficient and drag coefficient, and thus obtain the changes of the lift coefficient and drag coefficient with the tilt angle; the calculation formula of the lift and drag coefficient is as follows:

[0026]

[0027] Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient and S is the wing area.

[0028] S2_5: fitting the variation of the lift coefficient and the drag coefficient with the tilt angle into a polynomial, thereby obtaining a quadratic polynomial for the variation of the aerodynamic coefficient of the wing with the tilt angle at the stall angle of attack;

[0029] Change Model 1 to Model 2 and repeat the above steps to obtain the quadratic polynomial of the aerodynamic coefficient changing with the tilt angle when the wing has zero lift angle of attack.

[0030] Furthermore, step S3 calculates the wing stall angle boundary and the zero lift angle boundary: set the incoming flow velocity value, substitute the quadratic polynomial of the wing aerodynamic coefficient changing with the tilt angle at the stall angle obtained in step S2 into formula (1), calculate the tilt angle in the transition state, and finally obtain the wing stall angle boundary curve as the left boundary of the transition corridor; set the incoming flow velocity value, substitute the quadratic polynomial of the wing aerodynamic coefficient changing with the tilt angle at the zero lift angle obtained in step S2 into formula (1), calculate the tilt angle at this time by Newton iteration method, and finally obtain the wing zero lift angle boundary curve as the upper boundary of the transition corridor.

[0031] Furthermore, step S4 calculates the rotor thrust boundary curve to obtain the right boundary of the transition corridor. In this step, the thrust limit of the electric propulsion rotor comes from the maximum allowable rotor speed. Because the maximum thrust of the rotor is affected by the incoming flow velocity, the law of maximum thrust variation with the incoming flow is fitted into a formula. The tilt angle is set, and the resulting formula is substituted into any one of the equations (1). The incoming flow velocity in the transition state is calculated to obtain the corresponding rotor thrust boundary curve, which is the right boundary of the transition corridor.

[0032] Combining the three boundaries of the transition corridor obtained in steps 3 and 4, as well as the lower boundary limit of the 0° tilt angle, the tiltrotor transition corridor is finally obtained, which reflects the balancing requirements of the transition process. The tilt trajectory needs to be set within this range to meet basic safety guarantees.

[0033] The advantages of the present invention are at least:

[0034] 1. This invention establishes a tiltrotor transition corridor calculation method based on aerodynamic simulation, combining simulation with flight control to complement each other;

[0035] 2. The transition corridor obtained by the present invention is based on aerodynamic parameters that vary with the tilt angle obtained through simulation calculation. Compared with transition corridors that use fixed aerodynamic parameter values, it is closer to the actual situation and more accurate.

[0036] 3. The present invention is applicable to electric tilt-rotor UAVs and has a simpler calculation method for the power system limitation part. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a flow chart of a method for calculating a tiltrotor transition corridor based on aerodynamic simulation according to the present invention;

[0038] Figure 2 Schematic diagram of the rotor / wing model used in the specific embodiment of the present invention, in which: (a) is the fixed-wing mode, (b) is the transition state, and (c) is the helicopter mode;

[0039] Figure 3This is a schematic diagram of the force analysis of the system in the transition state in the specific embodiment of the present invention;

[0040] Figure 4 A schematic diagram of the calculation domain of the simulation calculation step in the specific embodiment of the present invention;

[0041] Figure 5 This is a schematic diagram of a transition corridor obtained using the calculation method of the present invention in a specific implementation manner. DETAILED DESCRIPTION

[0042] In order to clearly and completely demonstrate the calculation method and its specific working process described in the present invention, the present invention is further described in detail in conjunction with specific embodiments.

[0043] This example takes the two important components of the tilt-rotor UAV, the rotor and wing, as an example. The three states of the rotor-wing system are as follows: Figure 2 The rotor tilt center O is at the same height as the wing leading edge, 0.08m from the wing leading edge, the tilt radius is 0.07m, and the propeller center is 0.15m from the wing leading edge in the initial fixed-wing mode. The specific rotor and wing parameters are shown in Table 1

[0044] Table 1

[0045]

[0046] The calculation method of the transition corridor during the tilting process of the rotor-wing system from fixed-wing mode to helicopter mode is as follows: Figure 1 , the specific steps are as follows:

[0047] S1: Perform force analysis on the rotor-wing system to obtain the aerodynamic equilibrium equation in the transition state;

[0048] In this step, considering the limitation of simulation calculation amount, the main goal of the present invention is to perform rotor-wing aerodynamic matching analysis during transition flight, so only the longitudinal trim characteristics of the tiltrotor are considered.

[0049] The flow environment and force analysis of the rotor-wing system in the transition state are as follows: Figure 3 As shown. Correspondingly, the aerodynamic balance equation of the tilt rotor and wing can be simplified as:

[0050]

[0051] Where T is the thrust generated by the rotor; α is the tilt angle; G is the system weight (in this example, G is 3.72N); L is the lift generated by the wing; and D is the drag generated by the wing. The formulas for calculating L and D are as follows:

[0052]

[0053] Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient and S is the wing area.

[0054] S2: CFD is used to calculate the variation trend of the aerodynamic coefficient with the tilt angle when the wing is in the stall angle of attack and zero lift angle of attack, and the quadratic polynomial of the variation of the aerodynamic coefficient with the tilt angle at the stall angle of attack and the quadratic polynomial of the variation of the aerodynamic coefficient with the tilt angle at the zero lift angle of attack are fitted;

[0055] First, the stall angle of attack and zero lift angle of attack of the wing are estimated using the B-29TIP wing profile. In this example, the stall angle of attack of the B-29TIP wing profile is 12°, and the zero lift angle of attack is -2°.

[0056] The rotor-wing system was modeled, and Model 1 was established: a calculation model when the wing was at a stall angle of attack of 12°, and Model 2 was established: a calculation model when the wing was at a zero lift angle of attack of -2°.

[0057] Models 1 and 2 were simulated using CFD software. In this example, openFOAM, the open-source software, was used, specifically the overPimpleDyMFoam solver. Overset meshing was used to achieve relative mesh motion, capturing the rotor's rotational and continuous tilting motions. Information was then interpolated with the background mesh through the overlapped region. The Reynolds-averaged NS equations were used as the governing flow equations, and the k-ω turbulence model was selected as the turbulence model. Taking Model 1 as an example, the specific steps are as follows:

[0058] ① Set the calculation domain, such as Figure 4 As shown, the model is meshed. In this example, the number of computational grids is 1.68 million.

[0059] ② Set the rotor motion trajectory. In the transition state, the rotor rotates at 9000 rpm and tilts 90° to simulate the process of the rotor tilting from helicopter mode to fixed-wing mode.

[0060] ③ Use openFOAM's overPimpleDyMFoam solver to calculate the changes in the vertical and horizontal forces along the wing's chord length, which is how the wing's lift and drag change during the pitching process. In this example, the force in the z direction is the wing's lift, and the force in the y direction is the wing's drag.

[0061] ④ Calculate the lift coefficient and drag coefficient, and thus obtain the changes of lift coefficient and drag coefficient with the tilt angle; the calculation formula of lift and drag coefficient is as follows:

[0062]

[0063] Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient and S is the wing area.

[0064] ⑤ Fit the lift coefficient and drag coefficient as a function of the pitch angle into a polynomial formula. In this example, the fitting is a quadratic polynomial. This yields a quadratic polynomial for the aerodynamic coefficient of the wing as a function of the pitch angle at a stall angle of attack and a quadratic polynomial for the aerodynamic coefficient of the wing as a function of the pitch angle at a zero lift angle of attack.

[0065] Change the calculation model to Model 2 and repeat the above steps to obtain the formula for the change of aerodynamic coefficient with tilt angle when the wing has zero lift angle of attack.

[0066] S3: Calculate the wing stall angle of attack boundary and the zero lift angle of attack boundary to obtain the left boundary and the upper boundary of the transition corridor;

[0067] Set the incoming flow velocity value, substitute the quadratic polynomial of the wing's aerodynamic coefficient at stall angle of attack obtained in step S2 into formula (1), calculate the tilt angle in the transition state by Newton iteration method, and finally obtain the wing stall angle boundary curve; as the left boundary of the transition corridor;

[0068] Set the incoming flow velocity value, substitute the quadratic polynomial of the aerodynamic coefficient of the wing at zero lift angle of attack obtained in step S2 into formula (1), calculate the tilt angle at this time by Newton iteration method, and finally obtain the wing zero lift angle of attack boundary curve; it serves as the upper boundary of the transition corridor.

[0069] S4: Calculate the rotor thrust boundary curve and obtain the right boundary of the transition corridor:

[0070] In this step, since the maximum thrust of the rotor is limited by the maximum rotational speed allowed by the rotor, the rotor in this example is assumed to be in the optimal propeller-engine matching state;

[0071] Because the maximum thrust of the rotor is affected by the incoming flow velocity, we first fit the maximum thrust variation with the incoming flow into a formula. In this example, the variation of the rotor thrust with the incoming flow is fitted to a sine function. By setting the tilt angle, substituting the sine function into any one of the equations in (1), and calculating the incoming flow velocity at that time, we obtain the corresponding rotor thrust boundary curve, which serves as the right boundary of the transition corridor.

[0072] Combining the three boundaries of the transition corridor obtained in steps 3 and 4, and the lower boundary limit of the 0° tilt angle, the tilt rotor transition corridor of this example is finally obtained, as shown in FIG. Figure 5The shaded area in the middle reflects the balancing requirements of the transition process. The tilt trajectory of the rotor-wing system needs to be set within this range to meet basic safety guarantees.

[0073] The specific implementation method described above provides a detailed description of the purpose, calculation method and beneficial effects of the present invention. It should be understood that the above is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A tiltrotor transition corridor calculation method based on aerodynamic simulation, characterized in that: The steps include: S1. Perform force analysis on the rotor-wing system and obtain the aerodynamic equilibrium equation under the transient state; S2. Using CFD software, calculate the variation of the aerodynamic coefficients with the pitch angle at both the stall and zero-lift angles of attack. Fit the quadratic polynomials for the variation of the aerodynamic coefficients with the pitch angle at both the stall and zero-lift angles of attack to obtain the quadratic polynomials. S3. Calculate the wing stall angle of attack boundary curve and the zero lift angle of attack boundary curve to obtain the left and upper boundaries of the transition corridor: Set the incoming flow velocity value, substitute the quadratic polynomial obtained in step S2 for the variation of the aerodynamic coefficient of the wing with the tilt angle at the stall angle of attack into the aerodynamic balance equation in the transition state, calculate the tilt angle in the transition state using the Newton iteration method, and finally obtain the wing stall angle of attack boundary curve as the left boundary of the transition corridor; Setting the incoming flow velocity value, substituting the quadratic polynomial obtained in step S2 for the variation of the aerodynamic coefficient of the wing with the tilt angle at zero lift angle of attack into the aerodynamic balance equation in the transition state, calculating the tilt angle in the transition state using the Newton iteration method, and finally obtaining the wing zero lift angle of attack boundary curve as the upper boundary of the transition corridor; S4. Calculate the rotor thrust boundary curve to obtain the right boundary of the transition corridor: fit the law of change of the maximum rotor thrust with the incoming flow into a formula; set the tilt angle, substitute the formula into the aerodynamic balance equation under the transition state, calculate the incoming flow velocity under the transition state, and obtain the corresponding rotor thrust boundary curve as the right boundary of the transition corridor.

2. The method for calculating the tiltrotor transition corridor based on aerodynamic simulation according to claim 1, characterized in that: In step S1, the aerodynamic balance equation in the transition state is expressed as: Where T is the thrust generated by the rotor; α is the tilt angle; G is the system weight; L is the lift generated by the wing; and D is the drag generated by the wing. The calculation formulas for L and D are as follows: Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient and S is the wing area.

3. The method for calculating the tiltrotor transition corridor based on aerodynamic simulation according to claim 1, characterized in that: In step S2, the CFD software is used to calculate the variation trend of the aerodynamic coefficient of the wing with the tilt angle when the wing is in the stall angle of attack and the zero lift angle of attack, respectively, thereby obtaining a quadratic polynomial for the variation of the aerodynamic coefficient of the wing with the tilt angle at the stall angle of attack and a quadratic polynomial for the variation of the aerodynamic coefficient of the wing with the tilt angle at the zero lift angle of attack. The specific process is as follows: First, the stall angle of attack and zero lift angle of attack of the wing are determined, and Model 1: a calculation model when the wing is at the stall angle of attack, and Model 2: a calculation model when the wing is at the zero lift angle of attack are established; then, CFD calculation software is used to simulate and calculate Model 1 and Model 2.

4. The method for calculating a tiltrotor transition corridor based on aerodynamic simulation according to claim 3, characterized in that: The CFD calculation software is used to simulate the model 1 and the model 2, and the specific steps are as follows: S2_1: Set the computational domain and mesh the model; S2_2: Set the rotor motion trajectory. In the transition state, the rotor rotates at a certain speed and tilts at the same time, simulating the process of the rotor tilting from helicopter mode to fixed-wing mode. S2_3: Use CFD software to perform simulation calculations to obtain the changes in the vertical force and the horizontal force along the chord length of the wing, that is, the changes in the lift and drag of the wing during the tilting process; S2_4: Calculate the lift coefficient and drag coefficient, and thus obtain the changes of the lift coefficient and drag coefficient with the tilt angle; the calculation formula of the lift and drag coefficient is as follows: Where ρ is the density; V ∞ is the incoming flow velocity, C L is the lift coefficient, C D is the drag coefficient, S is the wing area; S2_5: Fitting the variations of the lift coefficient and the drag coefficient with the tilt angle into a polynomial, thereby obtaining a quadratic polynomial for the variations of the aerodynamic coefficient with the tilt angle at the stall angle of attack of the wing and a quadratic polynomial for the variations of the aerodynamic coefficient with the tilt angle at the zero lift angle of attack of the wing.

5. The method for calculating the tiltrotor transition corridor based on aerodynamic simulation according to claim 1, characterized in that: The limitation of the maximum thrust of the rotor in step S4 comes from the maximum rotation speed allowed by the rotor.

6. The method for calculating the tiltrotor transition corridor based on aerodynamic simulation according to claim 1, characterized in that: The tilt angle of zero is taken as the lower boundary of the transition corridor, and the three boundaries of the transition corridor obtained in steps S3 and S4 are combined to obtain the transition corridor of the tilt rotor.

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