CN109693807B

CN109693807B - Design method of self-adaptive pneumatic variable-pitch propeller

Info

Publication number: CN109693807B
Application number: CN201811629819.9A
Authority: CN
Inventors: 周洲; 范中允
Original assignee: Northwestern Polytechnical University
Current assignee: Northwestern Polytechnical University
Priority date: 2018-12-28
Filing date: 2018-12-28
Publication date: 2021-11-09
Anticipated expiration: 2038-12-28
Also published as: CN109693807A

Abstract

The invention provides a design method of a self-adaptive pneumatic variable-pitch propeller, which utilizes the concept and method of longitudinal trim of an aviation airplane to design the aerodynamic characteristics of a propeller blade per se under the condition of no redundant additional mechanism, designs the blade into a double-blade layout with aerodynamic static stability by simulating the trim principle of double-wing or canard layout of the airplane, can automatically vary the pitch regardless of the change of the advancing ratio caused by the change of the flying speed or the change of the rotating speed while providing high blade disc loading capacity, and ensures that the propeller has better automatic variable-pitch characteristic in a wider flying envelope.

Description

Design method of self-adaptive pneumatic variable-pitch propeller

Technical Field

The invention relates to the technical field of aviation aircrafts, in particular to a design method of a self-adaptive pneumatic variable pitch propeller for a power system of a small aircraft.

Background

In view of the design contradiction of propellers of an airplane in different flight states, a large airplane can usually adopt a controllable variable-pitch device. However, for a small airplane or an unmanned aerial vehicle, the controllable propeller pitch-variable system mechanism is too complex, and the weight cost is large. In order to find a cheaper design solution for the wide envelope of the propeller, there have been some solutions proposed by related studies. For example, Liu industry, Song Chaofeng et al, studied a variable diameter propeller (Liu C, Song B, Wang H. Design and optimization of a variable diameter propeller [ C ]// International Conference on Information Science, Electronics and electronic engineering. IEEE,2014:291-295.) to extend the working range of tiltrotor propellers; wangwei invented a device for automatically changing the pitch by centrifugal force (Wangwei a propeller pitch-changing mechanism and rotorcraft equipped with said device, CN 105947183A [ P ] 2016), and can make the propeller automatically adjust the pitch according to the difference of rotating speed; the invention relates to an automatic pitch-changing device (Bo Jing Yun, Van jin hong, Liu Shuang, Yinlan. a self-adaptive pitch-changing propeller and airplane, CN205499338U [ P ]. 2016), wherein the pitch-changing is that a spring assembly is used to automatically adjust the propeller pitch according to the pulling force, and the propeller pitch is small when the pulling force is large; hanwei invented an automatic pitch-changing device (pitch-changing propeller and unmanned aerial vehicle, CN201710853787.X [ P ].2017), which can utilize the reaction torque provided by the torque of a motor to automatically adjust the pitch, and when the torque is large, the pitch is reduced; the inventors invented a device for automatically changing the pitch and diameter of propeller by centrifugal force (Mat-Bright, Song pen front, King Hai mountain, a propeller capable of changing pitch and diameter of propeller, CN105620728B [ P ]. 2017). Hong jiang et al studied a device for varying pitch using the pressure difference between the front and rear edges of a blade (hong jiang, bailin, xiejonge. a variable pitch propeller for high altitude unmanned aerial vehicles and unmanned aerial vehicles, CN104973236A [ P ]. 2015).

Overseas, Heinzen S B et al have studied a Propeller using a recurved airfoil (Heinzen S B, Halljr C E, Gopalaartham A. development and Testing of a Passive Variable-Pitch Propeller [ J ]. Journal of Aircraft,2015,52(3):1-16.) so that the blades can be automatically pitched according to the aerodynamic force generated by their own flow field.

At present, most automatic variable pitch devices are based on an additional mechanism, automatic variable pitch is carried out by utilizing rotating speed (centrifugal force), torque or pulling force and the like, and the contradiction of propeller efficiency under different working conditions can be relieved to a certain extent. But is limited by the pitch change principle, and is difficult to consider various complex working conditions. For example, in the scheme of automatically changing the pitch by using a centrifugal force assembly, the pitch changing angle depends on the rotating speed, and the pitch changing can be performed only when the rotating speed changes, so that when the flight speed of the airplane changes but the rotating speed does not change, the propeller cannot automatically change the pitch, which occurs in the vertical take-off and landing and transition stages of the airplane, the propeller always operates in the maximum rotating speed state, but the flight speed of the airplane greatly changes; the scheme of automatically changing the pitch by using the tension and the torque also cannot solve the problems caused by the flight speed, for example, when the airplane flies at high altitude and low altitude, the change of the lift-drag ratio of the airplane is not large, and the change of the tension required to be provided by the propeller is not large, but due to different air densities, if the dynamic pressure is kept unchanged, the cruising speed of the high-altitude airplane and the low-altitude airplane can be greatly changed, so that the propeller automatically changing the pitch by using the tension cannot automatically change the pitch according to the situation.

The root of these conflicts is that the operating parameters that most mainly affect the performance of the propeller are the forward ratio, not the rotational speed, the tension, the centrifugal force, etc. The advancing ratio reflects the local attack angle of the section of the propeller blade, and the performance contradiction of the propeller is an aerodynamic contradiction at the bottom, so that the blade can be balanced by using the aerodynamic force of the blade and is designed instead of an additional mechanism. Heinzen S B et al have proposed the concept of aerodynamic pitch change of a recurved airfoil profile and have used it in power plants for certain types of conventional take-off and landing fixed wing drones. However, the inherent defects of the recurved airfoil are that the trim lift coefficient is too low, the stall characteristic of a large attack angle is poor, the designed propeller has low absorption power, and the designed propeller cannot be used in a high-paddle-disc load state, so that the recurved airfoil is not suitable for situations such as a vertical take-off and landing aircraft which need to provide high paddle disc load.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a design method of a self-adaptive pneumatic variable-pitch propeller, which utilizes the concept and method of longitudinal trim of an aviation aircraft to design the aerodynamic characteristics of a propeller blade per se under the condition of no redundant additional mechanism, and designs the blade into a double-blade layout with aerodynamic static stability by simulating the trim principle of double-wing or canard layout of the aircraft.

The design method provided by the invention can design the propeller blade into the pneumatic variable-pitch propeller blade with static stability. The static stability means that when the ratio of the incoming flow speed (vertical to a blade disc) of the blade to the rotating speed of the propeller is increased (the incidence angle of the section of the blade is reduced), the blade automatically increases the torsion angle (variable pitch angle); when the ratio of the incoming flow speed of the blade to the linear speed of the rotating speed is reduced (the section attack angle of the blade is increased), the blade automatically reduces the torsion angle (variable pitch angle).

The specific design scheme of the invention is as follows:

the design method of the self-adaptive pneumatic variable pitch propeller is characterized by comprising the following steps of: the method comprises the following steps:

step 1: determining the diameter D and the maximum tension T of the propeller according to the load requirement of a propeller disc of the vertical take-off and landing aircraft_max；

Step 2: selecting an airfoil profile with the existing lift coefficient and lift-drag ratio meeting the initial setting requirement, and determining the design attack angle alpha of the airfoil profile;

and step 3: determining the maximum rotating speed according to the Mach number limit of the blade tip;

and 4, step 4: designing the chord length and torsion angle distribution of the blade:

step 4.1: according to the maximum rotating speed obtained in the step 3, a micro incoming flow hypothesis is adopted, the number of the blades is four, and T is 1.05-1.2 times that of the blades_maxAs a design tension, increasing the design attack angle by 1-2 degrees compared with alpha in the step 2, and carrying out radial distribution design on the chord length and the torsion angle of the propeller to obtain the chord length and torsion angle distribution of the No. 1 paddle;

step 4.2: according to the maximum rotating speed obtained in the step 3, a micro incoming flow hypothesis is adopted, the number of the blades is four, and the T is 0.8-0.95 times that of the blades_maxAs a design tension, the design attack angle is reduced by 1-2 degrees compared with alpha in the step 2, and the radial distribution design of the chord length and the torsion angle of the propeller is carried out to obtain the chord length and the torsion angle distribution of the No. 2 propeller blade;

step 4.3: judging whether the ratio of the chord length of the No. 1 blade to the chord length of the No. 2 blade obtained through the design in the steps 4.1 and 4.2 is between 0.7 and 0.9 or not, and the torsion angle of the No. 1 blade is 1 to 2 degrees larger than the corresponding torsion angle of the No. 2 blade; if yes, ending the step 4; if the chord length ratio is smaller, increasing the design tension or reducing the design attack angle in the range set in the step 4.1, redesigning the chord length and torsion angle distribution of the No. 1 blade, or reducing the design tension or increasing the design attack angle in the range set in the step 4.2, redesigning the chord length and torsion angle distribution of the No. 2 blade; if the torsion angle of the No. 1 blade is smaller, increasing the design attack angle in the range set in the step 4.1, and redesigning the chord length and the torsion angle distribution of the No. 1 blade, or reducing the design attack angle in the range set in the step 4.2, and redesigning the chord length and the torsion angle distribution of the No. 2 blade;

and 5: establishing an initial configuration of a balancing blade:

establishing an initial configuration of four blades; two adjacent blades in the four blades are used as a group of blades, and the two groups of blades are formed; establishing variable-pitch rotating shaft axes for each group of blades, wherein two groups of variable-pitch rotating shafts are formed, and the variable-pitch rotating shafts mutually form an included angle of 180 degrees and are centrosymmetric with the rotating shaft of the propeller; each blade is arranged in a straight line at the 1/4 position of the chord length of the section airfoil, and the 1/4 chord length connecting line of the blade is used as the focal line of the blade;

for each group of blades, the rotation direction of the propeller is taken as the positive direction of the angle, the front blade is arranged in the front, and the rear blade is arranged in the rear, wherein the front blade uses the No. 1 blade, and the rear blade uses the No. 2 blade; recording the tension direction of a rotating shaft of the propeller as the positive direction of the height, wherein the front edge of the blade rotates upwards to form blade raising, and the rear edge of the blade rotates upwards to form blade lowering; the included angle between the focal point line of the front blade and the variable pitch axis is a front blade angle theta 1, and the included angle between the rear blade and the variable pitch axis is a rear blade angle theta 2; the distances between the focal line of the front blade and the focal line of the rear blade and the axis of the variable-pitch rotating shaft are d1 and d2 respectively;

in the initial configuration, the front blade is located forward and above the pitch axis, with θ 1 positive and d1 positive, the rear blade is located rearward and below the pitch axis, with θ 2 negative and d2 negative, and with d1/d2 being sin θ 1/sin θ 2, the absolute value of θ 1 being less than the absolute value of θ 2;

taking the leaf elements at any radial position r, and calculating the moment arms L1 and L2 of the leaf elements to the variable-pitch axis according to the geometric relationship between the blades, wherein L1 is rsin theta 1 and L2 is rsin theta 2, and the moment of each leaf element to the variable-pitch rotating shaft is the sum of the aerodynamic moment of the leaf element itself and the moment of lift force and resistance to the variable-pitch rotating shaft; calculating to obtain the moments M1 and M2 of the front blade and the rear blade to the variable-pitch rotating shaft, wherein the moment of one group of blades to the variable-pitch rotating shaft is the sum M of M1 and M2;

taking the position of the paddle with the initial configuration as a variable pitch reference position, wherein the variable pitch angle beta is 0; when one group of blades is subjected to pitch change around a pitch change axis, the front blade and the rear blade generate mounting angle changes beta 1 and beta 2, and the actual torsion angles of the front blade and the rear blade are respectively beta 1 ═ beta cos theta 1 and beta 2 ═ beta cos theta 2;

step 6: establishing a blade performance function:

establishing a function of a moment M of the blade to the variable-pitch rotating shaft and a blade tension T:

(T,M)＝f(a1,a2,δ1,δ2,θ1,θ2,V,n,β,ρ)

a1, a2, delta 1, delta 2, theta 1 and theta 2 are trim adjustment parameters, and V, n, beta and rho are state parameters; wherein a1 and a2 are chord length variation coefficients of the front and rear blades, and delta 1 and delta 2 are torsion angle variation of the front and rear blades; v is the flying speed, n is the rotating speed of the propeller, beta is the variable pitch angle, and rho is the air density under the flying height;

and 7: acquiring a flying speed V and an air density rho under a flying height according to a set cruising state, calculating a tension-rotating speed curve of an initial configuration at different variable pitch angles, and then selecting a use rotating speed n and a required variable pitch angle beta of the cruising state as state parameters during next leveling adjustment according to a tension requirement Tr;

and 8: optimizing and balancing:

for the performance function (T, M) established in the step 6, f (a1, a2, delta 1, delta 2, theta 1, theta 2, V, n, beta, rho), under the state parameters V, n, beta, rho given in the step 7, optimizing to obtain a group of a1, a2, delta 1, delta 2, theta 1 and theta 2, so that M is 0 and T is more than or equal to Tr;

and step 9: and (5) obtaining the geometrical parameters of the propeller according to the optimized balancing result obtained in the step 8 and the geometrical relation of the initial configuration in the step 5, and finishing the design of the pneumatic variable-pitch propeller.

Further preferred scheme, said a self-adaptation pneumatic variable pitch propeller design method, characterized by: the clamping groove is arranged to enable the pitch-variable angle of the paddle blade to be not lower than 0 degree.

Advantageous effects

The propeller blade designed by the invention has aerodynamic static stability, and can automatically adjust the propeller pitch according to different incoming flow and rotating speed conditions. For example, at the same rotation speed, as the flying speed increases (the advancing ratio increases), the local attack angle of the blades decreases, and since the focal points of the two blades are located behind the variable-pitch rotating shaft, the moment reduction of the front blade to the variable-pitch shaft is smaller than that of the rear blade to the variable-pitch shaft, the head-lowering moment of the rear blade is not enough to overcome the head-raising moment of the front blade, so that the resultant moment variation is head-raising, and the propeller pitch angle increases; the increase of the pitch angle increases the local attack angle of the blades, and as the focuses of the two blades are positioned behind the pitch-variable rotating shaft, the moment increment of the front blade to the pitch-variable shaft is smaller than that of the rear blade to the pitch-variable shaft, and the head-lowering moment of the rear blade can gradually overcome the head-raising moment of the front blade and is balanced at a certain angle; at the same speed, as the propeller speed decreases (the forward ratio increases), the propeller pitch angle will likewise increase and stabilize at an angle under the action of aerodynamic damping, based on the same principle. Because the propeller with high advancing ratio usually has a larger torsion angle, and the propeller with low advancing ratio usually has a smaller torsion angle, the invention can automatically adjust the propeller to the optimal pitch state according to the advancing ratio, thereby realizing the efficient use of the wide envelope.

Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

Drawings

The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1: a schematic top view of a double-blade layout;

FIG. 2: blade height (side view) schematic

FIG. 3: a blade circumferential cross-sectional schematic diagram;

FIG. 4: blade radial position section diagram;

FIG. 5: a set of blade schematics;

FIG. 6: another set of blade schematic diagrams;

wherein, 1-front blade, 2-rear blade, 3-pitch-changing lower limit clamping point, 4-motor shaft hole, 5-motor shaft hub, 6-pitch-changing shaft, 7-front and rear blade connecting hub, and 8-pitch-changing shaft hole;

FIG. 7: a four-blade schematic;

FIG. 8: a variable pitch axis torque coefficient along with a variable pitch axis angle change diagram;

FIG. 9: the torque coefficient of the variable pitch shaft changes with the rotating speed;

FIG. 10: the forward ratio curve of the propeller at a stable angle.

Detailed Description

The following detailed description of embodiments of the invention is intended to be illustrative, and not to be construed as limiting the invention.

The specific design method comprises the following steps:

(1) aiming at the high-propeller-disc load requirement of a vertical take-off and landing aircraft, the diameter D and the maximum tension T of a propeller are determined_max。

(2) Selecting an airfoil with higher lift coefficient and better lift resistance (the maximum lift coefficient should be more than 1.2), such as a Clark Y airfoil or an ARA-D airfoil and other mature airfoils; the design angle of attack a of the airfoil is selected (typically near the maximum lift-to-drag ratio, optionally before the maximum lift coefficient if the blade disc loading requirement is too high).

(3) The maximum speed is determined by the tip mach number limit (typically the tip mach number is less than 0.7 Ma).

(4) The distribution of the chord length and the torsion angle of the propeller along the radial direction is calculated by adopting a reverse strip theory and an optimal circular distribution theory (or other conventional propeller design methods) according to the maximum rotating speed, the maximum tension, the tiny incoming flow (such as the incoming flow speed of 1 m/s) and four blades (the propeller is designed according to the maximum tension state, because the propeller designed according to the highest cruising efficiency is difficult to meet the maximum tension requirement of the vertical take-off and landing state). Setting a tension offset and executing twice, wherein the first design tension is larger than the maximum tension, for example, 1.05-1.2 times Tmax, the airfoil design attack angle alpha in the step (2) is increased by 1-2 degrees, and the design result is called the chord length and torsion angle distribution of the No. 1 blade; and (3) the design tension of the second time is lower than the maximum required tension, for example, 0.8-0.95 times Tmax, the design attack angle alpha of the airfoil profile in the step (2) is reduced by 1-2 degrees, and the design result is called the chord length and torsion angle distribution of the No. 2 blade. The designed ratio of the chord length of the No. 1 blade to the chord length of the No. 2 blade is about 0.7-0.9, and the torsion angle of the No. 1 blade is slightly larger by 1-2 degrees. If the chord length ratio is smaller, the design tension of the first blade can be increased, or the airfoil design attack angle can be reduced, or the No. 2 blade can be reversely operated; if the torsion angle of the No. 1 blade is smaller, the design attack angle of the No. 1 blade airfoil can be increased (or the design attack angle of the No. 2 blade airfoil can be reduced). The step can obtain the distribution of the chord length and the torsion angle of the No. 1 and No. 2 blades along the radial direction.

(5) Establishing an initial configuration of a balancing blade:

among 4 paddles, get wherein relative (diagonal) two and use paddle distribution No. 1, two use paddle distribution No. 2 in addition relatively, adjacent two as a set of paddle, two sets of paddles altogether establish displacement pivot axis for every paddle, and two sets of displacement pivots altogether are each other 180 degrees contained angles and use screw pivot central symmetry. Each blade is arranged in a straight line at 1/4 (approximate focus position of the airfoil) of the chord length of the cross-section airfoil, namely, a 1/4 chord length connecting line of the blade is used as a 'focus line' of the blade.

The rotation direction of the propeller is the positive direction of an angle, the front blade is a front blade, the rear blade is a rear blade, the front blade uses a No. 1 blade (the design tension is larger, the chord length is smaller), and the rear blade uses a No. 2 blade (the design tension is smaller, the chord length is larger). The direction of the tension of the rotating shaft of the propeller is recorded as the positive direction of the height. The blade front edge is called blade 'raising' when rotating upwards, and the blade rear edge is called 'lowering' when rotating upwards.

The included angle between the focal point line of the front blade and the variable pitch axis is called a front blade angle theta 1, and the included angle between the rear blade and the variable pitch axis is called a rear blade angle theta 2. The distances between the focal line of the front blade and the focal line of the rear blade and the axis of the variable-pitch rotating shaft are d1 and d2 respectively (the pulling force direction is a positive direction). As shown in fig. 1 and 2.

Establishing an initial configuration: the front blade is positioned in front of and above the pitch changing axis, namely theta 1 is a positive value, and d1 is a positive value; the rear blade is located behind and below the pitch axis, namely theta 2 is negative and d2 is negative. Wherein d1/d2 is sin theta 1/sin theta 2, which enables a focal point connecting line at the same radial position of the front blade and the rear blade to pass through the variable-pitch rotating shaft; the absolute value of theta 1 is smaller than the absolute value of theta 2, so that the front blade is closer to the variable-pitch rotating shaft, and the area of the front blade is smaller than that of the rear blade, that is, the total focus of the variable blade is located behind the variable-pitch rotating shaft (representing that the variation of the front blade to the moment of the variable-pitch shaft along with the attack angle is smaller than that of the rear blade), and static stability is realized. In this case, when the blades generate positive lift, the front blades will provide a head-up moment and the rear blades will provide a head-down moment. Taking blade sections (phylls) at any radial position r, the moment arms L1 and L2 of the phylls to the variable pitch axis can be calculated by the geometrical relationship between the blades (a right-angle triangle is formed by the focal point lines of the front and rear blades, the variable pitch axis and the moment arm), wherein L1 is rsin theta 1, and L2 is rsin theta 2. The moment of each leaf element on the variable-pitch rotating shaft is the sum of the aerodynamic moment and the lift force of the leaf element and the moment of the resistance on the variable-pitch rotating shaft. The moments M1 and M2 of the front blade and the rear blade to the variable-pitch rotating shaft can be obtained by using a strip theory or other calculation methods, and the moment of one group of blades to the variable-pitch rotating shaft is the sum M of M1 and M2.

The blade position of the initial configuration is taken as a pitch reference position, and the pitch angle beta is 0. When a group of blades are pitched around a pitch axis, the front blade and the rear blade generate the mounting angle changes β 1 and β 2, and the actual torsion angles of the front blade and the rear blade are β 1 ═ β cos θ 1 and β 2 ═ β cos θ 2, respectively.

This step is completed to obtain the full geometry of the initial blade configuration (not yet trimmed).

(6) Establishing blade performance functions

Establishing adjustable parameters of front and rear blades: the chord length change coefficients a1 and a2 of the front blade and the rear blade are used for integrally zooming the chord length of the blades (the chord length change coefficients are multiplied on the whole under the condition that the original chord length distribution is not changed); the torsion angle variation delta 1 and delta 2 of the front blade and the rear blade (under the condition that the original torsion angle distribution is unchanged); when a1 and a2 are 1 and δ 2 are 0, the initial configuration is obtained.

And establishing a function of the blade to the moment of the variable-pitch rotating shaft. The moment of the blades on the variable-pitch shaft is influenced by the geometric parameters of the blades and the flight state. In order to trim the blade in a given state, the adjustable parameters of the function of the blade on the moment of rotation are selected as follows: chord length change coefficients a1 and a2 (generally 0.8-1.2), blade torsion angle change quantities delta 1 and delta 2 (generally-2 degrees), and blade angles theta 1 and theta 2. These parameters are trim adjustment parameters, and different trim adjustment parameters will determine different static stability, trim pitch angle and trim pull. Wherein a1, a2, theta 1 and theta 2 can be used for adjusting the static stability margin (representing the static stability moment) of the blade, and can also influence the displacement angle during the leveling; δ 1 and δ 2 are mainly used for adjusting the pitch angle during leveling. Any set of trim adjustment parameters will determine a blade layout profile, and then at a given flight speed V, rotation speed n, pitch angle β, and flight height (air density ρ), the blade moment M and blade tension T of the blade to the pitch rotating shaft can be calculated by using the propeller strip theory or other calculation methods, that is, (T, M) ═ f (a1, a2, δ 1, δ 2, θ 1, θ 2, V, n, β, ρ), where a1, a2, δ 1, δ 2, θ 1, θ 2 are trim adjustment parameters, and V, n, β, ρ are state parameters.

(7) Selecting the rotating speed n and the variable pitch angle beta in a design state: determining cruise states (V and rho are known), calculating tension-rotation speed curves at different pitch angles for an initial configuration (which is not balanced yet) by using a strip theory or other calculation methods, and selecting the use rotation speed n and the required pitch angle beta of the cruise states according to a tension demand Tr as state parameters during later leveling adjustment. (since it is not yet trimmed, the moment of the pitch axis calculated from this state is usually not zero. although the trim adjustment parameters a1, a2, δ 1, δ 2 affect the tension, it usually does not cause irreconcilable tension variations due to the limited variation of the adjustment parameters.)

(8) Optimizing and balancing: setting the tension requirement in a cruising state as a constraint condition (T is more than or equal to Tr), wherein the moment of a group of blades to a variable pitch shaft is zero (M is 0), and optimizing the balancing parameters a1, a2, delta 1, delta 2, theta 1 and theta 2 by using an optimization method under the state of the rotating speed n and the variable pitch angle beta selected in the step (7), so that the moment M of the variable pitch shaft of the blades under the given rotating speed n, variable pitch angle beta and flight state (V and rho) is zero (namely balancing), the tension is still not less than the required tension, and the static stability is provided (the moment of the variable pitch shaft is low when the variable pitch angle beta is increased by 1 degree, and the moment of the variable pitch shaft is high when the moment is increased by the reverse degree). That is, for the propeller performance function (T, M) ═ f (a1, a2, δ 1, δ 2, θ 1, θ 2, V, n, β, ρ), given an arbitrary set of a1, a2, δ 1, δ 2, θ 1, θ 2 under the design V, n, β, ρ (given by the cruise state and step (7)), one set T, M can be calculated, and a set of a1, a2, δ 1, δ 2, θ 1, θ 2 is optimized by an optimization method so that M is 0 while T is ≧ Tr, and M (β +1) <0, M (β -1) >0 (static stability requirement).

If the tension is always smaller during balancing, the design rotating speed or the pitch angle can be increased under the condition of meeting the tension requirement Tr in the step (7); otherwise, the design rotating speed or the pitch angle is reduced.

(9) And (4) determining all balancing parameters and propeller geometric parameters according to the balancing result in the step (8), and basically completing the design of the pneumatic variable-pitch propeller. Because the advancing ratio of the propeller in the vertical take-off and landing state is too low, the low head moment of the blades is usually too large, and the clamping grooves can be arranged to ensure that the pitch-variable angle of the blades is not lower than 0 degree.

Certainly, the performance of the propeller in the vertical take-off and landing state and the cruise state (and other task states focused on mainly) can be checked through a strip theory or other calculation methods, whether the performance of the propeller meets the actual use requirement is judged, if the balancing tension and the maximum tension of the propeller are too much, the wing profile in the step (2), the design attack angle and the design tension offset in the step (4) can be adjusted, and the iterative designs of the steps (4) to (9) are repeatedly and sequentially executed until the performance of the propeller meets the requirement.

According to the propeller established by the design method, the front blade provides head-up torque for the variable pitch shaft, and the rear blade provides head-down torque. That is, the resultant lift increase produced by the front and rear blades acts behind the variable pitch rotor shaft (with a focus behind the rotor shaft). The higher the advancing ratio (or the smaller the pitch angle of the blade), the smaller the local angle of attack of the blade, the smaller the resultant lift force of the blade, i.e. the negative increment of the lift force acts on the focal position (behind the rotating shaft), so that the blade generates a head-up moment to the pitch rotating shaft, and the angle of attack of the blade is increased; when the advancing ratio is reduced (or the pitch variation angle of the blade is increased), the local attack angle of the blade is increased, the combined lift force of the blade is increased, namely the positive increment of the lift force acts on the focal position, so that the blade generates head lowering moment on the pitch variation rotating shaft, and the attack angle of the blade is increased. Therefore, no matter the forward ratio is changed by the rotating speed or the flying speed, as long as the local attack angle of the blade is changed, the blade can generate aerodynamic moment to automatically change the pitch, and the aerodynamic moment is stabilized at the trim pitch changing angle under the action of aerodynamic damping.

The invention is different from the prior art in that the blades are pneumatically designed without an additional automatic pitch-changing mechanism, and the wide-envelope automatic pitch-changing of the propeller is realized by adopting a blade design idea of double-blade layout.

The following design is based on the no-inflow condition (vertical take-off and landing), and the design example is as follows. Wherein the tension in the vertical take-off and landing stage is larger than 110N, the concerned cruise state is divided into two states of 30m/s and 85m/s, and if the lift-drag ratio of the airplane is 6 and 2.2 respectively, the tension of the propeller in the two cruise states is larger than 18N and 50N.

The diameter of the paddle disk is 0.3m, and both the two paddles adopt Clark Y airfoil profiles. The front and rear blades are integrated and can freely rotate around a variable pitch axis, and the chord length c and the torsion angle are distributed as shown in the following.

The front blade and the rear blade are linearly arranged according to 1/4 (airfoil focus) of the airfoil chord length, the front blade and the rear blade are respectively 25 degrees and-30 degrees away from a variable pitch axis, the front blade is 0.01m higher than the variable pitch axis, and the rear blade is 0.0118m lower than the variable pitch axis.

To prove that the blade has aerodynamic static stability to both rotating speed and flying speed, a pitch axis torque coefficient curve (the blade head-up is positive) is calculated by exemplifying the strip theory.

As can be seen from the variable pitch axis torque coefficient plots of fig. 8 and 9, at different speeds, the blades all have aerodynamic static stability (the slope of the curve is negative), the blades will increase the pitch angle at high forward ratios, decrease the pitch angle at low forward ratios, and self-stabilize (the variable pitch axis torque is zero) at the appropriate variable pitch angle positions (except for the 1m/s condition). For example, when the propeller flies at 85m/s and rotates at 10000rpm, if the pitch angle of the propeller is 28 degrees, the head-up moment of the front blade is greater than the head-down moment of the rear blade, and the resultant moment of the two blades is a positive value (head-up), so that the blades will head up; with the raising of the blades, the raising moment of the front blade is gradually overcome by the rear blade, and when the blades raise to 31 degrees, the moments of the two blades are balanced; if the pitch angle of the blades continues to increase due to inertia effect (or disturbance), the head-lowering moment of the rear blade exceeds the head-raising moment of the front blade, the resultant moment of the blades is a negative value (head-lowering), and the head-lowering moment prevents the pitch angle of the blades from continuing to increase and lowers the head to about 31 degrees; finally, the blade will be stable around the 31 degree position, subject to aerodynamic damping. If the rotating speed is increased to 10500rpm, the local angle of attack of the blades is increased, the lift force of the front blade and the rear blade is increased, but the moment increment of the front blade is smaller than that of the rear blade, so that the head raising moment of the front blade cannot overcome the head lowering moment of the rear blade, the resultant moment is the head lowering moment, and the variable pitch angle of the blades is reduced.

Under the condition of 30m/s, the pitch angle of the blades is about 12deg, the rotating speed is 7000rpm, the pulling force is 20.46N, the power is 826.885W, and the efficiency of the propeller is 74%; under the condition of 85m/s, the pitch angle of the blades is about 31deg, the rotating speed is about 9800rpm, the pulling force is about 51.48N, the power is about 5330.5W, and the efficiency is 82.09%. In the 1m/s (vertical take-off and landing) state, due to the fact that the advancing ratio is too low, although the blade has static stability (the slope of a moment curve is negative), the blade cannot be balanced, the blade always has a head lowering moment, only a clamping point needs to be set at the moment, the blade is prevented from continuously lowering the head, namely the variable pitch angle of the blade is zero, and when the rotating speed is 13500rpm, the blade tension is 113.54N, the power is 3915.59W, and the force effect is 2.9 kg/kw.

The forward ratio curve for the propeller at steady angle at 85m/s is plotted, as well as the forward ratio curve for the fixed pitch propeller (pitch angle 31.5 degrees) as compared to fig. 10.

The curves show that the automatic variable pitch propeller can achieve high efficiency (eta is larger than 0.8) in a wider advancing ratio range, and the efficiency changes smoothly along with the advancing ratio; the high efficiency range of the fixed-pitch propeller is narrow, and the efficiency changes obviously along with the change of the advancing ratio.

The calculation shows that the propeller can provide higher blade disc load in the vertical take-off and landing stage so as to meet the power requirement of vertical take-off and landing, and simultaneously can provide higher tension in the high-speed stage and has higher propeller efficiency. Meanwhile, the pneumatic variable pitch propeller can find a self-stabilizing point (except for a vertical take-off and landing stage) at various speeds and rotating speeds, and the wide-envelope wire is used.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.

Claims

1. A design method of a self-adaptive pneumatic variable-pitch propeller is characterized by comprising the following steps: the method comprises the following steps:

step 4.2: according to the maximum rotating speed obtained in the step 3, a micro incoming flow hypothesis is adopted, the number of the blades is four, and the T is 0.8-0.95 times that of the blades_maxAs design tension, the design attack angle is reduced by 1-2 degrees compared with alpha in the step 2, the radial distribution design of the chord length and the torsion angle of the propeller is carried out, and the No. 2 blade is obtainedChord length and twist angle distribution;

and 5: establishing an initial configuration of a balancing blade:

step 6: establishing a blade performance function:

(T,M)＝f(a1,a2,δ1,δ2,θ1,θ2,V,n,β,ρ)

and 8: optimizing and balancing:

2. The design method of the adaptive pneumatic variable pitch propeller of claim 1, wherein the method comprises the following steps: the clamping groove is arranged to enable the pitch-variable angle of the paddle blade to be not lower than 0 degree.