A longitudinal heavy-load unmanned aerial vehicle through-shaft transmission and overload protection system

Abstract

The application provides a longitudinal heavy-load unmanned aerial vehicle through-shaft transmission and overload protection system, which comprises a fuselage, a single gasoline engine, a through-shaft transmission system, a synchronous reversing gear train and an overload protection module. The through-shaft transmission system comprises a hollow main through-shaft extending along the longitudinal direction of the fuselage, the front end of the main through-shaft is connected with a front rotor reducer assembly through a front universal joint, the rear end of the main through-shaft is connected with a rear rotor reducer assembly through a rear universal joint, and rigid synchronous transmission of front and rear rotors is realized; the synchronous reversing gear train enables the front rotor output shaft and the rear rotor output shaft to output in reverse at a speed ratio of 1:1; the overload protection module comprises a controllable slipping unit of a primary magnetic powder clutch and a mechanical fuse unit of a torque limiting coupling with replaceable shear gaskets, the system realizes high synchronous reverse transmission and graded overload protection under a single power source, reduces weight and improves efficiency, and improves flight safety.

Description

Technical Field

[0001] This invention relates to the field of unmanned aerial vehicle technology, and in particular to a through-shaft drive and overload protection system for a tandem heavy-duty unmanned aerial vehicle. Background Technology

[0002] Tandem-rotor UAVs employ two sets of rotors arranged longitudinally along the fuselage, offering significant advantages such as a large center of gravity range, high lift efficiency, good hovering stability, and a large payload capacity. They hold broad application prospects in heavy-duty transport, material delivery, and emergency rescue. However, existing tandem UAV power transmission schemes suffer from the following prominent technical defects, hindering their reliability and practicality: 1. Weight and efficiency issues caused by power redundancy Currently, most heavy-duty UAVs adopt a dual-power source solution of "front motor + rear motor" or "front engine + rear engine," with some even using four engines to drive two rotors, resulting in excessive power redundancy. This dual-power source solution significantly increases the overall weight (typically 15%–30%), accelerates fuel or electricity consumption, exponentially increases the complexity of the control system, and makes the power matching and fault switching logic between the two power sources extremely complex. Other solutions use a single engine connected to the front and rear rotors via belt drive; however, belt drives are subject to elastic deformation (the elastic slip rate is typically 1%–3%), making it difficult to guarantee high-precision synchronization between the front and rear rotors.

[0003] 2. Problem of poor synchronization between the front and rear rotors Tandem rotor UAVs require extremely high synchronization between their front and rear rotors. If the rotor speeds are not synchronized, it will lead to an imbalance in counter-torque, causing yaw drift and seriously threatening flight safety. Traditional belt-driven or chain-driven systems have inherent drawbacks: belts elongate under load, resulting in a significant phase and speed difference between the front and rear rotors; while chain drives offer high rigidity, they suffer from polygonal effects and wear elongation, leading to a significant decrease in synchronization accuracy over long-term use. Furthermore, belts and chains require additional tensioning mechanisms at high speeds, further increasing system complexity and weight.

[0004] 3. Inadequate or missing overload protection mechanisms. During takeoff and landing, or at low altitudes (such as traversing jungles or operating close to the ground), the rotor of a drone is highly susceptible to striking tree branches, power lines, buildings, or ground obstacles. Existing transmission systems often employ rigid connections; if one rotor becomes jammed or experiences a severe impact, the resulting enormous reaction torque is transmitted in reverse along the transmission chain, instantly destroying reducer gears, drive shafts, and even causing engine crankshaft breakage, often resulting in catastrophic crashes. While some existing solutions incorporate clutches (such as centrifugal clutches or overrunning clutches), these are only used for engine start-stop decoupling and are not designed to protect against sudden overloads during flight. Furthermore, they do not disclose replaceable shear shim-type mechanical fuse mechanisms.

[0005] 4. Lubrication and thermal management issues The transmission system of tandem UAVs runs through the entire fuselage, with a long drive shaft. Traditional external lubrication lines not only increase weight and wind resistance but are also prone to problems such as pipe aging and leakage. Especially when the rotor reducers are located at the front and rear ends of the fuselage, how to efficiently and reliably supply oil to the remote reducers is a problem that current technology has not solved well.

[0006] 5. Lack of thermal expansion compensation for the drive shaft. Metal drive shafts undergo axial thermal expansion during operation due to frictional heat and ambient temperature. The through-shaft length of tandem UAVs typically exceeds 2 meters, and the axial elongation caused by temperature rise can reach several millimeters. Without an axial compensation mechanism, thermal expansion will generate enormous axial internal stress, leading to abnormal bearing preload, changes in gear meshing clearance, accelerated wear, and even shaft seizure.

[0007] In summary, there is an urgent need for a tandem heavy-duty UAV transmission system that can achieve single-power-source drive, ensure high-precision synchronization of the front and rear rotors, and possess a comprehensive multi-level overload protection mechanism. Summary of the Invention

[0008] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a tandem heavy-duty UAV through-shaft drive and overload protection system, which solves the following technical problems: 1. Replacing dual power sources with a single gasoline engine, significantly reducing system weight and improving payload ratio; 2. Achieving strict synchronous reverse rotation of the front and rear rotors through a through-type rigid shaft, eliminating fuselage yaw moment; 3. Providing graded overload protection through a dual-stage overload protection architecture of "electronically controlled recoverable slippage + mechanically irreversible fuselage", addressing various overload conditions from light impact to severe collision; 4. Achieving centrifugal self-lubrication through a hollow through-shaft with built-in lubrication oil channels, simplifying the lubrication system; 5. Absorbing thermal expansion and protecting shaft integrity through a spline-type axial expansion compensation mechanism.

[0009] (II) Technical Solution The purpose of this invention is to provide a through-shaft drive and overload protection system for a tandem heavy-duty unmanned aerial vehicle, which includes a fuselage, a front rotor reducer assembly at the front end, and a rear rotor reducer assembly at the rear end. A single gasoline engine, fixed to the engine body, has an engine output shaft; The through-shaft drive system includes a main through-shaft extending longitudinally along the fuselage. The front end of the main through-shaft is connected to the input end of the front rotor reducer assembly via a front universal joint, and the rear end is connected to the input end of the rear rotor reducer assembly via a rear universal joint. The synchronous reversing gear system is at least partially housed within the housing of the front rotor reducer assembly or the rear rotor reducer assembly, such that the front rotor output shaft and the rear rotor output shaft output in a speed ratio of 1:1 and in opposite directions of rotation. The overload protection module is connected in series in the power transmission path consisting of the engine output shaft, the front rotor reducer assembly, the through-shaft transmission system and the rear rotor reducer assembly, and includes a primary overload protection unit and a secondary overload protection unit connected in series along the power flow direction. The primary overload protection unit is located between the engine output shaft and the input end of the front rotor reduction gear assembly; The secondary overload protection unit is located on the main through shaft, between the front universal joint and the rear universal joint.

[0010] Specifically, the first-level overload protection unit includes a magnetic powder clutch or an electromagnetic friction clutch, which has an active side and a driven side. The active side is coaxially fixed to the engine output shaft, and the driven side meshes with the primary reduction gear set in the front rotor reducer assembly through an input pinion. The magnetic powder clutch or electromagnetic friction clutch is configured to reduce the excitation current or cut off the power when the speed change rate |dω / dt| of the engine output shaft exceeds the calibrated value, so as to generate controlled relative slip between the active side and the driven side.

[0011] Specifically, the main shaft is a hollow cylinder with an axially penetrating internal lubrication channel inside. The internal lubrication channel is connected to the first lubrication oil chamber in the front rotor reducer assembly and the second lubrication oil chamber in the rear rotor reducer assembly through radial oil holes in the front universal joint and the rear universal joint, respectively.

[0012] Specifically, the secondary overload protection unit includes a torque limiting coupling, which consists of a left half of the coupling, a right half of the coupling, and at least one replaceable shear shim; the replaceable shear shim is sandwiched between the left half of the coupling and the right half of the coupling and is pre-tightened by a bolt pair; the replaceable shear shim has a pre-formed annular weakening groove; The replaceable shear pad is made of alloy steel, the cross-sectional shape of the annular weakening groove is a V-shaped or U-shaped groove, and the replaceable shear pad has at least two sets of specifications with different rated torques.

[0013] Specifically, the synchronous reversing gear system includes a bevel gear reversing assembly and a spur gear reduction stage. The bevel gear reversing assembly is housed within the front rotor reducer assembly and includes a driving bevel gear fixed on the intermediate shaft and a driven bevel gear fixed on the front rotor output shaft. The spur gear reduction stage is located within the rear rotor reducer assembly and transmits the torque at the rear end of the main shaft to the rear rotor output shaft.

[0014] Specifically, the tooth ratio of the driving bevel gear to the driven bevel gear is 1:1, and the tooth ratio of the driving gear to the driven gear of the spur gear reduction stage is also 1:1.

[0015] Specifically, the through-shaft transmission system further includes an axial telescopic compensation mechanism, which is located on the main through shaft and includes a spline sleeve structure consisting of an external spline shaft section and an internal spline sleeve. The external spline shaft section and the internal spline sleeve are axially slidingly engaged and maintain circumferential torque transmission, with an allowable axial travel δ of 2mm to 10mm.

[0016] Specifically, it also includes a flight control electronic controller, which is electrically connected to the magnetic powder clutch or electromagnetic friction clutch and is configured to receive the engine crankshaft speed signal ωe acquired by the Hall speed sensor, the front rotor speed signal ωf acquired by the encoder, and the rear rotor speed signal ωr acquired by the magnetic encoder. The flight control electronic controller has a pre-stored synchronization tolerance ε, where ε ≤ 5%. When |ωf-ωr| / ωf>ε, the magnetic powder clutch or electromagnetic friction clutch is de-energized and disengaged.

[0017] Specifically, the flight control electronic controller is also configured to perform a pre-takeoff self-check: during engine idling, it compares whether the transmission ratio of ωe and ωf satisfies |ωe / ωf - G| / G ≤ 2%, where G is a preset gear transmission ratio; if it does not satisfy the condition, it is determined to be a transmission system fault, takeoff is prohibited and an alarm is triggered.

[0018] This application also provides a method for overload protection of the power transmission of a tandem heavy-load unmanned aerial vehicle, including: Under normal operating conditions, the first-level overload protection unit is engaged, the second-level overload protection unit remains intact, and the power of the gasoline engine drives the front rotor output shaft and the rear rotor output shaft to rotate in opposite directions synchronously via the through-shaft transmission system. When the instantaneous torque rises to between the rated torque Tn and the first predetermined threshold T1, and T1 = 1.5 to 2.0 × Tn, the first-level overload protection unit enters a controllable slippage state to limit the transmitted torque; if the overload continues to exceed the preset time window Δt, Δt = 50ms to 200ms, the first-level overload protection unit is completely disconnected. When the transmitted torque exceeds the second predetermined threshold T2, and T2 = 2.0 to 3.0 × Tn, the shear pad of the secondary overload protection unit breaks, cutting off the power transmission; after replacing the shear pad, the system resumes transmission.

[0019] (III) Beneficial Effects This application has the following technical effects: 1. Weight Reduction and Efficiency Improvement: By using a single gasoline engine to rigidly connect the front rotor reduction gear assembly and the rear rotor reduction gear assembly through the main shaft, power is synchronously transmitted to the front and rear rotors through the same shaft, eliminating the speed difference and phase difference caused by dual power sources or flexible transmission. Therefore, the output shafts of the front and rear rotors can rotate stably in a speed ratio of 1:1 and in opposite directions, effectively eliminating the fuselage yaw moment. At the same time, the weight of the whole aircraft is reduced by 15% to 20%, so that the ratio of the maximum takeoff weight to the design empty weight of the tandem heavy-load UAV reaches more than 1.8, which greatly improves the payload and range.

[0020] 2. High synchronization: The through-shaft rigid connection (hollow steel shaft + universal joint) ensures strict synchronization of the front and rear rotor phases, with a speed synchronization accuracy of better than 0.5%, effectively eliminating fuselage yaw moment and improving flight stability.

[0021] 3. Tiered Safety Protection: The first-level overload protection unit uses a magnetic powder clutch or electromagnetic friction clutch, whose slippage torque can be precisely controlled by the excitation current. Therefore, under slight overload, it can generate controlled slippage, absorbing peak impact torque and protecting gears and shafts in the transmission chain. It also automatically recovers after the impact, achieving recoverable protection. The second-level overload protection unit uses replaceable shear shims with annular weakening grooves. When the transmitted torque exceeds a second predetermined threshold, the shear shims undergo pure shear fracture at the weakening groove, causing the left and right halves of the coupling to disengage. This irreversibly cuts off the power path under heavy overload, preventing engine stalling or further damage to the transmission system. This dual-level protection architecture of "electronically controlled recoverable slippage + mechanically irreversible fuse" achieves a complete safety protection chain from buffering to isolation, balancing safety and maintainability.

[0022] 4. Extremely low maintenance costs: After a severe overload, only the shear shims need to be replaced to restore the system. No major repairs are required. The system can be quickly restored by simply replacing the shear shims afterward, which greatly reduces maintenance costs.

[0023] 5. Integrated self-lubricating: Since the main shaft is a hollow cylinder with an internal lubrication channel, the lubricating oil can be delivered to the oil chambers of the front and rear reducers by using the centrifugal force of rotation. Therefore, the external oil pump and pipeline are eliminated, simplifying the structure and improving lubrication reliability.

[0024] 6. Thermal expansion self-adaptation: Due to the axial expansion and contraction compensation mechanism, the external spline shaft section and the internal spline sleeve can slide relative to each other along the axial direction, thus absorbing the thermal expansion and contraction displacement (2mm~10mm) of the main shaft and avoiding axial internal stress damage to the bearings and gears.

[0025] 7. Intelligent Fault Diagnosis: Because the flight control electronic controller monitors three rotation speed signals in real time and commands the clutch to de-energize and disengage when the speed difference exceeds the synchronization tolerance ε, it can promptly identify rotor asynchrony faults and control the UAV to enter autorotation descent, improving flight safety. The flight control system also supports pre-flight self-checks and real-time in-flight monitoring, enhancing system maintainability. Attached Figure Description

[0026] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the technical solutions of the present invention, and do not constitute a limitation on the technical solutions of the present invention.

[0027] Figure 1 This is a general longitudinal sectional view of the through-shaft drive and overload protection system for the tandem heavy-duty UAV in this embodiment; Figure 2 This is a structural diagram of the secondary overload protection unit of the through-shaft drive and overload protection system for the tandem heavy-duty UAV in this embodiment; Figure 3 This is a simplified diagram of the gear meshing principle of a synchronous reversing gear system; Figure 4 This is the control logic block diagram of the flight control electronic controller;

[0028] Explanation of reference numerals in the attached figures: 10. Gasoline engine; 11. Engine output shaft; 20. Front rotor reduction gear assembly; 21. Primary reduction gear set; 22. Front rotor output shaft; 30. Rear rotor reduction gear assembly; 32. Rear rotor output shaft; 40. Fuselage; 50. Through-shaft drive system; 51. Main through-shaft; 51a. Internal lubrication channel; 52a. Front universal joint; 52b. Rear universal joint; 52a1. Radial oil hole; 52b1. Radial oil hole; 60. Synchronous reversing gear system; 60a. Bevel gear reversing set; 60b. Spur gear reduction stage; 61. Driving bevel gear; 62. Driven bevel gear; 63. Intermediate shaft 70. Overload protection module; 71. Primary overload protection unit; 710. Magnetic powder clutch; 711. Input pinion; 72. Secondary overload protection unit; 720. Torque limiting coupling; 721. Left half of coupling; 722. Right half of coupling; 723. Replaceable shear shim; 723a. Annular weakening groove; 724. Bolt pair; 80. Flight control electronic controller; 81. Hall effect speed sensor; 82. Photoelectric encoder or magnetic encoder; 83. Magnetic encoder; 90. Axial telescopic compensation mechanism; 91. Spline sleeve structure; 91a. External spline shaft section; 91b. Internal spline sleeve. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] Example 1 The following is in conjunction with the appendix Figures 1 to 4 Further description reveals that this invention discloses a tandem heavy-duty unmanned aerial vehicle (UAV) through-shaft drive and overload protection system, comprising a fuselage 40, a gasoline engine 10, a front rotor reducer assembly 20, a rear rotor reducer assembly 30, a through-shaft drive system 50, a synchronous reversing gear system 60, and an overload protection module 70. The fuselage 40 is a longitudinal beam / truss structure extending longitudinally, with the front rotor reducer assembly 20 fixed at the front end and the rear rotor reducer assembly 30 fixed at the rear end. The components are described in detail below.

[0031] The gasoline engine 10 is a single two-stroke or four-stroke piston gasoline engine, fixedly mounted in the engine compartment at the upper middle section of the fuselage 40, with its engine output shaft 11 extending horizontally forward. In a preferred embodiment, a 120cc two-stroke piston gasoline engine is selected, with a maximum output power of approximately 15 kW. The engine output shaft 11 is coaxially fixed to the active side of the magnetic powder clutch of the first-stage overload protection unit 71 via a flange coupling. The engine displacement can be selected within the range of 80cc to 500cc according to the payload requirements of the UAV. Two-stroke engines have the advantage of high power density, while four-stroke engines have the advantages of fuel economy and low emissions, both of which are applicable.

[0032] The front rotor reducer assembly 20 is fixed to the nose truss at the front of the fuselage 40. An input pinion 711 is located on the driven side of the magnetic powder clutch. This pinion meshes with the primary reduction gear set 21 within the front rotor reducer assembly 20, introducing power into the front reducer. The reduction ratio of the primary reduction gear set 21 is set to approximately 3:1 to 5:1, determined based on the engine speed and the rotor's design speed. Inside the housing of the front rotor reducer assembly 20, a bevel gear reversing assembly 60a is also provided. The bevel gear reversing assembly 60a consists of a driving bevel gear 61 and a driven bevel gear 62. The driving bevel gear 61 is fixed to a horizontally mounted intermediate shaft 63, and the driven bevel gear 62 is fixed to the lower end of the vertical front rotor output shaft 22. Power is converted from a horizontal to a vertical direction and output to the front rotor via the bevel gear reversing assembly 60a, simultaneously changing the direction of rotation. In the preferred embodiment, the tooth ratio of the driving bevel gear 61 to the driven bevel gear 62 is set to 1:1 to minimize commutation losses.

[0033] The through-shaft drive system 50 includes a main through-shaft 51 arranged along the longitudinal axis of the fuselage 40. The main through-shaft 51 is an alloy steel shaft (such as 40Cr or 42CrMo material) with a hollow cylindrical structure, an outer diameter of approximately 30mm to 50mm, a wall thickness of approximately 5mm to 8mm, and a length determined according to the fuselage size (usually 1500mm to 3000mm). The hollow structure not only reduces the weight of the shaft (by approximately 40% to 50% compared to a solid shaft), but also serves as a lubricating oil channel, forming an axially continuous internal lubrication channel 51a. The front end of the main through-shaft 51 is connected to the output end of the front rotor reducer assembly 20 (i.e., the rear end of the intermediate shaft 63) via a front universal joint 52a (preferably a cross-type universal joint or a constant velocity universal joint), and the rear end is connected to the input end of the rear rotor reducer assembly 30 via a rear universal joint 52b. The front universal joint 52a and the rear universal joint 52b are respectively provided with radial oil holes 52a1 and 52b1, which are connected to the lubrication channel 51a inside the main shaft. The universal joints are made of high-strength alloy steel, and the rated torque is not less than 1.5 times the maximum output torque of the engine. The internal lubrication channel 51a is connected to the first lubrication oil chamber in the front rotor reduction assembly 20 through the radial oil hole 52a1, and to the second lubrication oil chamber in the rear rotor reduction assembly 30 through the radial oil hole 52b1.

[0034] The rear rotor reducer assembly 30 is fixed to the tail truss at the rear end of the fuselage 40. The rear end of the main through shaft 51 passes through the rear part of the housing of the front rotor reducer assembly 20 (supported by bearings and oil seals) and extends into the rear rotor reducer assembly 30 via the rear universal joint 52b. A spur gear reduction stage 60b is provided within the rear rotor reducer assembly 30. This reduction stage reduces and increases the torque at the rear end of the main through shaft 51 before transmitting it to the rear rotor output shaft 32. The reduction ratio of the spur gear reduction stage 60b is set to be the same as the reduction ratio of the front primary stage, ensuring that the front and rear rotor speeds are consistent. Since the power is reversed by the front bevel gear and then transmitted to the rear stage via the through shaft, the rotation direction of the rear rotor output shaft 32 is naturally opposite to that of the front rotor output shaft 22, eliminating the need for an additional reversing mechanism. The gear ratio of the driving bevel gear 61 to the driven bevel gear 62 is 1:1, and the gear ratio of the driving gear to the driven gear of the spur gear reduction stage 60b is also 1:1, thereby ensuring that the front rotor output shaft 22 and the rear rotor output shaft 32 have strictly equal speeds and strictly opposite directions.

[0035] Among them, such as Figure 1As shown, the primary overload protection unit 71 is located between the engine output shaft 11 and the input end of the front rotor reduction gear assembly 20, and includes a magnetic powder clutch or electromagnetic friction clutch 710. The magnetic powder clutch 710 consists of an active side (rotor), a driven side (magnetic powder cup), and an excitation coil. The active side is coaxially fixed to the engine output shaft 11, and the driven side meshes with the primary reduction gear set 21 in the front rotor reduction gear assembly 20 through the input pinion 711. The working principle of the magnetic powder clutch is as follows: when the excitation coil is energized, the magnetic powder forms a magnetic powder chain under the action of the magnetic field, coupling the active side and the driven side together to transmit torque; the larger the excitation current, the larger the transmitted torque; when the load torque exceeds the maximum transmitted torque corresponding to the current excitation current, slippage occurs between the active side and the driven side, thereby limiting the transmitted torque from exceeding the set value. The flight control electronic controller 80 can precisely control the upper limit of the transmitted torque of the magnetic powder clutch by adjusting the magnitude of the excitation current (typical range 0.5A to 4A). In this embodiment, the rated transmission torque Tn of the magnetic powder clutch 710 is set to 100 N·m (calculated based on the engine's rated power and reduction ratio), the excitation current under rated operating conditions is set to the rated value, and the upper limit of the transmission torque is slightly higher than the engine's maximum output torque (e.g., 1.2 × Tn). The first predetermined threshold T1 is set to 1.5 to 2.0 times the rated torque Tn. The calibration value of the engine output shaft speed change rate |dω / dt| is determined based on the UAV's takeoff weight and the rotor system's moment of inertia, with a typical range of 500 rad / s² to 2000 rad / s². For light platforms under 10 kg, the calibration value can be 500 rad / s² to 1000 rad / s²; for heavy-duty platforms over 20 kg, the calibration value can be 1000 rad / s² to 2000 rad / s². The flight control electronic controller 80 calculates |dω / dt| in real time at a sampling frequency of not less than 1kHz. When the calculated value exceeds the above-mentioned calibration value, it immediately triggers the controlled slip or power-off separation action of the first-level overload protection unit 71. In an alternative embodiment, the controllable slip element of the first-level overload protection unit 71 can also be replaced by an electromagnetic friction clutch instead of a magnetic powder clutch. The electromagnetic friction clutch includes an active friction plate group, a driven friction plate group, an electromagnetic coil, and an armature. The active friction plate group is coaxially fixed to the engine output shaft 11, and the driven friction plate group meshes with the primary reduction gear group 21 in the front rotor reduction gear assembly 20 through the input pinion 711. When the electromagnetic coil is energized, it generates an electromagnetic attraction, which attracts the armature and presses the active friction plate group and the driven friction plate group together. The torque is transmitted by the friction between the friction plates. The larger the excitation current, the stronger the electromagnetic attraction, the greater the pressing force of the friction plates, and the greater the maximum torque that can be transmitted. When the load torque exceeds the maximum static friction force corresponding to the current excitation current, relative slippage occurs between the active friction plate group and the driven friction plate group, thereby limiting the transmission of torque.When the rate of change of the engine output shaft 11, |dω / dt|, exceeds the calibrated value, the flight control electronic controller 80 reduces the excitation current of the electromagnetic friction clutch or completely de-energizes it, causing the friction plate clamping force to decrease or be completely released, resulting in controlled slippage or complete disengagement. Its control logic is the same as that of the magnetic powder clutch. The electromagnetic friction clutch has the advantages of simple structure, low cost, and strong anti-pollution capability, and is suitable for cost-sensitive platforms where the requirements for slippage smoothness are not high.

[0036] Among them, such as Figure 2 As shown, the secondary overload protection unit 72 is located on the main shaft 51, between the front universal joint 52a and the rear universal joint 52b, and includes a torque limiting coupling 720. The torque limiting coupling 720 consists of a left half 721, a right half 722, and at least one replaceable shear shim 723. The left half 721 is fixed to the front end of the main shaft 51 via a spline or interference fit, and the right half 722 is fixed to the rear end of the main shaft 51 via a spline or interference fit. The replaceable shear shim 723 is an annular thin sheet (outer diameter approximately 60mm to 100mm, thickness approximately 2mm to 5mm), made of high-strength alloy steel (such as 65Mn, 40Cr, or tool steel), sandwiched between the left half 721 and the right half 722, and pre-tightened by bolts 724. The shear shim 723 has a pre-formed annular weakening groove 723a. The cross-sectional shape of the annular weakening groove 723a is a V-shaped or U-shaped groove, continuously distributed along the entire circumference, reducing the effective cross-sectional area of ​​the shear shim at the weakening groove to the design value, making it a "weak link" in the entire transmission chain. When the transmitted torque exceeds the second predetermined threshold T2 (T2≈2.0~3.0×Tn, such as 250N·m), the shear shim 723 undergoes pure shear fracture at the annular weakening groove 723a, and the left half 721 and the right half 722 of the coupling disengage in the circumferential direction, irreversibly releasing the mechanical connection between the rear and front sections of the main shaft 51. The shear shim 723 is designed with at least two different rated torque specifications (such as 200N·m and 300N·m specifications), each with a different cross-sectional size of the annular weakening groove 723a. By simply replacing the shear shims with different specifications, the system can be adapted to UAV platforms with different takeoff weights (such as 10kg, 20kg, and 50kg classes) without replacing the coupling itself, greatly improving the system's versatility and adaptability. Replacing the shear shims is easy and can be completed within 10 minutes under field conditions.

[0037] Among them, such as Figure 1As shown, the through-shaft transmission system 50 also includes an axial expansion compensation mechanism 90, located in the middle of the main through-shaft 51, adjacent to the secondary overload protection unit 72. The axial expansion compensation mechanism 90 includes a spline sleeve structure 91, consisting of an outer spline shaft section 91a and an inner spline sleeve 91b. The outer spline shaft section 91a is fixed to the front extension of the main through-shaft 51, and the inner spline sleeve 91b is fixed to the rear receiving section of the main through-shaft 51. The outer spline shaft section 91a and the inner spline sleeve 91b slide and engage axially, allowing a relative displacement of a predetermined axial stroke δ (δ=2mm~10mm, preferably 5mm). In the circumferential direction, the spline engagement ensures the continuity of torque transmission; in the axial direction, the sliding fit allows the elongation of the main through-shaft due to thermal expansion to be absorbed, avoiding additional axial thrust on the front and rear bearings. The splined surfaces undergo heat treatment (such as carburizing and quenching to HRC58-62) and precision grinding to ensure wear resistance and transmission accuracy. The inner spline sleeve 91b has grease grooves on its inner wall to store grease and reduce sliding friction.

[0038] Among them, such as Figure 4 As shown, the system also includes a flight control electronic controller 80, which is fixed in the fuselage electronics bay and electrically connected to the magnetic powder clutch or electromagnetic friction clutch 710. The sensing system includes: a Hall effect speed sensor 81 fixed near the engine output shaft 11 to collect the engine crankshaft speed signal ωe; a photoelectric encoder or magnetic encoder 82 fixed near the front rotor output shaft 22 to collect the front rotor speed signal ωf; and a magnetic encoder 83 fixed near the rear rotor output shaft 32 to collect the rear rotor speed signal ωr. All sensor signals are transmitted to the flight control electronic controller 80 via shielded cables, with a sampling frequency of not less than 1kHz. The flight control electronic controller 80 has a pre-stored synchronization tolerance ε (ε≤5%, preferably 2%). When |ωf-ωr| / ωf>ε, the flight control electronic controller 80 determines a rotor desynchronization fault, instructs the magnetic powder clutch or electromagnetic friction clutch 710 to de-energize and disengage, causing the through-shaft transmission system 50 to enter an idle state, and the UAV to switch to rotor autorotation descent mode. In addition, the flight control electronics controller 80 also performs a pre-takeoff self-check: during engine idling (approximately 2000 RPM), it compares whether the transmission ratio of ωe and ωf satisfies |ωe / ωf - G| / G ≤ 2%, where G is the preset gear transmission ratio; if it does not satisfy this condition, it is determined to be a transmission system malfunction, takeoff is prohibited, and a ground alarm is triggered.

[0039] The front rotor reducer assembly 20 has a first lubricating oil chamber (for storing lubricating oil), and the rear rotor reducer assembly 30 has a second lubricating oil chamber. The internal lubrication channel 51a of the main shaft 51 is a central hole that runs through the entire shaft. The front universal joint 52a has a radial oil hole 52a1, which connects the internal lubrication channel 51a to the first lubricating oil chamber; the rear universal joint 52b has a radial oil hole 52b1, which connects the internal lubrication channel 51a to the second lubricating oil chamber. During operation, the centrifugal force generated by the high-speed rotation of the main shaft 51 (typically 3000 RPM to 6000 RPM) draws lubricating oil from the front and rear reducers into the internal lubrication channel 51a, and then throws it into the gear meshing area inside the front and rear reducer housings through the radial oil holes 52a1 and 52b1, achieving a combination of splash lubrication and forced lubrication. The preferred lubricant is a high viscosity index aviation gear oil (such as SAE 80W-90 or synthetic helicopter transmission oil) to ensure lubrication performance over a wide temperature range (-40℃ to 120℃). This design eliminates the need for an external oil pump, oil lines, and nozzles, simplifying the system structure and improving reliability. It also utilizes centrifugal force to achieve an adaptive effect of "the faster the rotation, the stronger the lubrication."

[0040] The working principle and usage process of this embodiment are as follows: Under normal operating conditions, the magnetic powder clutch 710 is in the rated excitation engagement state, and the shear shim 723 remains intact. The power of the gasoline engine 10 is transmitted to the main shaft 51 via the engine output shaft 11, the magnetic powder clutch 710, and the primary reduction gear set 21 in the front rotor reducer assembly 20, and is then synchronously transmitted to the rear rotor reducer assembly 30 by the main shaft 51. The synchronous reversing gear system 60 ensures that the front rotor output shaft 22 and the rear rotor output shaft 32 rotate in opposite directions at a speed ratio of 1:1.

[0041] When the rotor is subjected to a sudden impact such as a gust of wind or a rapid change in torque, causing the torque to rise to between the rated torque Tn and a first predetermined threshold T1 (T1 = 1.5 to 2.0 × Tn), the flight control electronic controller 80 detects that the rate of change of the engine output shaft 11 speed |dω / dt| exceeds the limit. It then actively reduces the excitation current of the magnetic powder clutch 710 to the micro-slip zone (e.g., 50% to 80% of the rated current), allowing it to enter the controllable micro-slip zone, absorbing the peak impact torque and protecting the gear teeth from pitting. After the impact (|dω / dt| returns to normal), the rated excitation is automatically restored.

[0042] If the overload condition persists for more than the preset time window Δt (Δt = 50ms ~ 200ms), the flight control electronics controller 80 determines it to be a continuous overload rather than a transient impact (such as foreign objects entangled in the propeller blades), and actively controls the magnetic powder clutch 710 to completely de-energize and disengage, unloading the engine. At the same time, it records the fault code and notifies the ground station via the data link. The UAV then switches to rotor autorotation descent mode.

[0043] When the transmitted torque exceeds the second predetermined threshold T2 (T2 = 2.0~3.0×Tn), the shear shim 723 undergoes shear fracture at the annular weakening groove 723a, causing the left half 721 of the coupling to disengage from the right half 722. The mechanical connection between the rear and front sections of the main shaft 51 is irreversibly severed, and the power path is physically broken. The flight control system detects a sudden drop in ωr (such as loss of the rear rotor speed signal or a significant drop exceeding 30%), immediately determining that secondary protection has been triggered. It then de-energizes the magnetic powder clutch, unloads the engine, and the UAV enters a rotor autorotation descent mode. Afterwards, only stopping the aircraft to replace the shear shim 723 and retightening the bolt assembly to the calibrated torque is required to restore transmission continuity.

[0044] This embodiment achieves high-precision synchronous reverse transmission between the front and rear rotors through a single gasoline engine 10 and a rigid hollow main shaft 51, reducing the overall weight by 15% to 20% and achieving a mechanical transmission efficiency of ≥92%. The dual overload protection is achieved through a first-stage magnetic powder clutch with controllable slippage and a second-stage shear pad with mechanical fusing, balancing flight safety and ease of maintenance. The built-in lubrication channel 51a and axial telescopic compensation mechanism 90 further improve the system's reliability and environmental adaptability.

[0045] Example 2 Based on Example 1, for heavy-load platforms with a takeoff weight of over 50 kg, the gasoline engine 10 is replaced with a four-stroke piston gasoline engine with a displacement of 300cc to 500cc, and a maximum output power of approximately 30 kW to 50 kW. The outer diameter of the main shaft 51 is correspondingly increased to 50mm to 80mm, and the wall thickness is increased to 8mm to 12mm. The rated transmission torque Tn of the magnetic powder clutch 710 is increased to 200 N·m to 400 N·m. The second predetermined threshold T2 of the shear shim 723 is correspondingly calibrated to 600 N·m to 1200 N·m. The gears in the front rotor reducer assembly 20 and the rear rotor reducer assembly 30 are changed from spur gears to helical gears to improve load-bearing capacity and reduce noise. The mechanical transmission efficiency of the shaft drive system 50 remains ≥92%. The sampling frequency of the flight control electronic controller 80 is increased to over 2 kHz. The remaining structures and working principles of this embodiment are the same as in Example 1, and will not be repeated here.

[0046] Example 3 Based on Example 1, the following components are adapted for use in extremely cold environments (below -40℃): the magnetic powder clutch 710 is filled with low-temperature magnetic powder (such as low-temperature magnetic powder material doped with molybdenum disulfide) to ensure that the fluidity of the magnetic powder is not lost at low temperatures; the lubricating medium in the front and rear rotor reducer assemblies 20 and 30 is replaced with low-temperature synthetic gear oil (such as PAO-based synthetic oil with a pour point below -50℃); the material of the shear shim 723 is replaced with low-temperature toughness alloy steel (such as low-temperature steel with added nickel) to prevent false triggering due to low-temperature brittle fracture; the flight control electronic controller 80 is equipped with a temperature compensation algorithm to automatically adjust the excitation current compensation value of the magnetic powder clutch 710 according to the ambient temperature. The lower the temperature, the greater the excitation current required to achieve the same transmitted torque. The remaining structure and working principle of this embodiment are the same as those of Example 1 and will not be described again.

[0047] Example 4 Based on Example 1, for scenarios requiring long-duration cruises such as high-speed patrols and border patrols, the gasoline engine 10 is replaced with a four-stroke electronic fuel injection gasoline engine, equipped with an electronic fuel injection system to improve fuel economy and emissions performance. The flight control electronic controller 80 adds cruise mode control logic: during constant altitude and speed cruise, it automatically optimizes the engine throttle opening and the excitation current of the magnetic powder clutch 710, ensuring the transmission system always operates within its highest efficiency range. The magnetic powder clutch 710 is equipped with a temperature monitoring function; when the heat generated by clutch slippage causes the temperature rise to exceed a safe threshold, it automatically reduces the upper limit of the transmitted torque to prevent magnetic powder aging. The remaining structure and working principle of this embodiment are the same as in Example 1 and will not be repeated here.

[0048] Although embodiments of the present invention have been shown above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications and variations to the above embodiments, but such modifications are all included within the broad scope of the foregoing disclosure, drawings and claims.

Claims

1. A through-shaft drive and overload protection system for a tandem heavy-duty unmanned aerial vehicle (UAV), characterized in that, include: The fuselage (40) has a front rotor gearbox assembly (20) at its front end and a rear rotor gearbox assembly (30) at its rear end; A single gasoline engine (10) is fixed on the fuselage (40) and has an engine output shaft (11); The through-shaft drive system (50) includes a main through-shaft (51) extending longitudinally along the fuselage (40). The front end of the main through-shaft (51) is connected to the input end of the front rotor reducer assembly (20) via a front universal joint (52a), and the rear end is connected to the input end of the rear rotor reducer assembly (30) via a rear universal joint (52b). Synchronous reversing gear system (60) is at least partially housed in the housing of the front rotor reducer assembly (20) or the rear rotor reducer assembly (30), so that the front rotor output shaft (22) and the rear rotor output shaft (32) output in a speed ratio of 1:1 and in opposite directions of rotation; The overload protection module (70) is connected in series in the power transmission path consisting of the engine output shaft (11), the front rotor reducer assembly (20), the through shaft transmission system (50) and the rear rotor reducer assembly (30), including a first-level overload protection unit (71) and a second-level overload protection unit (72) connected in series along the power flow direction; The primary overload protection unit (71) is located between the engine output shaft (11) and the input end of the front rotor reducer assembly (20); The secondary overload protection unit (72) is located on the main through shaft (51) and between the front universal joint (52a) and the rear universal joint (52b).

2. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 1, characterized in that: The first-level overload protection unit (71) includes a magnetic powder clutch or an electromagnetic friction clutch (710), which has an active side and a driven side. The active side is coaxially fixed to the engine output shaft (11), and the driven side meshes with the primary reduction gear set (21) in the front rotor reducer assembly (20) through the input pinion (711). The magnetic powder clutch or electromagnetic friction clutch (710) is configured to reduce the excitation current or cut off the power when the rotational speed change rate |dω / dt| of the engine output shaft (11) exceeds the calibrated value, so as to generate controlled relative slip between the active side and the driven side.

3. The tandem heavy-load UAV through-shaft drive and overload protection system according to claim 1, characterized in that: The main shaft (51) is a hollow cylinder with an axially penetrating internal lubrication channel (51a) inside. The internal lubrication channel (51a) is connected to the first lubrication oil chamber in the front rotor reducer assembly (20) and the second lubrication oil chamber in the rear rotor reducer assembly (30) through radial oil holes in the front universal joint (52a) and the rear universal joint (52b), respectively.

4. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 1, characterized in that: The secondary overload protection unit (72) includes a torque limiting coupling (720), which consists of a left half (721) of the coupling, a right half (722) of the coupling, and at least one replaceable shear shim (723). The replaceable shear shim (723) is sandwiched between the left half (721) and the right half (722) of the coupling and is pre-tightened by bolts. The replaceable shear shim (723) has a pre-formed annular weakening groove (723a). The replaceable shear pad (723) is made of alloy steel, the cross-sectional shape of the annular weakening groove (723a) is a V-shaped or U-shaped groove, and the replaceable shear pad (723) is provided with at least two sets of specifications with different rated torques.

5. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 1, characterized in that: The synchronous reversing gear system (60) includes a bevel gear reversing assembly (60a) and a spur gear reduction stage (60b). The bevel gear reversing assembly (60a) is housed in the front rotor reducer assembly (20) and includes a driving bevel gear (61) fixed on the intermediate shaft (63) and a driven bevel gear (62) fixed on the front rotor output shaft (22). The spur gear reduction stage (60b) is located in the rear rotor reducer assembly (30) and transmits the torque at the rear end of the main through shaft (51) to the rear rotor output shaft (32).

6. The tandem heavy-load UAV through-shaft drive and overload protection system according to claim 5, characterized in that: The tooth ratio of the driving bevel gear (61) to the driven bevel gear (62) is 1:1, and the tooth ratio of the driving gear to the driven gear of the spur gear reduction stage (60b) is also 1:

1.

7. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 1, characterized in that: The through-shaft transmission system (50) also includes an axial telescopic compensation mechanism (90), which is located on the main through shaft (51) and includes a spline sleeve structure (91) consisting of an outer spline shaft section (91a) and an inner spline sleeve (91b). The outer spline shaft section (91a) and the inner spline sleeve (91b) are axially slidingly engaged and maintain circumferential torque transmission. The allowable axial travel δ is 2mm to 10mm.

8. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 2, characterized in that: It also includes a flight control electronic controller (80), which is electrically connected to the magnetic powder clutch or electromagnetic friction clutch (710) and is configured to receive the engine crankshaft speed signal ωe collected by the Hall speed sensor (81), the front rotor speed signal ωf collected by the encoder (82), and the rear rotor speed signal ωr collected by the magnetic encoder (83); The flight control electronic controller (80) has a pre-stored synchronization tolerance ε, where ε ≤ 5%. When |ωf-ωr| / ωf>ε, the magnetic powder clutch or electromagnetic friction clutch (710) is de-energized and disengaged.

9. The tandem heavy-duty UAV through-shaft drive and overload protection system according to claim 8, characterized in that: The flight control electronic controller (80) is also configured to perform a pre-takeoff self-check: during engine idling, compare whether the transmission ratio of ωe and ωf satisfies |ωe / ωf - G| / G ≤ 2%, where G is a preset gear transmission ratio; if it does not satisfy, it is determined to be a transmission system fault, takeoff is prohibited and an alarm is triggered.

10. A method for overload protection of the power transmission of a tandem heavy-load unmanned aerial vehicle, employing the system as described in any one of claims 1 to 9, characterized in that, include: Under normal operating conditions, the first-level overload protection unit (71) is engaged, the second-level overload protection unit (72) remains intact, and the power of the gasoline engine (10) drives the front rotor output shaft (22) and the rear rotor output shaft (32) to rotate in opposite directions synchronously via the through-shaft transmission system (50). When the instantaneous torque rises to between the rated torque Tn and the first predetermined threshold T1, and T1 = 1.5 to 2.0 × Tn, the first-level overload protection unit (71) enters a controllable slippage state to limit the transmitted torque; if the overload continues to exceed the preset time window Δt, Δt = 50ms to 200ms, then the first-level overload protection unit (71) is completely disconnected. When the transmitted torque exceeds the second predetermined threshold T2, and T2 = 2.0 to 3.0 × Tn, the shear pad of the secondary overload protection unit (72) breaks, cutting off the power transmission; after replacing the shear pad, the system resumes transmission.

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