We have developed a dedicated rope-hook recovery device to enhance the applicability of recovery methods for small fixed-wing drones. This device utilizes a combined energy absorption mechanism integrating a “recovery rope and elastic deformation” strategy. The development process involved deriving the system’s dynamic equations using the Lagrange method, establishing an equivalent spring-damper model, and solving the response characteristics via the MATLAB platform. Static tensile tests were prioritized to determine the recovery rope parameters, followed by finite element analysis of the device’s core components. The methodology for calculating overturning loads was also detailed. Finally, we conducted recovery tests in ascending order of energy to validate the device’s performance.
The rope-hook recovery method is a classical technique for retrieving small fixed-wing drones. During the recovery phase, assisted by differential GPS positioning, the wingtip hook of the fixed-wing drone can precisely engage a rope deployed by the recovery device, enabling point-specific retrieval. This method is known for its non-destructive nature, safety, and cost-effectiveness, making it especially suitable for shipboard or island-based operations. Systems like the RQ-21 fixed-wing drone already employ this recovery approach.
To meet the requirements of a small long-endurance fixed-wing drone with an endurance exceeding 20 hours, our goal was to develop a recovery device with a higher energy absorption capacity than existing similar systems. While laboratory systems have achieved maximum recovery masses of 60 kg, longer endurance typically implies a greater take-off mass. Our design target was to create a device capable of handling emergency recovery scenarios for fixed-wing drones with kinetic energy exceeding 28.9 kJ, the theoretical value for certain systems.
System-Level Design
System Composition
The rope-hook recovery process for a fixed-wing drone consists of three phases: First, guided by differential GPS, the fixed-wing drone’s wing accurately strikes the recovery rope. The rope slides along the wing to the wingtip hook, which locks onto the rope and triggers the drone’s engine to stop, thereby capturing the fixed-wing drone. Second, due to inertia, the fixed-wing drone continues moving forward, tensioning the recovery rope, which absorbs the drone’s forward kinetic energy. The drone’s motion on the rope gradually transitions from linear forward movement to a rotational motion. Third, when the recovery rope’s absorption capacity is exhausted, the remaining kinetic energy of the fixed-wing drone is absorbed via elastic deformation of the recovery device’s mechanical structure. The drone’s motion fully converts into rotation relative to the device.
The recovery device comprises several key assemblies: a hose reel for automatic management of high-pressure hoses, a telescopic boom assembly for Y-direction extension, a turntable assembly acting as the pitch hinge, a multi-way valve for control, a rotary assembly for device rotation, a mounting bracket for vehicle installation, a cable drum for storing the recovery rope, a main luffing cylinder for controlling the boom’s elevation, a crossbar assembly driven by a hydraulic motor, and a folding arm assembly for deployment of the upper crossbar.
Hydraulic System Principle
The hydraulic system schematic details the power unit, directional control valves, cylinders (for folding arms, extension, main luffing), a telescopic motor, and a rotary table. The system is powered by a power take-off (PTO) from the vehicle’s transfer case, operating with the vehicle’s hydraulic oil tank.
Recovery Rope
Dynamic Modeling
A core principle in engineering is to simplify the system structure and reduce failure sources without compromising performance. Using a buffer rope with a suitable elastic modulus to directly absorb impact energy allows for a simpler system compared to using accumulators or motor-driven systems. To quickly obtain the dynamic characteristics, we derived the system’s dynamic equations using the Lagrange method and established an approximate dynamic model. This model equates the rope damping system to a spring-damper system.
In the model, \( k_3 \) and \( k_2 \) represent the lateral stiffness of the upper and lower crossbars, respectively. \( m_3 \) and \( m_2 \) are their equivalent masses. \( c_1/k_{l1} \) and \( c_2/k_{l2} \) are the spring-damping at the upper and lower ends. \( m_1 \) is the equivalent recovery mass of the fixed-wing drone. The parameters \( R_{10} \) and \( R_{20} \) divide the rope into upper and lower segments at the point of impact.
The total span between the upper and lower crossbar tips of our device is approximately 17 m. Assuming \( R_{10} \) is 8.5 m and a fixed-wing drone with 31.5 kJ of energy (mass 70 kg, velocity 30 m/s) is to be recovered, we solved the model using the central difference method in MATLAB. The resulting acceleration and velocity profiles provided crucial data. At the moment the fixed-wing drone experiences maximum overload, the corresponding rope tension was calculated to be approximately 5300 N. This value serves as a key mechanical parameter for rope design.
Static Tensile Tests
The maximum tension in the rope occurs at the moment of maximum rope extension during the buffering process, corresponding to a maximum rope displacement of 6 m. We decomposed this rope tension (5300 N) into X and Y components at the crossbar tips (folding arm assembly), also considering a Z-direction force.
During the recovery process, the maximum overload on the fixed-wing drone is about 9 g, and the rope experiences a tension of about 5300 N. When designing dynamic systems with static load criteria, a safety factor of 1.5 to 2 is typically chosen. Static tensile tests showed that a 10 mm diameter rubber rope had a maximum static breaking force of 16200 N, which meets the required safety margin.
Engineering Development
The tension of the recovery rope is determined by factors such as length, material, and elastic modulus. Synthetic rubber offers a higher elastic modulus. Ropes reinforced with a fiber wrap can achieve a higher tension limit. The length must be appropriate for the fixed-wing drone’s wingspan and recovery scenario. The material must ensure sufficient strength and wear resistance.
Our engineering design for the recovery rope assembly consists of a static rope and a rubber rope. The static rope is made of nylon (polyurethane) with an elongation of 3.6%, forming the main framework. The rubber rope, made of TPU wrapped in polypropylene yarn, is used for energy absorption.
A key feature is the arresting rope assembly, the part that makes direct impact contact with the fixed-wing drone’s wingtip hook. This assembly is a two-section design. It comprises a central section of static rope and locking buckle, with rubber ropes at both ends. This two-stage design not only enhances energy absorption but also facilitates inspection and replacement of the most wear-prone components. The rubber rope is shorter than the static rope. During recovery, the rubber rope stretches to absorb energy until it is stretched to the same length as the static rope, at which point its energy absorption function ceases.
| System | Velocity (m/s) | Mass (kg) | Recovery Energy (kJ) |
|---|---|---|---|
| FLAＲES 2.0 | 38.6 | 28.1 | 20.9 |
| ScanEagle | 25 | 28.1 | 8.8 |
| ＲQ21 | 30.8 | 61 | 28.9 |
| Hongyan | 30 | 29.2 | 12.8 |
| SC30 | 27.8 | 32 | 12.4 |
Recovery Device
Finite Element Analysis
After the recovery rope absorbs its portion of the energy, the remaining kinetic energy is absorbed through the elastic deformation of the device’s mechanical frame. This entire recovery process subjects the mechanical skeleton to impact overloads under elastic deformation. To ensure the safety of onboard equipment during recovery of the fixed-wing drone, we set a peak recovery overload of 10 g. Using the force relationships derived from the dynamic model, we applied constraints and loads for a finite element analysis (FEA). This analysis did not account for the rope’s buffering effect but included a level 7 wind load, representing a stringent condition for evaluating the device’s components.
| Assembly | Component | Max Stress (MPa) |
|---|---|---|
| Telescopic Boom | 1st Boom Section | 626.38 |
| 2nd Boom Section | 691.64 | |
| 3rd Boom Section | 685.00 | |
| 4th Boom Section | 853.38 | |
| Turntable | Turntable Body | 913.89 |
| Link 1 | 450.85 | |
| Link 2 | 476.52 | |
| Link 3 | 218.82 | |
| Link 4 | 483.65 | |
| Crossbar | End Boom Section | 714.56 |
| Base Boom Section | 904.47 | |
| Folding Arm | End Boom Section | 573.44 |
| Base Boom Section | 458.00 |
The FEA identified stress concentrations in several components: the 4th boom section’s head and slider contact area, the turntable’s local arc transition, and the crossbar base boom’s connection bushing with the turntable. For instance, the stress concentration in the 4th boom section, caused by holes for fastening wire ropes, resulted in a stiffness discontinuity and a calculated maximum stress of 853.38 MPa. Excluding these localized stress concentration effects, the overall stress levels were found to be within acceptable limits for the selected materials.
Strength Calculation
The primary material for the recovery device is Q690E steel, with a yield strength \( \sigma_s = 690 \) MPa and tensile strength \( \sigma_b = 770 \) MPa (minimum). The allowable stress \( [\sigma] \) was calculated using the permissible stress method:
$$ \frac{\sigma_s}{\sigma_b} = \frac{690}{770} = 0.896 $$
$$ [\sigma] = \frac{0.5\sigma_s + 0.3\sigma_b}{n} $$
Where \( n \) is the safety factor, taken as 1.34 considering wind load. This gives a basic allowable stress of approximately 458.58 MPa. For components with identified stress concentrations, a higher allowable contact stress was used. The stress levels for other components were all below 450 MPa, indicating the overall structural design meets the strength requirements.
Overturning Load Calculation
When deployed, the device reaches a height of 17 m, or 20 m when mounted on a vehicle. The overturning load \( M \) is a critical parameter for the design of the rotary assembly and anti-overturning features. It is comprised of three main components:
$$ M = M_1 + M_q + M_F $$
$$ M = m_1 g L_1 + (F_2 L_2 + F_3 L_3) + \frac{C_p q A L_3}{2} $$
Where \( m_1 \) is the equivalent mass of the device’s center of gravity, \( L_1 \) is the distance from the center of gravity to the base center axis, \( F_2 \) and \( F_3 \) are the Y- and X- direction forces at the upper crossbar tip, \( L_2 \) and \( L_3 \) are the corresponding distances to the base axis, \( C_p \) is the wind pressure correction factor, \( q \) is the standard wind load value, and \( A \) is the projected area of the device perpendicular to the wind direction. The total calculated overturning load was 137.3 kN·m with an axial force of 32000 N.
Operational Procedure
The recovery device is operated in a sequential procedure: Step 1: release the transportation lock, rotate the device to the predetermined angle using the turntable assembly. Step 2: raise the telescopic boom, attach the recovery rope, and deploy the lower crossbar driven by the hydraulic motor. Step 3: deploy the upper crossbar by actuating the folding arm assembly. Step 4: extend the telescopic boom to the specified height, completing the full deployment.
Recovery Testing and Validation
We conducted a preliminary test using a HY30 fixed-wing drone. The data acquisition system was an NI cRIO with an ICP force sensor sampled at 12800 Hz. The fixed-wing drone had a recovery mass of 25 kg and a recovery speed of 25 m/s. The force sensor, placed between the arresting rope and the recovery rope, recorded the force profile during recovery. The maximum force on the rope was approximately 500 N, corresponding to an overload of about 2g. These parameters were well below the system’s design limits.
Following the preliminary test, we conducted the final validation using a military-trade fixed-wing drone with an 87 kg take-off weight. The recovery process was monitored. The onboard fiber-optic inertial navigation system recorded the fixed-wing drone’s recovery speed and synthetic acceleration. After landing, the recovery mass was measured using an electronic scale. Six successful recovery tests were conducted, and the data from a third-party test report is summarized below.
| Mass (kg) | Ground Speed (m/s) | Synthetic Overload (g) | Recovery Energy (kJ) |
|---|---|---|---|
| 65.85 | 23.00 | 4.4 | 17.4 |
| 62.22 | 23.50 | 4.6 | 17.2 |
| 63.40 | 25.25 | 5.4 | 20.2 |
| 68.75 | 28.25 | 6.9 | 27.4 |
| 66.55 | 29.25 | 7.1 | 28.5 |
| 72.00 | 30.00 | 7.5 | 32.4 |
Conclusion
This study successfully developed a rope-hook recovery device for small fixed-wing drones using a combined energy absorption method of “recovery rope and elastic deformation.” The research covered the entire process from theoretical modeling and simulation to component design and experimental validation.
The key conclusions are:
1. Solving the response characteristics of a spring-damper equivalent model using the central difference method in MATLAB effectively provided the static and dynamic mechanical indicators (e.g., maximum rope tension) required for recovery rope design.
2. An engineering design for a composite recovery rope, consisting of “static rope and rubber rope,” was presented. The two-stage design of the arresting rope assembly is a practical solution that balances functional performance with maintenance needs.
3. Finite element analysis under stringent conditions successfully identified stress concentration areas in core components like the boom sections, turntable, and crossbar. A strength check based on the material properties confirmed the structural integrity of the device. The calculation of the overturning load provided crucial data for vehicle integration.
4. Recovery tests performed in ascending energy levels demonstrated the device’s impressive performance. The measured maximum recovery energy reached 32.4 kJ, which surpasses the theoretical 28.9 kJ of other systems. Importantly, the maximum synthetic overload during these high-energy recoveries was only 7.5 g, well within the tolerance limits of typical fixed-wing drones. This device is now capable of fully supporting the normal and emergency recovery requirements of a specific small long-endurance fixed-wing drone system, successfully solving its point-recovery challenge.