In the development of small fixed-wing drones, the ability to recover the aircraft in confined spaces is a critical technical challenge. Among the various recovery techniques—parachute, net, rope-hook, and short-distance hook—the rope-hook method offers a unique combination of non-destructive capture, safety, and cost-effectiveness, especially for naval or island-based operations. This paper presents our engineering development of a rope-hook recovery device tailored for small fixed-wing drones, with a focus on achieving high recovery energy while maintaining structural integrity and operational simplicity.
Our device employs a hybrid energy absorption strategy combining recovery ropes (a mix of static and elastic ropes) with elastic deformation of the mechanical frame. We derived the system dynamics using the Lagrange equation, built a spring-damper equivalent model, solved for response characteristics in MATLAB, and prioritized rope parameters through static tensile tests. Finite element analysis was conducted on all key components, and an overturning load calculation method was established. The development process and design principles provide engineering reference for similar devices. Finally, we performed a series of recovery tests with increasing energy levels, demonstrating a maximum recovery energy of 32.4 kJ (measured)—exceeding the 28.9 kJ theoretical value of certain RQ21 fixed-wing drone systems—while limiting the maximum overload to less than 7.5 g.
Background and Motivation
Small fixed-wing drones operating in restricted areas, such as ship decks or remote islands, require reliable and compact recovery systems. The rope-hook (or skyhook) method, used by platforms like the RQ-21 and ScanEagle, relies on differential GPS guidance to engage a wingtip hook with a suspended rope. However, publicly available data indicate that the recovery energy capacity of existing systems is limited. For instance, the RQ21 system theoretically handles about 28.9 kJ, while the ScanEagle handles ~8.8 kJ. To meet the demands of longer-endurance fixed-wing drones (20+ hours flight time, greater takeoff mass), we aimed to develop a device capable of absorbing more than 28.9 kJ of kinetic energy. The following table summarizes recovery capabilities of representative fixed-wing drones and our target:
| 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 (domestic) | 30 | 29.2 | 12.8 |
| SC30 (domestic) | 27.8 | 32 | 12.4 |
| Our target | 30 | 72 | 32.4 |
To achieve this, we designed a vehicle-mounted recovery device with a hydraulic power system derived from the vehicle’s power take-off (PTO). The core idea is to first absorb energy through the elastic extension of a recovery rope assembly, then dissipate remaining energy via elastic deformation of the mechanical structure. No complex hydraulic accumulators or active control systems are used, ensuring reliability and ease of maintenance.
System Architecture
The recovery process consists of three phases:
- Engagement: The fixed-wing drone’s wingtip hook precisely catches the recovery rope. The rope slides along the wing until the hook locks, triggering engine shutdown.
- Rope stretching: The drone continues forward, tensioning the rope. Kinetic energy is partially converted into rope elastic strain. The drone’s motion transitions from linear to rotational.
- Structural absorption: Once the rope reaches its maximum stretch (limited by the non-elastic static rope), the remaining energy is absorbed by flexure of the mechanical frame. The drone’s motion becomes purely rotational around the device.
The major mechanical subsystems include:
- Folding arm assembly (upper crossbar)
- Telescopic arm assembly (lower crossbar with hydraulic cylinder extension)
- Turntable assembly (provides pitch and yaw)
- Rope winding drum and hydraulic motor
- Hydraulic control block with proportional valves
The hydraulic schematic is simplified: a gear pump driven by the vehicle PTO supplies oil to a directional valve, which controls folding cylinders, telescopic cylinders, and a rotary motor. This configuration allows the device to deploy from a transport configuration to a working height of ~17 m.
Recovery Rope Design and Dynamics
Dynamic Modeling
To minimize mechanical complexity, we opted for a passive absorption system using ropes with appropriate elasticity. The recovery rope assembly consists of a static nylon rope (polyurethane, elongation ~3.6%) and a rubber-cord (TPU core wrapped with polypropylene filaments) that provides the primary elastic recovery. The static rope sets the maximum allowable stretch; the rubber cord absorbs energy until it reaches the same length as the static rope.
We built a simplified dynamic model using Lagrange’s equation. The total kinetic and potential energies of the system (rope + drone + structure) were expressed, and the resulting differential equations were solved via a central difference scheme in MATLAB. The model considered the rope as a spring-damper element with stiffness k and damping c.
Let m be the effective mass of the fixed-wing drone, l the initial rope length from the impact point to the upper crossbar, y(t) the displacement of the drone along the rope direction, and F(t) the rope tension. The equation of motion is:
$$ m \ddot{y} + c \dot{y} + k y = 0 $$
with initial conditions: y(0) = 0, ẏ(0) = v0 (recovery speed). The rope stiffness k was estimated from the combined rubber cord and static rope characteristics. The simulation for a 70 kg fixed-wing drone at 30 m/s (kinetic energy ~31.5 kJ) yielded a maximum rope tension of approximately 5300 N and a maximum overload of about 9 g. The maximum rope elongation was about 6 m.
The acceleration and velocity profiles from the MATLAB solution are summarized below:
| Parameter | Value |
|---|---|
| Recovery speed (m/s) | 30 |
| Mass (kg) | 70 |
| Initial kinetic energy (kJ) | 31.5 |
| Peak rope tension (N) | 5300 |
| Maximum overload (g) | 9 |
| Maximum rope elongation (m) | 6 |
Static Tensile Tests
Based on the predicted tension, we selected a 10 mm diameter rubber cord with a breaking strength of 16,200 N (1.5× safety margin). We conducted static tensile tests to verify the rope’s load-elongation behavior. The results confirmed a linear elastic region up to about 8000 N, well within the peak requirement.
The rope assembly is designed with two levels of energy absorption: the main recovery rope (static + elastic) and a shorter “arresting rope” directly contacting the drone’s hook. The arresting rope also contains a rubber section for initial impact mitigation and is easily replaceable after wear.
Structural Design and Finite Element Analysis
After the rope absorbs its maximum energy (~6 m stretch), the residual kinetic energy is taken by elastic deformation of the mechanical frame. The entire supporting structure (telescopic arms, folding arms, turntable, and crossbars) must withstand peak loads without permanent deformation. We performed finite element analysis (FEA) on the complete assembly using an overloading condition: 10 g combined with a steady 7‑level wind load (according to local standards). The loads were derived from the rope tension decomposition at the crossbar tips (see Figure in original paper, not reproduced here).
The FEA results identified several stress concentration regions:
| Component | Max Stress (MPa) | Location |
|---|---|---|
| 4th telescopic arm | 853.4 | Arm head – slider contact |
| Turntable | 913.9 | Local fillet radius |
| Base crossbar (inner) | 904.5 | Bushing connection region |
| 3rd telescopic arm | 685.0 | Slider joint |
| Folding arm (tip) | 573.4 | Edge diagonal |
| All other components | < 500 | Various |
The main material used is Q690E steel (yield strength 690 MPa, tensile strength 770 MPa). The allowable stress was calculated as:
$$ [\sigma] = \frac{0.5\sigma_s + 0.3\sigma_b}{n} $$
where σs = 690 MPa, σb = 770 MPa, and n = 1.34 (safety factor including wind). This gives [σ] ≈ 458 MPa. For localized stress concentrations, we used a higher allowable of 1.4[σ] ≈ 641 MPa. The FEA peaks (e.g., 913.9 MPa) are attributed to mesh singularities and model simplifications (e.g., fillets and welds not modeled). After refining the model in those regions, the true stress remained below 520 MPa, within the acceptable range.
Overturning Load Calculation
Since the device is mounted on a vehicle, we computed the total overturning moment to ensure stability during recovery. The overturning moment M has three contributors:
- Self-weight of the device: M1 = m1 g L1
- Rope tension forces: Mq = F2L2 + F3L3
- Wind load: MF = Cp q A L3/2
Using the force decomposition from the dynamic analysis (F2 ≈ 4000 N, F3 ≈ 2800 N), we obtained:
$$ M = m_1 g L_1 + (F_2 L_2 + F_3 L_3) + \frac{C_p q A L_3}{2} = 137.3\ \text{kN·m} $$
The axial force on the turntable bearing was about 32,000 N. These values dictated the sizing of the turntable bearing and the outriggers for the vehicle.
Operational Procedure
The device is deployed in four steps:
- Release transport locks and rotate the turntable to the desired azimuth.
- Raise the telescopic arm, attach the recovery rope, and extend the lower crossbar using the hydraulic motor.
- Fold out the upper crossbar (folding arm) using two hydraulic cylinders.
- Extend the telescopic arm to full height (~17 m) for final deployment.
All movements are controlled from a remote panel. After recovery, the process is reversed for stowage.
Recovery Tests
We first conducted a preliminary test using a HY30 fixed-wing drone (25 kg, 25 m/s) to validate the rope sensor setup. The measured rope force was about 500 N (overload ~2 g)—well within the design envelope.
The final qualification tests used a military-trade fixed-wing drone with a takeoff mass of 87 kg and endurance of 20 h. The drone was equipped with an onboard fiber-optic inertial navigation system (FINS) recording velocity and acceleration. Six successful recoveries were performed with increasing mass and speed. The test results are summarized in Table 4:
| Recovery mass (kg) | Ground speed (m/s) | Max 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 |
The maximum recovery energy of 32.4 kJ (measured) exceeded the theoretical capacity of the RQ21 system (28.9 kJ). The maximum synthetic overload of 7.5 g is well below the typical structural limit of fixed-wing drones (often >20 g). The device successfully recovered all test drones without damage.
Conclusion
We have developed and validated a rope-hook recovery device for small fixed-wing drones using a combined passive energy absorption approach: elastic ropes for primary absorption followed by structural deformation for residual energy. Key findings include:
- A dynamic model using Lagrange equations and spring-damper equivalence accurately predicted rope tension (~5300 N) and overload (~9 g) for a 70 kg fixed-wing drone at 30 m/s.
- The two-stage rope assembly (static + elastic) simplifies maintenance while providing sufficient energy absorption (up to ~6 m elongation).
- FEA identified stress hot spots that were mitigated by design refinements; all components remained within allowable stress limits.
- Overturning moment calculation guided the vehicle integration design.
- Full-scale tests demonstrated a maximum recovery energy of 32.4 kJ, a 12% improvement over the state-of-the-art RQ21 system, with peak overloads below 7.5 g.
This device has been delivered as part of a small long-endurance fixed-wing drone system and successfully solves the challenge of pinpoint recovery in confined areas. The engineering methodology presented here can serve as a reference for future developments of rope-hook recovery systems for fixed-wing drones.
