Beyond Conventional Drones: A Review of Unconventional Rotary-Wing UAV Design

Abstract

This paper explores unconventional configurations of rotary-wing unmanned aerial vehicles (UAVs), focusing on designs that transcend the limitations of traditional ones. Through innovative rotor arrangements, refined airframe structures, and novel flight mechanisms, these advanced designs aim to significantly enhance performance, versatility, and functionality. Rotary-wing UAVs that deviate markedly from conventional models in terms of mechanical topology, aerodynamic principles, and movement modalities are rigorously examined. These unique UAVs are categorized into four distinct groups based on their mechanical configurations and dynamic characteristics: (1) UAVs with tilted or tiltable propellers, (2) UAVs featuring expanded mechanical structures, (3) UAVs with morphing multirotor capabilities, and (4) UAVs incorporating groundbreaking aerodynamic concepts. This classification establishes a structured framework for analyzing the advancements in these innovative designs. Finally, key challenges identified in the review are summarized, and corresponding research outlooks are derived to guide future development in rotary-wing drone technology.

1. Introduction

In recent decades, significant advancements have been achieved in rotary-wing unmanned aerial vehicles (UAVs) for industrial, agricultural, and recreational applications. Though fixed-wing or flapping-wing drones and vertical takeoff and landing (VTOL) drones have demonstrated their unique advantages, rotary-wing UAVs, such as quadcopters, have gained more attention and been widely adopted due to their compact sizes, vertical takeoff capabilities, stable hovering performance, and operational simplicity [1]. Lift and control moments are generated through horizontally rotating rotors, with their design principles and flight dynamics having been extensively studied and optimized [2]. However, as technological progress continues and application requirements evolve, increasing attention has been directed toward unconventional rotary-wing UAVs [3]. These designs aim to overcome the limitations of traditional systems by modifying rotor layouts, optimizing airframe structures, or employing unique flight mechanisms to enhance performance, adaptability, and functionality. More specifically, these designs significantly differ from traditional designs in the following aspects:

• Topological Mechanical Mechanisms: Structural modifications, including unique rotor arrangements, innovative airframe morphologies, and novel configurations of constituent elements.
• Flight Control Principles: Utilization of distinct aerodynamic models and control strategies, such as the integration of fixed-wing elements to enhance payload capacity, flight efficiency, and stability.
• Task Execution Modes: Operational methods and task execution strategies that differ from traditional systems, enabling multiple motion modes, improved environmental adaptability, and expanded capabilities in complex scenarios.

Among the three aspects mentioned above, mechanical design is regarded as fundamental, as it directly influences both control principles and task execution modes. Consequently, unconventional rotary-wing UAVs are categorized into four primary types based on their mechanical structures and dynamic characteristics, and recent related works are thoroughly reviewed. Definitions for each category are outlined as follows:

• Tilted or Tiltable Propeller Design: The rotor tilt angle is adjusted to enhance UAV maneuverability and speed.
• Expanded Mechanical Structure Design: Additional mechanical components are integrated into conventional rotorcraft to expand motion modes or introduce new functionalities.
• Morphing Multirotor Design: The overall UAV shape is adjusted according to specific flight missions or environmental conditions.
• Revolutionary Design: The UAV configuration is significantly distinct from multirotor designs, characterized by fuselage autorotation, minimal actuator configurations, and/or lifting-wing structures.

To establish a structured framework for analyzing advancements and potential in innovative rotary-wing drone designs, this review focuses on unconventional rotary-wing configurations, excluding fixed-wing, flapping-wing, and VTOL architectures. Through synthesizing current research trends and future prospects, emerging opportunities in aerospace engineering, robotics, and intelligent systems are clarified. The remainder of this review is organized as follows: Section 2 outlines the review methodology. Section 3, Section 4, Section 5 and Section 6 analyze each design category, examining principles, advantages, challenges, and representative examples. Section 7 explores the potential impact of these unconventional designs on redefining drone capabilities and applications. Finally, Section 8 summarizes current trends and outlines future research directions.

2. Methods of Review

This review was conducted using scientific research databases, including Google Scholar, Web of Science, and IEEE Xplore. An initial title search was performed using the keywords “all in title: Rotary wing UAV OR Rotary wing drone OR Bio-inspired UAV OR Morphing UAV OR Tilt-rotor UAV OR Aerial-ground UAV OR Transformable UAV” to identify relevant publications, including review articles, research articles, and conference papers. The search yielded 954 publications from 2010 to 2025. Given the rapid advancements in this field, a rigorous manual screening process was conducted, excluding studies on path planning [4,5], mobile communication networks [6,7], and mobile object detection platforms [8,9], among others. This reduced the number of publications to 98, sourced from renowned journals and highly recognized conferences. References cited in these publications were then examined, and relevant articles, particularly from the past 5 years, were added to expand the final list beyond the initial search constraints.

The annual publication trends in this field, as well as the four subcategories identified, are illustrated in Figure 1. It is evident that the number of research publications on unconventional rotary-wing UAVs has increased steadily, from fewer than 20 in 2010 to over 150 in recent years. This trend can be attributed to the matured understanding of the dynamics and controls of conventional rotary-wing UAVs [10], prompting researchers to focus on designing unconventional UAVs for specific applications.

3. Tilted/Tiltable Propellers Design

Conventional rotary-wing UAVs are equipped with propellers installed at fixed tilt angles. To address this limitation, researchers have proposed the use of propellers with various tilted angles or adjustable tilt mechanisms to generate multidirectional thrust, prioritizing simplicity and stability for structured environments [11]. In general, tilted systems excel in static thrust efficiency, while tiltable designs trade mechanical complexity for reconfigurable dynamics. Together, these approaches demonstrate the balance between robustness and flexibility, addressing diverse UAV challenges—from stable hovering to agile environmental interaction—and expanding mission-specific design possibilities.

3.1. Tilted Propellers

Tilted propeller platforms are characterized by a primary rigid framework that integrates multiple propellers fixed in various orientations. These propellers are strategically positioned to direct thrust in multiple directions, enabling diverse functionalities based on the number, orientation, and type—whether unidirectional or bidirectional—of the propellers. Such platforms can be categorized into several types based on their propeller configurations, such as Forward Advancing (FA), Omnidirectional Agility (OA), or Directional Precision (DP) platforms. The differentiation among these categories is determined by the specific arrangement and operational capabilities enabled by the propeller configurations.

(1) Forward Advancing Platforms: FA platforms are characterized by a specific arrangement and number of propellers that facilitate forward thrust. A notable example is the Tilt-Hex [12], which consists of six unidirectional propellers rigidly attached to its main body, each oriented in a distinct direction. The Tilt-Hex platform, shown in Figure 2, has been enhanced with several components: (1) a rigid tool for precise tasks, such as point contact [13], sliding maneuvers, and peg-in-hole tasks [14]; (2) a passive link and gripper system designed for pick-and-place operations and various manipulation tasks [15,16]; and (3) an articulated arm for tasks involving point contact and sliding maneuvers [17,18].

(2) Omnidirectional Agility Platforms: OA platforms are designed to direct thrust omnidirectionally, enabling maneuverability across multiple axes. A notable example is the AEROX [19], shown in Figure 3, which features eight unidirectional propellers rigidly attached to its main body, each oriented in distinct directions. This platform is equipped with an articulated arm specifically designed for tasks involving point contact and sliding maneuvers. Notably, it has been utilized for contact inspections in oil and gas plants as well as bridges [20].

(3) Directional Precision Platforms: DP platforms utilize propellers designed to achieve highly specialized directional capabilities, often tailored for specific tasks or operations. A notable example is the platform developed by Brescianin et al. [21], which features a pouch designed for pick-and-place operations and has been successfully employed for dynamically catching a thrown ball. Another example is the ODAR [22], shown in Figure 3, which is characterized by eight bidirectional propellers rigidly attached to the main body and oriented in diverse directions. This platform is equipped with a simple rigid tool suitable for tasks such as pushing, sliding, and peg-in-hole maneuvers. Some representative examples of tilted propellers platforms are shown in Figure 2.

Figure 2. Examples of tilted propellers platforms: A. Forward Advancing platforms: (a) Tilt-Hex consists six unidirectional propellers [12]. (b) Tilted-prop, designed for tasks requiring high precision [13]. (c) Tilted-prop, utilized for pick-and-place operations and various manipulation tasks [15]. (d) Tilted-prop equipped with an articulated arm [17]. B. Omnidirectional Agility platforms: (e) AEROX equipped with an articulated arm tailored [19]. C. Directional Precision platforms: (f) Omnidirectional octorotor vehicle equipped with a pouch designed for pick-and-place operations [21]. (g) ODAR equipped with a straightforward rigid tool suited for peg-in-hole tasks (the orientations of its six rotary wings are explicitly depicted to have a better view) [22].

Figure 2. Examples of tilted propellers platforms: A. Forward Advancing platforms: (a) Tilt-Hex consists six unidirectional propellers [12]. (b) Tilted-prop, designed for tasks requiring high precision [13]. (c) Tilted-prop, utilized for pick-and-place operations and various manipulation tasks [15]. (d) Tilted-prop equipped with an articulated arm [17]. B. Omnidirectional Agility platforms: (e) AEROX equipped with an articulated arm tailored [19]. C. Directional Precision platforms: (f) Omnidirectional octorotor vehicle equipped with a pouch designed for pick-and-place operations [21]. (g) ODAR equipped with a straightforward rigid tool suited for peg-in-hole tasks (the orientations of its six rotary wings are explicitly depicted to have a better view) [22].

3.2. Tiltable Propellers

The tiltable propeller platform consists of a primary rigid frame and multiple propellers mounted onto movable actuated elements, typically servo motors. This additional actuation allows the propellers to pivot independently or in synchronized motions toward various directions, determined by the specific actuation configuration. Numerous designs of tiltable propeller platforms have been proposed, featuring propeller counts ranging from two to eight, excluding configurations with five or seven propellers. Examples of such designs include those proposed by Ryll et al. [23] and Kamel et al. [24]. For a comprehensive analysis of platforms utilizing tiltable propellers, readers are referred to the work of Hamandi et al. [25]. While most tiltable propeller platforms primarily function as FA systems, only a few are designed for physical interaction. These limitations stem from the inherent characteristics of the servo motors used to adjust the tilt angle, which often lack precision, exhibit slow dynamics, and suffer from mechanical issues such as backlashes.

Papachristos et al. proposed a tri-rotor platform with a Tee-shaped structure [26]. This configuration enables the two primary frontal propellers to tilt radially by the same angle, while the tail rotor can tilt independently, allowing the platform to achieve multidirectional thrust (MDT). Equipped with a passive one-DoF revolute end-effector, this platform is capable of performing point-contact tasks. Bodie et al. introduced a twelve-rotor platform employing a double motor configuration, where each pair of propellers can tilt radially, providing the platform with both DP and OA capabilities [27]. This setup, enhanced with a rigid tool, has been utilized for point-contact and sliding tasks. Bodie et al. further advanced this design, demonstrating its applicability in complex aerial manipulation scenarios [28]. Ding et al. proposed a novel tilting-rotor UAV with an H configuration, enabling enhanced maneuverability and energy efficiency [29]. A dual-level adaptive robust control strategy was developed for precise motion tracking and thrust optimization, validated through experiments in challenging aerial locomotion and manipulation tasks. Singh et al. introduced QuadPlus, a quadrotor with independent biaxial propeller tilting (100° and 180°), enabling 12-DOF and independent position/attitude control [30]. A cascade control approach combining nonlinear model-predictive and PID controllers was employed, validated through simulations for complex 3D trajectory tracking in constrained environments. Lv et al. proposed a multivariable cascaded finite-time controller and an improved nonlinear control allocation law for a coaxial tilt-rotor UAV, enhancing robustness, stability, and efficiency [31]. These advancements were validated via simulations and experiments, demonstrating reduced velocity error and faster attitude settling times. Further refinements were presented in [32], showing improved performance in dynamic flight conditions. Some representative examples of tiltable propellers platforms are shown in Figure 3.

Figure 3. Examples of tiltable propellers platforms: (a) 6 tiltable propellers [24], (b) 4 tiltable propellers (with the mechanism of its tiltable propeller shown aside) [23], (c) a twelve-rotor platform employing a double motor configuration [27,28], (d) a tri-rotor platform configured in a Tee-shape structure (with the mechanism of its tiltable propeller shown aside) [26], (e) QuadPlus [30], (f) a tilting-rotor UAV [29], (g) CTRUAV [31,32]. Certain insets show the details of some tilting mechanisms.

Figure 3. Examples of tiltable propellers platforms: (a) 6 tiltable propellers [24], (b) 4 tiltable propellers (with the mechanism of its tiltable propeller shown aside) [23], (c) a twelve-rotor platform employing a double motor configuration [27,28], (d) a tri-rotor platform configured in a Tee-shape structure (with the mechanism of its tiltable propeller shown aside) [26], (e) QuadPlus [30], (f) a tilting-rotor UAV [29], (g) CTRUAV [31,32]. Certain insets show the details of some tilting mechanisms.

3.3. Brief Summary

Both tilted propeller and tiltable propeller platforms represent significant advancements in aerial propulsion systems. Tilted propeller platforms, characterized by fixed propellers oriented in multiple directions, demonstrate improvements in stability and maneuverability. In contrast, tiltable propeller platforms, featuring orientation-adjustable propellers controlled by servo motors, offer enhanced adaptability and task-specific optimization. However, these systems face notable challenges. Tilted propeller platforms may encounter limitations in maneuvering within confined spaces due to their fixed orientations. Meanwhile, tiltable propeller platforms, despite their adaptability, are often hindered by the precision and speed limitations of servo motor dynamics. Additionally, both systems may experience mechanical complexities and reliability issues in their tilting mechanisms. Despite these challenges, ongoing research and development efforts are focused on addressing these limitations, aiming to optimize both tilted and tiltable propeller systems for a wide range of applications, from agile maneuvering to specialized operations.

4. Expanded Mechanical Structure Design

The integration of additional mechanical components into conventional multirotor platforms represents an intuitive and effective approach for expanding drones’ capabilities. This method is favored due to its structural simplicity and ease of scalability, leveraging existing multirotor control frameworks. Current expanded mechanical structures typically include robotic arms and grippers, perching mechanisms, wheels or walking mechanisms, and anti-interference systems. These innovations demonstrate significant potential for enhancing drone–environment interactions, adapting to diverse operational states, and improving flight stability.

4.1. Robotic Arms and Grippers

In 2011, Pounds et al. successfully retrieved objects of varying masses, sizes, and shapes using a highly adaptable flexible gripper mounted on a helicopter [33], marking the first in-flight use of an onboard gripper for unstructured object retrieval. Orsag et al. embedded a dual-arm gripper on a quadrotor UAV for valve turning, demonstrating early applications of aerial manipulation [34]. Yuksel et al. developed a lightweight, flexible-joint arm for UAVs with elastic components, enabling safe interaction and high-speed tasks, validated through experiments [35]. Bartelds et al. proposed a novel aerial manipulator with active and passive joints, enabling stable impact absorption by converting kinetic energy into potential energy stored in elastic elements [36]. Suarez et al. designed a lightweight, compliant three-DOF robotic arm with spring-based transmission and a compliant finger module for stable aerial manipulation and contact force estimation [37].

Recently, Fishman et al. proposed a tendon-actuated gripper that replaces rigid ones, enabling dynamic grasping of unknown objects through advanced control algorithms and soft robotics models [38]. Zhang et al. integrated a delta manipulator with 3D printing nozzles on quadrotor UAVs to achieve aerial additive manufacturing [39]. Zhao et al. introduced a quadrotor featuring an articulated rigid-element morphing mechanism, enabling in-flight shape adjustment and dual-function grasping of diverse objects [40]. Experimental validation demonstrated minimal flight disruption and effective adaptability. Hsiao et al. developed a mechanically intelligent and passive (MIP) gripper for aerial perching and grasping, which uses impact forces to close and maintain a hold without actuators [41]. The design was validated through static models, design guidelines, and quadcopter experiments, demonstrating lightweight and effective performance. Liu et al. designed the Soft Aerial Gripper (SoAG), an open source aerial robot with a horizontally aligned soft gripper and onboard pneumatic system, enabling safe mid-air catching of micro-robots under aerodynamic disturbances [42]. The system was validated through static and dynamic grasping experiments. Some representative examples of robotic arms and grippers are shown in Figure 4.

4.2. Perching Mechanism

Coupled with advanced controllers [43,44], perching mechanisms for quadrotor UAVs enable stable attachment to surfaces or objects, allowing drones to conserve energy, perform tasks, or monitor environments without sustained hovering [45]. These systems often integrate bioinspired designs—such as claw-like grippers, adhesive pads, or compliant structures—to absorb impact forces, adapt to irregular shapes (e.g., branches, ledges), and maintain grip under dynamic conditions [46].

Hang et al. proposed a modular, actuated landing gear framework enabling UAVs to perch and rest on diverse structures, reducing power consumption, enhancing stability, and preserving mission continuity without complex maneuvering [47]. Roderick et al. developed a biomimetic robot with passive legs and an underactuated gripper for dynamic perching on complex surfaces [48]. The system was validated through experiments, demonstrating that avian toe arrangements minimally impact perching performance. Bai et al. presented a bioinspired UAV perching system mimicking bird feet, featuring elastic toes and gear-driven reconfiguration for multi-object perching, achieving 15× payload capacity and 98.5% energy reduction [49]. Additionally, some gripper designs are multifunctional, enabling tasks such as grasping and perching [41]. The same group further advanced this concept with a bat-inspired mechanism using ratchet/four-link self-locking and autonomous target tracking via sensor fusion, achieving 97.1% energy savings and reliable outdoor perching on unstructured surfaces like branches and crossbars [50]. Zheng et al. designed metamorphic bi-stable arms for rapid morphing and perching [51]. Kominami et al. proposed a passively driven perching mechanism with underactuated fingers and two passive drive modes for landing on flat surfaces and grasping various objects (e.g., bars, plates, spheres), achieving lightweight, actuator-free operation and validating gripping forces through theoretical and experimental analysis [52]. Additionally, a simple design utilizing magnets for perching and unperching on ferromagnetic surfaces has been demonstrated as a feasible approach [53]. Some representative examples of perching mechanisms are shown in Figure 5.

4.3. Locomotion Mechanism

Integrating legged or wheeled locomotion mechanisms into quadrotor UAVs has emerged as a promising approach to creating hybrid aerial–ground robots. These multimodal systems combine the agility of flight with the energy efficiency of ground traversal, enabling seamless adaptation to diverse terrains. While flight allows rapid coverage of large areas and navigation over obstacles, ground locomotion conserves power during extended missions. Current designs typically employ either wheels for efficient planar motion or legged mechanisms for enhanced adaptability to uneven surfaces.

(1) Walking Mechanism: Walking mechanisms, or legged structures, provide enhanced adaptability to diverse terrains. The Flying Monkey robot integrates a lightweight, single-DOF walking mechanism, enabling the combination of walking, grasping, and flight capabilities within a 30 g package, thereby extending mission life and versatility [54]. Pratt et al. equipped a quadcopter with passive–dynamic legs, allowing energy-efficient walking on inclined surfaces and active walking on flat or uphill terrain using rotor thrust [55]. This design eliminates the need for additional actuators during terrestrial locomotion. The groundbreaking multimodal robot, LEONARDO, integrates bipedal walking and aerial flight through the synchronized control of thrusters and multi-joint legs, enabling complex maneuvers such as slackline walking, skateboarding, and obstacle navigation [56].

(2) Wheeling Mechanism: Wheeling mechanisms, including both active and passive wheel designs, are simpler and more prevalent. The FCSTAR is a hybrid robot equipped with four active wheels that share motors with its propellers. Combined with thrust reversal, these wheels allow the robot to climb vertical surfaces and traverse steep slopes [57]. Although active wheels provide precise movement and speed control, they require additional components, resulting in increased weight [58]. Consequently, many researchers tend to use passive wheels. Zhang et al. equipped a quadrotor with two passive planar wheels on its sides, creating a terrestrial–aerial robot platform that combines the high mobility of aerial vehicles with the long endurance of ground vehicles [59]. The SytaB is a hybrid terrestrial/aerial vehicle featuring two independently passive spherical wheels and a bicopter. This design enables energy-efficient terrestrial and aerial modes, smooth transitions between modes, and reduced attitude regulation for sensor stability, as validated through experimental results [60]. Jia et al. introduced a rotorcraft with a passively deformable airframe and rolling cage, enabling efficient terrestrial locomotion through controlled rolling and turning, achieving up to 15× lower power consumption compared to flying, while maintaining precise control and a compact rolling profile [61]. To simplify the complex mechanisms used in related works, Pan et al. introduced Skywalker, an air–ground vehicle featuring a simple yet robust omnidirectional wheel mechanism [62]. This design enables high-speed hybrid trajectory tracking, smooth mode transitions, and up to 75.2% energy savings during ground locomotion. Some representative examples of locomotion mechanisms are shown in Figure 6.

4.4. Brief Summary

Research into drone designs incorporating expanded mechanical structures introduces both advantages and challenges. First, weight and balance adjustments must be carefully managed, as additional mechanical modules significantly alter the drone’s total mass and dynamics. These changes necessitate precise modifications to the drone’s design and power systems to ensure stable flight under augmented payloads. Second, integrating additional modules and functionalities increases energy consumption, requiring enhanced energy efficiency or expanded battery capacity. Third, the system’s heightened complexity demands more advanced control algorithms to ensure effective coordination of components such as manipulator arms or perching mechanisms. Finally, reliability concerns arise from the inclusion of additional mechanical components, which introduce additional failure points and necessitate rigorous durability testing throughout the design and manufacturing processes.

5. Morphing Multirotor Design

Morphing multirotor UAVs are equipped with adaptable components or structures capable of altering their configuration through mechanical, electrical, or other methods [63]. These mechanisms include adjustable connectors, movable frames, and rotatable propellers. Although they exhibit higher mechanical complexity compared to the tiltable propellers platform discussed in Section 3.2, morphing multirotor UAVs are specifically designed to modify their shape to meet distinct task requirements. This adaptability enhances their operational capabilities, enabling adjustments in confined spaces, improved maneuverability, and optimized flight efficiency. Recent research trends have categorized morphing multirotor UAVs into two primary types: frame-morphing multirotors and linkage-morphing multirotors.

5.1. Frame-Morphing Multirotor

This type of rotary-wing UAV actively adjusts its frames to navigate through confined spaces, enhancing environmental adaptability and expanding the potential applications of multirotor UAVs across diverse task scenarios. Within this category, frame-deformable multirotors are further classified into two types based on the form of frame deformation: foldable-frame morphing and rotatable-frame morphing.

(1) Foldable-Frame Morphing: Foldable-frame morphing mechanisms enable drones to adjust their structure dynamically, enhancing adaptability for tasks such as obstacle navigation, gap traversal, and space-constrained operations. Mintchev et al. introduced the first self-deploying folding-arm quadcopter, utilizing origami principles [64]. Generally, articulated mechanisms are designed to enable the rotation [64,65,66,67], extension [68,69,70], or bending [51,57,71,72,73] of the arms where the propellers are mounted.

Zhao et al. proposed a deformable quadrotor featuring scissor-like foldable structures, enabling dynamic volume adjustment for obstacle navigation and space adaptation [68]. Desbiez et al. developed X-Morf, a drone capable of actively adjusting the angle between its two frame arms [65]. Bucki et al. employed passive rotary joints and sprung hinges to achieve rapid aerial morphing, reducing the vehicle’s largest dimension by 50% for applications like gap traversal [71]. Falanga et al. designed a quadcopter with four independently rotating arms that fold around the main frame, facilitating stable flight and adaptability for tasks such as navigating narrow gaps and inspecting surfaces [66]. The FQR, inspired by origami mechanisms, enables in-flight arm folding for aggressive maneuvers and obstacle avoidance [69]. An innovative design, the SQUID, can be launched from a tube and deploys its rotary wings mid-air [70]. Bai et al. introduced the SplitFlyer, a transformable quadcopter composed of two bicopters capable of disassembling mid-air [74]. This concept was further refined in 2022 [75]. The FCSTAR, previously introduced in Section 4.3, achieves ground locomotion by bending the arms where the active wheels are mounted [57]. Soft and flexible arms, actuated with tendons, have been used to achieve the bending and grasping of industrial pipes [73] and perching on tree branches [51]. Zhao et al. incorporated a morphing mechanism with grippers to achieve adjustable aerial pick-and-place [40]. Recent advancements include a biomimetic morphing quadrotor by Xu et al. [76] and the Ring-Rotor by Wu et al. [77], which will be elaborated in Section 5.3. Some representative examples of foldable-frame morphing designs are shown in Figure 7.

(2) Rotatable-frame Morphing: Rotatable-frame morphing involves structural configurations capable of altering their shape and layout through the manipulation of rotating components. Such UAVs exhibit the flexibility to adapt their configurations to meet diverse flight environments and mission requirements.

The Bi² Copter, a robot with a novel morphing design that connects two bicopter modules, enables continuous 360° tilt angle adjustment and efficient thrust utilization for applications like wall flying and spherical coverage. Its design, control, and operational performance were validated through comprehensive testing [78]. Riviere et al. presented a Quad-Morphing robot with an actuated elastic mechanism, enabling it to fold its wingspan by half to pass through narrow gaps at high speeds while maintaining stability through a specialized control strategy and recovery procedure [79]. Sakaguchi et al. designed a novel quadcopter with a parallel link mechanism and a single servo motor, enabling tilt angle adjustment and vertical folding within user-specified ranges [80]. A stabilization strategy for extreme tilt angles around $\pm {180}^{°}$ was proposed, utilizing a reference tilt angle generator. Tang et al. [81] proposed a quadcopter with a non-driven rotor tilt mechanism (QUaRTM), which is not categorized under Section 3. Unlike with the active adjustable arms by Ding et al. [29], passive hinges are used to replace the rigid arm–body connection, allowing propeller tilt in the forward flight direction without central body movement. Springs at the hinges pull the arms into an untilted configuration: when net propeller thrust overcomes spring torque, the vehicle tilts; when thrust falls below the threshold, the arms return, restoring the untilted configuration. Some representative examples of rotatable-frame morphing designs are shown in Figure 8.

5.2. Brief Summary

Foldable-frame morphing presents several challenges. First, structural stability may be compromised during folding and unfolding processes, as changes in the frame’s geometry can reduce its strength and adversely affect flight performance. Second, the design and manufacturing of foldable frames require intricate mechanical structures and reliable folding mechanisms, increasing the risk of faults and failures that may impact the frame’s reliability and durability. Third, achieving precise foldable deformation demands advanced control systems, including highly accurate sensors and algorithms, to ensure stable and reliable frame adjustments while maintaining effective flight control throughout the process. With advancements in materials science, the materials used for manufacturing arm components are no longer limited to traditional carbon plates, as flexible materials are increasingly being experimented with and integrated into UAV body designs. It is anticipated that the synergistic evolution of unmanned aerial vehicle technology and materials science will emerge as a prominent area of research.

Similarly to foldable-frame morphing, the technical challenges of rotating deformable frames lie in reconciling mechanical structures with control schemes. However, compared to foldable designs, rotating frames exhibit notably lower mechanical complexity. Future research will focus on reducing the driving structure for arm rotation and enhancing UAV control precision.

5.3. Linkage-Morphing Multirotor

The linkage-morphing multirotor features a linkage-shaped frame capable of altering its form and structure as needed. This design enables adjustments in linkage layout, angles, or lengths through mechanical, electrical, or other control methods, adapting to diverse operational environments and task requirements. Its key characteristics include the following:

• A multilink structure interconnected by joints, enabling frame reconfiguration through adjustments in linkage lengths, angles, or connections.
• Task-specific adaptability, such as expanding/collapsing linkages or adjusting angles/positions, to navigate confined spaces or switch flight modes.
• Precise control systems and stable flight control to ensure optimal performance during morphological changes.

Zhao et al. designed a novel multirotor with two-dimensional multilinks for aerial morphing, integrating linear–quadratic–integral control and motion planning to achieve stable transformations and narrow-gap traversal in complex 3D environments [82]. An upgraded version, HALO, was later introduced as a transformable aerial robot featuring a closed-loop multilink structure and tilted propellers, enabling the shape-adaptive aerial grasping of large objects through optimized transformation planning and stable flight control [83]. However, these deformable aerial robots, with all rotor discs arranged in the same plane, were underactuated, resulting in uncontrollable singular configurations. These singular forms constrained the effective range of aerial transformations, often leading to invalid solutions along the transformation path. To address this, Zhao et al. eventually proposed the famous DRAGON, a transformable aerial robot with dual-rotor gimbal modules enabling multi-DoF aerial transformation and full SE(3) pose control [84]. This design was validated through dynamics modeling, decoupled control strategies, and prototype experiments. Thrust control strategies included minimizing force norms to avoid saturation and implementing rotor gimbal control methods to enhance translational and rotational stability during hover and large-scale transformations.

Shi et al. designed a transformable multilink aerial robot to achieve aerial regrasping, such as pivoting a long box [85]. This was accomplished by optimizing joint configurations, grasping forces, and employing impedance and admittance controllers to mitigate external disturbances and downwash effects. Zhao et al. presented a quadrotor with an articulated rigid-element morphing mechanism that enables in-flight shape adjustment and dual-function grasping of diverse objects, validated through experiments demonstrating minimal flight disruption and effective adaptability [40]. Wu et al. invented Ring-Rotor, a ring-shaped quadrotor with a single-servo morphing mechanism reducing its dimensions by 31.4%, enabling energy-efficient flight, traversal of narrow spaces, and whole-body aerial grasping of various objects without external manipulators [77]. This design was validated through prototype experiments using a nonlinear model predictive control strategy. Xu et al. designed a biomimetic morphing quadrotor with vertically folding arms inspired by an eagle claw, enabling the dynamic grasping of unknown objects and traversal of narrow spaces using a closed-loop multilink structure, adaptive sliding mode control, and admittance filtering for adaptive morphology [76]. Some representative examples of linkage-morphing designs are shown in Figure 9.

5.4. Brief Summary

The multilink design holds significant potential for applications in search and rescue, surveying, monitoring, military operations, and related domains. However, its implementation faces engineering challenges, particularly in achieving precise control and managing mechanical design complexity. Recent research highlights the difficulty of developing reliable and efficient deformable mechanisms for multilink multirotor UAVs to enable precise and stable morphological changes, a critical focus in current studies. Mission planning and execution for these UAVs primarily emphasize optimizing multimodal configurations and task-specific operations through adaptive transformation strategies.

6. Revolutionary Design

This section examines innovative unconventional rotorcraft drone designs characterized by fuselage autorotation, minimal actuator configurations (single or dual actuators), and lifting-wing structures leveraging aerodynamic principles. Actuator reduction simplifies designs, enabling weight reduction and structural optimization, while lifting-wing configurations enhance flight efficiency and performance through aerodynamic lift. These designs achieve superior energy efficiency and flight performance via optimized rotor layouts and fuselage shaping.

6.1. Single-Actuator Design

Rotorcraft UAVs with single or dual actuators and lift-wing configurations have drawn significant inspiration from plants such as the Samara seed, renowned for its autorotating descent, which slows its fall and enhances dispersal efficiency. The development of single-rotor aircraft began with a seminal MIT publication in 2008 [86]. These single-axis flyers leverage the Samara seed’s autorotation principle, enabling smooth descent even during power-off scenarios.

Piccoli et al. introduced a design methodology for a low-cost, passively stable micro-aerial vehicle with only two moving parts and a single actuator [87]. Mechanism stability was analyzed and validated through simulation and prototyping experiments, providing design guidelines for hover-capable flight without active attitude control. The same group later presented Piccolissimo, a small self-powered flying vehicle with one motor and two counter-rotating propellers, achieving passive stability and Cartesian velocity control through asymmetric rotation and thrust pulsing [88]. Two prototypes, measuring 28 mm and 39 mm, were manufactured and tested. Zhang et al. designed the Monospinner, a single-moving-part flying vehicle controllable in three translational and two rotational DOFs using a cascaded control strategy, validated through experimental demonstrations of hover stability and robustness to perturbations [89].

Recently, Win et al. developed a single-actuator monocopter (SAM) with a simplified design using one motor and a 2D wing, optimized via a Genetic Algorithm for stable hovering and controlled five-DOF flight, achieving efficient power consumption and minimal oscillations [90]. The SAM was later optimized with a semi-rigid foldable wing, reducing its footprint by 69% [91]. However, due to the impact of propeller size, these single-actuator flyers exhibit poor load capacity, limited control torque, and maneuverability. Chen et al. designed PULSAR, an ultra-underactuated, self-rotating UAV controlled by a single motor, achieving 26.7% power savings over quadrotors while enabling panoramic LiDAR-based obstacle detection and autonomous navigation in unknown environments [92]. Some representative examples of single-actuator designs are shown in Figure 10.

6.2. Dual/Triple-Actuator Design

Paulos and Yim presented a 227 g swashplateless coaxial micro-drone using only two motors and propellers for roll, pitch, yaw, and thrust control, demonstrating reduced mechanical complexity, lower actuator mass, and closed-loop trajectory tracking in free flight [93,94]. Bhardwaj et al. upgraded their SAM [90,91,95] into a dual-rotor drone FROW and presented a novel single-wing aerial vehicle capable of mid-air transitions between monocopter and bicopter modes, combining their dynamics and control strategies, validated through prototype flight demonstrations [96,97]. A similar design was proposed by Bai et al., featuring a bioinspired, dual-wing aerial robot weighing 35.1 g. This drone was optimized for hovering flight with passive stability, achieving a twofold reduction in power consumption compared to multirotors, and demonstrated capabilities in position-controlled flight and payload applications [98]. A recent study by Kirchgeorg et al. introduced AVOCADO, a dual-rotor drone capable of both aerial and tethered locomotion for navigating tree canopies. The design incorporates a protective shell, enhanced sensors, and a control framework for trajectory tracking and disturbance rejection, validated through simulations and experiments [99]. Lan et al. employed a similar design to enable a quadrotor tensile perch [100].

Employing a dual servo and motor configuration, Low et al. introduced THOR, an underactuated system capable of achieving controllability in four DOFs during horizontal cruising and five DOFs during hovering [101,102]. Building on this design, the same group later proposed Flydar, which utilizes a single laser and self-rotating dynamics for omnidirectional scanning [103,104]. Flydar incorporates a quaternion-based filter for attitude correction and dual-accelerometer scan rate estimation, enabling 2.5D SLAM with reduced error and improved loop closure compared to 2D SLAM. Recently, Cai et al. combined three of their SAM drones and created the ARROWs, a modular aerial robotic platform with revolving wings for enhanced lift, featuring a cascaded flight controller and inertial measurement units to achieve stable flight across 12 configurations, validated through experiments with an average hovering position error of 89 mm [105,106]. Some representative examples of dual/triple-actuator designs are shown in Figure 11.

6.3. Coaxial Design

The coaxial propeller drone is an innovative aerial vehicle designed to combine high thrust efficiency with enhanced maneuverability. Unlike traditional quadrotors, it features two contra-rotating propellers mounted on a single axis, enabling greater thrust per platform area [107]. This design achieves a balance between energy efficiency, agility and size, making it suitable for applications requiring both stability and dynamic flight performance within a small vehicle size.

The coxial design was first adopted by Koehl et al., who presented the design and modeling of a Gun-Launched Micro-Air Vehicle (GLMAV) with two-bladed coaxial contra-rotating rotors and a cyclic swashplate, validated through experimental load data and parameter estimation using a Kalman filter [108]. Gao et al. introduced a bullet-shaped Trans-Domain Amphibious Vehicle (TDAV) with a coaxial counter-propeller-tilting platform, foldable blades for reduced underwater drag, and a trans-domain attitude adjustment system, validated through simulations and outdoor tests for efficient air–water transitions and maneuverability [109]. Wei et al. designed a coaxial rotor aircraft (CRA), which used two independent motors to actuate propellers and two servo motors to adjust the swashpalte. The position and attitude tracking of the CRA were validated through simulations and experiments, demonstrating improved robustness and effectiveness [110]. Li et al. experimentally investigated the aerodynamic performance of a ducted coaxial-rotor system, identifying optimal rotor spacing (H/R = 0.40), tip clearance, and rotor position (P5) for improved thrust and efficiency, with the ducted coaxial-rotor system achieving the best hover performance at S1 spacing [111]. Chen et al. designed a novel coaxial drone with dual-axis rotor rotation for high thrust efficiency and maneuverability, using servo motors for thrust vectoring and a nonlinear control allocation strategy to stabilize position and yaw, validated through simulations and experiments [112]. Some representative examples of coaxial designs are shown in Figure 12.

Though the coxial design features compact size, it faces several critical challenges. Firstly, the mechanical complexity involves intricate mechanisms like swashplates or multiple servo motors, which increase failure risks and control complexity. Secondly, interaction between coaxially placed propellers causes 25–35% thrust loss, impacting energy efficiency. Thirdly, this is an underactuated configuration, which requires advanced control strategies to achieve system stability and six-DOF motion.

6.4. Brief Summary

Revolutionary and innovative designs of rotor-wing drones have been reviewed in this section. These designs significantly improve energy efficiency, miniaturization, and/or passive stability by utilizing fewer actuators or employing wing-shaped fuselages capable of generating aerodynamic lift. Such unique configurations have opened new research avenues, expanding the potential of this field. However, these aircraft also face common technical challenges: (1) limited payload capacity, as simplified structures restrict the ability to carry heavy sensors or additional equipment, potentially limiting their use in certain applications; (2) control and navigation complexity, as spinning aircraft, despite their inherent stability, require more sophisticated control and precise navigation compared to traditional drone designs; and (3) design optimization challenges, where the precise optimization of shape and weight distribution is essential to fully leverage spinning stability, posing significant challenges in design and manufacturing processes.

7. Discussion

Designing unconventional configurations of rotary-wing UAVs poses significant technological challenges, yet these challenges drive advancements in technology and innovative applications. As technology evolves, unconventional UAVs are anticipated to play critical roles in enhancing operational efficiency, expanding functional versatility, and enabling broader real-world adoption. Challenges and outlooks are presented below and shown in Figure 13.

7.1. Challenges

1. Technical Complexity: Unconventional UAV designs, including but not limited to the aforementioned categories, demand sophisticated technological integration across multiple domains. These include advanced mechanical design and manufacturing processes, precise dynamic modeling and control system development, and the implementation of high-performance sensor arrays with robust and fast data-processing capabilities. The successful realization of such innovative configurations necessitates the synergistic optimization of these interdependent technological components to achieve reliable operation under diverse mission scenarios.

2. Stability and Reliability: The operational deployment of unconventional UAVs necessitates robust stability and reliability across heterogeneous environments, particularly when navigating complex aerial conditions and irregular terrain. While current validation efforts primarily focus on controlled indoor testing environments, this approach inherently limits the assessment of system performance under realistic operational scenarios. Comprehensive field testing remains essential to fully characterize the dynamic response and failure modes of these novel configurations in practical deployment conditions.

7.2. Outlooks

1. Technological Innovations: The battery remains the most critical factor limiting UAVs’ efficiency and flight duration. Currently, widely used Li-Po batteries exhibit low energy density, long recharge times, short flight times, and environmental hazards [113]. Thus, advanced Li-Po batteries are urgently needed. Additionally, hybrid energy systems incorporating supercapacitors, solar power, or fuel cells could further enhance the endurance and performance of rotary-wing UAVs [114]. Another critical aspect of rotary-wing UAV design and manufacturing is material selection. Materials must exhibit deformation resistance, high tensile strength, low thermal gradient, vibration damping, and a high strength-to-weight ratio. Advanced material innovations will significantly improve UAV performance and adaptability [115].

2. Modular Expandability and Reconfigurability: As previously analyzed, numerous versatile expanded mechanical modules and morphing mechanisms have been designed for specific tasks. While these systems exhibit strong task-specific performance, their generalizability remains limited. In contrast, modular reconfigurable robot (MRR) systems demonstrate superior potential in applications requiring high operational flexibility compared to fixed-morphology systems [116]. The MRR concept may be adopted for unconventional rotary-wing UAVs in the future, integrating expandable and reconfigurable modules.

3. Integration of Artificial Intelligence: Recent breakthroughs in AI-driven rapid soft robotics design [117] suggest similar potential for unconventional rotary-wing UAV development. Also, embodied intelligence (EI) [118] could revolutionize these UAVs through three synergistic advances: (1) Design: incorporating modular or bioinspired morphing structures for real-time shape optimization and damage resilience; (2) Modeling: combining physics and AI to predict complex aerodynamic interactions (e.g., turbulent flows), reducing reliance on precise dynamic models; and (3) Control: using decentralized neuromorphic algorithms for self-stabilization (e.g., thrust redistribution during rotor failure) and adaptive mission planning. This integrated approach would enable UAVs that self-optimize through environmental feedback, operate resiliently in unpredictable scenarios, and collaborate effectively in swarms.

4. Other Innovations: Additional advancements for unconventional UAVs—including advanced sensors and perception systems, communication networks, flight control and navigation, and automated formation—are also critical for future development [119]. However, as these topics are not directly related to this review’s focus on design, they are not further elaborated here. With these unique advantages and improvements, unconventional rotary-wing UAVs exhibit significant potential in agriculture, environmental monitoring, disaster response, logistics, and urban management.

8. Conclusions

This review highlights recent advancements, challenges, and potential improvements in unconventional rotary-wing UAV configurations. While their design poses significant technological hurdles, these challenges drive technological and application innovations. As technology evolves, such UAVs are anticipated to enhance operational efficiency, expand functional versatility, and enable broader real-world deployment.