A survey of hybrid Unmanned Aerial Vehicles

Progress in Aerospace Sciences 98 (2018) 91–105

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Progress in Aerospace Sciences journal homepage: www.elsevier.com/locate/paerosci

A survey of hybrid Unmanned Aerial Vehicles Adnan S. Saeed a, *, Ahmad Bani Younes b, Chenxiao Cai c, Guowei Cai d a

Aerospace Engineering, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates Aerospace Engineering, San Diego State University, San Diego, CA 92182, USA c School of Automation, Nanjing University of Science and Technology, Nanjing, 210094, PR China d Robotics Institute, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates b

A R T I C L E I N F O

A B S T R A C T

Keywords: Hybrid UAVs Platform design Dynamic modeling Flight control Fixed wing VTOL Tailsitter

This article presents a comprehensive overview on the recent advances of miniature hybrid Unmanned Aerial Vehicles (UAVs). For now, two conventional types, i.e., fixed-wing UAV and Vertical Takeoff and Landing (VTOL) UAV, dominate the miniature UAVs. Each type has its own inherent limitations on flexibility, payload, flight range, cruising speed, takeoff and landing requirements and endurance. Enhanced popularity and interest are recently gained by the newer type, named hybrid UAV, that integrates the beneficial features of both conventional ones. In this survey paper, a systematic categorization method for the hybrid UAV's platform designs is introduced, first presenting the technical features and representative examples. Next, the hybrid UAV's flight dynamics model and flight control strategies are explained addressing several representative modeling and control work. In addition, key observations, existing challenges and conclusive remarks based on the conducted review are discussed accordingly.

1. Introduction During the last several decades, Unmanned Aerial Vehicles (UAVs) have experienced a tremendous development and gained fast-growing popularity worldwide. Nowadays, UAVs are extensively used in various critical military and defence applications such as reconnaissance, surveillance, and security reinforcement. According to [1,2], the total sector sales of the global military UAV market is expected to increase by more than 60% between 2011 and 2020. Nevertheless, UAVs applications are not limited to military and defence: the market of civilian UAVs has recently grown rapidly, covering a wide range of areas such as traffic surveillance, disaster management, infrastructure inspection, law enforcement, and vegetarian monitoring. Many studies (see, e.g. [1,3,4]) predict that there is a high chance that the utilization of civilian UAVs will eventually dwarf the military demand in the near future. The promising future and unlimited potential of UAVs have also ignited strong interest in academia: numerous research works have been carried out on UAVs that are either commercially available or customized and a large amount of algorithms and techniques has been developed aiming at enhancing the UAVs' intelligence in guidance, navigation, and control. UAV platforms are currently dominated by two types: fixed-wing UAV

and rotorcraft UAV. Each type has advantages but exhibits inherent limitations. The fixed-wing UAV generally advances in cruising speed, payload capacity, flight range, and endurance. However, it requires runways or launching/recovery equipment with special design for reliable takeoff and landing. In addition, it is not applicable to missions requiring ultra low flight speed or confined environment. On the other hand, the rotorcraft UAV has much looser requirements on takeoff and landing spots. It also features the unique hovering capability, which brings much enhanced versatility in executing a mission. However, the speed and endurance limit significantly trunks the rotorcraft UAV's capability in missions requiring wide-range coverage or long endurance. As such, a newly emerging and promising trend of UAV design, particularly for miniature UAVs, is to design an aerial system that integrates the advantages of both, operates in a wider envelope (i.e., vertical takeoff, transition, cruise, and vertical landing), and contributes to a much broader range of applications. Inspired by such demanding need, the hybrid UAV, or fixed-wing Vertical Takeoff and Landing (VTOL) UAV in other words, is born. Indeed, integrating the advantages of fixed-wing and rotary aircraft has long been a concern for the aerospace and aviation industries. Over the years, there have been quite a number of attempts to build manned hybrid aircraft. Several representative examples are shown in Fig. 1,

* Corresponding author. E-mail address: [email protected] (A.S. Saeed). https://doi.org/10.1016/j.paerosci.2018.03.007 Received 27 December 2017; Received in revised form 17 March 2018; Accepted 20 March 2018 Available online 30 March 2018 0376-0421/© 2018 Elsevier Ltd. All rights reserved.

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recent advances of the miniature hybrid UAVs. According to our knowledge, remarkable progress has been mainly achieved in three aspects: platform design, flight dynamics modeling, and flight control. Correspondingly, the remaining content of this paper is organized as follows: Section 2 addresses the platform design, in which a systematic categorization method for the hybrid UAVs is proposed and the technical features as well as the representative hybrid UAV platforms for each type are detailed. In Section 3, an overview of hybrid UAVs' flight dynamics modeling and flight control techniques is presented addressing a comprehensive analysis of the representative modeling and control research work. Finally, key observations, existing challenges and key conclusion remarks are drawn in Section 4. It should be highlighted that the scope of this paper is particularly limited to the miniature hybrid UAVs. Thus, the word ”miniature” will be omitted in the remaining contents.

List of acronyms UAV VTOL LTV RC GL QTW MTT CTT DTT FDCL PID LQR PD P PI SDRE

Unmanned Aerial Vehicle Vertical Takeoff and Landing Ling-Temco-Vought Radio-Controlled Greased Lightning Quad Tilt Wing Mono Thrust Transitioning Collective Thrust Transitioning Differential Thrust Transitioning Flight Dynamics and Control Laboratory Proportional-Integral-Derivative Linear Quadratic Regulator Proportional-Derivative Proportional Proportional-Integral State Dependent Riccati Equation

2. Platform design Generally, hybrid UAVs are categorized into two types: convertiplane and tail-sitter. A convertiplane maintains its airframe orientation in all flight modes, and certain transition or switching mechanisms are employed to achieve mode transfer. On the other hand, a tail-sitter is an aircraft that takes off and lands vertically on its tail, and the entire airframe needs to tilt to accomplish cruise flight. As shown in Fig. 2, both are further categorized into a few sub-types, depending on the specific transition mechanisms and airframe configurations. In what follows of Section 2, we will address all subtypes, analyze their design features, discuss the advantages and disadvantages of every subtype, and introduce representative examples.

including: Bell-Boeing V-22 Osprey [5,16], Vertol VZ-2 [6,17], Sikorsky X-wing [7,18], Harrier GR7 [8,19], Convair XFY-1 [9,20], Lockheed XFV-1 [10,21], Ling-Temco-Vought (LTV) XC-142 [11,22], and Canadair CL-84 [12,23]. Some of them did achieve remarkable success and a few manned hybrid aircraft such as Bell-Boeing V-22 Osprey and Harrier GR7 are still in service. Within the last five years, the hybrid aircraft design concept has gained increasing popularity in miniature UAV development, given 1) the increasing maturity of miniature UAV design and manufacturing, 2) the steady cost reduction of miniature UAV development, and 3) the saturation of the conventional miniature fixed-wing and rotary UAVs. As a result, a number of pioneer research work has been documented in literature, and a few designs such as BirdsEyeView FireFLY6 [13], X PlusOne [14] and MartinUAV V-Bat [15] have been commercialized successfully. For now, the hybrid UAV development is still in its infancy, and there is a huge space for the hybrid UAVs to become more mature in terms of many critical perspectives such as design philosophy, dynamics modeling, control, guidance, navigation, and robustness. Nevertheless, given their rapidly growing popularity, it is believed that the hybrid UAVs will have a bright future and promptly form an essential pillar of the UAV market. In this paper, we intend to provide a comprehensive overview on the

2.1. Convertiplane A variety of mechanisms have been implemented in developing the convertiplane UAVs to achieve the transition between vertical flight and cruise flight. In general, the convertiplane UAVs are classified into four sub-types: 1) tilt-rotor, 2) tilt-wing, 3) rotor-wing, and 4) dual-systems. 2.1.1. Tilt-rotor A tilt-rotor UAV has multiple rotors mounted on the tilting shafts or nacelles. During the hover-to-cruise transition, partial or all rotors tilt towards flight direction to provide the aircraft forward speed until the cruise flight is achieved. Since the birth of the first tilt-rotor UAV (i.e.,

Fig. 1. Examples of manned hybrid aircraft. 92

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Fig. 2. Categorization of miniature hybrid UAVs.

Fig. 3. Representative examples of bi-rotor convertiplane UAVs.

treated as the pioneer in the field of bi-rotor convertiplane UAVs by unveiling Bell Eagle Eye UAV shown in Fig. 3 (a) (i.e., a scaled-down, unmanned version of the manned hybrid aircraft V-22 Osprey shown in Fig. 1(a)) to the world [24,25,30]. It was later adopted by a number of small-scale bi-rotor convertiplane UAV prototypes such as the NUAA tilt-rotor UAV prototype shown in Fig. 3(b) [26]. It is noted that for such design the two rotors produce constant rolling torques to both wings and fuselage, resulting in relatively shorter wing span and thicker airfoil. As a consequence, the aircraft has a relatively poor aerodynamic performance in terms of aspect ratio and drag. An alternative bi-rotor convertiplane solution was unveiled in 2011 by AgustaWestland Project Zero shown in Fig. 3(c), which integrates flying-wing fuselage and twin embed-in-fuselage motor design [27]. Such combination theoretically eases the request on the lift and thrust in the cruise mode but makes the attitude stabilization in hover more challenging. Furthermore, the fuselage-rotor interaction effect is a critical issue for such UAV's operation, particularly in transition stage and cruise mode. So far, only its basic hovering capacity has been initially flight-demonstrated in 2013. There are two small-scale convertiplanes (the Navig8 and the UPAT UAV shown in Fig. 3) which adopted the bi-rotor design methodology

Bell Eagle Eye [16]) in 1993, the tilt-rotor design concept has nested a number of aerospace enterprises and research institutes, and as a result tens of tilt-rotor UAVs have been developed and flight-tested. Accordingly, the tilt-rotor UAVs are further categorized into three branches: 1) bi-rotor, 2) tri-rotor, and 3) quad-rotor, which will be all detailed in what follows. In the description of each branch, three main aspects will be addressed: 1) main feature(s), 2) operation principle, and 3) representative design cases. A bi-rotor convertiplane UAV employs two tilting rotors to provide lift in hover and thrust in cruise. When operating in the cruise mode, a birotor convertiplane works very similarly to a conventional fixed-wing aircraft (i.e., thrust is provided by rotors, and various control surfaces are used to achieve the roll, pitch and yaw motions). On the other hand, when operating in the vertical mode, pitch is generated by the forward longitudinal cyclic pitch produced by the forward tilting of the proprotor discs. Moreover, the availability of nacelle lateral tilting and collective and cyclic pitch are pretty important for generating yaw and roll motions. According to our survey, the most dominant rotor configuration for bi-rotor convertiplane UAVs is that the two rotors are mounted on the tilting nacelles or shafts that are located on the wing tips. Such design concept was first introduced in 1993 by the Bell Helicopter Inc., which is 93

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design features relatively good payload capacity. For instance, according to [24,31–33] the TURAC can approximately carry the same payload capacity as the IAI Panther with an obvious wing-span reduction. It was also noted that a few of additional design features have been employed to improve the performance of the tri-rotor flying-wing convertiplanes. For instance, in order to simplify the rolling and yawing control in the hover mode, TURAC adopts a co-axial rear rotor whereas FireFLY6 adopts the Y-6 (i.e., three co-axial rotors) configuration. Furthermore, aerodynamical feature was intended to be optimized by all designers through selecting appropriate airfoils [34], Computational Fluid Dynamics (CFD)-analyzing the performance [31,34], and winglet utilization. The third but relatively rare convertiplane UAV design is the quadrotor configuration, which can be further divided into two sub-types: “ þ ” style and “  ” style. According to our survey, very few hybrid UAVs were constructed based on quad-rotor configuration, and only two representative prototypes have been found, that is, Phantom Swift developed by Boeing and Quantum Tron developed by Quantum Systems [35,36] shown in Fig. 4. The former mainly features “ þ ” style rotor configuration and tilting shrouded rotors mounted on the wing tips for thrust generation during cruise flight. In the hover mode, Phantom Swift generally operates as a “ þ ” style quadcopter with the enhanced yawing controllability provided by the tilting rotors. The Quantum Tron integrates the “  ” style quad-rotor configuration, sailplane aircraft design, and retractable rotor blades. During hover, the Quantum Tron works as a “  ” style quadcopter while in cruise flight mode, the front two rotors tilt forward for thrust generation, and the two rear rotors are not in usage. Furthermore, the blades of rear rotors are naturally retracted to minimize the drag caused for cruising. Currently, the critically challenging issue for both aforementioned prototypes is the transition of techniques developed for the miniature scale demonstrators to the full-scale counterparts.

[28,29]. The Navig8 UAV features an aerodynamical fuselage design that generates additional lift in cruise flight. The UPAT UAV is built with a highly simplified mechanical design for initially examining the proposed low-level control algorithms but shows the emerging interest in the academia. Another popular convertiplane design, which is particularly dominant in the small-scale convertiplane UAV market, is the tri-rotor configuration. In the vertical mode, all three rotors are directed upwards and the operation principle is analogous to the Y-configuration VTOL drones. The transition to cruise mode occurs by tilting one or two rotors forward to generate thrust and gain airspeed. While in the cruise mode, the tri-rotor convertiplane UAV works very similarly to a conventional fixed-wing aircraft (i.e., thrust is provided by rotors, and various control surfaces are used to achieve the roll, pitch and yaw motions). Tri-rotor convertiplane generally features the following three advantages: 1) the lift generation requirement on each rotor can be reduced roughly from 50% to 33% of the total UAV weight, 2) rotor configuration is geometrically symmetric with respect to the UAV's center of gravity, which can effectively ease the attitude stabilization of the tri-rotor convertiplane in hover mode, and 3) the rear rotor is constantly fixed to the fuselage and thus causes minor additional mechanical design complexity. The first world-acknowledged remarkable success in the tri-rotor convertiplane UAV development is the Panther UAV shown in Fig. 4(a), which was developed by IAI Inc [24]. It is currently served for military tactical usage. As the pioneer, Panther UAV adopts sailplane fuselage and wing design and low-wing configuration, aiming at desired tradeoff between the flight endurance and versatility. However, such design is indeed rarely followed by other designers worldwide for either scientific research or commercialization. Instead, the combination of tri-rotor and flying-wing fuselage has recently gained emerging popularity, which is reflected by the fact that three of the four representative tri-rotor convertiplane UAVs (i.e., Orange Hawk, TURAC and FireFLY6 shown in Fig. 4) addressed in this survey fall into this line. Such trend following is mainly caused by three reasons: 1) miniature or small-scale flying-wing aircraft is low-cost and easily available in the Radio-Controlled (RC) communities, 2) the enhanced capability of the low-cost auto-pilot systems (e.g., PixHawk or DJI-NAZA) is sufficient to handle the hybrid UAV control, and 3) aerodynamically flying-wing

2.1.2. Tilt-wing The primary feature of the tilt-wing convertiplane is that partial or the entire wing is required to be tilted together with the rotors during flightmode transition. The tilt-wing convertiplane design can be traced back to the period 1957 to 1965, when Boeing Vertol pioneered this area by developing a manned tilt-wing aircraft named Vertol VZ-2 shown in Fig. 1(b) [6]. Compared with the tilt-rotor, a tilt-wing convertiplane

Fig. 4. Representative examples of tri-rotor and quad-rotor convertiplane UAVs. 94

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sophisticated speed-control mechanism of the multiple rotors and enhancing controllability in case of the malfunction of one or few rotors. Another very successful example is the DHL parcelcopter which is targeted for parcel delivery in mountainous regions. As previously mentioned, the hybrid UAVs pave the way for increasing payload capacity while maintaining high cruise speed and long flight range. DHL took advantage of this as the latest version 2.2 m wingspan hybrid UAV (parcelcopter 3.0) is able to carry a payload of 2 kg and have a cruise speed of 70 km/h with a flight distance of around 8 km and is loaded and unloaded automatically [42]. Moreover, it has successfully completed the three-month test and has been logistically integrated into the delivery chain [43]. Two tandem-wing convertiplane UAVs prototypes shown in Fig. 5(f) and (g) have been developed by academic institutes as an extension of their research on routine quad-copter aerial vehicles. Mechanically, both surveyed tandem-wing convertiplane platforms follow an almost identical configuration: four fixed-pitch rotors are mounted on the edge of the tandem wings, and two tilting mechanisms are employed, each of which is attached to a pair of the tandem wings for collective tilting. During hover, a tandem-wing convertiplane UAV is operated similar to an “  ” style quad-copter with the assistance of the on-board attitude stabilization system. Differential tilting function to a wing set (front or rear) is not required and thus can significantly simplify the design of the tilting drive train. Furthermore, it should be noted that airfoil selection and the interaction between the front and rear wings are highlighted in all the documentations related to both UAVs (i.e. [44–47]), which indicates the critical importance of these two issues for achieving desired flight performance.

generally features more complicated and sophisticated design in on-board components such as tilting drive train. Furthermore, in low-speed operation (i.e., hover, takeoff, and landing), the wings of a tilt-wing convertiplane need to be directed upwards, which makes the aircraft more vulnerable to cross wind and thus requires additional effort in developing control mechanisms to handle the attitude stabilization. Due to such inherent challenges, the tilt-wing design concept did not regain sufficient popularity until the year of 2000, when the manufacturing technology of miniature RC aircraft became more mature and its potential in the miniature UAV market was gradually dug out by UAV designers. Over the last decade, tilt-wing convertiplane UAVs have been researched actively, and a number of representative cases (shown in Fig. 5) have been found through our survey, including: HARVee [37], AVIGLE [38–40], Greased Lightning (GL) VTOL Drone [41,48], DHL parcelcopter [42,43], AT-10 Responder [24], Quad Tilt Wing (QTW) VTOL UAV [44,45], and SUAVI [46,47]. . Generally, tilt-wing convertiplane UAVs fall into two main categories: single-wing configuration and tandem-wing configuration. Single-wing configuration is treated as the most popular solution to the miniature tilt-wing convertiplane UAV development, as five of the aforementioned seven representative examples (i.e., HARVee, AVIGLE, GL VTOL Drone, AT-10 Responder and DHL parcelcopter) adopt such concept. HARVee is an early work developed by a young research group [37]. The aerodynamic performance is relatively coarse, given the two facts: 1) only two fixed-pitch motors are integrated with a sailplane aircraft, and 2) the entire wing is required to tilt for realizing the transition. In hover-mode, the HARVee operates similar to a bi-rotor convertiplane as explained previously. Additional mechanical design innovations have been employed by all the subsequent efforts, aiming at improving the stabilization performance of the tilt-wing convertiplane UAVs, particularly in hover-mode. More specifically, the designers of the AT-10 Responder invented a unique tilting mechanism for drag/disturbance reduction; the wing of the AT-10 Responder is separated into two parts. Only the inner part with the fixed-pitch motors mounted on the leading edge is able to tilt. As for AVIGLE, another novel mechanism featuring the integration of variable blade pitch and aileron control on top of differential wing tilting has been adopted, providing rich freedom for attitude control in hover mode. On the other hand, GL VTOL Drone adopts a substantially different control mechanism by employing ten fixed-pitch rotors (eight are located on the leading edge of the wings and the remaining two are with the horizontal stabilizers). Consequently, the attitudes of GL VTOL Drone are well controlled by implementing a

2.1.3. Dual-system The third type of convertiplane UAVs is referred to as dual-system. It utilizes two sets of propulsion systems: one contains upward mounted rotor (or rotors) for vertical operation and another adopts tractors or pushers for cruise flight. As tilting mechanism is not required for dualsystem convertiplane, such aircraft, compared with the two aforementioned convertiplanes, features simplified mechanical design and enhanced reliability. However, during cruise flight, the multiple nonoperational rotors for vertical lift generation cause extra aerodynamic drag due to their fixed mounting, resulting in additional burden to the tractors or pushers. Six representative dual-system examples have been discussed in this section, including: HADA [50], Quadcruiser [51], Arcturus JUMP [52], Hybrid Quadcopter [53], SLT VTOL UAV [54], and

Fig. 5. Representative examples of tilt-wing convertiplane UAVs. 95

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Fig. 6. Representative examples of dual-system convertiplane UAVs.

Stop-Rotor aircraft [60], respectively. The main reason for such inactivity is that the unique rotor-wing feature poses over critical challenges to a qualified solution to balance the design complexities in terms of aerodynamics and mechanics. For the X-50 DragonFly, the symmetric rotor design with an elliptic airfoil section was inherited from its predecessor S-72 X-Wing [61] to simplify the mechanical drive train. However, such feature leads to the usage of additional lift-canard and enlarged horizontal tail to assist lift generation during the hover-to-cruise conversion. To avoid the tail-rotor usage, the X-50 DragonFly applied a complicated tip-jet-driven mechanism to the main rotor. The rotation speed is controlled by the exhaust from a jet engine through thrust nozzles located at the rotor tips. As for the electrically powered NRL Stop-Rotor aircraft, a highly complicated drive train which was detailed in Ref. [60], was compulsorily adopted to pivot the rotor. None of the aforementioned prototypes have succeeded in completing the transition from the vertical flight mode to the cruise mode. Largely due to the inherently too challenging design, both the Sikorsky S-72 XWing project and the Boeing X-50 project have been canceled. However, Singapore University of Technology and Design recently developed a rotor-wing hybrid UAV called THOR with a tailless flying wing configuration and a single-axis rotor [62]. As shown in Fig. 7, the UAV completed a full envelope flight including transition to and from cruise mode. Although controllers were only implemented for cruise and hover flights, this work can be considered a breakthrough in advances in this branch with further development of the modeling dynamics, design parameters and control strategies to be implemented.

TU-150 [55]. These UAVs are illustrated in Fig. 6 for convenient reference. Based on the number of rotors employed for vertical flight, the dual-system convertiplane UAVs consist of three sub-types: compound helicopter (one rotor) as in Ref. [56], bi-rotor dual-system and quad-rotor dual-system. The operation principle of the compound helicopter is basically the same as that of the helicopter but has additional wings with several mounted rotors to allow cruise flight. Further details about the operation principle can be found in Ref. [56]. Only one of the aforementioned dual-system prototypes (HADA) has adopted this configuration in which it performs as a helicopter for takeoff and the transition occurs by unfolding the wings and powering up the pusher [50]. The project is currently being developed by Embention jointly with INTA (Spanish National Institute of Aerospace Technology) for the employment of the flight control system [57]. Among all sub-types, the quad-rotor configuration prevalently dominates dual-system convertiplane development, given the fact that all the other examples except TU-150 adopt the quad-copter configuration. Such prevalence is mainly due to the desired controllability and attitude stability provided by the “  ” style quad-copter operation in vertical flight mode. Among the four quad-rotor dual-system UAVs, the Quadcruiser and the Arcturus further share the same tandem-wing design feature, whereas the Hybrid Quadcopter and the SLT VTOL UAV adopt conventional fixed-wing fuselage with either T-tail or V-tail. Several representative dual-system UAVs demonstrated the flight capacity over the full envelope which indicates the practicality of quad-rotor dual-system UAVs [52,54,58]. On the other hand, TU-150 is the only bi-rotor dual-system UAV that has been found in our survey. Two three-blade rotors are adopted and mounted on the tips of the wings. For sufficient lift generation, the rotor size is significantly enlarged. The rotor radius is over 30% of the wing-span based on the current prototype. For now, the authors have found poor resources on flight demonstration on TU-150. The feasibility of implementing such design methodology is still questionable.

2.2. Tail-sitter The other type of hybrid UAV is the tail-sitter which takes-off and lands vertically on its tail and the entire airframe tilts to achieve cruise flight. Such intuitive configuration is traced back to the 1950s, when two pioneering trials on manned tail-sitter aircraft, XFY-1 [63] developed by Convair shown in Fig. 1(d) and XFV-1 [10] developed by Lockheed shown in Fig. 1(e), occurred. With the increasing maturity and cost reduction of miniature UAV development over the last two decades, this concept was commonly adopted in developing tactical UAVs. Based on the transition mechanism, these tail-sitter UAVs can be classified into three sub-types: 1) Mono Thrust Transitioning (MTT), 2) Collective Thrust Transitioning (CTT), and 3) Differential Thrust Transitioning (DTT).

2.1.4. Rotor-wing The fourth type of the convertiplane UAVs, named rotor-wing or stoprotor, employs a rotary wing that spins to provide lift during vertical flight and stops to act like a fixed-wing during cruise flight. Such design methodology was initially exploited by Sikorsky Aircraft [7] in developing their first manned rotor-wing prototype (a modified Sikorsky S-72 integrated with X-Wing composite blades shown in Fig. 1(c)). Later, it was only researched by Boeing and Naval Research Laboratory in designing two rotor-wing UAVs, that is, X-50 DragonFly [59] and NRL 96

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Fig. 7. Rotor-wing UAV (THOR) flight test [62].

capacity when operating in fixed-wing mode. The third and most risky issue is that, due to the single rotor configuration, the hover-to-cruise transition is commonly achieved via a Stall-and-Tumble procedure (illustrated in Refs. [67,68]) during which control loss could easily occur. Consequently, few trajectories were proposed to mitigate the steep altitude loss by introducing trajectory-optimization techniques as in Refs. [69–72]. To date, the development of the MTT tail-sitter UAVs is still in an immature stage and no traces on mass production and remarkable implementations were recorded. Lastly, some effort on developing miniature MTT tail-sitter UAVs are identified in some academic work such as [73] but the results presented are only limited to conceptual design.

2.2.1. Mono Thrust Transitioning (MTT) MTT tail-sitter UAVs usually employ a single rotor located either at the nose or at the rear side of the aircraft fuselage for thrust generation. The transition to and from cruise flight is usually achieved via vectored thrust, ducted fan vanes, cyclic or variable blade pitch propeller or swashplates. Four representative cases shown in Fig. 8 and identified in our survey include 1) SkyTote [64] developed by AeroVironment, 2) Flexrotor [65] developed by Aerovel Cooperation, 3) V-Bat [15] developed by MartinUAV, and 4) Hybrid tail-sitter UAV [66] developed at KAIST Flight Dynamics and Control Laboratory (FDCL). Despite falling into the same category, the methods of realizing attitude stabilization at hover are rather different. More specifically, SkyTote features the usage of a co-axial rotor for thrust generation and automatic yawing torque cancellation in vertical flight. A sophisticated control scheme involving rotor spin speed, 3 blade pitch, aileron collective and cyclic pitch, and stabilizer (located at the cruciform tail) collective and cyclic pitch, was developed for attitude control. Flexrotor generates thrust via a rotor with variable blade pitch. The rolling and pitching movements are realized by cyclically changing the rotor's blade pitch. A novel anti-yawing mechanism is adopted by Flexrotor: two tiny rotors are installed at the wing tips with opposite orientations to counter the yawing torque generated by the main rotor. As for the V-Bat and FDCL's tail-sitter UAV, a shrouded ducted-fan installed at the rear side of the fuselage is employed to generate thrust. The deflection of the ducted-fan's vanes is controlled individually and further combined to achieve desired attitude change [66]. Besides, control surfaces such as aileron and/or canard also contribute to a small portion of moment generation. Generally speaking, designing a functional MTT tail-sitter UAV, compared with other hybrid aircraft, is indeed more challenging, and the complexity can be summarized in the following three aspects. First, the size, airfoil selection, and blade pitch control of the rotor must be carefully determined to cover a fairly wide range of thrust generation that is suitable for both vertical and cruise flight modes. Secondly, slender wing design with relatively high aspect ratio is commonly adopted by MTT tail-sitter UAVs to minimize the disturbance caused by cross wind during vertical flight. However, such configuration poses an inherent limitation to the payload

2.2.2. Collective Thrust Transitioning (CTT) CTT tail-sitter UAVs are usually equipped with single or multiple fixed-pitch non-cyclic blade rotors. The transition between the vertical and cruise flight modes is realized mainly via the deflection of various control surfaces coupled with collectively increased or decreased thrust. Although the usage of multiple rotors increases the control freedom and can consequently outperform the MTT tail-sitter UAVs, only two CTT tailsitter prototypes achieved remarkable progress in terms of platform design, that is, the T-Wing prototype developed by the University of Sydney and the VD200 developed by China's Chengdu Aircraft Research and Design Institute (CARDI). Both of them fall into miniature UAV category, and adopt twin-rotor and fixed-pitch blade configurations. The research on the T-Wing is a pioneer work conducted in the academia that explores the potential of applying CTT tail-sitter UAVs into defense and civilian missions [74,75]. This prototype features a conventional low-wing airplane configuration with assistance from a pair of lifting canards and flight trials over the full envelope have been successfully conducted [76]. As for the VD200, the flying-wing fuselage design was chosen, which, in principle, can result in an enhanced payload capacity and eliminate the Stall-and-Tumble procedure [77]. However, no flight-test record is archived in the literature for this platform as a proof of the aforementioned claim. Like the MTT tail-sitter UAVs, the development of the CTT UAVs is also still in an immature stage and no traces

Fig. 8. Representative examples of MTT tail-sitter UAVs. 97

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[81,87,88,126]

[113–125]

DTT

CTT

No extra actuators, Efficient forward flight, High Cruising speed, Various design options for wing geometry, Common in Research and Industry No extra actuators, Controllability and stability, Common in Research and Industry, Various design options for wing geometry, Easy takeoff and landing

Unstable vertical flight, Low cruising speed, complex control systems, Low payload capacity, Low Endurance Unstable vertical flight, Difficult to land in moving decks, Strong tail landing mechanisms required, Vulnerable to cross winds, Difficult to land in moving decks Reduced efficiency in horizontal flight, Sufficient differential thrust to be provided, Vulnerable to cross winds, Difficult to land in moving decks

No systematic work found in the literature [66,71,72,107–112] Extra unnecessary weight, Very few examples in industry

RotorWings DualSystems MTT

Controllability and stability, Easy takeoff and landing, Various design options for wing geometry, Simple transition mechanism No extra actuators required

Vulnerable to cross winds, Difficult to land in moving decks, Heavy and powerful actuators required Unsuccessful previous attempts, complex transition mechanism, unstable due to single rotor Tilt-Wings

Controllability and Stability, Common in Research, Easy takeoff and landing, Simple transition mechanism Good aerodynamic performance, Common in Research and Industry, Simple transition mechanism Easy takeoff and landing, Lightweight

Disadvantages Advantages

A reliable model that accurately captures the flight dynamics over the flight envelope of interest is critically important for developing the autonomous flight control system. For the hybrid UAVs, the flight envelope can be generally divided into three modes, namely, vertical flight mode, transition mode, and level flight mode. As a result, developing a reliable flight dynamics model becomes more challenging compared with any conventional aircraft. Depending on the specific type of the hybrid UAV as well as the specified missions, the orientation kinematics can be expressed via two formulations, namely, Euler angles and Quaternion. Generally, Euler angle representation dominates the convertiplanes and single mode (e.g., hover or cruise with constant speed) of partial tail-sitters because the fuselage does not change with large amplitude. On the other hand, the tail-sitters operating over full flight envelope commonly adopt quaternion representations because the transition between hover and level flight modes leads to approximately 90-degree pitch angle change which has a high chance to raise the singularity of Euler-angle representation. According to our survey, Newton-Euler formulation is commonly adopted to represent the rigid body dynamics of hybrid UAVs. However, only few research works (i.e. [118–120,125]) adopt Euler Lagrange formulation without highlighting the particular reason for their selection.

Tilt-Rotors

3. Flight dynamics modeling and control

Type

Table 1 Advantages and Disadvantages of the types of hybrid UAVs and the corresponding Representative Modeling and Control Work (RMCW).

RMCW

2.2.3. Differential Thrust Transitioning (DTT) A primary design feature of DTT tail-sitter UAVs is that partial rotors are installed above and below the vehicle's horizontal plane. The differential change of these rotors' thrust generates a pitching torque and further leads to the mode transition. Our survey has found that six UAV platforms shown in Fig. 10 belong to this category, including: ATMOSUAV developed by ATMOS [80], VertiKUL developed by the University of Luvine [81], Project Wing developed by Google X [82,83], Heliwing developed by Boeing [84], two RC-fan-developed prototype X PlusOne [14]and QuadShot [85,86]. It is until the last decade that the DTT tail-sitter UAV gained significantly enhanced popularity, which is mainly due to the rapid maturity and prevalence of miniature multi-rotor aerial vehicles and the potential of embedding the multi-rotor concept into the fixed-wing aircraft design for enhanced capacity and endurance. The resulting hybrid UAVs inspired by such intensive interest ended up sharing several key design similarities such as 1) flying wing for high payload and 2) quad-rotor configuration for structural simplicity. Compared with the CTT type, DTT UAVs have two distinguished advantages: 1) no involvement of control surfaces in VTOL operation, and 2) enhanced thrust-to-weight ratio. According to our survey, all the aforementioned DTT UAVs have four rotors to provide differential thrust to make the transition to horizontal flight and back. However, ATMOS and Quadshot have control surfaces or tilting rotors for control during horizontal flight unlike VertiKUL which depends on differential thrust to control the aircraft in all modes as well as performing the transition. Both ways seem to operate well in both flight modes but those using the control surfaces during horizontal flight require extra actuators to control the ailerons, rudders and elevators which increases the weight of the aircraft. On the other hand, VertiKUL requires more complex control strategies since it only uses differential thrust for control in both modes. Consequently, further development of the parameter selections and control approaches are being carried as illustrated in Refs. [87,88]. The authors found few technical information on the other aforementioned DTT tail-sitter UAVs. Table 1 summarizes the advantages and disadvantages of each type of the hybrid UAVs.

Poor aerodynamic performance, Structural complexities, Actuators required

on mass production and remarkable implementations were recorded. Some academic work such as [78,79] developed miniature CTT tail-sitter UAVs but the results presented are only limited to either conceptual design such as in Ref. [79] or preliminary implementation level such as [78].

[38,39,44,46,47,49, 102–104] [62,105,106]

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[26,29,33,89–101]

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The main and most critical part of the control systems is the control law which can be classified into linear and nonlinear. As previously mentioned, hybrid UAV models are nonlinear but are commonly linearized by applying relative equilibrium conditions around a steady state operating point allowing the implementation of linear controllers. However, although linear control laws are simple, easy to implement, reduce the computational effort and minimize the design time but their performance degrade when operating away from the local equilibrium point or while performing agile maneuvers. This is very critical during the transition flight for the case of hybrid UAVs because changing from vertical flight mode to horizontal flight mode and vice versa results in operation far away from the relative equilibrium condition. That is the reason behind which some current hybrid UAVs implement nonlinear controllers or three separate linear controllers, one for the horizontal mode, one for the vertical mode and one for the transition. From the survey conducted, it was found that the classical Proportional-IntegralDerivative (PID) controller and the Linear Quadratic Regulator (LQR) are the most common linear control laws applied in hybrid UAVs while the gain-scheduling, back-stepping, and Nonlinear Dynamic Inversion (NDI) are the common nonlinear laws. Table 2 shows a summary of the control laws with their advantages, disadvantages and implementations on different types of hybrid UAVs [127–129]. From the review, it was found that the PID controller gain values were mostly determined by empirical tuning until some preconceived ideal response of the system is achieved. Since PID control strategy only requires appropriate adjustment of the control gains, it serves as a concrete starting design point for many hybrid UAVs as it does not require extensive knowledge of the model. Regarding nonlinear controllers, it was found that gain-scheduling was mainly utilized to enhance the control during transition and back-stepping method was mostly coupled with using Euler-Lagrange approach for the dynamic modeling. There are several other control laws implemented in the hybrid UAVs [66,91,113]. apply adaptive control techniques which account for the nonlinearities and uncertainties present in the model. J.A. Guerrero et al. [112] presents a robust control design based on sliding mode of a mini birotor tail-sitter for the hovering mode. The work in Ref. [130] shows the control of hovering flight and vertical landing using optical flow. Fault tolerant flight control system for a tilt-rotor UAV was discussed by S. Park et al. in Ref. [100]. Moreover, other control strategies based on Lyapunov stability concepts can be found in Refs. [93,99,118–120]. The last column of Table 1 provides a complete list of the documentations related to the dynamics modeling and flight control systems of hybrid UAVs following the categorization method introduced in Section 2. According to the review conducted, no systematic modeling or control work on dual-system hybrid UAVs have been documented in the literature. The remaining part of this section will focus on analyzing some

Regarding the propulsion system of hybrid UAVs, two coupled sub components (i.e., the propeller aerodynamics and the motor dynamics) are involved. For the former, the majority of the modeling works adopt highly simplified dynamics model which involves very fundamental aerodynamic analysis. For instance, in Refs. [110] and [114] only quasi-steady equations are utilized to model the aerodynamic forces and moments. For the latter, no research work has particularly paid attention to the motor dynamics. Thus, the response of propulsion systems to the actuator input is assumed instantaneous. Particularly for the case of hybrid UAVs, the motor dynamics might have a significant effect during the transition phase and therefore it should be studied intensively. The transition mode requires careful attention from the modeling aspect due to two reasons. Firstly, the sources of required lift change. The lift in vertical flight comes from the thrust generated by the rotors whereas the lift in horizontal flight comes from the aerodynamics of the wings and tails. Secondly, due to the change in UAV's configuration, the direction of some of the forces changes accordingly. For example during transition of tilt-rotor or tilt-wing, the direction of the thrust from the tilting rotors changes during the transition phase and therefore it should be modeled appropriately. As introduced in Section 2, the tilting mechanism uniquely belongs to two types of convertiplane UAVs: tilt-rotor and tilt-wing. Similar to the propeller aerodynamics part, the current tilt-rotor and tilt-wing modeling works adopt highly simplified models to account for the tilting motion of the propulsion systems. For instance, a common method has been documented in Refs. [29,89,101] in which two instantaneous shaft tilting angles, αL and αR , are defined for the rotation of the left and right front propulsion systems and the tilting motion is reflected by a rotation matrix based on the tilting angles defined. Given the configuration of the dual-system UAVs, no specialized modeling for the transition phase is required because the transition to and from cruise flight is achieved by operating directed rotors accordingly. No systematic transition phase modeling has been documented for rotor-wing UAVs. On the other hand, the tail sitters are viewed differently since no specialized mechanisms are employed for achieving the transition. The UAV does not change its configuration and therefore no extra modeling parameters are required. Instead, certain control strategies have to be employed in order to achieve a safe transition to and from cruise flight. These strategies are discussed later in this section. The core of the control system depends on the derived dynamics model. Particularly speaking, the dynamics of the hybrid UAVs, which is highly complicated and nonlinear, can be inherently unstable because it inherits the operation of a fixed-wing and VTOL UAVs. Even if horizontal and vertical modes are analyzed separately, the transition phase remains a critical part of the control system due to the multiple nonlinearities in the model. Table 2 Control Laws Classification of the hybrid UAVs. Control Law

Advantages

Disadvantages

TiltRotor

TiltWing

Rotorwing

MTT

CTT

DTT

PID

Easy implantation, very common control scheme design in real life applications, does not require the knowledge of the UAV model

[26,29, 91,92]

[46,47, 103, 104]

[62]

[81, 126]

Handles complex dynamic systems and multiple actuators, robust w.r.t process uncertainty, asymptotically stable for controllable systems, very large stability margins to errors in the loop Very robust for external disturbances and irregular parameter uncertainties, deals with all the states of the system and accounts for the nonlinearities Allows easy understanding and simple implementation of the control laws over the full flight envelope Closed loops can be easily tuned

[97, 100]

[39,47]

–

[66,71, 107, 109, 111] [108]

[116,118, 122–124]

LQR

Poor robust ability compared with the robust controller when the system encounters to multiple challenges, not optimal solution Requires access to the full state which is not always possible

[117,124]

–

Not optimal, computationally expensive for operation in real time

[90,93, 98,99, 101]

–

–

–

[113]

–

computationally expensive for operation in real time

–

[104]

–

[71]

[116,117, 124]

–

Requires a precise knowledge of the aerodynamic coefficients

[89]

–

–

[66,72]

–

–

Backstepping

GainScheduling NDI

99

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issue by treating the equations of motion as inner and outer loop states and inputs. The inner loop, which uses SDRE control, considers the rotational and translational velocities and the outer loop considers the Euler angle kinematics. Simulation results showed that the system successfully controls the tilt duct but experimental verification should follow [98]. and [90] synthesized a nonlinear controller based on back-stepping control strategy. Both works showed simulations which indicate the stability and operability of the control system. However, no experimental validation was carried out [89]. presented a control strategy for the transition mode based on the dynamic inversion technique using reference model tracking to linearize the system and extended state observer to compensate for external disturbances. Simulations showed that the method is robust and the conversion can be controlled successfully but again, the control strategy was not verified experimentally. Only two works [92], and [91], demonstrated the control of Tri-rotor tilt-rotor UAV. Regarding [91], although a PD controller was sufficient for altitude control but PID was used to ensure safe flight and another PID controller was also implemented for attitude control. Moreover, an adaptive control using neural network was used to enable the adaptability of control gains of the PID controller which therefore minimizes the position error. This unique strategy was only demonstrated in Ref. [91] and it was shown by simulations using MATLAB that the proposed strategy can be implemented in reality. For [92], the system was linearized around its hovering point and PID controllers were implemented for the same reasons as [29]. However, no experimental verification was found for both Tri-rotors. Regarding Quad tilt-rotors, Flores and Lozano worked on the transition flight control of a Quad tilt-rotor from hover to level flight [93,99]. They have proposed, designed and simulated a nonlinear back-stepping approach for the attitude dynamics and tilting mechanism and nested saturation control approach for altitude and velocity control. In Ref. [99], they have completed the work in Ref. [93] by adding the pitch dynamics. Level to hover flight control and experimental verification are required in the next phase. Apart from that, a very unique work presented by Park and his colleagues in Ref. [100] shows a proposal for a fault control scheme for the actuator and sensor faults for the tilt-rotor UAV system. An LQR controller that makes the Euler angles track the desired command values was applied for actuator faults and a fault detection and isolation algorithm and a fault tolerant federated Kalman filter were presented for the sensor faults. Numerical simulations carried out on the linearized model in the airplane mode only showed the effectiveness and robustness of the scheme. However, simulations for others modes and experimental verification should follow.

unique features of the work given in Table 1. 3.1. Tilt-rotor Starting with the tilt-rotor hybrid UAVs, most of the works (e.g. [29, 89,90,101], for bi-rotor convertiplane [91,96], for tri-rotor convertiplane, and [93] for quad-rotor convertiplane) employ highly simplified motor dynamics and tilting mechanisms to minimize the complexity of the overall model. An exception that can be treated as a benchmark is the modeling work documented in Ref. [96], in which a fairly complete flight dynamics model for a bi-rotor convertiplane has been proposed. The propulsion system is modeled in depth by introducing additional coordinate systems (such as Nacelle axis system, hub-axis system, and blade axis system) and including the flapping motion of the propellers. Furthermore, the aerodynamics of the control surfaces and fuselage are carefully determined via variety of wind-tunnel experiments. Model validation in both time- and frequency-domains is presented and the results indicate the relative high fidelity of the proposed model. In another work documented in Ref. [94], the essential role of the wind tunnel usage in determining various aerodynamic coefficients is clearly demonstrated via both large amount of data and model validation results. Instead of using the experimental results collected in the wind-tunnel, the authors of [33,95] have explored the possibility of using CFD to determine the aerodynamic coefficients for a 0.15-scale MV-22 bi-rotor convertiplane and a custom-built tri-rotor convertiplane TURAC respectively. Validation results have also been presented in Ref. [95] to prove the efficiency of the CFD-based estimation. It is vital to mention that the use of wind tunnels, CFD techniques or any other method for aerodynamics modeling is subject to several factors such as project funding, particular interest, etc. Most of the works reviewed about tilt-rotors concerned the control systems of bi-rotors (e.g. Refs. [26,29,89,90,97,98]) since the bi-rotor is an underactuated system in hover mode where there are only four actuators for six degrees of freedom. In Ref. [29], which presents the experimental attitude control of UPAT tilt-rotor prototype shown in Fig. 3(e), the model was linearized about the hovering point and PID loops were utilized for controlling the system because of the simplicity in terms of implementation at high frequency rates using an embedded microprocessor and the requirement of high bandwidth and zero steady state error. It was noted in Ref. [29] that a Proportional-Derivative (PD) controller will lead to system stability as well but a simple Proportional (P) controller will not because of the inherent underactuated critically unstable dynamics. Moreover, to minimize noise from attitude rate measurements and improve the performance of the PID controller a Low-Pass filter with time being constant was convoluted with the derivative term in Ref. [29]. Experimental verification was done for the hovering mode but yet to be done for level flight and transition [26]. presents another work related to the control of a bi-rotor shown in Fig. 3(b) where the attitude control was done by an inner and an outer feedback control loop. The objective of the inner loop, which considers angular rates as the controlled variables, is to decouple the system and improve the frequency response and stability characteristics by adopting state feedback combining the compensable matrix, whereas, the outer loop, which targets the attitude control, focuses on the control quality of the controlled variables based on Proportional-Integral (PI) control. Experiment tests verified that the linearization trim control point was valid and the control law testing is yet to be done [26]. Following the work presented in Refs. [131] and [132] which demonstrate the concept design study and the flight control of a tilt duct UAV showing that the system is overactuated with redundant controls and is unstable in conversion mode, a State Dependent Riccati Equation (SDRE) control law, which is considered as an improved LQR, was studied for the implementation in the tilt duct UAV in Ref. [97]. The controller proposed is nonlinear and does not require the linearization of the system but it requires that the system remains controllable with the appropriate choice of the state factorization used. Tekinalp and his colleagues avoided this

3.2. Tilt-wing Compared with tilt-rotor hybrid UAVs, less interest in modeling and control of tilt-wing hybrid UAVs has been observed. Furthermore [47,49, 102], are based on an identical custom-built miniature tandem tilt-wing hybrid UAV and only one modeling work [39] on single tilt-wing hybrid UAV has been found. All the proposed models adopt Euler-angle expression, Newton-Euler formulation, and highly simplified motor dynamics and tilting mechanisms [39]. shows the control concept of a single wing tilt-wing hybrid UAV called AVIGLE shown in Fig. 5(b). Their control system contained three different controllers, one for the horizontal mode (consisting of P and PI sub-controllers), one for the vertical mode (consisting of P, PI and PID sub-controllers) and one for the transition mode where the horizontal speed controllers of the vertical mode sub-controllers are deactivated and the cruise control is assigned to the transition control. Also, a supervisor controller is proposed to decide the activation of the controllers and sub-controllers. Several simulation tests were carried out and the concept was satisfactory but no hardware test was performed yet. Regarding tandem tilt-wing UAVs, the control of SUAVI shown in Fig. 5(g) is demonstrated in Refs. [47,103] where [47] shows the simulation of LQR based position control for vertical flight and [103] shows the hierarchical control system design and vertical flight 100

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ducted-fan design code is employed to account for the unique duct fan feature of the custom-made MTT developed at the KAIST shown in Fig. 8(d) and a Navier-Stokes solver integrated into the FLUENT toolkit is used to determine aerodynamic control coefficients. In another two documentations [113,115] based on a miniature CTT hybrid UAV developed at BYU, aerodynamic coefficients are determined by maximally matching the flight test data collected in experiments. Furthermore [113], also addresses a technique of modeling the angular dynamics as a combination of one bias acceleration term and one actuator-based input term which aims at reducing the computational load of physical parameter estimation. Moreover [117,121], additionally considered motor dynamics aiming at covering the key dynamic features of the CTT proposed model. Validation results and analysis are again rarely addressed with the exception of [108,110]. In Ref. [108], identification results for the longitudinal motions are provided and in Ref. [110], a comparison between the model responses and actual experimental data is conducted where non-ignorable deviations have been observed for all channels which indicates that the model accuracy can be further enhanced. The transition is usually achieved by what is known as stall-andtumble maneuver or optimal trajectory. However, an alternative maneuver (Continuous ascent) was demonstrated by Refs. [71] and [72] for MTT tail-sitters where in Ref. [72] they developed a dynamic inversion control law and consistent simulations were done and in Ref. [71] they used those simulations to determine several design points for intermediate PID controllers which are linked using gain-scheduling. In addition to that, the development and application of an adaptive controller which can track the desired attitude despite the uncertainties and unexpected variations was presented. The designed controller was tested by the MTT tail-sitter prototype for hovering flight only. However, it was verified experimentally using a testbed tail-sitter UAV which entered level flight autonomously and then the transition controller was activated to make the transition from level flight to hover and back. Moreover, the control architecture of the V-Bat MTT tail-sitter shown in Fig. 8(c) is demonstrated in Ref. [107] where separate controllers are applied for each mode, namely, hover, level, level to hover and hover to level. One issue with their proposed controller is that their transition controller does not minimize the altitude error. Currently, their control system is being implemented in hardware. In Ref. [113], the complete flight envelope of the CTT tail-sitter is illustrated in which an adaptive quaternion controller using the back-stepping method was designed based on regularized date-weighted recursive least squares parameter estimation algorithm enabling the controller to adapt to the rapid changing dynamics. The control strategy was tested experimentally which makes this work considered as one of the benchmarks in the control of single-rotor CTT tail-sitters. Moreover, few research works [117,123,124], investigated the control of transition phase of bi-rotor CTT tail-sitters. More specifically [117], and [123] have developed separate controllers for different modes of flight (vertical, horizontal, horizontal to vertical, etc..) where they have set linearization operating points for each controller (LQR for [117] and PID for [123]) and simulated their control strategies successfully but the control laws were not implemented on any platform. A nonlinear control law was also studied in Ref. [117] and it was concluded that both, the linear and nonlinear control laws, were feasible for the control of the tail-sitter; however, this is without taking into account the wind disturbances, sensor noise and experimental implementation. Stone, the main developer of T-wing shown in Fig. 9(a), considered the control architecture of his tail-sitter by analyzing separate control strategies for vertical flight (using gain-scheduling LQR technique), horizontal flight (using classical SISO root-locus techniques) and transition flight (using the horizontal controllers as basis and changing the guidance technique) [124]. The T-wing performed fully autonomous vertical flight but the full envelope flight (including transition and horizontal flight) is yet to be done.

experiments implementing a PD controller, both where successful. Transition control and Horizontal control are yet to be done and verified experimentally. 3.3. Rotor-wing Only two research works [62], and [105,106], were found regarding the modeling and control of rotor-wing UAVs. Both implemented a simplified model of the rotor-wing dynamics implementing rotation matrices and Newton-Euler formulations [105,106]. presented and analyzed a combination of linear and nonlinear control methods for the hovering flight of the rotor-wing where the linear control (LQR) works when operating close to the equilibrium point and a Lyapunov approach based nonlinear controller was implemented to achieve global stability. No experimental tests were carried out to verify the model and flight control fidelity. In Ref. [62] which is employed for the hybrid UAV called THOR shown in Fig. 7, the cruise and hover modes were modeled separately and tests were conducted to verify the model using PID controller for the cruise mode and P controller for the hover mode. However, each test was carried out for a single mode, therefore, the full envelope model verification is yet to be done. Because of their uniqueness, these works can be considered as a benchmark in this sub-category; however, more accurate modeling and control techniques are to be further employed. 3.4. Mono Thrust Transitioning (MTT) and Collective Thrust Transitioning (CTT) A number of research works on MTTs and CTTs have been carried out and documented in the literature. Part of them only focuses on vertical flight mode and attitude stabilization. For instance [109,111,112,122], show the hovering control strategies for MTT tail-sitters whereas [118–120,125] focused on the hovering control and attitude stabilization of bi-rotor CTT tail-sitters. Although Newton-Euler formulation was commonly adopted but [118–120,125] which present the development of two types of bi-rotor CTTs considered Euler-Lagrange formulation for modeling their dynamics. As attitude stabilization in hover mode is the focus, Euler angle instead of quaternion is commonly used for more straightforward attitude representation. In Refs. [109,111,116,118], the system was linearized about the hovering point and PID controllers were used because of their good performance and easy implementation and regulation for attitude stabilization (roll, pitch and yaw). Given the difficulty of adjustment for big and complicated systems such as the MTT or CTT tail-sitters, several research works adopted modified control laws for attitude stabilization during hover flight. In Ref. [122], two control strategies, quaternion feedback control and resolved Tilt-Twist Angle Feedback control, were applied to the PID controller and experimental tests were carried out and each strategy was evaluated accordingly [112]. implemented a control law based on the sliding mode control technique which is applied to stabilize the decoupled attitude control systems [119]. demonstrated a control scheme based on saturations to achieve stability of the CTT by studying lateral, longitudinal and axial dynamics separately for simplicity. All works presented valid simulations for their control strategies; however, with the exception of [109,116,119,122], no other work verified their control design experimentally. The transition of tail-sitters from vertical flight to horizontal flight and back attracted researchers' interests as several works investigated the modeling and control methods of tail-sitters covering the full envelope; that is, hover, transition, and level flight. More specifically [71, 107–110], which concentrate on the modeling of MTT hybrid UAV, and [113–117,121], which concentrate on the modeling of CTT hybrid UAVs, adopt 1) quaternion formulation for avoiding the singularity in pitch angle expression and 2) simple expression for propulsion systems. In order to enhance the accuracy of the proposed model, additional effort has been made in some documented works, mainly on motor dynamics and aerodynamic coefficients determination. For instance, in Ref. [71], a 101

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Fig. 9. Representative examples of CTT tail-sitter UAVs.

Fig. 10. Representative examples of DTT tail-sitter UAVs.

the advantages, disadvantages and the corresponding modeling and control work of each type of hybrid UAVs. Table 2 highlights the advantages and disadvantages of the control laws adopted for hybrid UAVs and their implementation on each type. Throughout this survey, several key observations have been concluded as follows:

3.5. Differential Thrust Transitioning (DTT) For DTT, little work on dynamical modeling and control has been documented in the literature, as DTT-based UAV is still a relatively new topic to the academia and even less systematic research has yet been conducted. One representative work on DTT dynamics modeling is presented in Ref. [81], in which a quaternion-based Newton-Euler formulation model is proposed for the custom-made quadcopter tail-sitter named VertiKUL shown in Fig. 10(b). Detailed descriptions of the parameters selection and control approaches for VertiKUL followed where several components where modeled and the sensitivity of important parameters is analyzed [87,88]. For its control, PID controllers were adopted for the attitude, velocity and transition stabilization and a separate specialized controller was implemented for the heading of the vehicle. Flight tests and validations were illustrated leading to a remarkable progress on the advances of this subcategory [126]. is another work targeting DTT but it focused mainly on power consumption and energy efficiency instead of the flight control system (which is beyond the scope of this article).

 The concept of hybrid aerial vehicles has long been investigated by aeronautical industries for designing and manufacturing manned aircraft to enable a wider range of missions. Many of these trials were not successful. However, with the low-cost of the miniature UAVs, enhanced control techniques, and the thriving demand of the civilian and military markets, the concept has regained a huge interest and increasing popularity as the survey showed plenty of designs featuring the horizontal and vertical flight capabilities together. Moreover, many of the prototypes were successful and are documented in the literature and some others have been commercialized successfully. As an example, manned tail-sitters (i.e. such as the Convair XFY-1 [9] and Lockheed XFV-1 [10] shown in Fig. 1(e) and (f) respectively) did not achieve remarkable success and some projects in that regard were cancelled mainly because of the control and piloting difficulties. This problem is, of course, overcome by the utilization of unmanned tail-sitters which eliminates the need of the pilot and integrates advanced control techniques to ensure stability and safe transition to and from cruise flight. That is why the authors found several Tail-sitter prototypes, many of which were successful.

4. Key observations, existing challenges and concluding remarks A technical overview of the hybrid UAVs has been provided in this paper. The common platform design types were first categorized followed by explanation and representative examples of the flight modeling techniques and the flight control strategies implemented. Table 1 shows 102

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 The convertiplane hybrid UAVs were more dominant than the tailsitters. This is mainly due to the simple mechanism and control, smooth transition, and enhanced vertical flight stability of some subtypes of the convertiplanes. Furthermore, the most common sub-type among all the platform designs was the tilt-rotor convertiplane for which the authors found plenty of prototypes and, as mentioned earlier, many of which were successfully commercialized. This is mainly due to the enhanced controllability and stability, easy takeoff and landing and simple transition mechanism.  The majority of the current modeling work adopts highly simplified models, in which very fundamental motor dynamics and preliminary or even no aerodynamics for the propeller dynamics are contained. Such method mitigates the workload on the dynamics modeling and further enables researchers with little aerodynamics background to quickly proceed to the stage of flight control law design. However, very rare work has addressed the model validation of such simplified model in a systematic manner, which make the utilization of the simplified model questionable to a certain level. Furthermore, due to the oversimplification, the accuracy of the developed model has a high chance to decrease significantly, and may deteriorate the performance of the corresponding control law design.  The system identification approach, which has been widely utilized for deriving dynamic models for both fixed-wing and rotary aircraft, is rarely employed for hybrid UAV research. According to [133], a systematic integration of the system identification approach and the first-principles modeling (i.e., the method adopted by the current modeling work listed in Table 1) can generate a high-fidelity dynamics model for desired flight conditions or envelopes and can be reliably utilized in the subsequent control law design. The implementation of the aforementioned method on various hybrid UAVs is a promising trend.  Very rare work achieved the full autonomy of the hybrid UAV as most of the works only investigated a single flight mode. For instance, for the case of tail-sitters, some works addressed the hovering control only and for the case of all hybrid UAVs, most works concentrated on the transition phase only. Therefore, developing a control strategy for the full envelope including all the flight modes is rarely addressed and remains a promising trend.  The majority of the current control work did not verify the validity of their approaches experimentally. Although simulations using MATLAB mostly were common but the implementation of the control laws on a hardware (platform) were rarely addressed. Furthermore, this point is critical because when designing the model and control laws, many simplifications were carried out which means that the robustness, effectiveness and performance of the controller remains questionable.  Most of the works implemented simplified linear controllers applying different configuration of PID controllers. This is because of their simplicity and easiness of implementation. However, it is important to note that the performance of linear controllers such as PID deteriorates when operating away from the linearization point.  Few works demonstrated a comparison between the different linear and nonlinear controllers that can be implemented in different types of hybrid UAVs. A work that compares those linear and nonlinear controllers with software simulations and, more importantly, hardware implementations will be considered as pioneer in the field of UAVs in general and hybrid UAVs specifically since it might validate the possibility of many simplifications that are done whilst designing the control laws and strategies for simplicity purposes.

hybrid UAVs. Similar to the VTOL UAVs, the vertical mode of hybrid UAVs consumes a high amount of power to keep the UAV airborne. This discharges the batteries quickly and therefore reduces the endurance and range of the hybrid UAV in general. Consequently, some research works considered solar powered hybrid UAVs [134] and gasoline-electric hybrid propulsion system [135] as potential solutions to this issue. Power system optimization and experiments were conducted for the solar-powered tilt-wing UAV in Ref. [134] but successful flight tests are required to validate the concept. On the other hand, flight tests were carried for the gasoline-electric tilt-rotor UAV in Ref. [135]; however, further testing, modeling and control optimization is still to be done as well as a detailed analysis of the enhancement achieved by the novel hybrid propulsion system. Given their enhanced popularity and recently gained interest, the hybrid UAVs are becoming more mature in terms of many critical aspects such as design philosophy, dynamic modeling and control. As a result, it is expected that they will soon dominate the civilian as well as military applications. In fact, several research works already considered the application of hybrid UAVs for vision-aided tracking in Ref. [136], utilization in dense urban environments in Ref. [137] and for automated external defibrillator (AED) transport in Ref. [138]. Moreover, the DHL parcelcopter is being integrated for parcel delivery in mountainous regions [42]. To this end, the hybrid UAVs will have a bright future and will promptly be an essential pillar of the UAV market. The review presented in this paper is expected to be informative to the researchers who are interested in the promising hybrid UAV development. Acknowledgments The authors would like to thank Khalifa University of Science and Technology and Khalifa University Robotics Institute (KURI) for their continuous support and assistance. References [1] Uav roundup 2013, Aero. Am. 51 (7) (2013) 26–36. [2] C. Drubin, Uav market worth $ 8.3 b by 2018, Microw. J. (2013) 37. [3] J.T.K. Ping, A.E. Ling, T.J. Quan, C.Y. Dat, Generic unmanned aerial vehicle (uav) for civilian application-a feasibility assessment and market survey on civilian application for aerial imaging, in: Sustainable Utilization and Development in Engineering and Technology (STUDENT), 2012 IEEE Conference, 2012, pp. 289–294, https://doi.org/10.1109/STUDENT.2012.6408421. [4] B. I. For Society, Security, Unmanned Aircarft Systems for Civilian Missions, February 2012. [5] V-22 osprey, [Online, cited 5 December 2017] (August n.d.). URL http://www. boeing.com/defense/v-22-osprey/. [6] D.C. Dugan, Thrust control of vtol aircraft part deux, in: The 5th Decennial AHS Aeromechanics Specialists Conf, 2014. [7] B. Handy, Harrier Gr7, Royal Air Force Aircraft and Weapons, 2003, pp. 8–9. [8] J. Richmond, It's a Helicopter! It's a Plane, Military Aerospace Technology, High Technology, 1985, pp. 68–69. [9] U. S. N. C, Newsletter, Rollout Week, 2009. [10] Back to the drawing board: The lockheed xfv-1 salmon, [Online, cited 5 December 2017] (n.d.). URL http://www.military-history.org/articles/back-to-the-drawingboard-the-lockheed-xfv-1-salmon.htm. [11] Ling-temco-vought xc-142a tri service, [Online, cited 5 December 2017] (n.d.). URL http://www.aviastar.org/helicopters_eng/ling_xc-142.php. [12] Canadair cl-84 dynavert, [Online, cited 5 December 2017] (n.d.). URL http:// www.aviastar.org/helicopters_eng/canadair_dynavert.php. [13] Firefly6, [Online, cited 5 December 2017] (n.d.). URL http://www.birdseyeview. aero/products/firefly6. [14] Introducing the xplusone, [Online, cited 5 December 2017] (n.d.). URL http:// xcraft.io/. [15] V-bat, MartinUAV[cited 5 December 2017]. URL http://martinuav.com/productsv-bat/. [16] [Online, cited 5 December 2017], Bell Eagle Eye, 2017, https://en.wikipedia.org/ wiki/Bell_Eagle_Eye. [17] Vertol vz-2 (model 76), [Online, cited 5 December 2017] (n.d.). URL https:// airandspace.si.edu/collection-objects/vertol-vz-2-model-76. [18] [Online, cited 5 December 2017], Sikorsky S-72, 2017, https://en.wikipedia.org/ wiki/Sikorsky_S-72. [19] [Online, cited 5 December 2017], A Harrier Gr7 of 1 Squadron Raf Took Part in Deck Operations On-board Hms Illustrious, 2017, https://commons.wikimedia.

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