Abstract
In order to enhance the stability of the tilt transition process, a new configuration of Quad-Tiltrotor UAV was presented in this paper. Firstly, numerical simulation was used to calculate and analyze the aerodynamic interaction between the front rotor/fuselage/rear rotor during the transition state mode. The calculation model of the isolated rotor, front-rear rotor, front rotor-fuselage, and front rotor-rear rotor-fuselage combination states are established. Besides, the effects of pitch, roll, and yaw moment on the fuselage at different tilt angles are analyzed. It is concluded that the front rotor is the leading factor in the aerodynamic interference of the whole UAV in the different combination states. The research results can provide a reference for the optimization design of the overall layout, structure, and flight control strategy of the cross-shaped quad-tiltrotor UAV, and can also provide solutions for the logistics application of urban air traffic.
Keywords: Quad-Tiltrotor UAV, Urban air traffic, eVTOL, Tiltrotor, Flow field
Subject terms: Aerospace engineering, Mechanical engineering
Introduction
The tremendous growth in Unmanned Aerial System (UAS) technology has enabled its application to a wide range of military as well as civilian applications such as surveillance, surveying, crop monitoring, precision agriculture, cinema, and TV applications1. Several of these applications require the UAS to have hovering and vertical flight capabilities. Several innovative hybrid Vertical Take Off and Landing (VTOL) capable fixed wing type concepts have also been attempted, such as monoplane wing tail-sitter, biplane quadrotor, tail-sitter, tiltrotor, and tiltwing concepts2.
So far, tilt-rotor aircrafthave developed very well. A tiltrotor is an aircraft that combines the hovering capability of a helicopter with the high-speed cruising capability, just like a propeller-driven aircraft. It represents a concrete possibility to overcome the main limitations of helicopters and propeller aircraft by matching together the peculiarities of both of them. Therefore, tilt-rotor UAV (TRUAV) has great development prospects. As a novel aircraft, the TRUAV has potential applications with the merits of high-efficiency cruise of fixed-wing aircraft and vertical takeoff and landing(VTOL) of multi-rotor aircraft. In this paper, a new configuration of quad-tiltrotor UAV is proposed, which is a kind of TRUAV. It is interesting to note that the idea of a quad-tiltrotor was several decades old when Curtis-Wright X-19 was built as a full scale quad-tiltrotor VTOL aircraft in the early 1960s. The X-19 has high-mounted tandem wings with two propellers mounted on each wing which could be rotated through 90◦ allowing the aircraft to take off and land like a helicopter. Later, Bell developed its model D-322 as a quad-tiltrotor concept in 1979. We can see that the technology of constructing the conceptual model is according to the increasing demands and has reference value to some extent.
Most of the quad-tiltrotor UAVs that can be seen on the market are H-shaped3. The layout of the quad-tiltrotor UAV presented in this paper is cross-shaped (Fig. 1). Cross-shaped quad-tiltrotor UAV is an improved scheme to overcome the shortcomings of the H-shaped quad-tiltrotor UAV. A pair of rotors are added to the front and rear of the fuselage, using a step-by-step tilt strategy (the first step is to tilt the rotors of the left and right wings, the second step is to tilt the rotors of the front and rear fuselage). This strategy enhances the stability of the longitudinal plane of the UAV in the process of tilt transition and improves the flight safety. Compared with dual-tiltrotor UAV and H-shaped quad-tiltrotor UAV Cross-shaped quad-tiltrotor UAV contains not only aerodynamic interference between rotors and wings but also aerodynamic interference between rotors and fuselage. The downwash and wake generated by the front rotor of the fuselage will continuously impact the fuselage in the tilt transition state, thus affecting the pitch moment, rolling moment and yawing moment of the whole aircraft. Therefore, it is necessary to study the aerodynamic interference between the front rotor/fuselage/rear rotor of this configuration.
Figure 1.
Cross-shaped quad-tiltrotor UAV.
Flight conditions for quad-tiltrotor UAVs are categorized into three modes: hover, transition, and forward flight. While extensive research has been conducted on the hover and forward flight modes through both experimental and numerical approaches, the transition mode remains relatively understudied due to its complex aerodynamic interactions. Nevertheless, understanding the transition mode is crucial for the aerodynamic design and optimization of tiltrotors. Additionally, investigating the aerodynamic interactions of tiltrotor components during the transition mode is essential for informing the design of flight controllers4.
Currently, significant research has focused on the aerodynamic interactions between the rotor and wing of tiltrotor aircraft in hover and forward flight modes. Tests on the aerodynamic characteristics of a scaled V-22 model in hover were conducted at the Ames Research Center5,6. CHEN et al. Experimental and analytical analysis of rotor-wing interaction in hover for low Reynolds number flow7–9.
However, the complexity of aerodynamic interactions in transition modes increases compared to hover and forward flight modes. And there is limited literature on the subject. Hyeonsoo et al. numerically simulated the transient flow field of a tilting four-rotor aircraft and analyzed the aerodynamic interference between the front and rear rotors10. Sheng et al. numerically simulated the flow field interference of all components during the transition state of tilt-rotor aircraft11. Droandi et al. investigated the aerodynamic interaction between the rotor and tilting wing in hovering conditions and later studied the proprotor-wing aerodynamic interaction during the conversion from hover and forward flight12,13. Garcia and Barakos studied airloads on a model-scale Enhanced Rotorcraft Innovative Concept Achievement (ERICA) tiltrotor in three flight configurations using full computational fluid dynamics (CFD) simulations14. Zhenlong Wu et al. conducted numerical simulations of rotor-wing transient interaction for a tiltrotor in the transition mode15. Marcin fight analyzed the aerodynamic interference of rotor-wing with an H configuration quad-tiltrotor UAV16.
In summary, most researchers have studied the aerodynamic interference between the rotor and the wing, while few have studied the aerodynamic interference between the rotor and fuselage of the quad-tiltrotor UAV with the Cross-shaped configuration described in this paper. The tilting transition process is the most typical flight state of tiltrotor aircraft. In this stage, the cabin will tilt, the plane of the rotor disc will change from the horizontal state before tilting to the vertical state after tilting, and the rotor will produce strong aerodynamic interference with the fuselage. Therefore, it is of great significance to study the aerodynamic interference characteristics in this transition state for the analysis of the aerodynamic interference mechanism of the UAV with this configuration. In this paper, the CFD method is used to study the aerodynamic interference of the rotor/fuselage caused by the tilt angle under the five combined states of the front rotor, the fuselage, the front rotor-fuselage, the rear rotor-fuselage, and the front and rear rotor-fuselage.
Calculation model and method
The Cross-shaped quad-tiltrotor UAV described in this paper was shown in Fig. 2a. The geometric parameters are shown in Table 1. The geometric structure model constructed by CATIA V5 R20 software (https://www.technia.com/support/software-downloads/) is shown in Fig. 2b to d. The model for numerical calculation is shown in Fig. 2e. The governing equation used in this paper is the incompressible viscous N-S equation, the turbulence model is the k-epsilon model, the coupling of pressure and velocity is the SIMPLE algorithm, the convection term is discretized by second-order upwind method, the whole flow field is calculated by finite volume method, the flow field is defined and meshed by ANSYS ICEM software (ANSYS workbench 2019 R1, https://www.ansys.com/products/), and the commercial software ANSYS FLUENT(ANSYS workbench 2019 R1, https://www.ansys.com/products/) is used for numerical calculation. Figure 2f shows the calculation domain, where the distance between front and back is 103.45c, up and down 68.97c, left and right 82.76c, and the distance between the front end of the UAV and the inlet is 34.48c.The shape of the airframe (fuselage, wing, tail wing, etc.) is regular, and the structural mesh is divided (Fig. 2g–i). The sliding mesh is used in the rotor flow field (Fig. 2j,k). Specific partitioning methods are as follows: the geometric model of the rotor, fuselage, wing, tail, and the far field was composed, and the mesh was divided separately. The thickness of the first layer of the hexahedral mesh near the rotor wall is set to 0.1 mm, and the height of the first layer of the mesh of other components is set to 0.3 mm. This makes it easier to capture turbulence near the walls. Not only the high-precision mesh partition, but also the time step and the number of iterations are one of the factors that affect the calculation accuracy. Through calculation attempts, the time step is set to 1/1440 of the time it takes for the rotor to rotate once, and the lift was basically stable and does not change when the rotor rotates 20 times. grid independency studies were also conducted with three successive grid: coarse, medium, and fine grid. The coarse grid has 8.591 million elements. The medium grid has 15.521 million elements.The fine grid has 31.117 million elements. The study has shown that differences in results from the coarse and medium meshes are significant, while differences in results between the medium and fine meshes are almost negligible. Hence, it was decided to use the medium mesh as the baseline for further simulation.
Figure 2.
Introduction of Cross-shaped quad-tiltrotor UAV.
Table 1.
The geometric parameters of cross-shaped quad-tiltrotor UAV.
| Parameters | Value |
|---|---|
| Fuselage length (m) | 2.04 |
| Wingspan (m) | 2.35 |
| Wing chord length (m) | 0.29 |
| Wing airfoil | SAUTER1 |
| Wing area (m2) | 0.625 |
| Rotor type | T-motor 12 × 6 |
| Tail wing airfoil | NACA0012 |
| Tail wing length | 0.865 |
Using the numerical simulation method mentioned above, the pressure distribution on the upper and lower surfaces of the rotor is calculated (Fig. 3a). It can be seen in the figure that there is a low pressure zone near the tip of the upper wing of the rotor, and the pressure value gradually increases from the tip to the root of the rotor. On the contrary, there is a high pressure area on the lower wing surface. The high pressure area was near the tip of the propeller, and the pressure value from the tip to the root of the propeller decreases gradually. Figure 3b shows the pressure cloud and streamline distribution of the rotor cross-section. It can be seen that the upper part of the rotor is high pressure zone, and the lower part is low pressure zone, which accords with the flow field characteristics of the rotor. In order to verify the validity of the numerical calculation method, a test platform for verifying the thrust and power of the rotor was constructed (Fig. 3c). A rotor with the same geometric size as the numerical calculation is used as the test object. The measured thrust and power values are shown in Fig. 3d. It can be seen that the calculated values agree well with the experimental values at low rotational speed. As the speed increases, the relative value of thrust begins to deviate from the experimental value, although the error is still small, but there are many reasons for this error. Firstly, the establishment of the geometric model itself has certain errors. Secondly, the numerical calculation method also has errors, mainly because when the same grid is used to calculate different speeds, the same grid cannot make the numerical simulation results consistent at all speeds. Finally, the measuring sensor also has errors. It can be seen from Fig. 3d that the rotational speed point with the smallest calculation error is around 2000 rpm, and the calculated values at all rotational speeds are within the acceptable range (error value < 5%). According to the actual flight test, the rotor speed is 2000 rpm when hovering, and the maximum flight speed in fixed-wing mode is 20 m/s. The flight height remains unchanged during the tilt transition, and the forward flight speed changes.
Figure 3.
Rotor test and numerical verification.
Calculation results and analysis
Aerodynamic effects on the front rotor
Figure 4 shows the variation curve of the relative thrust and torque of the front rotor with the tilt angle in the combined states of the isolated front rotor, the front and rear rotor, the front rotor-fuselage, and the front and rear rotor-fuselage. It can be seen from Fig. 4a that the thrust of the front rotor decreases with the increase of the tilt angle during the tilting process. This is mainly because with the tilt of the rotor disc, the axial inflow velocity of the rotor disc will gradually increase. The increased value of the axial inflow velocity of the rotor disc will decrease with the increase of the tilt angle of the rotor disc. Analyze the combined state of the front and rear rotors: Because the presence of the rear rotor has little influence on the upstream flow field of the front rotor, the rear rotor has no influence on the thrust of the front rotor. Considering the slipstream theory, it is shown that the influence of the rear rotor on the curl and velocity of the upstream flow field is very small. Analyzing the combined state of the front rotor and fuselage, it can be seen that the presence of the fuselage increases the thrust of the front rotor relative to the first state, because the downwash of the rotor disk will directly impact the position of the fuselage head (Fig. 5). This results in a reduction in the induced speed at the rotor disc, which in turn increases the effective Angle of attack and the thrust generated by the rotor. The change of the front rotor thrust in the front and rear rotor-fuselage combination is basically the same as that in the front rotor-fuselage combination.
Figure 4.
The change curve of the thrust and torque of the front rotor with the angle of the rotor disc.
Figure 5.
The flow field of the front rotor- fuselage.
As can be seen from Fig. 4b, the rotor torque does not change much with the tilt of the rotor disc, but it increases with the presence of the fuselage. The torque variation of the front rotor is basically the same when a single front rotor is combined with a front and rear rotor. When the tilt angle is large, the small increase of the torque is mainly due to the combined effect of the axial velocity and the induced velocity of the rotor disk. The change of the front rotor's torque in the combined state of the front rotor-fuselage and the front and rear rotor-fuselage is basically the same as that of the single front rotor. In addition, due to the interference of the fuselage on the flow field behind the fuselage, the induced velocity at the rotor disc decreases, and the effective power increases.
Aerodynamic effects on the rear rotor
Figure 6 shows the variation curve of the thrust of the rear rotor with the tilt angle in the combined states of the isolated rear rotor, the front and rear rotors, the rear rotors-fuselage, and the front and rear rotors-fuselage. Generally speaking, with the increase of the tilt angle of the rotor disc, the thrust of the rear rotor decreases gradually, and its decreasing trend is the same as that of the front rotor. As can be seen from Fig. 7, there are two main reasons for the influence of the front rotor on the rear rotor thrust.
The wake disturbance mainly occurs when the tilt angle was less than 45 degrees (Fig. 7a). When the tilt angle of the current rear rotor is small, the wake of the front rotor has been falling off continuously, and it escapes backward under the effect of incoming flow. A large number of tip vortices that fall off from the front rotor will hit the plane of the rear rotor disc, causing the tension of the rear rotor to decrease.
Downwash disturbance: According to the slip flow theory, when the tilt Angle is greater than 45 degrees (Fig. 7b), the rear rotor is in the slip flow area of the front rotor, resulting in an increase in the flow velocity of the rear rotor and a decrease in thrust.
Figure 6.
Relative thrust and velocity curves of rear rotor at different tilt angle.
Figure 7.
The interference of the front rotor- fuselage on the rear rotor.
The influence of the fuselage on the rear rotor thrust is also related to the tilt angle. when it is greater than 25°, the influence of the fuselage on the aerodynamic force of the rear rotor is small. This is mainly due to the disturbance of the fuselage wake region on the flow field of the rear rotor, while the influence of the fuselage wake region on the axial flow velocity of the rotor disc is not significant, which indicates that the interference of the fuselage on the thrust of the rear rotor is limited. When the tilt angle is large, the wake region behind the fuselage will disturb the flow field at the rear rotor (Fig. 7c). The axial inflow velocity of the rear rotor disc will directly affect the thrust of the rear rotor, thus increasing the thrust of the rear rotor. The disturbance of the front rotor-fuselage combination state to the rear rotor pull is similar to that of the front rotor. When the tilt angle was small, the wake of the front rotor is scattered by the fuselage due to the obstruction of the fuselage, which has little influence on the thrust of the rear rotor. When the tilt angle is large, the wake of the fuselage and the downwash flow of the front rotor will flow to the rear rotor, resulting in a decrease in the thrust of the rear rotor.
Aerodynamic effects on the fuselage
The influence on lift and drag of fuselage
Figure 8a shows the variation curve of the relative lift of the fuselage with the tilt angle in the combined states of the isolated fuselage, front rotor-fuselage, rear rotor-fuselage and front rotor-rear rotor-fuselage. It can be seen from the figure that the lift varies greatly under the two states of the front rotor-fuselage and the front and rear rotor-fuselage. Analysis of the combination state of the rear rotor-fuselage, the lift of the fuselage varies little. Figure 8b shows the change curve of the fuselage relative drag. It can be seen from the figure that with the tilt of the rotor disc, the influence of the rear rotor on the fuselage drag is not great. The interference of the front rotor on the fuselage drag is relatively large. And with the increase of the tilt angle, the fuselage drag increases gradually.
Figure 8.
The change curve of the relative lift and drag of the fuselage with the angle of the rotor disc.
From Fig. 9, it can be seen that with the increase of the tilt angle, the interference of the front rotor on the fuselage lift is more obvious. Overall, with the tilt of the rotor, the lift of the fuselage changes from negative to positive due to the impact of downwash flow on the fuselage. According to the slipstream theory, the downwash flow of the front rotor will also change. When the tilt angle is small, the area of the downwash flow sweeping the fuselage is smaller, but the impact force per unit area is larger. With the increase of the tilt angle, the area of the downwash flow from the rotor to the fuselage gradually increases, but the impact force per unit area also decreases. The influence of the front and rear rotors on the lift of the fuselage is basically the same as that of the single front rotors on the lift of the fuselage.
Figure 9.
High pressure zone change of body during tilting.
When the tilt angle is small, a small part of the downwash flow of the front rotor hits the fuselage, which increases the drag of the fuselage, while the other part downwash flows downward and backward, which reduces the drag of the fuselage. When the tilt angle is large, a large part of the downwash flow of the front rotor will directly hit the fuselage, which will increase the velocity of the flow field in the nose of the fuselage, and then increase the drag of the fuselage. The interference of the front and rear rotors on the fuselage drag is basically the same as that of the front rotors. In addition, the difference in fuselage drag between "front rotor-fuselage" and "front and rear rotors-fuselage" is caused by the disturbance of the rear rotor to the fuselage flow field, but this disturbance has little effect on the fuselage drag and can be ignored Excluding.
The influence on moment of fuselage
Figure 10 shows the change curve of the relative pitch moment of the fuselage with the tilt angle. It can be seen that the pitch moment of the fuselage changes slightly in the combined state of the rear rotor-fuselage during the tilting process. The pitch moment of the fuselage varies greatly in the combined state of the front-rotor-fuselage and front-rear-rotor-fuselage. This is mainly due to the interference of the front rotor flow field on the fuselage. The aerodynamic interference of the rear rotor to the fuselage is relatively small, mainly because the flow field of the rear rotor does not flow through the fuselage. The pitch moment of the fuselage increases first and then decreases with the change of the tilt angle due to the interference of the front rotor to the fuselage. With the increase of the tilt angle, the downwash flow of the front rotor will sweep the fuselage. When the downwash flow reaches the nose, the downwash flow will generate a downward load on the fuselage and the downwash moment at the same time. At this stage, the area of downwash flow to the fuselage will gradually increase, so the downwash moment will gradually increase. As the tilt angle continues to increase, the downwash flow of the propeller disc gradually approaches the horizontal and uniform flow to the nose. The downwash flow field of the rotor is gradually horizontal to the fuselage, and the main drag, not the pitch moment, is produced in the fuselage. Therefore, in the second half of the turning process, the pitching moment of the fuselage will gradually decrease. The pitch moment of the fuselage is similar to the disturbance mentioned above when the combined state of the front and rear rotors is analyzed.
Figure 10.
Curves of fuselage relative pitch moments with tilt angle.
Figure 11 shows the change curve of the relative roll moment of the fuselage with the tilt angle. It can be seen from the figure that the effect of the rear rotor on the roll moment of the fuselage is not significant. Analysis of combination states of the front rotor-fuselage and front and rear rotor-fuselage. With the tilt of the rotor disc, the change of the roll moment of the fuselage is more obvious, which shows the trend of increasing first and then decreasing. The influence of the rear rotor on the roll moment of the fuselage is not obvious, mainly because the rear rotor does not disturb the flow field around the fuselage. The influence of the front rotor on the roll moment of the fuselage tends to increase first and then decrease. This is because when the tilt angle increases gradually, the downwash flow of the front rotor flows to the left front of the nose, which causes the body to roll to the left. There is also a high pressure area in front of the left nose, which causes the rolling moment to increase first and then decrease. This is only due to the direction of rotation of the front rotor. The influence of the combined state of front and rear rotors on the roll moment of the fuselage is analyzed. It is mainly caused by the aerodynamic interference of the front rotor to the fuselage, while the interference of the rear rotor to the fuselage is relatively small.
Figure 11.
Curves of fuselage relative roll moments with tilt angle.
Figure 12 shows the change curve of the relative yaw moment of the fuselage with the tilt angle. The influence of the rear rotor on the yaw moment of the fuselage is not significant. The influence of the front rotor on the yaw moment of the fuselage increases first and then decreases during the tilt process. The influence of the front rotor on the yaw moment of the fuselage is the main factor. Under the combined action of forward rotor tilt and forward incoming flow, the rotor will disturb the flow field and makes the airflow from both sides of the fuselage to the rear of the fuselage (Fig. 13a,b). The pressure difference is formed on both sides of the fuselage, which generates the yaw moment of the fuselage.
Figure 12.
Curves of fuselage relative yaw moments with tilt angle.
Figure 13.
Interference of front rotor flow flied on fuselage yaw torque.
Analysis of Rotor/Wing aerodynamic interference
Interference of rotor to wing lift
It can be seen from Fig. 14a that the rotor downwash reduces the lift of the wing. With the increase of the tilt angle, the wing lift gradually returns to normal. When the tilt Angle is small (Fig. 14b) and the rotor disk has tilted to a certain Angle, the area of the rotor downwash impacting the wing will increase significantly compared with the vertical flight state, resulting in a significant decrease in wing lift.With the increase of tilt angle (Fig. 14c), the aerodynamic interference of rotor downwash flow on wing lift decreases gradually. When the rotor tilts to 85°, the wing lift returns to normal. The existence of the rotor disrupts the flow field environment on the upper surface of the wing (Fig. 14d–f).
Figure 14.
Rotor/Wing aerodynamic interference.
The disturbance of rotor to wing pitch and roll moment
It can be seen from Fig. 15a that the wing produces bow moment without rotor interference. When there is rotor interference, the pitch moment generated by the wing changes with the increase of the rotor tilt angle, which may be mainly caused by the rotor downwash impinging on the wing and the downwash disturbing the flow field on the wing surface. When the tilt angle increases to close to level flight, the rotor's interference with the wing's pitch moment decreases gradually. It can be seen from Fig. 15b that the wing produces a roll moment without rotor interference. The interference of rotor to roll moment mainly comes from two aspects. On the one hand, when the rotor tilt Angle is small, the downwash flow of the rotor will impact the wing, resulting in downward load on the wing and reducing the rolling moment of the wing. On the other hand, the presence of the rotor will disturb the flow field of the wing surface, resulting in the reduction of the pressure difference between the upper and lower surfaces of the wing, thus reducing the lift force of the wing and the rolling moment of the wing.
Figure 15.
Disturbance of rotor to wing pitch and roll moment.
Conclusion and discussion
Aerodynamic interactions between front rotor, fuselage, rear rotor of quad-tiltrotor UAV with cross configuration were numerically simulated in this study. The following concise and insightful conclusions are drawn:
The impact of front and rear rotor-fuselage configurations on thrust and torque align with previous analyses. Negligible aerodynamic interference of rear rotor on front rotor was observed.
Front rotors exert greater influence on lift and drag of fuselage compared to rear rotors in combined state analysis. Minimal aerodynamic interference of rear rotors on fuselage was noted.
Front rotor significantly influences pitch, roll, and yaw moments of fuselage in combined rotor-fuselage states, highlighting its predominant role.
Design considerations for tilting UAV with this configuration should prioritize rotor selection and layout to mitigate or leverage aerodynamic interference on rotor/fuselage aerodynamic characteristics. Further analysis can be conducted on the impact of dynamic tilting transition time on aerodynamic interference.In addition, the role of tiltrotor aerodynamic interference in the design of tiltrotor eVTOL aircraft is multifaceted and pivotal in shaping the performance, efficiency, safety, and operational characteristics of these advanced vertical takeoff and landing platforms. Efforts to mitigate and leverage aerodynamic interference effects can lead to more optimal and capable tiltrotor aircraft designs for the future of urban air mobility.
Acknowledgements
Project funded by China Postdoctoral Science Foundation [Grant No. 2018M642241] and National Key Laboratory of Rotorcraft Aeromechanics. Thanks to Wang Ce, Huang Kai, Xu Pei and Tang Zhengfei for sorting out the relevant data in this paper.
Author contributions
Siliang Du and Yi Zha. wrote the main manuscript text and prepared all figures . All authors reviewed the manuscript.
Data availability
The data used to support the findings of this study are available from the corresponding author upon request.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher's note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Data Availability Statement
The data used to support the findings of this study are available from the corresponding author upon request.