Hover Performance Research Articles

Numerical simulation investigation on aerodynamic characteristics of rotor in plateau environment

To investigate the impact of high-altitude environments on helicopter rotor aerodynamic characteristics, this study developed a computational model that integrates CFD, BP neural networks, and the free wake method. The accuracy of the model was validated through wind tunnel experiments and comparisons with CFD results. Using this model, the effects of altitude and temperature at high altitudes on rotor aerodynamic characteristics during hover and forward flight were studied. The results indicate that with constant temperature, rotor thrust significantly decreases as altitude increases, primarily due to reduced air density. As altitude rises with unchanged temperature, the decrease in air density directly affects the lift and thrust generated by the rotor, leading to a decline in hover performance. Furthermore, the study found that rotor aerodynamic coefficients do not vary significantly across different altitudes. Further analysis suggests this stability is due to the high Reynolds number in the calculated state. In the high Reynolds number range, changes in Reynolds number have minimal impact on aerodynamic coefficients, resulting in relatively stable aerodynamic performance across varying altitudes. At a constant altitude, lower temperatures lead to slight increases in rotor thrust and torque, while the thrust coefficient and power coefficient remain almost unaffected. This phenomenon is attributed to changes in air density and viscosity caused by the lower temperatures. The increase in air density positively impacts thrust and torque, while the reduction in air viscosity decreases the rotor's surface frictional resistance. This partially offsets the increase in lift and drag caused by the higher air density, leading to negligible changes in thrust and power coefficients.

Low-Order Modeling of Stacked Rotor Performance in Hover

Coaxial, corotating (stacked) rotors have shown improved performance compared to conventional rotors, but the optimum configuration is highly sensitive to the rotor parameters. This paper reports the development of a low-order model, blade interaction prediction (BLIP), to efficiently predict the thrust and power of closely spaced rotor blades and quantify the effects of rotor geometry on total and individual stacked rotor performance. BLIP couples a vortex panel method and blade element momentum theory to combine the effects of bound circulation and rotor inflow. A cascade effect in the two-dimensional vortex panel method was implemented to incorporate the effect of airfoils rotating in a circle. Validation was performed with measurements of a 1.108-m-radius fixed-pitch stacked rotor with variable axial and azimuthal spacing, and excellent correlation was observed. It was found that small azimuthal angles and axial spacings are most effective in varying total thrust. The variation of thrust and power with azimuthal spacing was found to be a result of bound circulation, while the variation with axial spacing is due to rotor inflow interactions.

Aerodynamic Investigation of Shrouded Rotors with Dual Exit Channels

The growing need for rotary-wing aerial vehicles with high-speed forward movement and dependable hover performance is critical in various applications. Shrouded rotors enhance aerodynamic performance, create more overall thrust with the same power consumption as open rotors, and have a more uniform induced velocity flow field. This paper presents a parametric computational investigation centered on the hypothesis, that dividing the shroud exit channel into convergent inner and divergent outer channels would enhance flow uniformity, reducing power losses, and preventing airflow separation from the main shroud’s inner walls. The validity of this hypothesized concept is demonstrated through extensive Computational Fluid Dynamic (CFD) simulations. The paper includes a case analysis utilizing experimental data from a highly- maneuverable drone, named Navig8, equipped with a 9-inch shrouded propeller where various shrouded configurations are examined using Computational Fluid Dynamics. Results typically show an increase in total thrust with the incorporation of an inner shroud for a given power.

Aerodynamic characteristics simulation of helicopter rotor in hover using HeliX software

In order to predict hover performance of a new helicopter rotor, the aerodynamic performance and flow field of the rotor in hover is calculated by using HeliX software. The comparison between numerical results and wind tunnel test data of S-76 rotor as a validation case of CFD method demonstrate that HeliX software has the capability to consistently and reliably predict performance parameters for a helicopter rotor. The numerical results of the new rotor in hover suggest that the maximum hover efficiency of the new rotor is about 73% with collective pitch angle of 9.2°, while the shock induced separation and the reattachment phenomenon begin to appear on the upper surface near the blade tip. As the increase of collective pitch angle from 9.2° to 11.2°, hover efficiency decreases significantly due to the serious flow separation near the blade tip. The research in this paper can provide reference and guidance for aerodynamic optimization design of helicopter rotor.

Exploration of stacked rotor designs for aerodynamics in hover

The development of rotorcraft with multiple propulsors has increased interest in co-rotating coaxial rotors, also known as stacked rotors, which can generate high thrust while maintaining a compact aircraft design. By optimizing the pitch angle and the blade arrangement, stacked rotors can reduce aeroacoustic noise and vibration while improving aerodynamic performance. This study leveraged a high-fidelity numerical solver to analyze the aerodynamic performance and underlying physics of various stacked rotor configurations in hovering conditions. With three rotor design variables — index angle, stacked distance, and pitch angle difference — together with high-resolution simulations, two key physical phenomena were identified: the inflow effect and the blade-vortex interaction effect. Specifically, the blade-vortex interaction at the lower blade was found to play a critical role in improving hover performance by generating upwash, which increased the local power loading of the lower blade. Also, the unsteady simulations revealed that stacked rotors produced significantly lower peak-to-peak thrust values compared to counter-rotating coaxial rotors, indicating potential reductions in aeroacoustic noise level. Finally, a design optimization based on deep neural networks was performed to successfully extract design rules to take full advantage of inflow and blade-vortex interaction effects, which can be used as a guideline for the future design study of stacked rotors.

Experimental Investigation on Hover Performance of a Ducted Coaxial-Rotor UAV.

This paper presents experimental investigations on aerodynamic performance of a ducted coaxial-rotor system to evaluate its potential application as a small unmanned aerial vehicle (SUAV). Aimed at determining the influence of design parameters (rotor spacing, tip clearance and rotor position within the duct) on hover performance, a variety of systematic measurements for several correlative configurations (single/coaxial rotor with or without a duct) in terms of thrust and torque, as well as power, were conducted in an attempt to identify a better aerodynamic configuration. The experimental results for the coaxial-rotor system indicated that varying rotor spacing affected the thrust-sharing proportion between the two rotors, but this had no significant effect on the propulsive efficiency. The optimal H/R ratio was identified as being 0.40, due to a larger thrust and stronger stability in the case of identical rotation speeds. As for the ducted single-rotor configuration, the tip clearance played a dominant role in improving its thrust performance, especially for smaller gaps (δ≤0.015R), while the rotor position made subordinate contributions. The maximum performance was obtained with the rotor located at the P5 position (0.31Cd from the duct lip), which resulted in an enhancement of approximately 20% in power loading over the isolated single rotor. When the coaxial rotors were surrounded within the duct, the system thrust for a given power degraded with the increasing rotor spacing, which was mainly attributed to the upper rotor suffering from heavier leakage losses. And hence, the ducted coaxial-rotor system with S1 spacing had the best propulsion efficiency and hover performance with a figure of merit of 0.61.

Simulation of Rotorcraft Fuselage with Rotor Effects Using An Immersed Boundary Method

Computational fluid dynamics simulations of the flow around the ROtor Body INteraction (ROBIN)-mod7 fuselage with pressure-sensitive paint rotor are conducted using an immersed boundary method and an actuator surface model in OpenFOAM. The ROBIN-mod7 fuselage is represented by the immersed boundary method, while the unsteady rotor is modeled using the actuator surface model. A comprehensive analysis of the generic helicopter configuration is carried out for the hovering flight condition; the isolated fuselage is simulated to provide its baseline aerodynamics, and the isolated rotor and rotor–fuselage cases are studied to measure the rotor performance in hover and the fuselage effect on the performance. The validation of each test case is conducted against both experimental measurements and computational data from the literature. The surface pressure data from the isolated fuselage case shows good agreement with the experimental measurements. Also, the rotor performance predicted on the isolated and installed rotors (rotor–fuselage) has excellent agreement with the reference data; in particular, the performance data on the installed rotor agree with the experimental data better than the previous numerical study does. The fuselage effect has been analyzed by comparing the isolated rotor and rotor–fuselage datasets. The computational effort for different grid levels of each test case is provided. Overall, the results have demonstrated an equivalent level of accuracy compared to the previous high-fidelity simulation results at their fraction of setup and computational expenses.

Hover Dynamics and Flight Control of a UAM-Scale Quadcopter With Hybrid RPM and Collective Pitch Control

Hover analysis is performed on a 1200-lb gross weight UAM-scale quadcopter with both variable rotor speed and collective pitch control. With these redundant controls, the hover performance and flight dynamics are considered at three trim points, where power consumption can be increased to improve authority of the pitch inputs for changes in rotor thrust. An explicit model following control laws is optimized using CONDUITR to meet ADS-33E-PRF handling qualities specifications, with design margin optimization on each axis. The responses of the linearized system are examined with either control type, and pitch control is shown to outperform RPM-control in heave, while the opposite is true for yaw. Trim in axial climb is simulated, where the collective pitch can be scheduled with the climb rate to maintain effective stall margin. Hybrid control mixing is implemented using a complementary filter, allowing the aircraft to use pitch control for short-term responses and RPM control for trim. The benefits of this hybrid control scheme are demonstrated through simulation of hot/high/heavy conditions, where trimming with RPM control allows the pitch actuators to maintain margin for maneuvers. It is concluded that hybrid control allows the aircraft to reap the benefits of pitch control for maneuverability while maintaining stall margin by using RPM control for trim.

CFD-based pesticide selection for a nozzle used in a six-rotor UAV in hover mode for tea spraying.

A key challenge for unmanned aerial vehicle (UAV) spraying sometimes used in tea plantations is the downwash flow structure there stronger than in crops. In addition, the UAV spray is affected by the relationship between the nozzle design and the pesticide. However, there is little current research on this aspect. As a preliminary step this study focuses on the most appropriate pesticide for a designated nozzle in a six-rotor UAV according to the nozzle-pesticide relationship using a three-dimensional computational fluid dynamics model. This model considers the downwash flow structure effect and nozzle spray performance in hover. Nozzle FVP110-02, widely used in six-rotor UAVs, is used as a representative nozzle and bifenthrin and tea saponin water, commonly used in tea plantations, are used as the pesticides. The downwash flow structure of the six-rotor UAV in hover was conveniently controlled by the flight height and rotational speed, thereby causing the turbulence to be more stable. For nozzle FVP110-02, bifenthrin was more appropriate than tea saponin water at the same concentration, whilst bifenthrin and tea saponin water at a concentration of 1:1000 showed the best performance under identical working conditions. The numerical model developed here was shown to be effective for investigating the relationship between nozzle and pesticide. Our findings will help to not only improve UAV spraying for tea cultivation but also provide guidelines for pesticide selection in crops. Further work will address the comparison of the rigorous qualification of the numerical simulations with the measurements by the field test. © 2023 Society of Chemical Industry.

On genetic algorithm and artificial neural network combined optimization for a Mars rotorcraft blade

The Mars rotorcraft can help the Mars rover to optimize the driving route and explore more efficiently. However, the thin and cold Martian atmosphere can heavily increase the consumed power of the rotorcraft blades and reduce the generated thrust, leading to the reduction in power loading. The aerodynamic force generated by the Mars rotorcraft blade can be significantly influenced by the blade structure, which has several main structural characteristics and needs to be well designed. A multivariable optimization design method of blade structure is proposed based on neural network, genetic algorithm, and computational fluid dynamics simulations. Experiments conducted in the Martian atmosphere simulator indicate that the Mars rotorcraft blades with the hyperbolic chord length distribution and twist have higher power loading in the constant collective pitch angle. • A model was built to predict rotor hover performance at low-Reynolds-number. • Established an optimization model of the planform of the Mars rotorcraft blade. • The optimized blade consumes 29% less power than the conventional blade. • The optimization rusult were verified experimentally in Mars simulated environment.

Flight Performance with Respect to Rotor Rotation Directions of Multirotor Aircraft

A multirotor electric vertical takeoff and landing (EVTOL) aircraft is specialized for intracity point-to-point urban air mobility services due to its advantages of efficient hover performance, high gust resistance, and relatively low noise. Because of its multiple rotors, rotor–rotor interference inevitably affects flight performance, mainly depending on inter-rotor distance and rotor rotation directions. It is assumed that there exists an optimal rotation direction for a given lot of multiple rotors. In this study, the aim is to investigate rotor–rotor interference for various rotation directions and the resulting flight performance. We propose a concept of rotor rotation direction that achieves the desirable flight performance in forward flight. The concept is called front-retreating/rear-advancing (FRRA), in which the retreating side of the front rotor and the advancing side of the rear rotor are aligned in a straight line. To this end, a multidisciplinary flight simulation framework was developed to predict the flight performance of a general multirotor EVTOL aircraft. It was found that the FRRA concept minimizes thrust loss due to rotor–rotor interference in high-speed forward flight. For a generic mission profile, the rotation direction set by the FRRA concept reduces the battery energy consumption by 7% in comparison to the rotation direction of unfavorable rotor–rotor interference in operation.

ホバリングにおけるロータと主翼の空力干渉に対する主翼翼幅の影響

The effect of wingspan of the compound helicopter was experimentally investigated since the aerodynamic interaction between the rotor and wing affects the hovering performance of the compound helicopter. The thrust and torque of the rotor and the wing download were measured by varying the wingspan and vertical distance between the rotor and wing using a 1/14 scaled experimental model of the rotor and wing. The results show that the wing download increases as the wing span increases and it is maximum as the wingspan is 90% of the rotor diameter. Also, the wing download is maximum as the vertical distance between the rotor and the wing is 40% of the rotor radius.

Effects of flapping deviation on the hovering performance of tandem pitching-plunging foils

Dragonfly-like flapping wings and foils take an in-line arrangement and their performance is strongly affected by the wing–wing interaction. In this study of tandem pitching-plunging foils, the effects of flapping deviation on the hovering performance are studied numerically. A two-dimensional model is established, and the flapping deviation is represented by the non-dimensional deviation distance ΔL/c, the effects of which are investigated, together with those of the phase angle φ in plunging motion and the non-dimensional inter-foil spacing G/c. In the hovering status, our results support that the wing–wing interaction at ΔL/c = 0 is detrimental to the lifting capacity of tandem pitching-plunging foils. More importantly, we prove that a modification of ΔL/c in which the fore foil is below the hind foil during hovering can compensate for the loss of lift and may even provide a lift greater than that generated by a single foil. The mechanism of this compensation is dependent on the interaction between the wake vortices of the fore foil and the leading-edge vortex of the hind foil. Our results also indicate that the effects of ΔL/c and φ are coupled. The lift enhancement by a negative ΔL/c is maintained within the ranges φ = 45°–90° and G/c<2, beyond which an abrupt drop may occur. These findings suggest that the control of flapping deviation may enhance the performance of dragonfly-like micro air vehicles.

Analytical and Computational Study of Hover Performance of Novel Dissimilar Coaxial Rotor

This paper studies a novel dissimilar coaxial rotor concept with significantly reduced rotor–rotor aerodynamic interaction during hovering flight. The proposed concept’s performance is systematically compared with regular coaxial and conventional single rotor configurations using 1) analytical and 2) blade element momentum theory (BEMT)-based analysis for hover flight condition. The hover performance among different rotor configurations is compared using a baseline main rotor combined with various antitorque mechanisms. Through this study it is established that the dissimilar coaxial rotor configuration shows improved performance for torque equilibrium cases when compared to both conventional and regular coaxial configurations for moderate- to high-thrust conditions with power reduction of 16–37% when compared to a conventional single main rotor–tail rotor configuration and 14–17% of power saving when compared to a regular coaxial rotor during hovering flight with rotor thrust in the range of . The analytical method from momentum theory and BEMT results correlate well for the induced and profile torque balanced conditions. The proposed concept appears to be an efficient alternative to the conventional single main rotor with tail rotor and regular coaxial rotor for hovering flight conditions within the scope of the analysis.

Performance Optimization of Helicopter Rotor in Hover Based on CFD Simulation

An optimization method of helicopter rotor hovering performance is proposed, which is mainly carried out by means of CFD simulation. This method can be used to obtain the rotation speed of any blade model under the designed hovering tension through several iterations, and then, the tension, torque, power, and power load of the rotor under the rotation speed can be obtained. By comparing different blade pitch, radius, twist, and chord length, the blade parameters at the highest power load can be selected. A series of new rotor models are obtained by changing the model parameters on the basis of C-T rotor. NASA has carried out a lot of experiments on the C-T rotor model and published detailed experimental reports. The transient simulation is carried out by the embedded grid method.

A Study on Hover Performance of Ducted Fans for an Unmanned VTOL Aircraft

In this study, ground tests and numerical analysis were performed to verify the hover performance of the second ducted fan model designed by this research team. Shape of the duct inner surface was asymmetric in longitudinal direction, and considering various aspects such as test site or costs for test operation, a 40% scaled-down model of the ducted fan was adopted for this study. Both the results of the tests and the analysis were found to be coherent, and it was verified that the ducted fan was verified to be well designed to achieve the targeted performance. Furthermore, the performance of this ducted fan was compared with that of the symmetric one to determine the effects of the difference in the duct shape on the performance. It was observed that the symmetric ducted fan performs better in hovering flight, although the asymmetric ducted fan was sufficiently satisfied with the design intention.

A Dimensionality Reduction Approach in Helicopter Hover Performance Flight Testing

The power required to hover a helicopter is fundamental to any new or modified performance flight-testing effort. The conventional method of relating two nondimensional variables (coefficients of power and weight) is overly simplified and neglects compressibility effects in the power required to hover under a wide range of gross weights and atmospheric conditions. An alternative flight-test method for assessing hover performance while addressing this deficiency of the conventional method is proposed. The method uses an original list of 15 corrected variables derived from fundamental dimensional analysis, which is further reduced by means of dimensionality reduction to include only the most essential and effective predictors. The method is demonstrated using data of a Bell Jet-Ranger and shows that at the 95% confidence level; the averaged prediction error is only 0.9 hp (0.3% of the maximum continuous power). Using the same data, the conventional method yields a much larger averaged prediction error of 1.7 hp.

HOVER PERFORMANCE ANALYSIS OF COAXIAL MINI UNMANNED AERIAL VEHICLE FOR APPLICATIONS IN MOUNTAIN TERRAIN

Due to its compactness, agility, good hover performance, and ease of carriage, coaxial rotor Mini UAV is apt for various military and civilian applications in mountain terrain. This paper examines various factors to arrive at viable configurations of coaxial rotor Mini UAV for applications in mountain terrain. A consideration of the coaxial rotor Mini UAV to analyse the suitability for mountain terrain is presented. Coaxial rotor design is evaluated to assess the design requirements of mountain terrain. Various design parameters are analysed to arrive at viable design configurations for coaxial rotor Mini UAVs to operate in mountain terrain. Due to mechanical complexities, more than three blades per rotor for a small coaxial rotary wing aircraft is not recommended. The compact frame of the coaxial rotor Mini UAV is a key advantage, so rotor blades with a radius bigger than 1 m are not desirable. With a radius smaller than 1 m, a range of 0.9 m to 1.2 m, and an rotor speed between 900 RPM and 1200 RPM for 3-blade and 2-blade coaxial rotors, the Mini UAV offers a variety of options for applications in mountain terrain.

Study on the effect of transition process on rotor hovering simulation

Hovering is one of important statuses to evaluate the aerodynamic performance of a rotor. With the development of the computer technology and CFD technique, the numerical methods based on the first principle are usually employed to evaluate the hovering performance of the rotor. The transition process will evidently affect the results from the RANS-based numerical simulations in some steady cases for the fixed wing aircrafts, which should be taken into consideration in the design process. But it's not clear whether the transition process would affect the numerical results for the rotor simulation. To provide the reference in designing and evaluating the rotorcraft, the effect of the transition process in the rotor simulation needs to be discussed further. The PSP rotor proposed by NASA is calculated using the in-house solver based on the overset grid in this paper. Simulations are performed with fully turbulent model as well as the transitional model and the results are compared to the experimental data. The results prove the superior ability to simulate the flow around a hovering rotor of the in-house solver. The relative errors of the numerical results are under 5%. The range of the laminar flow on the blade is proportional to the rotor thrust, which causes a higher Figure of Merit in transition simulation than the fully turbulent simulation. The sectional pressure distribution and torque distribution along the blade apparently suffer from the transition process, which doesn't affect the thrust distribution along the blade and the blade vortex wake flow under the rotor disk. An obvious flow separation on the surface of the blade can be observed in the transition simulation compared to the fully turbulent simulation.

Multi-Objective Design of a Distributed Ducted Fan System

The distributed propulsion system applied to the vertical take-off and landing aircraft must maintain the high performance in both hover and cruise flight. The gap of the power unit has an adverse effect on the distributed ducted fan system, especially in cruise flight. Therefore, a new distributed ducted fan system was proposed, which eliminated the power gap in design, and adjusted the contraction and expansion of the wake through the deflectable induced wing arranged behind the ducted fan to ensure the high efficiency of the distributed ducted fan system in different flight phases. Then, a multi-objective design method of the distributed ducted fan system was proposed, and the feasibility of the design method was verified by designing the inlet and outlet of the duct and the induced wing. Design results show that the performance change of the distributed ducted fan system mainly came from the change of the inlet. By increasing the length and height of the inlet, the flow separation was alleviated and the duct thrust was increased in hove flight, but the cruise drag became larger. The increase of the inlet height made the operating point of the blade far away in hover and cruise flight, which increased the difficulty of the multi-objective design. Compared with the distributed ducted fan system composed of the traditional circular ducted fan, the hovering power load was reduced by 3.703%, but the cruise efficiency was increased by 17.372%, and the spanwise space was reduced by 20% in the final design.