Design, experimental investigation, and nonlinear flight dynamics with atmospheric disturbances of a fixed-wing micro air vehicle

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Abstract

In this paper, the design process of a fixed-wing Micro Air Vehicle (MAV) is presented. The design cycle begins with mission requirements, followed by collecting statistical data from available MAVs. Next, MAV's sizing and weight estimation are conducted based on mission requirements. Finally, the aspect ratio, taper ratio, and airfoil selection are performed. As the MAV has a low aspect ratio and low Reynolds number airflow, the aerodynamics is not fully understood. The propeller wash increases the complexity of the aerodynamic flow; therefore, the MAV is manufactured for wind tunnel testing. The absent of actual flight performance and flight dynamics of fixed-wing MAVs are the main driven factors for this research. The wind tunnel experimental data are used in the flight performance equations to evaluate the MAV's actual flight performance and compare it with the estimated values. As MAV is very sensitive to the external disturbances which make the prediction of its dynamics a hard task, a nonlinear flight simulation model is created for the MAV's dynamics prediction. This model uses the experimentally measured data to predict the MAV's dynamics and its stability against wind disturbances. The results in this paper indicate that using aerodynamics historical data will give a huge error in estimating MAV's flight performance. The measured data in this research can be used to eliminate this error. Also, the flight dynamics of fixed-wing MAVs cannot be assumed linear, and the technique used in this paper gives a very accurate flight dynamics estimation.

Introduction

The first studies on fixed-wing MAVs were conducted in 1993 by DARPA corporation, suggesting that MAVs can be applied for military missions since they could gather important information and could be small enough to fit in soldiers' pockets [1]. In 1996 AeroVironment developed the first MAV project (Black Widow). Lockheed Martin also started developing the Micro-Star which became operational at an altitude of several hundred feet and had a flight endurance of 20 minutes [2]. The definition of MAV is an unmanned aerial vehicle (UAV) that its largest linear dimension is no more than six inches [3]. Another definition [4] states that the MAV is a UAV weighing less than 5 kg. Based on [5], MAV's weight is changing in the range between 50 gram to 2 kg. Fixed-wing MAVs, rotary wings, and helicopters have many applications in both civilian and military operations. These drones have been employed for photography, remote sensing, rescue operations, etc. [4], [5], [6], [7]. Sachs in 2016 discussed the power consumption comparison of fixed and flapping wing MAVs [8]. Based on this study, it has been shown that the flapping wing configuration can be considered as the best choice for small size drones and the fixed-wing configuration is more proper for larger ones.

Hassanalian et al. [9] presented a design methodology for fixed-wing MAVs, which starts with sizing process, including the mission requirements, planfrom and aspect ratio determination, constraint analysis, weight estimation, and airfoil selection. XFLR5 as an open-source software was used for computational fluid dynamics analysis, using a panel method [9]. XFLR5 software generally does not consider the high nonlinear aerodynamics of these micro drones and the propeller effects are ignored in the analysis. Brandt proposed a sizing and weight estimation techniques in [4], in which there is a strong interaction between sizing and weight estimation in order to improve the solution accuracy.

To compute the aerodynamic forces of MAVs, several techniques have been attempted. Simple vortex lattice methods have been tested in [10] and [11]; however, they are not accurate when compared to experiments. Vortex lattice Method cannot predict the separation of flow and considers that the lift increases linearly with the angle of attack which is not the case for low aspect ratios and low Reynolds number flows.

To measure the aerodynamic performance of MAVs, the experimental setup must be sensitive to small forces and well-calibrated. Methodologies have been described for accurately measuring the forces on a fixed-wing MAV in low Reynolds number flows [12], [13], [14], [15], [16], [17]. Roberts et al. in 2011 [14] combined a numerical model of a MAV with experimental results of the same MAV to produce a linear aircraft model that is parameterized with respect to turbulence intensity. Zhan et al. in 2006, proposed an iterative design process to generate several concept designs and the most promising concept design was fabricated and tested in a wind tunnel. Experimental results were used to further refine the design [16]. Torres and Mueller in 2000, presented a detailed design procedure for MAVs based on wind tunnel data [15]. In 2012, Babcock and Lind conducted a complete study for a MAV with an inverse Zimmermann planform, including an investigation of the i) the aerodynamic characteristics of the vehicle in the range of incidence and sideslip angles expected during its flight, ii) an understanding of the propeller effect on the aerodynamic data and iii) control surface effectiveness with incidence [12].

The previous studies indicate the importance of MAV and how it is difficult to predict its aerodynamic forces and moments, and due to its small weight, it would require accurate sizing and weight estimation. As an initial stage in this work, the design procedure of BlueBird MAV is illustrated. This design methodology can be summarized by the chart in Fig. 1. The design process is started by the mission requirements. In the mission requirements the payload, stall speed, cruise speed, turning speed, and endurance time are chosen based on the MAV's flight mission. The statistical data will follow the mission requirements. In this phase, different geometrical and aerodynamics data from the previous MAV projects are collected. By using this statistical data a constraint analysis based on the mission performance requirements is constructed. The selected design point in this constraint diagram will determine the wing loading and thrust loading of the MAV. The weight estimation phase could be started based on wing loading and thrust loading values. In the weight estimation design phase, the different MAVs' weights could be determined, such as total weight, empty weight, and battery weight.

The selection of aspect ratio and the tapered ratio will follow the weight estimation phase; these two parameters have a strong effect on the MAV's flying efficiency. Airfoil selection based on the Reynolds number and the required aerodynamics performance will take part in this project after the aspect ratio and tapered ratio selection. Finally, the manufacturing process of the fixed-wing MAV is carried out. A model with scale 1:1 will be built for wind tunnel and flight testing. Due to its aerodynamic complexity, the MAV will be tested in the wind tunnel to obtain the MAV's aerodynamic data, and this wind tunnel data will be used to evaluate the MAV's stability and flight performance. Some work of the current paper was presented in the conference papers [18], [19].

A Simulink model will be used to examine the dynamics of BlueBird MAV. The experimental data of the MAV will be incorporated in the Simulink model to examine longitudinal stability against atmospheric wind disturbances. Simulink was chosen for the following reasons; (1) It can incorporate the aerodynamic measurements directly in Simulink lookup tables; (2) It is integrated with the Matlab numerical solvers (ODE45); and (3) It has a virtual reality interface that can be used to visualize the MAV during its flight. The rest of this paper is organized as follows: In Section 2, the design process of the fixed-wing MAV is presented. In Section 3, the aerodynamics tests of the fixed-wing MAV through the wind tunnel are shown. The BlueBird MAV's stability and flight performance are discussed in sections 4 and 5, respectively. Finally, the summary and conclusions are shown in section 6.

Section snippets

Mission requirements

The starting point in an airplane design project is the mission requirements, and based on the mission requirements the MAV's geometry and power plant are determined. BlueBird MAV's flight mission is divided into four segments. The first segment is the hand launch from the starting point. Next, are the climb and the cruise segment, when the MAV will reach its defined elevation. The third segment is loitering, when the MAV will fly around the objective target. The last segment is to return to

BlueBird MAV wind tunnel aerodynamics testing

BlueBird MAV was tested in a calibrated Aerolab wind tunnel to measure its aerodynamic performance. This aerodynamics data were used to evaluate the stability and performance of the MAV. The Aerolab open circuit subsonic wind tunnel was used (see Fig. 6). The tunnel entrance is fitted with a honeycomb flow straightener and turbulence reducing screens. The wind tunnel is driven by a 10 HP motor that can generate a maximum speed of 30 m/s with a turbulence intensity of 2%, in the 0.47×0.47 m2

BlueBird MAV stability

One of the most important aspects of airplane design is stability. Stability predicts airplane behavior after external disturbance. If the airplane has a perfect aerodynamic performance but is unstable, then it is useless. Based on [28] stability is divided into two sections. First is the static stability which is the tendency of the airplane to return to its initial equilibrium position after an external disturbance acting on it. Second is the dynamic stability which is the time history of the

BlueBird MAV's flight performance

Flight performance is the last step in any airplane design process. The weight of the MAV and the aerodynamics data are now available, which can be used to estimate the performance of the MAV before the flight tests. Based on the trim analysis which was illustrated in the stability section, the trim flap deflection is −6.5°. As the measured aerodynamic data does not include this flap deflection, Matlab lookup tables are used to interpolate and extrapolate the measured data. The results for

Conclusion

BlueBird MAV was designed based on Brandt sizing and weight estimation techniques. Based on the MAV's actual weight after manufacturing there was about 10% error. The main source of error is due to using available electronics heavier than used from the literature. MAV's actual weight is 188 grams with all the servos and electronics included. The wing uses RG14 section for the airfoil. The wingspan is 27.69 cm, the wing chord is 21.66 cm, and the aspect ratio is 1.25. A flat plate vertical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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