Wind Tunnel Test for Videogrammetric Deformation Measurement of UAV for Mars Airplane Balloon Experiment-1 (MABE-1)

This paper reports the results of the aerodynamic deformation measurements of the meter-scale, entire shape, actual UAV in the wind tunnel using a video grammetry technique. The measured airplane was the airplane for Mars exploration being developed by Japan Aerospace Exploration Agency (JAXA) and Japanese universities. Its main wing span length was 2.4 m. The video grammetry measurement was performed using VICON’s system. Retroactive markers and stickers were put on the airplane. JAXA’s 6.5 m × 5.5 m Low-Speed Wind Tunnel was used. The airplane was mounted on the strut support with pitch-free or pitch-locked conditions. The deformations of the main wing bending, the main wing twisting, the tail boom bending, and the elevator deflection angle change were revealed quantitatively. The bending stiffness of a main wing spar that was designed as a safety factor of 2.8 at load factor of 5 was sufficient. The main wing spar was located around a center of pressure of an airfoil and it showed enough stiffness for twisting at nominal condition. The effects of the main wing bending and twisting, and the tail boom bending on the aerodynamic performance were estimated but they were in an acceptable range from the standpoint of the controllability of the aerodynamic performance using control surfaces. Even though the servo motor was located near the elevator and the linkage between the servo motor and the elevator was short, the measured elevator deflection angle was at most 4% smaller than the angle at no-wind condition. The obtained results and presented method are useful for control, flight data analysis, and design of lightweight airplanes. *A part of this work was presented at 15th International Conference Fluid Dynamics (ICFD15) inSendai, 7-9 Nov. 2018. How to cite this paper: Fujita, K., Oyama, A., Kubo, D., Kanazaki, M. and Nagai, H. (2019) Wind Tunnel Test for Videogrammetric Deformation Measurement of UAV for Mars Airplane Balloon Experiment-1 (MABE-1. Journal of Flow Control, Measurement & Visualization, 7, 87-100. https://doi.org/10.4236/jfcmv.2019.72007 Received: December 17, 2018 Accepted: March 6, 2019 Published: April 4, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Airplane for Mars exploration (Mars airplane) is a new Mars observation platform that enables wide-range observation from low altitude. Since 2010, the working group for Mars exploration aircraft has been working on a conceptual design and various fundamental researches of the Mars airplane [1] [2] [3] [4].
Flight tests are an important step of the airplane development to confirm the actual performance. However, the flight test of the Mars airplane has several difficulties. First difficulty is an unconventional flight environment. Atmospheric density on Mars is only about 1% of the density on Earth. In addition, average atmospheric temperature on Mars is about −40 degC. To simulate such flight environment, the Mars airplane has to conduct flight tests at high altitude atmosphere on Earth. The flight test altitude is around 35 km. A special balloon is used to ascend the airplane to that altitude. Therefore the flight test is expensive and the chance of the test is limited. Second difficulty is the cost of the test airplane. The test airplane is expensive because the airplane is not a mass production and it has special payloads and avionics for academic research. Third difficulty is an aerodynamic deformation. To fly in such rare atmosphere, the Mars airplane has to be lightweight. Therefore the amount of the deformation of the Mars airplane may be relatively large. Due to such reasons, even though the flight test of the Mars airplane is important and possible, it is highly difficult and expensive.
In a conventional development process of Unmanned Aerial Vehicles (UAVs) and Micro Aerial Vehicles (MAVs), to conduct a flight test is relatively easy because it can be conducted at low altitude atmosphere on Earth and the cost of the airplane is not so high. Therefore the flight test is able to be conducted without detailed investigation of the uncertainties such as an aerodynamic deformation, with accepting some risk of a failure due to the uncertainties. However, the flight test of the Mars airplane is neither easy nor low cost. In such circumstance, it is important to reveal the amount of the aerodynamic deformation before the flight test to increase a success rate of the flight test.
In this research, the aerodynamic deformation of the airplane for a high-altitude flight test named "Mars Airplane Balloon Experiment-1 (MABE-1)" [5] [6] [7] was directly measured in a large wind tunnel using a video grammetry technique. Then, the effects of the deformation on the aerodynamic performance were estimated using the wing element theory to evaluate the impact on the entire aerodynamic performance.
There are many articles about aerodynamic deformation measurement for wing models and MAVs using a wind tunnel [8] [9]. However, the measurement Journal of Flow Control, Measurement & Visualization of the meter-scale, entire shape, actual UAV is very few. Therefore the results obtained in this research are useful for not only the development of the Mars airplane but also the structural and aerodynamic designs of the light-structure airplanes that are expected large aerodynamic deformation. In addition, this research provides a valuable instance of the part of the development process of the airplane that is hard to conduct flight tests such as an expensive custom-ordered airplane for academic use, an airplane that cannot conduct many flight tests, an airplane for unconventional flight condition, an airplane that may deform largely, an airplane that has large uncertainty, and an unconventional airplane that has no flight experience.

Experimental Setup
This section describes a specification of a tested airplane, a wind tunnel, a video grammetry system, and experimental conditions. Table 1 and Figure 1 show the principal dimensions and the outline drawing of the airplane for MABE-1. Figure 2 shows a three-view drawing of the airplane. The fuselage was made by Carbon Fiber Reinforced Plastic (CFRP). Figure 3 shows main wing structure. The main wing structure consisted of a main spar, ribs, a trailing edge spar, and a skin. These members were also made by CFRP. The main spar was designed as a safety factor of 2.8 for bending at a load factor of 5. The height, the width, and the thickness of the main spar were 27, 20, and 1 mm, respectively. The thickness of the rib was 0.25 mm. The skin covered the rib from the leading edge to main spar, i.e. the skin formed a D-box structure. The main spar was located on a center of pressure of the airfoil at nominal flight condition; therefore, the twisting moment was expected to be small when the center of pressure was around the main spar. The fuselage and the tail were connected by a tail boom, as shown in Figure 4. The tail boom was made by two CFRP pipes. One was a main boom and another was an additional reinforcing boom. The second pipe was added based on a static load test conducted beforehand. The diameter and thickness of the both pipes were 13 mm and 0.5 mm. The tail structure consisted of a main spar, ribs, leading and trailing edges. These members were made by wood. The servo motor for the elevator actuation was DS189HV-ALM-KU (VACUUM) [10]. Its rated torque is 3.5 kgf•cm. The servo motor was located near the elevator to keep a length of a linkage short.

Wind Tunnel and Support
The 6.5 m × 5.5 m Low-Speed Wind Tunnel of JAXA was used. Figure 5 shows appearance of the test section. The type of the wind tunnel is a closed-circuit and continuous-atmospheric type. The size of the test section is 6.5 m high, 5.5 m wide, and octagonal with corner. The maximum and minimum wind speeds are 70 and 1 m/s, respectively. The airplane was supported using a strut, as shown in Figure 6. The airplane and the strut were connected using a rotational joint at the airplane's center of gravity. Therefore the airplane can rotate in a pitch direction.

Video Grammetry System
A position of each part of the airplane was measured using a motion capture system of VICON [11]. In this test, four infrared cameras were used, as shown in Figure 8. The image data was processed by a software NEXUS. Two types of the marker were used: a ball type (see Figure 9) and a sticker type. The diameters of the ball and the sticker were both 14 mm. The markers were made by retroreflective material.
The ball type is good for marker detection because the shape of the marker in the image is always a circle; however, the ball type may change the flow around the test model. In contrast, for the sticker type, the detection is relatively difficult but the effect on the flow is small. This time we first tried to use the sticker type for Journal of Flow Control, Measurement & Visualization

Experimental Conditions
The targets of the deformation measurement were the followings:   For the measurement of the main wing deformation, the rotational joint was unlocked and the AoA was changed by the elevator deflection. The elevator deflection angles were set to every 2.5 degrees from −2.5 degrees to 10.0 degrees. For the measurement of the tail boom and the elevator, the rotational joint was locked to 6 or 9 degrees. The elevator deflection angle was set from −20 degrees to 16 degrees.

Results and Discussion
The aerodynamic deformations of the main wing bending, the main wing twisting, the tail boom bending, and the elevator deflection angle change of the airplane for Mars exploration were measured in the wind tunnel using the video grammetry system. Each result and discussion is described in the following subsections. The error bars in figures show a standard deviation of each measurement result. This deviation includes effects of an accuracy of the video grammetry system and small oscillations of the UAV due to flow.

Conclusion
The aerodynamic deformations of the MABE-1 (actual UAV) were measured in the wind tunnel using the video grammetry technique. The measured deformations were the main wing bending, the main wing twisting, the tail boom bending, and the elevator deflection angle change. The main wing bending was linearly changed with AoA. The amount of the bending deformation was sufficiently small; therefore the main spar had enough strength by designing as the safety factor of 2.8 at the load factor of 5. The main wing twisting was rapidly increased when the AoA exceeded 10 degrees. To locate the main spar on the center of pressure was effective; however designer should consider not only a nominal condition but also off-nominal condition. The elevator deflection angle had a non-negligible effect to the tail boom bending even if the AoA was fixed.
Even though the servo motor was located near the elevator to shorten the linkage, the maximum difference between command and actual elevator deflection angle in flow was 4%. These deformations were obtained at the representative dynamic pressure at horizontal flight phase, 65 Pa. Note that a larger deformation is expected at higher dynamic pressure environment such as pull-up phase.
The airplane should be operated considering the effects of these deformations.