# Calibration of a 3-Component External Balance for MAVs Wind Tunnel Research
## 1 Introduction  
A wind tunnel balance is a highly engineered and adaptable structural device specifically designed to measure aerodynamic loads on a scale model during wind tunnel testing. In their study, [PERSON] et al. [1] provide an extensive overview of these devices in Fluid Mechanics, offering insights into their types, components, and calibration techniques. These balances are typically divided into the following two primary categories: internal and external, based on their placement within the test setup. An internal balance is embedded within the model, while an external balance operates outside the model or test section [2]. External balances come in either multi-piece or one-piece configurations. The one-piece balance is tailored to measure aerodynamic loads in half-model tests [3; 4], whereas a multi-piece balance consists of separate force transducers linked together by a framework [5]. Positioned outside the model or test section, external balances can be optimized for various requirements, such as enhanced sensitivity, stiffness, and the decoupling of load interactions [6]. The accuracy and reliability of wind tunnel balance measurements are influenced by several factors. A well-thought-out design and meticulous calibration process are key elements in ensuring the device's performance. Selecting the most suitable balance type and tailoring its components to the specific testing requirements are fundamental to achieving accurate and repeatable measurements [7]. For example, the Institute of Aeronautics and Space in Brazil employs a specialized external balance equipped with six strain gauge load cells, which allows for precise measurements in both subsonic and transonic wind tunnel facilities. These balances are also widely used in full-scale automobile testing, highlighting their versatility and importance in a wide range of aerodynamic studies [8]. Another in-depth resource on wind tunnel balances and their applications can be found in [9; 10; 11; 12; 13]. [PERSON] [10] and [PERSON] [11] focus on the development of force balance systemsspecifically for educational wind tunnels. They use a design approach using flexible rods for measuring forces in wind tunnels that will enhance sensitivity and flexibility, making it suitable for educational purposes. [PERSON] [12] described the fabrication and testing of an external balance for measuring drag force in hypersonic shock tunnels. [PERSON] [13] investigated a load diagram technique for strain-gauge balances in wind tunnels, benefiting from the experimental and practical applications in aerodynamic testing. [PERSON] [14] investigated balance concepts specifically tailored for UAVs (Unmanned Aerial Vehicles), providing a detailed explanation of the calibration process and force measurements on a model designed and tested at INTA. These vehicles often experience low aerodynamic loads due to their small dimensions and low flight range, which is a challenge that becomes even more significant as the dimensions of these vehicles decrease, as seen in smaller UAVs and Micro Air Vehicles (MAVs) [15; 16]. In these cases, maximum aerodynamic loads are typically limited to 3-4 Newtons, for which commercial balances are generally unavailable. Standard balance systems lack the necessary sensitivity and precision to measure such small forces accurately, resulting in limited options for effective force measurement in MAV applications. Due to the absence of commercially available balances suitable for these low load ranges, it is essential to design, manufacture, and precisely calibrate a prototype. Such a prototype must be engineered with high sensitivity to reliably measure forces in this low range, ensuring accurate aerodynamic data collection for MAVs and other small-scale UAVs. This approach addresses the need for high-resolution force measurements in low-Reynolds-number environments, facilitating advancements in MAV performance evaluation and aerodynamic research.