**1. Introduction**

In order to follow the modern trend of using lightweight materials and optimized product design, it is necessary to focus on the individual construction elements and dimension them exactly according to the expected requirements. Nowadays, proven designs of parachute harnesses are commonly used, which do not differ greatly in the materials and construction elements used. [1]. The effective sizing of individual elements is only possible when detailed information about their loading is available. This is the aim of this research, which in principle, can be divided into three main stages. The first stage is to obtain the opening shock force, which characterizes the aerodynamic parameters of the canopy. This force is then applied to the harness worn by a dummy fixed in different positions. The final stage is to identify the decomposition of this total force into the individual structural elements. This information is then used for evaluating the load capacity of the elements and to gain insight into the possibility of subsequent optimization of the structure. In the results section of this research, it is shown that the safety margins of the separate construction elements differ significantly. This suggests a unification of the safety coefficients of the

**Citation:** Grim, R.; Popela, R.; Jebáˇcek, I.; Horák, M.; Šplíchal, J. Determination of the Parachute Harness Critical Load Based on Load Distribution into Individual Straps with Respect of the Skydiver's Body Position. *Aerospace* **2023**, *10*, 83. https://doi.org/10.3390/ aerospace10010083

Academic Editors: Spiros Pantelakis, Andreas Strohmayer and Jordi Pons-Prats

Received: 22 November 2022 Revised: 5 January 2023 Accepted: 11 January 2023 Published: 14 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

individual components when the maximum required load is reached. The outcome would be a lighter overall structure. However, the intention is not just to analyze the case for one specific loading force representing a particular canopy during its opening shock load test. The objective is to obtain a percentage value of the total force that will be transmitted to each node and to prove or disprove that this redistribution is constant over a certain range of loading. Expressing this dependence would imply the possibility of applying the presented results without restriction on the magnitude of the total force, in other words, for arbitrarily chosen canopies and activation parameters.

Nowadays, structural overload tests, which include the entire sequence of canopy activation, are considered standard [2,3]. The procedure is performed based on the Technical Standard 135 published by the Parachute Industry Association (PIA) [4]. These types of tests evaluate whether a harness shows signs of mechanical damage after activation under specific conditions. Capture 4.3.6, named "Structural Overload Tests", defines the general conditions for a drop test that the complete parachute assembly or separate components must withstand. This approach has a major disadvantage for the proposed force redistribution analysis, since dropping a dummy from an airplane or helicopter does not guarantee an exact position when the rescue system is activated. As will be shown in the results section of this document, not only does the opening shock load play a major role but also the direction of it does. In fact, the proposed test methodology is a combination of dynamic and static tests that are primarily used for the certification of paragliding harnesses and work at heights for safety harnesses [5–7]. They have a common main feature, namely, the fact that both force and position conditions are precisely defined.

The possibility to divide the parachute assembly test into separate tests of individual components allows one to obtain the dynamic opening force of the canopy first, and then apply this force to the harness as a static load. This makes it possible to position the dummy in the different setups and evaluate the required influence of the skydiver's body position, asymmetry during parachute activation, and the tightening or loosening of each strap.

To achieve the above goals, the following tasks were completed. The first step was the development of the drop test laboratory and methodology. The most important factor was reaching the activation speed exactly according to the specification. This was achieved using a real-time measurement of the speed together with a backup timer. Data logging of the forces in the connection between the parachute and ballast is performed at a frequency of 200 Hz. The opening force is recorded by measuring the carabiners, so it is possible to reach force on both attachment points. A similar logging of the force has been presented in this publication [8].

The next challenging step was to measure the force in flexible straps. Few studies have focused on the measuring tension in flexible structures, such as parachute fabric, but for the purpose of the proposed aim, this could not be used [9–11]. The requirement for strain gauges developed, especially for the purpose of this research, was that they be versatile enough to be used regardless of the exact type of strap on with which they were installed. The second important goal was to minimize the influence of structural rigidity. Preferably, the measuring components were made of parts from which the harness itself is assembled. The main advantage of this research is that it provides a very detailed analysis of the distribution of forces in the individual parts of the harness. By achieving dimensionally small load cells, it was possible to install the strain gauges in all the necessary places so that a load from the individual nodes was captured during one harness load cycle. In order to obtain comprehensive data, the different dummy positions that may occur when the rescue system is activated were also investigated.

Information about the forces in the individual parts of the harness gives the possibility to analyze the dimensioning of each part according to the required load. Thus, by evaluating the data, it was possible to identify the critical elements for different dummy configurations and harness settings. Subsequently, identifying non-uniformity in the sizing of the individual elements based on their actual loading is also very important information. The outcome is, therefore, a vision of significant weight savings without affecting the load

capacity of the whole system, once the above results are incorporated into the design of the new version.
