Dynamics of a Small Unmanned Aircraft Parachute System

doi: 10.5028/jatm.v10.752 original PaPer xx/xx Dynamics of a Small Unmanned Aircraft Parachute System Ashim Panta1, Simon Watkins1, Reece Clothier2 how to cite Panta A https://orcid.org/0000-0002-5452-1456 Watkins S https://orcid.org/0000-0001-5550-4941 Clothier R https://orcid.org/0000-0002-5264-3222 Panta A; Watkins S; Clothier R (2018) Dynamics of a Small Unmanned Aircraft Parachute System. J Aerosp Tecnol Manag, 10: e1218. doi: 10.5028/jatm.v10.752. aBStract: Parachute Systems (PS) can be readily used by small Unmanned Aircraft Systems (UAS) for risk mitigation and aircraft recovery. To date there has been limited research into the fundamental dynamics of parachutes at low Reynolds numbers, with existing studies focusing on larger parachutes. An understanding of the dynamics is needed to establish sound guidelines for parachute design and for their use during UAS operations. Existing design guidelines are reviewed and the key parachute design parameters are identified. The validity of the existing guidelines applied to lower Reynolds number parachutes is explored through a series of wind-tunnel tests. It was found that existing design guidelines underpredict the key parameters of inflation time and peak forces for parachute deployments at typical UAS operating speeds. The ramifications on the design and operation of small UAS are discussed. keyWordS: Parachute system, UAS, Dynamics. INTRODUCTION For UAS, the primary safety related hazards are: 1) a collision between an Unmanned Aircraft (UA) and other airspace users; and 2) the controlled or uncontrolled impact of the UA with terrain or objects on the terrain (e.g., people or structures) (Clothier et al. 2015). Hayhurst et al. (2006) further classify these hazards into the domains of design, flight crew and operation. As discussed in Clothier et al. (2015), parachute systems are one of a number of devices and procedures that can be employed by UAS operators to reduce the risk to people overflown, and secondarily, to the UA and its payload. Small commercial UAS currently exhibit a high unreliability due, in part, to the use of commercial off-the-shelf components, the limited redundancy in flight critical systems, and the uncontrollability of the UA given a failure. The high cost of payloads and the increasing use of UA in populous areas have motivated many commercial UAS operators to use parachutes. Historically, parachute design has utilised design guidelines based on empirical data. A principle resource is Knacke’s Parachute recovery systems design manual (Knacke 1992). The design guidelines established in Knacke’s design manual, hereafter referred to as Knacke’s guidelines, are derived from experimental testing of large parachutes at Reynolds numbers (Re) on the order of 107. Knacke established the boundary and performance ranges of parachute operations, which have been applied to parachute envelopes greater than 20 foot in diameter deployed at speeds greater than Mach 0.2 and from altitudes of 5,000 ft or higher. The vast majority of small UA (e.g., of maximum takeoff mass less than two kg) currently operate at much lower Re (in the order of 105) and at altitudes below 400 ft above ground level (this operational height limit is due to regulatory constraints as opposed to the operational capability of the UA). 1.RMIT University – School of Engineering – Aerospace Engineering and Aviation – Melbourne/VIC – Australia. 2.Brisbane Technology Centre – Boeing Research & Technology – Melbourne/VIC – Australia correspondence author: Ashim Panta | RMIT University – School of Aerospace Engineering and Aviation | Plenty Road - Bundoora 3083 – Melbourne/VIC – Australia | Email: received: Nov. 29, 2016 | accepted: Feb. 05, 2017 Section editor: Valder Steffen Jr. J. Aerosp. Technol. Manag., São José dos Campos, v10, e1218, 2018 02/14 xx/xx Panta A, Watkins S, Clothier R Despite these differences, Knacke’s guidelines have been applied in the design of parachute systems (PS) for UAS (Cartwright 2008; Butler and Montanez 2007; Wyllie 2001), with minimal consideration as to the validity of their use. Butler and Montanez (2007) address common issues associated with the design, testing and qualification of PS for a range of UAS scales. Based on Knacke’s guidelines, the authors illustrate the way key physical parameters of a parachute are sized for UA platforms with a clear layout of the design procedure. Cartwright (2008) again applies Knacke’s guidelines to design a PS for a 25 kg UA. Cartwright investigates the possibility of inflating a small parachute without losing altitude. The study used data from a series of parachute deployments from a moving motor vehicle. The inflation times were recorded to derive the altitude loss during deployment of the canopy. These existing studies make use of existing parachute design guidelines with minimal consideration as to the validity of their use. This paper explores the validity of existing parachute guidelines, and in particular those described in Knacke (1992), for small UA. The existing design guidelines for key parachute design parameters are presented. The validity of the existing design guidelines are explored through a comparison of results obtained from wind-tunnel experimentation. The experimental setup is also presented. The experiments utilised a representative parachute, which is applicable to UA of maximum takeoff mass of up to 2 kg. This mass restriction was chosen as: • • • It is representative of many common commercial fixed and multi-rotor UA types (Fig. 1); The applicable parachutes are of a size that can be readily tested within available wind-tunnel facilities; and Parachutes applicable to UA of this size can be readily bought off-the-shelf. Testing was undertaken for the three operational speeds of 48, 68, and 105 km/h. These speeds are representative of the average stall, cruise, and maximum speeds (respectively) determined for a wide range of fixed wing UA (Fig. 2). The results from wind-tunnel experiments xxxx are presented later in this paper. Number of aircraft 35 30 25 20 15 10 5 0 0 2 4 6 8 10 12 Maximum take-off weight (kg) Figure 1. Distribution of maximum takeoff weights of sub-10 kg fixed wing unmanned aircraft (Palmer and Clothier 2013). 40 Stall speed Cruise speed Maximum speed Mean stall speed Mean cruise speed Mean maximum speed Number of aircraft 35 30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 80 90 100 Operational Speed (km/hr) 110 120 130 140 150 160 Figure 2. Market assessment of typical operating speeds of sub-10 kg fixed wing unmanned aircraft (Palmer and Clothier 2013). J. Aerosp. Technol. Manag., São José dos Campos, v10, e1218, 2018 Dynamics of a Small Unmanned Aircraft Parachute System 03/14 xx/xx EXISTING PRINCIPLES anatomy oF a Parachute Detailed anatomy of PS is described in Knacke (1992). A typical simple PS assembly is shown in Fig. 3. The payload, in this instance, is the UA, which is at (...truncated)