Calvin K. Lee

Modeling of parachute opening - An experimental investigation

Scaling parameters for the peak opening force and opening time of solid cloth parachutes are investigated using the physical modeling technique. With Froude number and mass ratio as the two scaling parameters, correlation of the nondimensional peak opening force and opening time between the full-scale and the model parachutes made from the same 1.1 oz/yd ripstop nylon was not entirely satisfactory. Nylon fabrics with areal densities up to 14 oz/yd were then used to construct full-scale parachutes so that the 1.1 oz/yd model parachute would have a lighter fabric. This improved the correlation between the full-scale parachutes and the model parachute and established the importance of scaling canopy fabric as well as Froude number and mass ratio for modeling parachute opening.

Geometric Properties of Parachutes Using 3-D Laser Scanning

The technology of gliding parafoils is currently being pursued by the U.S. Army for precision airdrop of cargos and personnel. The performance of parafoils (lift and drag) and round parachutes (drag) is closely related to their 3-D geometry. There have been some studies on the geometry of round parachutes, but hardly any on parafoils. The technology of 3-D whole body scanning provides a viable tool to investigate the 3-D geometry of parachutes. In this paper, we present an investigation on the 3-D geometry, surface area and volume of small-scale models of parafoil, round parachute, ring-slot parachute, and cross parachute using a 3-D laser scanning apparatus. Scan data from these model parachutes were obtained in a climatic chamber with a steady air velocity. Surface areas and volumes of these parachutes were calculated from the scan data using specially developed mathematical methods. In addition, cross sections of the parachute canopies were obtained from the scan images. These cross sections provide valuable information on the relationship between model parachutes and full-scale parachutes, fabric properties on canopy geometry, and parachute canopy design and manufacturing.

Experimental Parachute Validation Research Program and Status Report on Indoor Drop Tests

The US Army Natick Soldier Research Development and Engineering Center (NSRDEC) is currently developing coupled fluid/structure computer codes to simulate the performance of parachutes and airdrop systems. Concurrently, NSRDEC is leading and managing an experimental parachute research program to obtain parachute performance data for validation of the computer codes. This program consists of detailed measurements of the aerodynamics and structural dynamics of small-, medium- and full-scale parachutes. Currently, measurements and instrumentation development are focused on the first two scale parachutes. These measurements include spatial canopy position and motion, the three-dimensional geometry of the canopy, canopy fabric strain, suspension line forces, air velocity, and pressure. For the small- and medium-scale parachutes, a test program has been developed in which flat round 3.5-ft., 7.0-ft., and 9.0-ft. diameter model parachutes are vertically dropped in an indoor controlled environment to obtain the measurements. The tests consist of dropping the parachutes in a large enclosed structure with a highly instrumented payload. To obtain repeatable and consistent data, a guided vertical drop system is being used. A thin steel guide wire is anchored at the floor of the structure and extends vertically through the centerline of the payload and the vent of the canopy to the ceiling. Such a test setup restricts the opening and the descent trajectory of the parachute to be on the vertical path of the guide wire. Repeatable and consistent parachute performance data have been obtained.

Controlled opening method for clustered parachutes

This paper presents a method for achieving simultaneous opening of clustered parachutes. The method involves connecting and partially reefing the parachutes in a cluster during the initial stage of the opening so that they open together as a single parachute; they are then disconnected and disreefed at almost full inflation, thereby controlling and opening the parachute simultaneously. This method, called the controlled opening method, was tested with clusters of 64-ft-diam G-12 cargo parachutes, 100-ft-diam G-ll cargo parachutes, and 28-ft-diam C-9 personnel parachutes. Their openings were significantly improved when compared to the openings of these clusters without using the controlled opening method. LUSTERED parachutes offer several advantages over a single large parachute. The important ones are shorter opening time and distance, easier fabrication and ground re- covery, and more stable descent. The major difficulty of clus- tered parachutes is that parachutes in a cluster generally open randomly and unevenly; the random opening results in a large variation in opening times and an uneven distribution of open- ing forces among the parachutes. Consequently, the lead- opening parachutes are often damaged, resulting in an un- satisfactory airdrop operation. In view of the importance of uniform opening of clustered parachutes, some fundamental studies of flowfields and open- ing of clustered parachutes were conducted by Braun and Walcott,1 Heinrich and Noreen,2 Heinrich et al., 3 Wolf and Spahr,4 and Nicuum and Kovacevic.5 Current techniques for improved cluster opening are generally those used for con- trolling the opening of a single parachute. They include reef- ing the canopy skirt, adding a secondary chute at the skirt, introducing a pull-down centerline, and applying tension force at the apex by a drogue chute. These techniques generally modify the opening of a single parachute, but they do not necessarily improve the opening of a cluster as a whole. This is evidenced by the current problems in opening 100-ft-diam standard U.S. Army G-ll cargo parachute clusters6 and the great difficulties in opening 137-ft-diam developmental cargo parachute clusters.7-8 Most recently, Johnson9 developed a central reefing/disreefing system that addressed the opening of a cluster as a whole. His system improved the opening of a cluster of three 52.5-ft-diam parachutes. This paper presents a method for improved opening of clustered parachutes. The method was tested extensively with clusters of various size and number of parachutes. Test results demonstrated that the method improves their opening significantly.

Continuous Disreefing Method for Parachute Opening

Although round parachutes have been used for airdrop for over 60 years, damage to canopy fabric and suspension lines still occurs during parachute opening due to the rapid canopy opening and the associated high opening force. Continuous disreefing of round parachutes to slow down the opening and decrease the peak opening force has been discussed in the literature, but no viable continuous disreefing method has ever been developed. In this paper, we present a practical, effective, and low-cost continuous disreefing method that does not use any external electrical or power source, only the opening force and the weight of the payload. The method was successfully demonstrated in a full-scale test from an aircraft using a 10.7-m (35-ft) diameter round parachute. The kinetic energy of the payload at parachute deployment of that test was 11 times higher than that of the standard deployment that the parachute is designed for. In spite of the severe deployment condition, no damage to the parachute was observed after ground impact.

Radial reefing method for accelerated and controlled parachute opening

Future Army airdrop systems will require aerial insertion of cargo and personnel from low altitudes to minimize ground-fire hazards. A radial reefing method was developed as a potential candidate to meet this requirement. The radial reefing method involves selecting equally spaced radials of a parachute canopy and reefing these radials near the skirt; concurrently, the canopy fabric adjacent to the reefed radials is puckered. Reefing the canopy this way creates large fabric pockets near the skirt during initial canopy inflation, resulting in an accelerated and controlled parachute opening. This was confirmed by full-scale airdrop testing using single Army personnel and cargo parachutes. In addition to demonstrating promises for single-canopy low-altitude airdrop applications, the radial reefing method also shows potential to improve clustered parachute opening by minimizing canopy enfolding and slumping. Nomenclature F0 = opening force, Ibf Fs = snatch force, Ibf ng - number of gores between reefed radials np = number of fabric pockets n, = total number of gores of canopy R = canopy radius, ft r = radial reefing ratio t = time, s tp = time when upper canopy begins to inflate, s ts = time when snatch force occurs, s

Low Cost HALO Cargo Airdrop Systems

A low cost high-altitude and low-opening (HALO) cargo airdrop system was developed to airdrop 2,500 - 10,000 lb payloads with an objective of providing precision airdrop capabilities. The system consists of the Army standard Low Cost Low Velocity parachutes and Low Cost Container that have been used in humanitarian and other Army airdrop applications. The system was successfully designed, developed and full-scale tested. Performance tests were first conducted to generate Calculated Aircraft Release Point (CARP) data to examine the basic system performance characteristics. Operation Utility Evaluation (OUE) tests were then performed using the CARP data to determine system airdrop accuracy. The mean value of 32 OUE tests was found to be 662 ft (202 m) with an uncertainty of 51 ft (16 m).

Effects of Inflation Dynamics on the Drag of Round Parachute Canopies

This paper addresses the issue of the source of peak opening force, and whether it can be determined solely by the instantaneous canopy geometry. A set of rigid canopy models were fabricated which had geometries resembling those of a flexible, round canopy model undergoing an infinite mass inflation. The drag coefficient of rigid models were measured in a wind tunnel and compared with that of the fabric canopy. Data indicated that the two drag coefficient trends were quite different. It was concluded that the canopy geometry alone cannot be responsible for the peak opening forces during inflation, even when the apparent mass is accounted for. The time history of the viscous flow field about the canopy determines the peak opening force for the most part.

Compact, Lightweight Pressure Sensors for Aerodynamic Parachute Measurements

Compact, lightweight sensors suitable for direct mounting onto parachute canopies are investigated to help verify and validate aerodynamic pressure and flow simulations for new parachute designs, and can provide real-time sensor data for airdrop studies and for parachute delivery systems having active control. Current efforts are focused on the development of a high accuracy, high dynamic-range pressure sensor for data extraction from experimental setups, real-time monitoring of conventional parachutes, and flight control for emerging decelerator systems. The present design is targeted at a full-scale pressure range of +/-100 Pa (+/-0.015 psi) having better than 1% accuracy while withstanding and measuring 200 kPa overpressure in either direction. A custom plastic package houses a custom sensor die having a pressure-deformable diaphragm with multiple optically coupled sealed-cavity resonant pickoffs. The sensor die is connected to an external interface circuit for signal extraction, formatting and data logging via a rugged, 1.0-mm diameter plastic optical fiber (including the jacket). The sensor package may be attached directly to the parachute fabric for differential pressure measurements. Fiber-tip versions of the sensor for absolute (barometric) pressure measurements are configurable for freestream wind and water tunnel testing, which allows measurement of fluid pressure throughout the flow stream as the sensor probe is repositioned.

Method for Steerable Clustered Round Parachutes

The U.S. Army is currently pursuing a low-cost parachute system for precision airdrop of cargo. Low-cost clusters of round parachutes are being considered as a potential candidate for such a system. A method is presented that provides a glide and steering capability for a cluster of round parachutes for precision airdrop. The method involves partially connecting two round parachutes that have openings and pulldown skirts on their canopies. Glide and steering are achieved by maneuvering the pulldown skirt. This method was developed and tested first using a cluster of two one-quarter-scale G12 cargo parachutes. System glide and turn were demonstrated on a cluster of two full-scale G12 parachutes. Currently, the method is being applied to a cluster of two G11 parachutes for precision airdrop of a 4,540 kg load