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The Omega sensor was developed for measuring stress in textile and other flexible materials by the Department of Aerospace Engineering and Mechanics of the University of Minnesota and sponsored by the Air Force Flight Dynamics Laboratory. Two studies were conducted by the University which indicated that the circumferential stresses of inflated parachute canopies indicated by Omega sensors agreed with theoretically predicted stresses and also that the stresses measured by an Omega sensor were not affected by dynamic loading. This particular study deals with an in-house test program, designed to measure the circumferential stresses of a model (five foot nominal diameter) ringslot parachute during inflation and at steady state using modified Omega sensors. Slight modifications to the original Omega sensor had to be made due to complications of the tabs tearing during preliminary testing. Five sensors were attached strategically to the canopy of a ringslot parachute and put through a series of low speed wind tunnel tests. The results are presented in detail and provide for the first time actual measurement of circumferential stresses on the surface of a model ringslot parachute. These results, however, can only present the general trend shown in the parachute's stress distribution and not actual stress values due to the inability to calibrate the sensor while attached to the canopy.
This paper describes the results of an experimental study of canopy stresses in bias constructed solid flat parachutes. Stresses were measured in the warp and fill directions during inflation and at steady state for different values of dynamic pressure. Omega sensors were used to measure stress. These sensors were mounted along the gore center lines so that the warp and fill stress distributions could be determined as a function of distance from the vent. It was found that stresses in the fill direction were substantially larger than stresses in the warp direction.
At the 4th Aerodynamic Deceleration Systems Conference, 1973, the development of the Omega Stress Sensor and initial canopy circumferential stress measurements were reported. Following the first success, the Omega sensor was tested under conditions of rapid loading. No dynamic effects were recorded at strain rates up to 100% per second. In the further pursuit of experimental stress analysis, initial measurements of radial canopy stress were made, and it was found that at certain locations the radial stress is as high or higher than the circumferential stress. Finally, exploratory tests were made in which circumferential stresses were measured on a 5 ft model parachute during the periods of inflation and at the following steady state. All tests were made under infinite mass conditions. The stresses were measured versus time, and their instantaneous values vary with respect to time and location on the parachute canopy.
This paper describes the results of an experimental study of canopy stresses in a model ribbon parachute. The distribution of circumferential stress was measured during inflation and at steady state for different values of dynamic pressure. Testing was performed in the wind tunnel at the infinite mass condition. Omega sensors were used to measure stresses and were mounted in different ribbons along the gore centerline. It was found that the steady state stress had two maxima of about equal value. One maxima was near the skirt, the other half-way between the vent and skirt. The distribution of maximum stress during inflation was similar to the steady state distribution and the ratio of maximum stress during inflation to steady state stress ranged from 1.25 to 1.75 and was essentially independent of dynamic pressure. The spectral density of the steady state stress measured at several points on the canopy.
For a design engineer strength calculation is a prerequisite for completion of design. The parachute designer however encounters great difficulty. Theory provides no means for calculation and almost no data is available of measurements under dynamic conditions to permit adequate strength calculation. The rapidly changing dynamic forces can vastly exceed the steady state forces. Great efforts have been made to predict stresses in a parachute canopy. The latest work for a parachute stress analysis was conducted by H.G. Heinrich and L.R. Jamison [1] and is a great advance. Contrary to former attempts it includes parachute shapes during inflation of the canopy. The great disadvantage of all theoretical attempts is that the actual stress distribution in a canopy is still unknown. No method has yet been found to measure stresses directly in many parts of a canopy.
An experimental investigation was conducted to determine the differential pressure distribution over the surface of parachute canopies during the period of inflation under mass conditions. Full scale parachute canopies of the circular flat, 10% extended skirt, ringslot and ribbon types were utilized during the free-flight test program, and differential pressures on the gore centerline and on the cord line were measured by means of four pressure transducers distributed over the canopy in equal distances from the skirt to the vent. In order to analyze the relationships and dependencies between the pressure distribution, projected canopy area, canopy shape, generated force, and dynamic pressure, graphical displays of these quantities were made as a function of time for each type of parachute canopy. The results of the pressure distribution measurements permit a better understanding of the physical nature of the dynamic process of parachute inflation. The stress distribution in a parachute canopy can be calculated if the corresponding canopy shape is known. For this purpose, the evolvement of the canopy shape with the corresponding time is presented for each of the canopy types. (Author).
A short opening time and moderate opening force are desirable characteristics of practially all parachute applications. But because of the interdependency between opening time and force, this is one of the most intricate functions in the dynamics of parachute inflation. Particularly, parachutes with rapid inflation usually develop a high opening force. It has been hypothesized that since theoretical and experimental analyses indicate that the maximum force occurs after the canopy has attained one-half or more of its fully inflated size, possibly an acceleration of the initial phase of opening might have the desired effect upon the total inflation.
The measured mass flows are analyzed relative to free stream velocity and density ratio. The results are presented as ratios of measured to inviscid flow through the canopy inlet area, thus giving a measure of the canopy's ability to accommodate the air mass entering the parachute mouth. Related to the mass flow studies, was the determination of pressure distributions on the internal and external canopy surfaces. Measurements were made at Mach 0.8, 1.2, and 3.0. In addition, several tests were conducted using an ogive forebody at two upstream locations. The wake of the ogive forebody with respect to pressure is analyzed. (Author).