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Why 18 pounds should be the maximum reserve free bag extraction force.

12/6/2012

Due to recent incidents indicating there might have been difficulty extracting the reserve free bag from its container during deployment we have initiated an investigation of the forces available and the forces required along with the mechanics of accomplishing the process. In simplest terms there are two forces involved. The drag force capability of the pilot chute and the force required to extract the bag. Certainly the ability of the pilot chute must be greater than the retention of the container. This demands that we know the capability of the pilot chute. To know this requires testing with instrumentation. However we can calculate the size requirement of the pilot chute and measure the bag extraction force without doing sophisticated testing. A rate of descent of twenty (20) feet per second defines a malfunction. If we assume this rate of descent to begin our problem we add to it the rate of acceleration due to gravity, 32 feet per second squared, which begins at cutaway. In the first second of this acceleration we travel 16 feet plus the 20 feet of the initial rate which means we traveled 36 feet. We are now traveling at a rate of 52 FPS and still accelerating. The rate of 52 FPS at 2000 feet will generate a dynamic pressure of 3 pounds per square foot. This benchmark is important as it is where a commonly used free bag and bridle will load for extraction. This means that at the point of bag extraction we have available, 3 pounds per square foot of dynamic air pressure for the pilot chute to drag. Pilot cutes are generally 36 inches in diameter which is 7.07 square feet. No parachute or flexible body will drag to its full size; the size is effectively reduced due to rigging and billow of the canopy. This is accounted for mathematically by applying a Coefficient which reduces the physical size to an “effective size”. It could be thought of as a percent of efficiency of the design. An MA-1 Military surplus pilot chute commonly found has a published Coefficient of Drag (Cd) of .65. When the Cd is applied to the physical size or plan form it identifies the “effective size” of 4.6 square feet. There are better more efficient pilot chutes. Let us assume there is a pilot chute with an effective size of 6 square feet (I don’t know of any that good). At bag extraction time there is 3 pound per square feet available dynamic pressure as explained above. 6 square feet times 3 Pounds per Square Foot equals 18 pounds drag. This is the maximum that could possibly be allowed and will be too high in some cases. Nothing over this limit should be acceptable on any rig without a letter from the manufacturer specifically allowing whatever force they specify. This bench mark should be set by National Aero Clubs as instruction to their riggers until the manufacturers provide a specification for their specific gear.
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