Experimental study of bridle loading and freefall terminal speed
during the drogue rides of Bureau of Land Management smokejumpers
The National Interagency Fire Center
The primary goals of this study were two-fold: first, to measure the freefall speeds of
BLM smokejumpers during the drogue-stabilized freefall that preceeds the deployment of their ram-air parachutes; and secondly, to measure the loads sustained by the drogue bridle. To accomplish these goals a series of test jumps by Boise, ID- and Ft. Wainwright, AK-based BLM smokejumpers was carried out at a commercial DZ located in Star, ID. The test jumpers were equipped with an electronic barograph built and maintained by the members of the Parks College Parachute Research Group. Moreover, the drogue bridle of each test parachute was fitted with a
PRG-built load cell designed to measure the loads sustained by the drogue bridle. Instrumentation maintenance and calibration, as well as preliminary data analysis was carried out in the field by a member of the PRG (Jean Potvin). The results of these test jumps and following data analysis are presented and discussed in this report.
The PRG barograph is an instrument designed to record air pressure every quarter of a second. The barograph uses this information to compute the corresponding altitude and instant descent speed (or vertical speed). These calculations are performed during the jump, after each pressure measurement. As a result, the output of the barograph includes a complete time history of the altitude and vertical speed of one's freefall and parachute glide.
Designed by Gary Peek, this one-of-a-kind instrument consists primarily in a microcontroller which records the electrical output of a solid state pressure sensor. The barograph is attached on the top of the jumper's hand, very much like a wrist altimeter. The data accumulated and computed by the barograph is downloaded into a laptop computer after each jump.
Bridle Load Measurement System
The bridle load measurement system consisted of two components: first, a load cell fitted to link the drogue bridle to the parachute harness. As shown in this diagram, the cell was located below the bridle three-ring release system. Attaching the load cell at this particular point insured that the cell remained connected to the harness after each drogue deployment, thus eliminating the need for a disconnect mechanism of the cell's electric cable as would be required by the cell being attached above the 3-ring release. The cell consists in a "dog-bone shaped" steel load link on which a four-strain gauge bridge is mounted. The elongation of the steel link during loading is recorded by the strain gauges which produce an output voltage proportional to the amount of load.
The second component of the load measurement instrumentation is an electronic data acquisition system (or "data box") which is connected by electric cable to the load cell and records the measured loads at a specified rate. This system is a PRG-built, 20Hz sampling rate, 2 analog-to-digital channels data logger which reads and stores the output voltage of the load cells. The relatively small size of the data box allowed it to be simply carried inside the test jumper's suit near the chest area.
Operation of the data box and load cell system was quite simple, and consisted first in turning the data box "on" and starting the "record" mode prior to the jump. Such a task was usually done by the the jumper or spotter. The data box automatically switched from the record mode to the data download mode 75 seconds later. The jump data was downloaded into a laptop computer after each jump.
The following data is from jump #4 in a series of 13 jumps done during the study.
Graphs from jump #4:
- File number: smjmp13
- Date: 6-17-97
- Test jumper: J. Raudenbush
- Drogue number/type: 76
- Exit weight: 207 lbs
- Freefall duration (ground timing): NA
- Freefall duration (load meas.): 27.7 sec
- Time to achieve force balance and constant vertical speed (load meas.): 10-12 sec
- Aircraft speed at exit: approx 103 mph
- Corrected terminal (vertical) speed: 94 mph
- Maximum force (during snatch or terminal descent): 341 lbs
- Maximum snatch force (during drogue deployment): 269 lbs
Terminal fall speeds:
Overall, the measured velocities are typically smaller than those occuring in sport parachuting which are in the 110-130 mph range. This is understandable since although BLM drogues are very similar in construction and size to sport tandem parachute drogues, they are used to stabilize freefall of loads which are smaller.
Maximum bridle loads (i.e. non snatch force):
During terminal freefall the balance of forces are such that bridle load balances exactly jumper weight. However, the measured bridle load values show large fluctuations during all four stages. Such fluctuations are generated by the turbulent wake of the jumper which causes the drogue to pulse and oscillate wildly. Such load fluctuations exceed average loading by approximately 75 to 150 pounds.
The load values illustrate the point more accurately by showing the maximum load measured during the "terminal" stage of the drogue ride. A clearly visible trend is the fact that maximum load values tend to increase with the weight of the test jumper. This is understandable since heavier jumpers fall faster and as a result generate more drogue drag.
Finally, it was observed that maximum loading appears to be independent of the drogue type.