U.S. Army Yuma Proving Ground testing impacts safety of parachutists

Yuma, Ariz. - In successfully conducting military operations, the United States has relied upon air superiority as an indispensible means of delivering equipment, supplies, and troops to remote areas.

Mass exit of paratroopers are useful for seizing isolated air fields deep behind enemy lines, which can then be used to receive and deploy more troops and armament. The tactic has been used by American forces since the World War II and remains relevant today. There is far more to a successful combat insertion than simply jumping out of an airplane, though: In flight, mass jumpers can be impacted by the air disturbance called "wing tip vortex" created by all aircraft, even military cargo planes with wings sporting special finlets to help dissipate this downwash. Another problem is that as the weight of the aircraft increases, the laminar flow from that aircraft changes and has been shown to effect the minimum separation distance between parachutists. The weight of the aircraft effects the resulting wing tip vortices.

To cope with this and other potential problems caused by aircraft weight, C-17 cargo airplanes have a maximum weight limit for flights that accommodate mass jumps, a weight limit that was increased thanks to a gross weight test conducted by U.S. Army Yuma Proving Ground (YPG) air delivery testers that wrapped up last autumn. Currently, YPG testers are breaking down data from the second phase, which is even more intricate.

"Our conclusion at the end of the first phase of the project was that increasing the weight of the aircraft by 15,000 pounds had no significant effect on the jumper interactions and minimum separation distance," said Keith Allen, test officer. "Now we are transitioning to the second phase of this project, which is to examine the wing tip vortices that come off the C-17 aircraft and analyze this problem in a formation airdrop. What we are primarly concerned with is the behavior of the vortices and the risk of jumper-vortex interactions, as well as their severity."

Air behaves like a fluid, and the air disturbance left by massive cargo aircraft speeding through the sky are extremely turbulent and fast, radiating off the tips of the wings at hundreds of feet per second. Though the wings of military cargo planes sport special finlets to help dissipate this downwash, the extreme turbulence is an invisible, but dangerous adversary to combat jumpers that must be mitigated.

"All aircraft create a wake vortex, just as a speed boat produces a water wake behind it," said Keith Allen, test officer. "It's the same principle with an aircraft, only you can't see it: it is an air disturbance. These vortices have been shown historically to be very strong and extremely turbulent. The commercial passenger and cargo aviation industry studies these vortices at airports worldwide every day, which drives take-off and landing times."

Thus, airborne Soldiers participating in a mass insertion must contend with this roiling aerial sea as they descend toward the ground. To negotiate these conditions safely, the formations of C-17s carrying jumpers are required to keep a minimum distance from each other: if this distance could be shortened, more Soldiers could reach the ground and enter the fight faster. However, vortices, hazardous air disturbances from previous planes, have the potential for deadly consequences for Soldiers in subsequent formations of planes.

"If the vortices that come off these aircraft don't get dissipated or blown off the drop zone, the parachutists that deploy from following echelons of planes have a risk to contact that vortices, which has been shown to actually collapse parachutists' canopies," said Allen.

To try to discover a shorter distance that safely avoids these dangers, YPG testers undertook an ambitious two week study of the life cycle of these vortexes, using a small DHC-6 Twin Otter aircraft equipped with LIDAR to fly above a massive C-17, scanning the wake it left behind. While pilots and jumpers in a combat situation would prefer to avoid aerial disturbances, the testers actively sought them out, changing their flight approaches in real time to accommodate different wind and weather conditions. The vortices dissipate as minutes elapse, which made this even more challenging.

"We changed the run-in of the C-17 through all these passes depending on where the winds were coming from," said Allen. "We wanted to get as many different atmospheric condtions with respect to the aircraft as we could to really measure the difference with what happens to the vortices."

Additionally, the testers flew missions across all hours of the day and night, and over different terrain features. The LIDAR used to scan the air disturbances caused by the vortices is sensitive enough to pick up everything within the scanned area, including wind, thermals, and ground effects that were unrelated to the C-17 passing through the area. To be useful, this data had to be separated out from the effects of the C-17 in time and space, a time-consuming task. Despite these challenges, Allen said 90% of the test's 158 flights yielded usable data.

"Your model is only as good as the data you are putting into it," he observed. "Something like this is so complex that you can't possibly simulate every scenario, so you have to be smart about how you do it. You have to understand what assumptions and biases you are making, and eventually you will have to manage your risks and validate your assumptions with actual flight testing, first with mannequins, then graduating to live jumpers."

YPG testers are still processing the data from these flights, but hope to conduct similar testing in different natural environments in the future. Allen says the information learned in this testing will be useful to other air drop activities, both at YPG and beyond.

"This is applicable not only to the military, but in a commercial sense," said Allen. "Our main goal is to characterize the behavior, strength and dissipation rates when the vortices encounter different atmospheric and flight configurations."