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AFRL tests helped define new rescue helicopter

  • Published
  • By John Schutte
  • AFRL Human Effectiveness Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --- --  Tests conducted in hurricane-force winds by the Air Force Research Laboratory's Human Effectiveness Directorate helped the Air Force define its next-generation combat search-and-rescue helicopter.

Test data showed that pararescuemen--known as PJs from their earlier designation as para-jumpers--can safely do their job in airflows up to 65 knots, or about 75 miles per hour. A Category 1 hurricane generates winds from 74 to 95 mph.

On Nov. 12, the Air Force awarded a $712 million contract to Boeing Co. for 141 Air Force search-and-rescue helicopters. Boeing will deliver an adaptation of its CH-47 Chinook helicopter, which has been in the military arsenal in various missions for nearly half a century.

The Air Force used the Human Effectiveness Directorate's data together with other data to specify requirements for a replacement for the aging HH-60G Pave Hawk rescue helicopter. They wanted a helicopter to accommodate more survivors and a larger pararescue team but were concerned that a larger helicopter could potentially generate higher velocity downwash in a rescue area.

Wind forces generated by rotor blades can range from about 52 mph with the HH-60G to more than 86 mph for some larger rotorcraft.

"Sixty-five knots is the maximum horizontal downwash velocity that PJs can accomplish their tasks in consistently," said John A. Plaga, a research aerospace engineer with the Human Effectiveness Directorate's Biosciences and Protection Division, Biomechanics Branch. "But one thing we learned about these guys, if they have to do something they'll find a way to do it."

The PJs are assigned to Search and Rescue Units which were recently transferred from the Air Force Special Operations Command to Air Combat Command. They provide recovery and medical treatment in combat situations and for survivors in humanitarian need or disaster situations; they're commonly seen "fast-roping" down from a hovering helicopter onto rooftops and into perilous situations.

A larger aircraft offers more space for transporting PJs and casualties but it also presents a new problem--the potential for increased airflow beneath the hovering aircraft, Mr. Plaga said. A rotorcraft hovers by pushing down a mass of air through its rotor blades. The heavier the aircraft, the more air must be pushed towards the ground to hold the aircraft up.

"For some helicopters, the downwash is so strong that no one can do anything beneath it," said Nathan Wright, aerospace engineer and associate investigator on the project.
As the air strikes the ground, it turns from a vertical direction to horizontal, making it difficult for PJs to approach the aircraft, particularly if they are carrying litters or toting gear weighing 55 pounds or more.

Researchers used a large test stand at the 46th Operations Group Aerospace Vehicle Survivability Facility where bypass air from up to five jet engines can generate wind speeds of more than 460 mph.

After conducting preliminary tests with a mix of non-rescue personnel, both Air National Guard and active duty PJs arrived for more realistic assessments, performing a total of 227 tests.

For horizontal airflow tests they walked into the airflow first with basic gear, then wearing 30-pound and 55-pound backpacks. Two-man teams carried a 130-pound mannequin on a litter, and then added 30 more pounds to their burden. Carrying an empty litter actually proved more difficult because it caught the airflow and acted much like a sail, Mr. Plaga said.

Working in a vertical airflow, PJs hoisted various-sized mannequins in a Stokes litter, climbed a rope ladder, and descended a fast rope.

Wind speed increased throughout testing to about 130 mph, the maximum velocity permitted by safety guidelines. Videos show test subjects straining to stay on their feet and, at higher airspeeds, being knocked down.

"We wanted to determine what the limits of the PJs were in high air velocities encountered near a hovering aircraft, with wind speeds similar to a Category 3 hurricane," Mr. Plaga said. "We found that PJs almost had no limits."

According to Plaga, the PJs lived up to their motto, That Others May Live. "The PJ mentality is that they get the mission accomplished no matter what it takes," Mr. Plaga said.

"This project tapped into our resources in aerodynamics," Mr. Plaga said. "It was definitely a unique project for us and we came up with data that helped the Air Force define its new CSAR helicopter platform."