Hit the dust


Have you ever wondered what would happen if you took an old CH-46E helicopter fuselage and dropped it from a height of about 30ft? Engineers at NASA Langley’s Landing and Impact Research Facility (LandIR), in Hampton, Virginia, certainly have – in fact their curiosity, fueled by a desire to improve the crashworthiness of seats and seatbelts, as well as to gather data on the odds of surviving a helicopter crash, led them to do exactly that at the end of August.

The US Navy provided the CH-46E Sea Knight helicopter fuselage, complete with seats, which was then fitted out with 15 occupants – 13 instrumented crash test dummies and two uninstrumented manikins.

The Navy also contributed five of the crash test dummies, one manikin and other equipment, while the US Army provided a manikin and a crash test dummy that was placed in a position representative of a patient in a medical evacuation litter.

The FAA provided a side-facing specialized crash test dummy and part of the data acquisition system; and NASA Langley added six of its own dummies, as well as lead technical expertise and the use of its own specialized facility, known as ‘the Gantry’.

Engineers then used cables to hoist the helicopter fuselage into the air and swing it above the ground like a pendulum. It was traveling at 30mph when pyrotechnic devices separated the cables, sending the fuselage smashing into the soil below.

“Four swing cables and two pullback cables were attached to the test article,” explains lead test engineer Martin Annett. “The pullback cables are pyrotechnically severed, and the swing cables guide the test article to the ground with a controllable horizontal and vertical impact velocity,” he continues. “We chose a soil impact surface, because the majority of mishaps occur on non-prepared surfaces. The velocity conditions are a trade-off between civilian and military requirements for what is considered severe but survivable.”

The test article was fitted out with 350 sensors to capture data on airframe accelerations and crash test dummy loads. Over 40 high-speed and high-definition cameras recorded onboard and external movements.

Joining the dots

Researchers also made use of a new photographic method available to them to help analyze the data collected from the crash test. Called ‘full field photogrammetry’, it saw the helicopter fuselage stripped of its usual coat of naval grey paint in favor of an altogether more eye-catching – or rather camera-catching – scheme. “We painted more than 8,000 dots on the side of the test article to measure global and local deformation on the fuselage skin,” explains Annett.

High-speed cameras filming at 500 images per second were used to track each dot, ensuring researchers were able to plot and ‘see’ exactly how the fuselage behaved under crash loads.

“Two cameras were positioned and calibrated against the large backboard to provide 6DOF motion for those points,” continues Annett. “The dots were over one inch in diameter to accommodate the necessary field of view and resolution, so localized strains were not computed. Relative deformations and the overall spatial and temporal response could be tracked.”

Another testing technique adopted for the first time involved researchers making use of some rather familiar ‘off-the-shelf’ technology – taken from a video game sensor: “We also tested a markerless tracking technique using an Xbox Kinect sensor,” continues Annett. “The sensor was aimed at one of the standing dummies and identified 19 joint locations to track.”

Preliminary observations from the test indicate good data collection, which the team at Langley will now take months to fully analyze, as well as informing their simulation models.

“We designed this test to simulate a severe but survivable crash under both civilian and military requirements,” says Annett. “It was amazingly complicated with all the dummies, cameras, instrumentation and collaborators, but it went well. The deceleration response of various locations on the airframe is now being correlated directly to the simulation models, and the models will be calibrated based on those results.”

This was the first of two planned tests using US Navy-provided CH-46E Sea Knight fuselages. A similar helicopter equipped with additional technology, including high-performance, lightweight composite airframe retrofits, will be used in a crash test next summer. Both are part of the Rotary Wing Project (RWP) in NASA’s Aeronautics Research Mission Directorate. “The overarching goal of the RWP is to develop and validate tools, technologies and concepts to overcome key barriers for rotary wing vehicles,” explains Susan Gorton, who heads up the RWP for NASA.

The project is one of four in the Fundamental Aeronautics Program (FAP) and contributes to the FAP goals of advancing vehicle technology capabilities for improved efficiency and increased mobility within the air transportation system. “To achieve the objectives, the project is organized around research themes that articulate the longer-term, important areas of research necessary to advance the state-of-the-art,” continues Gorton.

RWP has three research theme areas: advanced efficient propulsion; advanced concepts and configurations; and rotorcraft integration into NextGen. These three themes provide a framework to deliver research that addresses the main barriers for expanded use of rotary wing vehicles: efficiency, performance and public acceptance, including noise and safety. But where does the recent crash test fit into all of this?

“The recent full-scale crash test is part of the larger effort in rotorcraft crashworthiness research in the RWP,” responds Gorton. “RWP invests in fundamental crashworthiness investigations to improve the safety of rotary wing vehicles.”

Researchers want to increase industry knowledge and create more complete computer models that can be used to design better and safer helicopters. NASA says the ultimate goal of its rotary wing research efforts is to help helicopters and other vertical take-off and landing vehicles carry more passengers and cargo more quickly, quietly, safely and with less harm to the environment, ultimately leading to their wider use in the airspace system.

Mind the gap

However, Gorton admits the test was also focused on examining how lightweight composites withstand impact testing: “As composites become more widely used as primary structure in helicopters and advanced rotary wing vehicles, it becomes apparent that there is much that is still unknown about their characteristics in a crash environment, and the ability to design for specific crashworthiness criteria with composite structures is still an active research area,” she says.

Hence the decision to fit a composite component inside the fuselage to be used in the second test to be carried out next summer: “We will test another CH-46E airframe and will seek our partners’ input on the experiments they would like to include this time,” says Annett. “We intend to remove the cabin subfloor and replace it with composite concepts that can provide equal or better crashworthy capability compared with the metallic counterpart. We would crash with the same impact conditions and overall weight for comparison with the first test.”

But Annett isn’t waiting around until next year’s second test to begin making use of all the data collected so far – or the valuable testing observations already noted: “Many of the lessons learned from the test relate to test preparation and performing adequate checkout of the data acquisition system and the crash test dummies,” he says.

One particular problem identified involves instrument clutter: “Once the airframe is fully populated with instruments and dummies, it becomes difficult logistically to debug and diagnose. Also, having a full dry run was very beneficial as all the systems, with the exception of the pyrotechnics, could be tested out under normal procedural conditions.”

Asked how the test might be improved, Annett underlines the challenges of checking onboard equipment immediately prior to crash testing: “The biggest challenge is ensuring that our sensors, cameras and data acquisition systems are operating properly prior to the final swing and crash,” he says. “Also, experiments must be designed to allow the proper crash performance without affecting experiments nearby.” As for the anthropomorphic test devices (ATDs) used, Annett says work continues on improving their performance with regard to side impacts: “The ATDs have been designed to handle the typical vertical and forward load experienced in aerospace crashes,” he says. “There is ongoing work to evaluate side-facing ATDs.” Finally Annett confirms the ‘test, test, and test again’ mantra of all test engineers when asked how testing could be further improved: “Frequent testing is always recommended, as there are always lessons to be learned with every crash test,” he says. Helicopter passengers and pilots of the future couldn’t agree more!

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