Battling The Burble Pays Off In Increased Safety

Aircraft pilots have the third most dangerous job in the world (behind fishermen and loggers), according to a Bureau of Labor statistics report for 2005. Naval pilots who must land their aircraft aboard a moving ship have additional risks associated with their work.

Susan Polsky and the Airwake Analysis team at Naval Air Station Patuxent River, MD are working to reduce those risks, making at-sea landings of H-60s, F/A-18s -- and other Navy aircraft -- safer. Polsky is an aerospace engineer with the Naval Air Systems Command (NAVAIR) and leads the team as they work with the Naval Sea Systems Command to ensure future Navy ships are safe for aviation operations.

The Airwake Analysis team uses a combination of simulators, wind tunnels and Computational Fluid Dynamics (CFD – their computer analysis tool) to determine air flow behind and around a ship. That airflow, called a “burble” by fixed-wing pilots, varies in speed and turbulence levels.

"Airwake, or burble, is unavoidable," explained Polsky. "We are participating in future ship design programs to potentially lessen that burble since increased turbulence would mean more of a workload, and an increased risk, for the pilot."

Sid Hatcher is a retired Navy H-60 pilot with 700 small-deck landings and lots of experience with airwakes. He presently works as a test pilot for the Naval Air Warfare Center Aircraft Division’s HX21, the rotary wing test squadron at NAS Patuxent River.

Recently he has been testing simulations using CFD airwake data in support of the H-60 Program Office (PMA-299) and the Naval Aviation Training Systems Program Office (PMA-205).

"Airwake is an important factor in getting an aircraft aboard (a ship) safely," said Hatcher. "There are controllability challenges in maintaining a precise position, as well as performance issues for an aircraft transitioning in and out of the relative winds."

Previously, the operational capability of an aircraft landing on a ship was not known until the aircraft landed on the ship for the first time. Now, ship designers are able to assess potential geometries on the drawing board for adverse ship airwake effects before metal is even cut.

One of the ships the team is presently working on is the next class of carrier (the 21 Class) – specifically, USS Gerald R. Ford (CVN-78).

"The new carrier has a different superstructure and it’s in a different place than on existing carriers," said Polsky. The superstructure houses the command and control functions for the carrier and the entire battle group.

By using computer modeling, the team can determine where and how the airwake will flow behind the new ship. Since fixed-wing aircraft approach for landing from behind a carrier, this is vital information for pilots.

The team has developed a ship simulation which allows a pilot to "fly" through new airwake data, obtaining aircraft control surface activity and other data that help determine how much the airwake will affect the pilot and the aircraft.

"That data is key to determining any deviations from the old to the new configurations," explained Polsky.

With that information, the team can develop a training simulator for pilot orientations in preparation for landing on board CVN-78 and other ships, or to test new aircraft such as the V-22 Osprey.

Lt. Col. Bill Witzig is a US Marine Corps pilot with the V-22 program who has also flown a simulator using the CFD airwake analysis.

"The value of this effort has enormous potential to improve the quality of pilot training," said Witzig. "The control inputs needed close to the flight deck were very representative of those needed on actual flight decks."

According to Witzig, the Marine Corps’ V-22 training and readiness manual directs 51% of the total pilot training requirement be conducted in simulators. "It goes without saying that flight simulation devices, in this type of a training program, need to be as realistic as possible."

Hatcher agrees. "Especially when coupled with a full motion simulator, the aircraft response and flight control activity is as close to the real thing as I’ve seen," he said. "Of course there is nothing better than using real ships, real aircraft and the real environment. But sometimes all three are not available. This is especially true when looking to evaluate shipboard landings in high winds and sea states. We are discovering CFD airwake modeling is an excellent and recommended tool for training pilots in shipboard landing as well as for buildup prior to Dyanamic Interface (shipboard) testing."

The Airwake Analysis team also worked with USS Abraham Lincoln (CVN-72) immediately following the Indian Ocean earthquake/tsunami in December 2005. The carrier was using helicopters for humanitarian airlifts, but still needed to continue F/A-18 flight operations.

Polsky and her team analyzed the level of jet wash (air expelled from a jet engine) during a catapult launch and determined that the helicopters could safely land and take off using a platform on the other side of the super structure. Their work will impact most of the other Navy carriers as ship deck markings are changed and as the temporary Interim Flight Clearance becomes permanent, expanding operational envelopes beyond their previous capacity.

The team has also performed a general airflow evaluation on the latest destroyer design – the DDG 1000. They used the H-60 helicopter as their model for the evaluation of the Zumwalt Class Destroyer. The results of the team’s analysis and testing are being utilized by the program office in the design efforts for DDG 1000. The team's efforts have provided confidence to the DDG 1000 team that the unique deckhouse design does not adversely effect helicopter operations on the flight deck as the analyzed air flow is shown to be within acceptable operation limits for the H-60.

"Our job was to look at the operational envelope for the H-60 on the new ship and determine if it’s acceptable or unacceptable relative to current destroyers," explained Polsky. "We used CFD to predict the airflow around the ship and the results were provided to the program office to help identify any risk to aviation associated with the unconventional topside design."

"By getting it right the first time, the need for expensive modifications to the ship geometry are averted," said Polsky, "and the Fleet is provided with the ship-aircraft system that gets the job done in as wide a range of environmental conditions as possible."

Although influencing ship design in the earliest stages allows Polsky and her team to help enlarge the operational envelope of Navy ships, they are especially pleased to be increasing the safety of the pilots who land their aircraft aboard those ships.

"I’m thrilled that we’ve been able to positively impact the Fleet by reducing the risk of operating aircraft – both fixed and rotary wing," said Polsky.

(ANN thanks Vicky Falcón, NAVAIR Public Affairs Office, for this report.)

FMI: www.navair.mil