A recently-published patent application reveals one issue Boeing overcame to develop KC-46A Pegasus tanker’s wing aerial refueling pod (WARP) system.

The Cobham WARP mounted on the outboard wings of the tanker can refuel more than one aircraft simultaneously with 1,514l (400gal) of fuel per minute. The US Air Force will buy 94 KC-46 tankers through fiscal 2021 and 42 WARP sets.

But according to the 26 May patent, the airflow around the tanker during flight can create a vortex that lifts the hose and drogue upward, causing instability during extension and retraction. Although features such as chines or Gurney flaps could be designed on older tankers to counter vortices across the refueling envelope, the KC-46’s higher speed exacerbated the issue.

“The pods and its pylon may be designed to minimise buffet at maximum operating speed, but minimizing buffet will cause higher side loads, which result in stronger vortices at refueling speeds,” Boeing states in the patent application. “The existing mitigation designs are not effective for addressing significantly higher vortex strengths across the entire flight envelope.”

Boeing invented a fix for the problem under the original engineering and development contract awarded in 2011. The company developed a swept pylon flap which unloads the pylon and pod during flight, creating a downward flow that counters the vortex, according to the patent. The government has certain rights to the invention, the patent states.

The first 18 KC-46s are now going to be delivered by January 2018 with all the requirements except the WARPs, which will come in October of that year. While Boeing will equip the initial 18 aircraft with refueling boom and centerline drogue refueling capabilities, the WARPs fulfill the Air Force’s required assets available milestone.

Boeing maintains that prolonged conformity and qualification testing needed for Federal Aviation Administration certification — and not a functionality issue — caused the pods to fall a year behind schedule.

“The patent is reflected in the baseline integrated design of the WARPs and was in place at first flight of the KC-46 (Sept 2015),” Boeing says in an email to FlightGlobal. “There is not a problem with the WARPs functionality … the WARP and center line drogue systems performed as planned during Milestone C testing.”

An original version of this story misstated the specific contract associated with the patent.

Source: FlightGlobal.com