Downwash is the latest battlefield in the behind-the-scenes fight to build political pressure against the Chinook’s selection as the U.S. Air Force’s next-generation combat search-and-rescue (CSAR) helicopter.
Lockheed Martin, pitching an EH101 variant — or AW101 variant, we should now say — with teammates AgustaWestland and Bell Helicopter, is protesting the USAF re-bid of the contract. It succeeded in having the U.S. General Accountability Office overturn the November 2006 contract to Boeing. Sikorsky, offering an S-92 variant, also is protesting to GAO again.
Downwash isn’t known to be part of their protests. But press accounts have rehashed U.S. Army reports on Chinook downwash and brownout incidents. U.S. Sen. Mary Landrieu (D-La.) implied in a July 3 letter to the Air Force that rescuers avoided using Chinooks after 2005’s Hurricane Katrina for fear they would blow people off roofs. Boeing disputes that.
The HH-47 has a downwash velocity lower than its competitors, said Boeing HH-47 Program Manager Rick Lemaster, adding Boeing is committed to limiting it to 59 kt, as measured up to 6 ft off the ground while hovering at 80 ft. USAF’s limit is 65 kt. Lemaster said max velocity is below the overlap of the Chinook’s rotors. Below the forward hoist, he said, downwash is 35-45 kt. He added Boeing’s calculations, based on disc loading data for the EH101 and S-92, put the Chinook’s downwash velocity 3-10 percent below theirs.