Graham Warwick/WASHINGTON DC
Boeing and the US Navy are testing three solutions to the wing-drop problem encountered during flight testing of the F/A-18E/F. The Navy is now "extremely confident" that an expensive wing redesign will not be required, says a US Department of Defense official.
Wing drops of up to 30 degrees occur during manoeuvres at high subsonic Mach numbers, making it difficult for the pilot to track a target during an air-to-air gun engagement. Airflow over the fairings covering the F/A-18E/F's wing-fold hinges is believed to be the cause of the asymmetric stall which results in the wing drop.
Three "solution sets" have been demonstrated which overcome the problem, and flight testing continues in an effort to optimise the cure and determine its impact on performance, the official says.
The first solution involves fitting stall strips on the wing upper surface inboard of the hinge fairing. The spanwise strips are about 450mm long and "several" are mounted one behind the other on each side of the wing, beginning close to the leading edge.
The second solution involves a wing-fold fence which runs chordwise across the wing upper-surface just inboard of the hinge fairing. The fence is designed to keep the airflow over the outer section of the wing away from that over the mid-span area, where the stall occurs.
The third solution involves a "porous" hinge fairing. Slots in the fairing allow air to flow in both directions through the wing fold and effectively make the fairing "aerodynamically invisible". This cure was developed after flights with the hinge fairings removed showed no wing drop.
Flight tests are continuing to determine which solution has the least impact on weight, drag, mission radius and radar cross-section. The porous fairing looks the most promising, but the Navy "has not declared victory yet", the official says. Some modifications improve aircraft controllability, the official says, notably the 450mm inboard extension of the wing leading-edge "snags" which is part of all three solution sets.
By mid-December, testing was "about 90% complete" and had been accomplished without delaying the F/A-18E/F development flight-test programme, the official says. A solution set is expected to be selected in early January.
Source: Flight International