THE US AIR FORCE has lifted the 20¡ angle-of-attack (AoA) limit which was placed on McDonnell Douglas F-15Es with conformal fuel tanks after an Edwards AFB, California-based team successfully evaluated performance-handling improvements tested in the "Keep Eagle" programme.
The USAF test team says that the improved F-15E is "...now more resistant to departures than the F-15C, and is very resistant to spin".
It now plans to brief Air Combat Command on the modifications with a view to potential upgrades to the F-15A to D series.
The Keep Eagle project was begun in 1993 after several departures from controlled flight were experienced with operational F-15Es, at least two of which resulted in crashes after unrecoverable spins.
The unexpected handling problems were attributed to the larger weight of the F-15E over the basic A version, which had been found to be basically spin-resistant when last tested in 1975.
The F-15E is around 50% heavier than the first F-15s, at almost 37,000kg gross weight. It has large conformal fuel tanks scabbed onto the engine nacelles.
Testing revealed that the heavier weight and increased inertia of the larger aircraft, added to the modified aerodynamics, produced the new spin characteristics.
Since first revealing the spin problem at the Society of Experimental Test Pilots meeting in California in September 1994, the Keep Eagle team has undertaken 78 test flights and 164h of tests, including 141 spins. Some 18 different configurations have been tested, ranging from a "clean" airframe with no stores or conformal tanks, to a "worst case" configuration with centreline and conformal tanks, asymmetric fuel load and a LANTIRN pod.
Changes made to improve the handling include revised feed backs to the flight-control system (FCS), which is now biased to reflect changes in sideslip rate, lateral acceleration and vertical coupling.
New FCS control paths have also been developed, and include a connection between the rudder pedals and differential stabilator which activates at high AoA.
Another new control path is the direct electric link between the stabilator and the FCS in a spin.
Source: Flight International
