NASA will use its HARV F-18 to tackle "falling-leaf" problem

Guy Norris/LOS ANGELES

NASA's McDonnell Douglas F-18 HARV (high-alpha research vehicle) is to be converted for use in a joint US Navy/NASA Langley test programme aimed at solving a control problem experienced on F-18s and other high-performance aircraft.

"At certain points in the flight envelope they get into a 'falling leaf' condition where they just begin rocking back and forth and basically fall out of the sky," says NASA Dryden Flight Research Center HARV programme manager, Denis Bessette. The aircraft are normally recovered automatically at "around 11,000ft to 12,000ft", he adds.

Aerodynamicists at NASA Langley have developed a theory about the cause of the condition, which is, "...part of a continuum between Dutch roll and spin," according to Bessett. "If roll and yaw are in sequence then the aircraft can go into the falling leaf. If they are out of sequence it can develop into a spin which is pure yaw without Dutch roll."

NASA performed some falling-leaf spin-entry work with the USN in mid-1995 and the data that was collected "supports the theory", he adds. The HARV aircraft has been chosen because it is the only F-18 fitted with a spin parachute. The two-phase refurbishment will clear the way for the falling-leaf, or advanced-control-research, programme to start in 1998.

The HARV aircraft's distinctive thrust-vectoring paddles will be removed for the Navy programme but they will be retained for other research flights.

Source: Flight International