NASA and McDonnell Douglas (MDC) are upgrading the flight-control software in the X-36 tailless-fighter technology demonstrator before resuming flight tests of the remotely piloted subscale vehicle on 29 July. The team plans to increase angle of attack (AoA) to 35 degrees during the next phase of flights, having achieved 20 degrees AoA during the initial series of eight flights completed on 30 June.

The upgrade will optimise the scheduling and prioritising of control effectors to improve stability margins at higher AoAs. The X-36 - a 28% scale model of a tailless fighter - uses split ailerons and thrust vectoring for yaw control. The ailerons also provide roll control and the aircraft has forelplanes for pitch control.

NASA Ames project manager Mark Sumich says that the initial phase of flight testing exceeded expectations "…because stability margins were so good and the flight controls worked so well". The X-36 was flown to 2g in a variety of handling qualities manoeuvres. "We are extremely happy with the handling," says MDC programme manager Gary Jennings.

NASA-developed software enables the stability margin to be determined in real time, by stimulating the surfaces and downlinking data for processing to calculate the phase and gain margins, says Sumich. Furthermore, MDC-developed parameter-identification software allows the X-36 to be used as a "flying windtunnel", he says, by moving the surfaces and gathering data for off-line processing to determine the stability derivatives and control effectiveness, for comparison with computational fluid dynamics predictions and actual windtunnel results.

The last four flights of the initial phase were accomplished in four days, generating a "tremendous" amount of data, says Jennings, although according to Sumich, there were "challenges" on the first four flights. The first flight was cut short by overheating, while datalink failures resulted in loss of contact with the vehicle on each of the next three flights. The X-36 reverted to autopilot control until contact was re-established.

The next phase of flight testing will combine high AoA with high g, followed by high AoA with low speed. The aim is to show that the tailless X-36 can exceed the agility of current fighters, says Jennings. At least one more software optimisation cycle is planned, with the team hoping to minimise the checkout time to get the turn-around down to five to seven days.

NASA and MDC hope to fly the second X-36 in August, and to complete as many flights as possible before funding runs out by November/December.

Source: Flight International