The F-15 Active used a quad-redundant Digital Fly-By-Wire system. There original mechanical linkages to the control surfaces have been removed. The AI was paired with control algorithms for superior safety and performance. It used a 2nd-order equivalent system transfer function for the pitch rate to stick deflection tracking task. The low wing loading gave the F-15 active a low numerator time constant with a CAP value of 0.4. The FCS designers kept the same time constant and CAP but augmented to a higher time constant. This produced excellent tracking capabilities but at the expense of long-term flight path creep. These were implemented in the COMBAT and CONVENTIONAL flight control modes. The CRUISE mode had correct flight path responses to the pitch input at the cost of degraded tracking results. Stick deflection was the command input and all equivalent systems were defined in response to the stick deflection, not including the feel characteristics. There were interconnects from the lateral commands to the directional controls. All of the primary modes and secondary modes (sideslip excursion, roll oscillation) had Level 1 handling characteristics for all tasks besides tracking. A pitch-axis “ringing” problem was identified during the flight tests caused by a reduction in gain margin to around 3dB (Well under the 6dB gain margin specified by the MIL-F-9490D). A software fix restored the gain margins to 6dB.
The flight control design method was selected based on the control mode and the axis.
The design of this controller simplified the high-order compensator by using the more classical formulations of the design problem. This led to a good balance between performance and complexity. This was also more efficient than simply using the classical or the multivariate design. The classical design was easier to modify as it did not require an entire analysis for any small change in the feedback gains. Structural filters may have accounted for the differences between the conventional and combat modes. When an anti-slip breaking system was added without consulting the integration board, 19 software versions were required to achieve stable braking.
Three generations of the adaptive flight controller were built, Gen1 flew in 1999-2003, Gen2 flew from 2002-2006, and Gen3 flew in 2006
[[Intelligent Flight Control System (IFCS)]]
[[F-15 Active Flight Control System Requirements]]
[[F-15 Active Lateral Directional Model]] – used for lateral-directional control
[[F-15 Active Lateral Tracking Tasks]]
[[MIL-F-9490D]]
[[Wind Models]]
- rodriguezNASAIntelligentFlight2021
- RTO-TR-029
- AdaptiveFlightControl