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Dual Digital Flight Control System With 3 Axis Control Augmentation Plus Full Time Mechanical System

The F-20A used advanced aerodynamic technology to achieve high performance with an unrestricted flight envelope. Leading edge extensions (LEX) were carefully designed to control forebody vortices and to provide increased vortex induced lift. The maneuvering flap system provided continuous automatic flap operation to enhance wing efficiency. In the automatic mode, flaps were automatically positioned at the optimal setting as a function of airspeed, angle of attack, pitch rate, normal acceleration, and stick position. Excellent stability characteristics throughout the flight envelope eliminated the need for control system limiters, which otherwise would preclude attainment of maximum angle of attack or limit load factor.

The flight control system combined the advantages of an electronic, or fly by wire, system with the safety and reliability of a mechanical system. Primary flight controls included an all movable horizontal stabilizer, ailerons, and rudder. The flight controls in all three axes were controlled through a combination of pilot-commanded mechanical inputs and the dual digital three axis Control Augmentation System (CAS) commanded by the Flight Control Electronics Set (FCES). In the pitch axis, for example, the CAS system accounted for approximately 40 percent and the pilot's mechanical input for approximately 60 percent of the total control authority of the horizontal stabilizer.

Should the highly reliable FCES have failed for any reason, the mechanical system had sufficient authority under all loading conditions to control and land the aircraft safely.

The F-20A flight control system produced the following benefits:

  • Low pilot workload
  • Low maneuvering control stick forces
  • Neutral speed stability during gear up acceleration
  • Reduced control stick movement
  • Uniform aircraft response
  • Improved tracking capability
  • Operational enhancement
  • Allowed neutral stability center of gravity position
  • Eliminated ballast requirements.
AUTOPILOT

The F-20A autopilot provided the following HOLD modes:

  • Attitude
  • Heading
  • Altitude
  • Mach/airspeed
A control stick steering mode automatically disengaged the selected HOLD mode when the stick was moved and reengaged when the selected conditions were met and the stick was released.

HANDLING QUALITIES

Excellent handling qualities were provided throughout the flight envelope by a combination of aerodynamic characteristics and a flight control system optimized to allow precise control with reduced pilot workload. Above 350 KCAS, a constant maneuvering stick force gradient of approximately 4 pounds per g was provided. Below 350 KCAS, where the aircraft could maneuver to maximum lift conditions at high angles of attack, the gradient increased with decreasing airspeed to yield a constant stick force as a function of pitch rate. The F-20A's high degree of departure and spin resistance and its highly responsive flight system allowed the pilot to maneuver aggressively without fear of departing the aircraft.

Performance and operational reliability of the flight control electronics system (FCES) were achieved by a dual digital flight control computer (FCC). The FCES combined high reliability with a simple actuation scheme that capitalized on performance gains of relaxed static stability while achieving excellent handling qualities. In the event of multiple failures of the FCES, control reverted to the active mechanical control system, with handling qualities sufficient to allow the pilot to return to base and land safely.

Increased mission operational reliability was provided in the FCES through dual channels of air data computation by the digital air data unit (DADU). The DADU supplied Mach number, altitude, and dynamic pressure data to the FCC and the mission computer.

Required air data parameters were computed and transmitted by the DADU to the radar, HUD, and environmental control system through the FCC interface with the MUX bus.

OVERSTRESS REMINDER

The F-20A flight control system incorporated a computer controlled "g reminder" system designed to prevent inadvertent overstress of the aircraft. As the aircraft reached limit load factor, the normally low and constant stick force per g increased substantially, informing the pilot that he was at design limit load factor. Limit load factor was automatically computed by the pitch CAS computer as a function of external stores loading and fuel state. In an emergency situation, however, the pilot could override the system by pulling through the increased resistance.


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© Mark Wade, 1997 - 2006 except where otherwise noted.
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