Development and Flight Test of an Emergency Flight Control System Using Only Engine Thrust on an MD-11 Transport Airplane(14)

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In addition, an IBM mainframe computer hosted a non-real-time batch version of the full MD-11 simulation. This nonlinear simulation was used for control system development and evaluation.
For .ight control system and engine control system integration testing, the MD-11 bench simulation was used. The bench simulation allowed actual PCA software in FCCs and one FADEC to operate through the data buses, and used a database similar to the FDS. Pilot inputs could be simulated through a joystick interface. Linear models of the MD-11 were extracted from nonlinear simulations and used at MDA and NASA for control system design and analysis (refs. 12 and 13).

PRINCIPLES OF THROTTLES-ONLY FLIGHT CONTROL FOR THE MD-11
The following section uses examples from the MD-11 airplane to present the principles of throttles-only .ight control for the MD-11. First, basic lateral-directional and longitudinal modes will be discussed. The later sections discuss airspeed control, speed effects on propulsive control power, surface .oat with hydraulics off, and a control concept with only one wing engine and an offset lateral CG.
Lateral-Directional
Differential thrust is effective in producing roll for most airplanes, including the MD-11. Differential thrust generates yaw (sideslip), in the direction of the turn. In addition, rolling moments are developed from the dihedral effect. Swept-wing airplanes also have an additional rolling moment that is a function of twice the wing sweep angle and the wing lift. A rolling moment may also be contributed from the vertical tail. All of these rolling moments normally are in the same direction as the yaw and result in the airplane rolling in the direction of the yaw. Proper modulation of the differential thrust allows the airplane to be rolled to a desired bank angle, which results in a turn and change in aircraft heading.
An open-loop throttle step response for the MD-11 is shown in .gure 7 at 220 kn with gear down and .aps up. The commanded 10° throttle split results in about 20,000 lb of differential thrust and a roll rate averaging 1.5° /sec. The EPRs lag the throttle by about a second, and roll rate lags sideslip, which lags yaw rate. A lightly damped dutch roll mode is excited by this throttle step. Full differential thrust for the MD-11 at a speed of 150 kn yields a peak roll rate of approximately 8° /sec.

Longitudinal
Longitudinal pitch control from throttle changes is more complex, in part because more modes are being controlled. Several effects can occur:
1.
Flightpath angle change due to speed stability

2.
Pitching moment due to thrust-line offset

3.
Flightpath angle change due to the vertical component of thrust

4.
Phugoid

The following sections describe each effect in the order stated.
Bank
angle,
deg

Roll
and yaw
rate,
deg/sec

Angle of
sideslip,
deg

Throttle
angle,
deg

EPR

30
20
10

0
3
2
1
0

–1
.5
0

–
.5

–
1.0

60
50
1.4
1.3
1.2
1.1
–
1.5 70

Differential throttle step input 

 

 

Roll rate 
Yaw rate 

 

 

Left 
Right 

Left 
Right 

0 10 20 30
Time, sec

970578

Figure 7. Differential thrust open-loop step response, MD-11, 220 kn, 15,000 ft, .aps up, gear down, center engine idle, and LSAS and yaw dampers off.

Flightpath Angle Change Due to Speed Stability
Most stable airplanes, including the MD-11, exhibit positive speed stability. Over a short time (≈ 10 sec), a thrust increase causes a speed increase, which causes a lift increase. With the lift being greater than the weight, the airplane climbs. The long-term effect is oscillatory (see Phugoid section later).
　
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