2
1
0
–1
–2
Stabilizer position, – 3 deg
–4
–5
–6
–7
–8

CG  = 34 pe rcent MAC 

30 pe rcent 
25 percent 

21 pe rcent 
12 percent 

140  160 180 200 220 240 260 280 300 320 340 360 Airspeed, kn
970592
(k) Crossplot, GW = 560,000 lb. Figure 10. Concluded.

Speed Effects on Propulsive Control Power
The propulsive forces (differential thrust for lateral control and collective thrust for .ightpath control) tend to be relatively independent of speed. The aerodynamic restoring forces that resist the propulsive forces are, however, proportional to the dynamic pressure, which is a function of speed squared. This relationship would result in the propulsive control power being approximately inversely proportional to the square of the airspeed, as discussed in reference 1. On the MD-11, the slight EPR and thrust loss with increasing speed, shown earlier in .gures 3 and 9, reduces this effect somewhat.

Surface Float With Hydraulic Systems Off
With the hydraulic system failed, a surface will .oat to the zero hinge moment condition. For the MD-11 hinge geometry, this position is essentially the trail position. Simulator studies on the MD-11 indicated that a total hydraulic system failure would cause the ailerons to .oat trailing edge up, the amount depending on speed, thus reducing lift and increasing trim airspeed. Rudder .oat would only negligibly affect trim speed but would reduce directional stability. Elevators are usually trimmed to near zero force; hence elevator .oat would have a small effect. The stabilizer is usually moved with a jackscrew actuator that, in case of hydraulic failure, remains .xed due to friction. Flight data shown later do not agree with the simulator predictions.

Wing-Engine-Out With Lateral CG Offset Control
If an airplane without .ight controls and without any operating engine on one wing has the lateral CG offset toward the side with one or more operating engines, that engine thrust can be modulated to develop yaw and a rolling moment to counter the moment from the lateral CG offset. With proper thrust modulation, providing a degree of bank angle control is possible. If thrust is reduced from a wings-level condition, the airplane rolls toward the operating engine. Conversely, if thrust is increased above that needed for wings level, the airplane rolls away from the operating engine. The degree of lateral offset dictates the level of thrust required for wings level and, hence, the average .ightpath.

 CONTROL MODES USING ENGINE THRUST
With inoperative .ight controls, engine thrust can generate pitching and rolling moments, as discussed earlier. The pilot can manually move the throttles or use a PCA system to send commands and feedback parameters to generate throttle commands. Tests were performed to evaluate both methods on the MD-11 airplane.
Manual Throttles-Only Control
For these tests, the crew turned off the LSAS, yaw dampers, speed protect system (by lowering the AFS override switches on the FCP), and the fuel transfer system. The crew then trimmed the airplane, released the .ight controls, and used only the throttles for .ight control. In this mode, the airplane behaved much like it would with a total .ight control system failure. Differential thrust controlled bank angle, and collective thrust controlled pitch. Results are discussed later.

PCA System Control
In the PCA system, closed-loop control of engine thrust is provided to track pilot-commanded .ightpath and ground track. Figure 11 shows simpli.ed block diagrams of the PCA control laws. For the two-engine PCA system, the pilot uses the center engine only manually as a low-frequency speed trimmer. The control modes that used the center engine are discussed later. The lateral control system shows the track mode and a bank angle control mode which was also available.
　
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