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Figure 43. Analytical trim speed variation with GW, CG, landing gear, and center-engine thrust setting. MD-11 initial conditions: 285 kn high altitude cruise, 31 percent CG.
Other data also show that a suitable landing speed can be achieved for most conditions in a typical .ight. Some dif.culties arise if a total hydraulic failure occurs right after takeoff at a high gross weight. Aileron .oat and dumping fuel would cause the speed to be reduced to an unacceptably low value unless the CG can be transferred forward and the center-engine thrust increased above that of the wing engines. At maximum landing weight of 430,000 lb, as shown in .gure 6, fuel could be transferred to the center tank to move the CG forward to 21 percent. Figure 10(j) or 10(k) shows that moving the CG from 25 to 21 percent would increase speed by 30 kn.

Wing Engine and Lateral CG Offset
As discussed in the section, Principles of Throttles-Only Control, the MD-11 without .ight controls and without an operating engine on one wing can have a degree of control if the lateral CG is offset toward the side with the operating engine (ref. 22).
The MD-11 Flight Deck Simulator (FDS) was used to brie.y study the wing-engine-out thrust control capability. On the MD-11, a CG offset of up to 48 in. can be obtained using the existing fuel system. Figure 44 shows the maximum lateral CG offset as a function of fuel quantity. If one wing tank is full and the other is empty, a CG offset of 48 in. occurs. With all tanks full, obviously no offset is possible. As fuel is burned or dumped, the maximum offset occurs after the tanks in one wing are empty and can be maintained as long as fuel is in the center or tail tanks to keep the other wing tank full. After the center and tail tanks are empty, the lateral CG offset decreases until, with all fuel exhausted, it is again zero.
Figure 45 shows MD-11 FDS results of the time required for the fuel transfer, based on the normal fuel pump operational rates. Starting with all wing tanks equally full, about 7 min is needed to get a lateral CG offset of 25 in., transferring from the left wing to the right wing. At this time the right wing is full, and further transfer is from the left wing to the center tank, which is obviously less effective in shifting lateral CG offset. After a total of 13 min, the lateral CG was 40 in., and the maximum tested 45 in. was reached in 15 min. An average rate of change of lateral CG offset is ≈ 3 in./min.
50
40
30
CGY, in.
20
10

 
 
Centai ter and l tanks  Emptying 
 One wing  empty 
em pty  center antail tanks  d 

E mptying  right 

 Empwin tying left g tank  wing tank 
 

 All  tanks full 
All  tanks empty 
0 40 80 120 160 200 240 280 320 x 103 Total fuel quantity, lb

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Figure 44. MD-11 lateral CG offset compared with total fuel quantity and location.
50
40
30
CGY, in.
20
10

Transfeto cen r from left ter fuselage tank  wing 

Right wing tanks full 

Transfe r from left 
wing t o right wing 

Figure 45. Time history of fuel transfer and lateral CG offset, MD-11 FDS. Initial wing fuel tank quantities approximately 31,000 lb.

0 2 4 6 8 10 12 14 16 Time, min
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Well within this lateral CG offset, wings-level .ight on one engine is available over a range of speeds from 200 to 300 kn. Figure 46 shows simulation results of the engine 3 EPR required to hold wings level (with engine 1 off or at idle) as a function of speed. A level .ightpath is possible depending on speed and the degree of lateral CG offset.
Figure 47 shows a time history of a right-engine throttle step increase followed by a step to idle at 205 kn with gear and .aps up, and the left and center engines at idle and a lateral CG offset of 35 in. The initial sideslip is 2° . With the thrust increase, sideslip increased and the roll rate was 5 deg/sec. Angle of attack also increased because the engine was below the vertical CG as well as to the right of the lateral CG. As the bank angle passed through 40° , the right-engine thrust was reduced to idle, which caused the sideslip to go to zero and the roll rate to reverse to ≈ 4 deg/sec. Maximum roll rates up to 4 to 5 deg/sec are possible, although depending on speed, it may not be equal in each direction. These rates should be adequate for runway lineup in light turbulence.
　
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