getting to grips with aircraft performance 如何掌握飞机性能 2(12)

When the aircraft flies at the optimum altitude, it is operated at the maximum lift to drag ratio corresponding to the selected Mach number (as in Figure F9).

When the aircraft flies at high speed, the polar curve depends on the indicated Mach number, and decreases when Mach increases. So, for each Mach number, there is a different value of (CL/CD)max, that is lower as the Mach number increases.
When the aircraft is cruising at the optimum altitude for a given Mach, CL is fixed and corresponds to (CL/CD)max of the selected Mach number. As a result, variable elements are weight and outside static pressure (Ps) of the optimum altitude. The formula expressing a cruise at optimum altitude is:
Weight
= constant
Ps
The optimum altitude curve, illustrated in Figure F10, is directly deduced from Figure F8.

Getting to Grips with Aircraft Performance  CRUISE
Summary: 
weight For a given PA : .  ..... . specific range optimum altitude . . 

ISO Mach number optimum altitude curves are all quasi-parallel (Figure F11).

3.1.2. Wind Influence
The MMR (or MLRC or MECON) value varies with headwind or tailwind, due to changes in the ground SR. Figure F12 shows the Maximum Range Mach number versus wind variations.

 CRUISE  Getting to Grips with Aircraft Performance 
As a result: 
tailwind   Ground SR M MR . ... ..  . . 
headwind  M MR Ground SR . . .. .. . . 

The wind force can be different at different altitudes. For a given weight, when cruise altitude is lower than optimum altitude, the specific range decreases (Figure F8). Nevertheless, it is possible that, at a lower altitude with a favorable wind, the ground specific range improves. When the favorable wind difference between the optimum altitude and a lower one reaches a certain value, the ground-specific range at lower altitude is higher than the ground-specific range at optimum altitude. As a result, in such conditions, it is more economical to cruise at the lower altitude.
Figure F13 indicates the amount of favorable wind, necessary to obtain the same ground-specific range at altitudes different from the optimum:

 IN FLIGHT PERFORMANCE CRUISE  3.05.15  P 7 
SEQ 020  REV 24 

WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE

GIVEN : Weight : 68000 kg (150 000 lb)
Wind at FL350 : 10 kt head FIND : Minimum wind difference to descend to FL310 : (26 . 3) = 23 kt RESULTS : Descent to FL310 may be considered provided the tail wind at this
altitude is more than (23 . 10) = 13 kt.

3.2. Maximum Cruise Altitude
3.2.1. Limit Mach Number at Constant Altitude
Each engine has a limited Max-Cruise rating. This rating depends on the maximum temperature that the turbines can sustain. As a result, when outside temperature increases, maximum thrust decreases (see Figure F14).
　
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