getting to grips with aircraft performance 如何掌握飞机性能 2(13)

For a given weight: Temperature . . Limit Mach number .
For a given temperature: Weight . . Limit Mach number .

3.2.2. Maximum Cruise Altitude
On the other hand, when an aircraft flies at a given Mach number, the higher the altitude, the more the thrust must be increased. The maximum cruise altitude is defined for a given weight, as the maximum altitude that an aircraft can maintain at maximum cruise thrust when the pilot maintains a fixed Mach number.

.

m2 m1 Figure F15: Maximum Altitudes at Maximum Cruise Thrust
From Figure F15, it can be deduced that:
.  
At m1, the maximum altitude is PA1 for temperatures less than ISA + 10

.  
At m2, the maximum altitude is PA2 for temperatures less than ISA + 10, but PA1 for temperatures equal to ISA + 20.

Maximum cruise altitude variations can be summed up as:
weight . . Maximum cruise altitude . temperature . . maximum cruise altitude . Mach number . . maximum cruise altitude ì
 Figure F16 illustrates how maximum and optimum altitudes are shown in an A330 FCOM:

 IN FLIGHT PERFORMANCE CRUISE  3.05.15  P 6 
SEQ 055  REV 06 

3.3. En route Maneuver Limits
3.3.1. Lift Range
In level flight, lift balances weight and, when CL equals CLmax, the lift limit is reached. At this point, if the angle of attack increases, a stall occurs.
Lift limit equation:  
mg = 0.7 S P C M2
S Lmax
CLmax M2

At a given weight, depending on the lift limit equation, each CLmax.M2 value corresponds to a static pressure (Ps) value. That is, a pressure altitude (PA). Therefore, there is a direct relationship between CLmax.M2 and PA.

Figure F18: Lift Area Definition

3.3.2. Operating Maneuver Limitations

3.3.2.1. Buffet phenomenon
Concerning the low Mach number limit, when speed decreases, the angle of attack must be increased in order to increase the lift coefficient, which keeps the forces balanced.

In any case, it is not possible to indefinitely increase the angle of attack (AoA). At a high AoA, the airflow separates from the upper wing surface. If the AoA continues to increase, the point of airflow separation is unstable and rapidly fluctuates back and forth. Consequently, the pressure distribution changes constantly and also changes the lift’s position and magnitude. This effect is called buffeting and is evidenced by severe vibrations.
When the AoA reaches a maximum value, the separation point moves further ahead and total flow separation of the upper surface is achieved. This phenomenon leads to a significant loss of lift, referred to as a stall.
The high Mach number limit phenomenon is quite different. In fact, at high speed, compressibility effects produce shock waves on the upper wing surface. When Mach number, and/or AoA increase, the airflow separates from the upper surface behind the shock wave, which becomes unstable and induces buffeting of the same type as encountered in the low speed case.
　
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