getting to grips with aircraft performance 如何掌握飞机性能 2(14)

3.3.2.2. Buffet limit When maneuvering, the aircraft is subject to a load factor expressed as:
Lift
n =
Weight
During turns, the load factor value mainly depends on the bank angle, as shown in Figure F21. In fact, in level flight, n = 1/cos(bank angle).

0.7 S P C M2
S Lmax
n =
At the lift limit,  
m g
At a given pressure altitude (Ps) and given weight (mg), one load factor corresponds to each CL max M2. Therefore, a curve representing load factor versus Mach number will have the same shape as the one observed in Figure F17.
In fact, the useful limit Mach numbers in operation are the ones for which buffeting occurs.
Figure F22 represents the buffet limit, and for n = 1 (level straight flight), a minimum Mach appears for low speed buffet and a maximum Mach for high speed buffet. When n increases, the Mach number range decreases, so that when n = n max, Mmin = Mmax.
So, nmax is the maximum admissible load factor at this weight and altitude, and the corresponding Mach number M allows the highest margin regarding buffet limit.

3.3.2.3. Pressure altitude effect
 Figure F23 illustrates the effects of pressure altitude on the lift area. It appears that, for a given weight:
n max .
Pressure altitude .
lift range .
When nmax = 1, the aircraft has reached the lift ceiling. For example, in Figure F23, PA3  corresponds to the lift ceiling at a given weight.

At pressure altitude PA1 (Figure F23), nmax = 1.3. That is to say, it is possible to bear a load factor equal to 1.3, or make a 40° bank turn before buffeting occurs.

In order to maintain a minimum margin against buffeting and ensure good aircraft maneuverability, it is necessary to determine an acceptable load factor limit below which buffeting shall never occur. This load factor limit is generally fixed to
1.3. This value is an operating limitation, but not a regulatory one. The corresponding altitude is called the “1.3g buffet limited altitude” or “buffet ceiling”.
For a given Mach number, Figure F24 represents the 1.3g buffet limited altitude versus weight. At a given Mach number, when weight ì. the buffet limited altitude ê.

As a result, the maximum recommended altitude indicated by the FMGS, depending on aircraft weight and temperature conditions, is the lowest of the:
.
Maximum certified altitude,

.
Maximum cruise altitude,

.
1.3g buffet limited altitude,

.
Climb ceiling (see the “Climb” chapter).

3.3.2.4. A320 example
 Figure F25 shows how buffet limitations are illustrated in an A320 FCOM.

 OPERATING LIMITATIONS GENERAL LIMITATIONS  3.01.20  P 5 
SEQ 001  REV 27 

BUFFET ONSET
R

In practice, for a given weight, the load factor limitation (1.3g) is taken into account as follows:
　
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：getting to grips with aircraft performance 如何掌握飞机性能 2(14)