getting to grips with aircraft performance 如何掌握飞机性能 1(15)

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Two minimum unstick speeds must be determined and validated by flight tests:
- with all engines operatives : VMU (N)
-with one engine inoperative : VMU (N-1)
In the one-engine inoperative case, VMU (N-1) must ensure a safe lateral control to prevent the engine from striking the ground.
It appears that : VMU (N) ≤ VMU (N-1)

1.3.5. Stall Speed
Air velocity over the wing increases with the angle of attack, so that air pressure decreases and the lift coefficient increases.
. Air pressure .

Angle of Attack .
.
.

ê.Air velocity over the wing .

. Lift coefficient .

Therefore, the lift coefficient increases with the angle of attack. Flying at a constant level, this lift coefficient increase implies a decrease of the required speed. Indeed, the lift has to balance the aircraft weight, which can be considered as constant at a given time.
Angle of Attack .
.
CL .
Weight = . ρ S (TAS)2 CL = constant ρ = constant
S = constant

CL .
.
TAS .

Lift = constant
The speed cannot decrease beyond a minimum value. Above a certain angle of attack, the airflow starts to separate from the airfoil (Figure B5).

Figure B6 shows that the lift coefficient increases up to a maximum lift coefficient (CLmax), and suddenly decreases when the angle of attack is increased above a certain value.
This phenomenon is called a stall and two speeds can be identified :
-VS1g, which corresponds to the maximum lift coefficient (i.e. just before the lift starts decreasing). At that moment, the load factor is still equal to one (JAR 25 reference stall speed).
-VS, which corresponds to the conventional stall (i.e. when the lift suddenly collapses). At that moment, the load factor is always less than one (FAR 25 reference stall speed).
CL
n = 1g
n < 1g
Stall area (n ≤ 1g)

Angle of Attack CAS
VS1g VS

Figure B6: CL versus Angle of Attack

“JAR 25.103 Stall speed
(a) The reference stall speed VSR is a calibrated airspeed defined by the applicant. VSR may not be less than a 1-g stall speed. VSR is expressed as:

Where: VCLMAX = [speed of maximum lift coefficient, i.e. VS1g] nzw = Load factor normal to the flight path at VCLMAX”
Change 15 of JAR 25 (October 2000) introduced this notion of reference stall speed VSR, which is the same as Vs1g. In the previous version of JAR 25, a direct relationship between VS and VS1g was provided, in order to ensure the continuity between aircraft models certified at Vs, and aircraft models certified at VS1g.

For JAR, this rapport between Vs and Vs1g is:

As an example (refer to the “Takeoff” chapter):
.
For aircraft models certified at VS (A300/A310), V2min = 1.2 VS

.
For aircraft models certified at VS1g (Fly-By-Wire aircraft), V2min = 1.13 VS1g
　
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