getting to grips with aircraft performance 如何掌握飞机性能 2(37)

曝光台 注意防骗 网曝天猫店富美金盛家居专营店坑蒙拐骗欺诈消费者

Moreover, it has been demonstrated that a V1 speed less than 84% of VR renders the takeoff distances too long and doesn’t, therefore, present any takeoff performance advantages. Consequently, the minimum V1/VR ratio is equal to 0.84 (manufacturer value).
This is why the V1/VR ratio is used in the optimization process, since its range is well-identified:

2.3.1.2. V2/VS Range
The minimum V2 speed is defined by regulations (Part 25.107):

V2min = 1.2 VS (A300/A310) V2min = 1.13 VS1g (Fly-By-Wire aircraft) . (V2/VS)min = 1.2 or 1.13
The stall speed depends on weight. So, the minimum V2 speed is not a fixed value, whereas the minimum V2/VS ratio is known for a given aircraft type.
Moreover, a too high V2 speed requires long takeoff distances and leads to the reduction of climb performance (Figure J4). As it doesn’t present any advantage, the V2/VS ratio is limited to a maximum value (V2/VS maxi), which depends on aircraft type:
Figure J4 : 2nd Segment Climb Gradient versus V2/VS ratio
V2max = 1.35 VS (A300/A310)
V2max = 1.35 VS1g (A320 family)
V2max = 1.40 VS1g (A330)

. (V2/VS)max = 1.35 or 1.4 or 1.5
V2max = 1.50 VS1g (A340)

The V2/VS ratio is used in the optimization process, since its range is well-identified:
(V2/VS)min ≤
 V2/VS ≤
 (V2/VS)Max
Any V2/VS increase (resp. decrease) should be considered to have the sameeffect on takeoff performance as a V2 increase (resp. decrease).
2.3.2. V1/VR Ratio Influence
The purpose of this paragraph is to study the influence of V1/VR ratio variations on takeoff performance, while the V2/VS ratio remains constant. For that purpose, it is assumed that the following parameters are fixed:

2.3.2.1. Runway Limitations

2.3.2.2.   Climb and Obstacle Limitations
The V1 speed (decision speed on ground) has no influence on climb gradients (first, second and final takeoff segments).
On the contrary, the obstacle-limited weight is improved with a higher V1, as the takeoff distance is reduced. Therefore, the start of the takeoff flight path is obtained at a shorter distance, requiring a lower gradient to clear the obstacles.

Any V1/VR increase leads to (Figure J6):
.  
An  increase in MTOW limited by:

.  Obstacles

.  
Not influencing the MTOW limited by the:

. 
 First segment

. 
 Second segment

.  
Final takeoff segment

 

2.3.2.3. Brake Energy and Tire Speed Limitations
A maximum V1 speed, limited by brake energy (VMBE), exists for each TOW. To achieve a higher V1 speed, it is necessary to reduce TOW.
On the contrary, the decision speed doesn’t influence the tire speed limit.

2.3.2.4. All limitations
The following Figure (J8) shows that the highest of the maximum takeoff weights can be achieved at a given optimum V1/VR ratio. This optimum point corresponds to the intersection between two limitation curves.
　
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(37)