On ground, with hydraulic systems not pressurized, the normal position of the elevators is the droop down position to the servo control stop.
答案解析
解析：
### 题干解析
题干的意思是：在地面上，当液压系统没有增压时，升降舵的正常位置是向下偏转到伺服控制的止动位置。
1. **地面状态**：飞机在地面上，通常指的是飞机没有进行飞行操作。
2. **液压系统未增压**：液压系统是飞机控制系统的重要组成部分，负责操控各种舵面。在未增压的情况下，液压系统的压力不足，可能无法完全控制舵面。
3. **升降舵的正常位置**：升降舵是控制飞机俯仰（上下运动）的关键部件。在液压系统未增压的情况下，升降舵会自然下垂到一个特定的位置。
### 选项分析
- **A选项**：停在地面时，在液压系统没有增压的情况下，方向舵的正常位置是低垂位到伺服控制止挡。
- 这个选项的描述与题干相似，但“方向舵”通常指的是控制飞机左右转向的舵面，而题干提到的是“升降舵”。因此，这个选项不够准确。
- **B选项**：在地面模式，在液压系统没有增压的情况下，方向舵的正常位置是向下偏转到伺服控制止动位。
- 这个选项使用了“地面模式”这个术语，强调了飞机在地面上的状态，并且正确地描述了升降舵在液压未增压时的状态，即向下偏转到止动位。因此，这是正确答案。
- **C选项**：在地面时，在液压系统增压的情况下，方向舵的正常位置是向下偏转到伺服控制止动位。
- 这个选项与题干的条件相悖，因为题干明确指出是在液压系统未增压的情况下。
### 知识点总结
升降舵的工作原理可以通过一个简单的例子来理解：想象一下你在游泳池里，水面平静。你用手掌向下推水，水会向下流动，形成一个向下的力。这就像升降舵在飞行中通过改变气流来控制飞机的俯仰。
而在地面上，当液压系统未增压时，升降舵就像是放松的手掌，自动下垂到一个特定的位置。这是为了确保在飞机停放时，舵面不会因为液压失效而处于不稳定状态。
### 结论
相关知识点：
地面液压下方向舵位置记
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