词汇学习：
lift 升力
weight 重力
thrust 推力
drag 阻力
During flight, there are four forces acting on an airplane. These forces are lift, weight, thrust, and drag. Lift is the upward force created by the wing, weight is the pull of gravity on the mass, thrust is the force created by the airplane’s propeller or turbine engine, and drag is the friction caused by the air flowing around the airplane.
在飞行过程中，有四个力作用在飞机上。 这些力是升力，重力，推力和阻力。 升力是机翼产生的向上力，重力是作用在质量上的地心引力，推力是飞机的螺旋桨或涡轮发动机产生的力，阻力是飞机周围空气流动引起的摩擦。
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Any time the forces are not in balance, something about the airplane’s condition is changing. The possibilities are as follows:
每当力量不平衡时，有关飞机状况的事情就会改变。 可能性如下：
1. When an airplane is accelerating, it has more thrust than drag.
2. When an airplane is decelerating, it has less thrust than drag.
3. When an airplane is at a constant velocity, thrust and drag are equal.
4. When an airplane is climbing, it has more lift than weight.
5. When an airplane is descending, it has more weight than lift.
6. When an airplane is at a constant altitude, lift and weight are equal.
1.飞机在加速时，推力大于阻力。
2.飞机减速时，其推力小于阻力。
3.当飞机以恒定速度行驶时，推力和阻力相等。
4.飞机爬升时，升力大于重力。
5.当飞机下降时，它的重力大于升力。
6.当飞机处于恒定高度时，升力和重力相等。
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When an airplane is in straight-and-level flight at a constant velocity, all the forces acting on the airplane are in equilibrium. If that straight-and-level flight is disrupted by a disturbance in the air, such as wake turbulence, the airplane might pitch up or down, yaw left or right, or go into a roll. If the airplane has what is characterized as stability, once the disturbance goes away, the airplane will return to a state of equilibrium.
当飞机以恒定速度直飞并水平飞行时，作用在飞机上的所有力都处于平衡状态。 如果直航飞行因空气扰动（例如尾流）而中断，则飞机可能会上下颠簸，向左或向右偏航或翻滚。 如果飞机具有所谓的稳定性，则一旦干扰消失，飞机将返回平衡状态。
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The initial response that an airplane displays after its equilibrium is disrupted is referred to as its static stability. If the static stability is positive, the airplane will tend to return to its original position after the disruptive force is removed. If the static stability is negative, the airplane will continue to move away from its original position after the disruptive force is removed. If an airplane with negative static stability has the nose pitch up because of wake turbulence, the tendency will be for the nose to continue to pitch up even after the turbulence goes away. If an airplane tends to remain in a displaced position after the force is removed, but does not continue to move toward even greater displacement, its static stability is described as being neutral.
飞机在其平衡被破坏后显示的初始响应称为其静态稳定性。 如果静态稳定性为正，则消除干扰力后，飞机将倾向于返回其原始位置。 如果静态稳定性为负，则消除干扰力后，飞机将继续从其原始位置移开。 如果具有负静态稳定性的飞机由于尾流而使机头向上倾斜，则即使在湍流消失后，机头仍会继续向上倾斜。 如果飞机在消除力后趋于保持在位移位置，但没有继续朝更大的位移方向运动，则其静态稳定性称为中性。
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