How do airplanes actually fly?_Raymond Adkins

How do airplanes actually fly?

 

By 1917, Albert Einstein had explained the relationship between space and time. But, that year, he designed a flawed airplane wing. His attempt was based on an incomplete theory of flight. Indeed, insufficient and inaccurate explanations still circulate today. So, where did Einstein go wrong? And how do planes fly?

1917 年，阿尔伯特 · 爱因斯坦解释了 时间和空间的关系。 但就在那年，他却设计了一个 有缺陷的机翼。 他的尝试是基于 一个不完备的飞机原理。 事实上，不充分和不准确的解释 至今仍在流传。 那爱因斯坦错在哪儿呢？ 飞机又是怎么飞的？

 

Though we don’t always think of it this way, air is a fluid medium— it’s just less dense than liquids like water. Things that are lighter than air are buoyant within it, while heavier objects require an upward force, called lift, to stay aloft. For planes, this force is mostly generated by the wings.

尽管我们不常从这个角度思考， 但空气是一种流体介质—— 其密度小于水等液体。 轻于空气的物体 即可飘浮其中， 而较重的物体则需要一个向上的力， 称为升力，以保持在高空。 飞机的升力主要是由机翼产生的。

 

One especially pervasive false description of lift is the “Longer Path” or “Equal Transit Time” explanation. It states that air molecules traveling over the top of a curved wing cover a longer distance than those traveling underneath. For the air molecules above to reach the wing’s trailing edge in the same instance as those that split off and went below, air must travel faster above, creating a pocket of lower pressure that lifts the plane. This explanation has been thoroughly debunked. Air molecules floating above and below the wing don't need to meet back up. In reality, the air traveling above reaches the wing’s trailing edge much faster than the air beneath.

一个关于升力 特别普遍的错误描述 是“更长的路径”和 “相同传输时间”的解释。 其指出空气分子 在弧形机翼上方滑过的距离 大于其滑过机翼下方的距离。 为了使上方的空气分子到达机翼后缘， 并且保持和下方空气分子同时相遇， 空气必须在上方划过的更快， 创造出一个低压区 从而使飞机升空。 这个解释已经被完全推翻。 飘浮在机翼上方和下方的空气分子 不需要重新相遇。 实际上，上方的空气 到达机翼边缘的时间， 远快于下方的空气。

 

To get a sense of how lift is actually generated, let's simulate an airplane wing in motion. As it moves forward, the wing affects the movement of the air around it. As air meets the wing’s solid surface, a thin layer sticks to the wing. This layer pulls the surrounding air with it. The air splits into pathways above and below the wing, following the wing’s contour. As the air that’s routed above makes its way around the nose of the wing, it experiences centripetal acceleration, the force you also feel in a sharply turning car. The air above therefore gathers more speed than the air traveling below. This increased speed is coupled with a decrease in pressure above the wing, which pulls even more air across the wing’s upper surface. The air flowing across the lower surface, meanwhile, experiences less of a change in direction and speed. The pressure across the wing’s lower surface is thus higher than that above the upper surface. This pressure difference results in the upwards force of lift. The faster the plane travels, the greater the pressure difference, and the greater that force. Once it overcomes the downward force of gravity, the plane takes off.

为了明白升力到底是如何产生的， 我们来模拟下飞行中的机翼。 随着其前进，机翼会影响 其周围的空气运动， 当空气接触到机翼坚硬的表面， 一层的空气薄层附着在机翼上。 该薄层会拉动周围空气。 空气会根据机翼的轮廓， 在机翼上方和下方形成不同通道。 当上面的空气绕过机翼的前端时， 会有一个向心加速度， 这种力同样可以在 车辆急转弯时体会到。 所以上方空气的速度会大于下方的空气。 机翼上方速度增加的同时 会伴随着一个压强的减小， 进而拉动更多空气通过机翼上表面。 与此同时，穿过机翼下方的空气， 在方向和速度的改变较小。 因此，机翼下表面的压强 高于机翼上表面。 这种压强差进而产生了 向上的升力。 飞机飞行速度越快， 压强差越大，升力也就越大。 一旦克服重力的向下作用力， 飞机就飞起来了。

 

Air flows smoothly around curved wings. But a wing’s curvature is not the cause of lift. In fact, a flat wing that’s tilted upwards can also create lift— as long as the air bends around it, contributing to and reinforcing the pressure difference. Meanwhile, having a wing that’s too curved or steeply angled can be disastrous: the airflow above may detach from the wing and become turbulent. This is probably what happened with Einstein’s wing design, nicknamed “the cat’s back.” By increasing the wing’s curvature, Einstein thought it would generate more lift. But one test pilot reported that the plane wobbled like “a pregnant duck” in flight.

空气在弯曲的翅膀上顺利流过。 但升力的产生并不是因为机翼的弧度。 实际上，向上倾斜的平坦机翼 也可以造成升力—— 只要机翼周围的空气弯曲， 形成并加强压强差。 而过于弯曲或者角度过陡的机翼 是十分灾难的； 机翼上方的气流 可能会脱离机翼而变成湍流。 这可能就是 爱因斯坦机翼设计失败的原因， 从此有了“猫背翼”的美名。 通过增大机翼的弯曲弧度， 爱因斯坦认为会产生更大的升力。 但以为测试飞行员反馈 飞机飞行中会摇晃， 就像一个“怀孕的鸭子”。

 

Our explanation is still a simplified description of this nuanced, complex process. Other factors, like the air that’s flowing meters beyond the wing’s surface— being swept up, then down— as well as air vortices formed at the wing’s tips, all influence lift. And, while experts agree that the pressure difference generates lift, their explanations for how can vary. Some might emphasize the air’s behavior at the wing’s surface, others the upward force created as the air is deflected downwards. However, there's no controversy when it comes to the math. Engineers use a set of formulas called the Navier-Stokes equations to precisely model air’s flow around a wing and detail how lift is generated.

我们的解释仅是 对这一细微、复杂过程的简化描述。 其他的因素，比如机翼上方 几米处流动的空气—— 被卷起向上，后又向下—— 以及在机翼尖处形成的涡流， 全都影响着升力。 而且，虽然专家们认同升力 是由压强差产生的， 但他们对产生过程的阐述却不尽相同。 一些专家会强调空气 在机翼表面的表现， 其他专家则认为向上的力 是由空气向下偏转时产生的。 然而，说到数学原理时 则毫无争议。 工程师们采用了名为 纳维-斯托克斯方程 （Navier-Stokes equations）的一组公式 精准地建立空气绕机翼流动的模型， 并详细说明升力是如何产生的。

 

More than a century after Einstein’s foray into aeronautics, lift retains its reputation as a confounding concept. But when it feels like it’s all going to come crashing down, remember: it’s just the physics of fluid in motion.

在爱因斯坦 涉足航空领域的一个多世纪之后， 升力仍是一个未解之谜。 当你感觉当要坠落的时候， 请记住： 这只是流体运动的物理现象。

 

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