Status
已完成
已完成
How do airplanes stay in the air?
飞机如何在空中停留？
听力音频：
听力材料：英文原文及中文翻译
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?
到1917年，阿尔伯特-爱因斯坦已经解释了空间和时间之间的关系。但是，这一年，他设计了一个有缺陷的飞机机翼。他的尝试是基于一个不完整的飞行理论。事实上，不充分和不准确的解释至今仍在流传。那么，爱因斯坦在哪里出了问题？飞机又是如何飞行的呢？
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.
虽然我们并不总是这样想，但空气是一种流体介质--它的密度比水等液体小。比空气轻的东西在空气中具有浮力，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。对于飞机来说，这种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.
对升力的一个特别普遍的错误描述是 "较长路径 "或 "相等运输时间 "的解释。它指出，空气分子在弧形机翼的顶部行进的距离比在下面行进的距离要长。为了使上面的空气分子与下面的空气分子一样到达机翼的后缘，空气在上面必须走得更快，从而产生一个较低压力的口袋，使飞机升空。这种解释已经被彻底推翻了。漂浮在机翼上方和下方的空气分子不需要重新相遇。在现实中，上面行驶的空气到达机翼后缘的速度比下面的空气快得多。
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.
为了了解升力是如何实际产生的，我们来模拟一个运动中的飞机机翼。当它向前移动时，机翼会影响它周围空气的运动。当空气遇到机翼的固体表面时，会有一个薄层粘在机翼上。该层将周围的空气拉到一起。空气沿着机翼的轮廓，在机翼的上方和下方分成若干条通道。当上面的空气绕过机翼的前端时，它经历了向心加速度，也就是你在急转弯的汽车中感受到的力量。因此，上面的空气比下面的空气收集了更多的速度。这种速度的增加伴随着机翼上方压力的下降，这将拉动更多的空气穿过机翼的上表面。同时，流经下表面的空气在方向和速度上的变化较小。因此，机翼下表面的压力比上表面的压力高。这种压力差导致了向上的升力。飞机的速度越快，压力差就越大，这个力也就越大。一旦克服了重力的向下作用力，飞机就会起飞。
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.
空气在弯曲的机翼上平稳地流动。但机翼的弧度并不是升力的原因。事实上，一个向上倾斜的平坦机翼也能产生升力--只要空气在它周围弯曲，促成并加强了压力差。同时，一个过于弯曲或倾斜的机翼可能是灾难性的：上面的气流可能会从机翼上分离出来，变得湍流。这可能就是爱因斯坦的机翼设计所发生的情况，它被昵称为 "猫的背部"。通过增加机翼的曲率，爱因斯坦认为它将产生更多的升力。但一位试飞员报告说，飞机在飞行中像 "怀孕的鸭子 "一样摇摆不定。
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.
我们的解释仍然是对这个细微、复杂过程的简化描述。其他因素，如流过机翼表面数米的空气--被卷起，然后向下--以及在机翼顶端形成的空气涡流，都会影响升力。虽然专家们同意压力差会产生升力，但他们对如何产生升力的解释可能有所不同。有些人可能会强调空气在机翼表面的行为，有些人则强调空气向下偏转时产生的向上的力量。然而，在谈到数学时，并没有争议。工程师们使用一套被称为纳维-斯托克斯方程的公式来精确模拟空气在机翼周围的流动，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一个多世纪后，升力仍然是一个令人困惑的概念。但是，当感觉一切都要崩溃的时候，请记住：这只是运动中的流体物理学。
笔记区