How do airplanes actually fly? - Raymond Adkins

譯者: Lilian Chiu 審譯者: Helen Chang

By 1917, Albert Einstein had explained the relationship between space and time.

到了 1917 年時，

亞伯特‧愛因斯坦已經解釋了 空間和時間之間的關係。

But, that year, he designed a flawed airplane wing.

但那一年，他設計出了 有問題的機翼。

His attempt was based on an incomplete theory of flight.

他嘗試的做法是以一個 不完整的飛行理論為依據。

Indeed, insufficient and inaccurate explanations still circulate today.

的確，不充份和不正確的解釋 現今仍然在流傳著。

So, where did Einstein go wrong?

那麼，愛因斯坦錯在哪裡？

And how do planes fly?

飛機又是怎麼飛起來的？

Though we don’t always think of it this way, air is a fluid medium—

雖然我們不見得都用這種角度思考，

但空氣是一種流體介質——

it’s just less dense than liquids like water.

只是它的密度比水這類液體還要低。

Things that are lighter than air are buoyant within it,

比空氣輕的東西會浮在空氣中，

while heavier objects require an upward force, called lift, to stay aloft.

而較重的東西則需要向上的力量，

稱為升力，才能持續留在空中。

For planes, this force is mostly generated by the wings.

就飛機來說，升力 多半是由機翼產生。

One especially pervasive false description of lift

關於升力，最普遍的錯誤說法之一

is the “Longer Path” or “Equal Transit Time” explanation.

是「更長的路徑」 或「相等通過時間」這套解釋。

It states that air molecules traveling over the top of a curved wing

這個說法是：空氣分子從 有弧度的機翼上方流過，

cover a longer distance than those traveling underneath.

要行經的距離 比從下方流過的分子更長。

For the air molecules above to reach the wing’s trailing edge

從上方流過的分子 要抵達機翼的後緣，

in the same instance as those that split off and went below,

且要和被機翼分開而從下方 通過的分子同時抵達，

air must travel faster above,

上面的空氣一定得 以更快的速度行進，

creating a pocket of lower pressure that lifts the plane.

創造出一個壓力較低的 空間，讓飛機被抬起。

This explanation has been thoroughly debunked.

這個解釋已經被完全拆穿了。

Air molecules floating above and below the wing don't need to meet back up.

從機翼上方和下方通過的空氣分子

並不需要再相會。

In reality, the air traveling above reaches the wing’s trailing edge

實際上，從上方通過的 空氣分子抵達機翼後緣的時間

much faster than the air beneath.

會比下方的空氣早許多。

To get a sense of how lift is actually generated,

為了了解升力實際上是如何產生的，

let's simulate an airplane wing in motion.

咱們來模擬飛行中的機翼。

As it moves forward, the wing affects the movement of the air around it.

機翼向前移動時， 會影響其周圍空氣的移動。

As air meets the wing’s solid surface, a thin layer sticks to the wing.

在空氣碰到機翼的堅固表面時，

會有薄薄一層空氣附著在機翼上。

This layer pulls the surrounding air with it.

這一層會將周遭的空氣朝它拉過去。

The air splits into pathways above and below the wing,

空氣會分成兩路， 分別從機翼的上方和下方

following the wing’s contour.

沿著機翼的外輪廓通過。

As the air that’s routed above makes its way around the nose of the wing,

當上方的空氣沿著機翼前緣通過時，

it experiences centripetal acceleration,

會發生向心加速。

the force you also feel in a sharply turning car.

也就是坐在急轉彎的車上 會感受到的那股力量。

The air above therefore gathers more speed than the air traveling below.

因此上方空氣的移動速度 會比下方空氣更快。

This increased speed is coupled with a decrease in pressure above the wing,

而速度提升也會伴隨著 機翼上方的壓力下降，

which pulls even more air across the wing’s upper surface.

導致更多空氣從機翼上方流過。

The air flowing across the lower surface, meanwhile,

同時，從下方流過的空氣

experiences less of a change in direction and speed.

相對沒有那麼多方向和速度的改變。

The pressure across the wing’s lower surface is thus higher

因此，機翼下方的壓力會較高，

than that above the upper surface.

比上方的壓力還高。

This pressure difference results in the upwards force of lift.

這種壓力差造成向上的升力。

The faster the plane travels,

飛機飛得越快，

the greater the pressure difference, and the greater that force.

壓力差就會越大，升力也就會越大。

Once it overcomes the downward force of gravity,

一旦升力大於向下的重力，

the plane takes off.

飛機就能起飛了。

Air flows smoothly around curved wings.

空氣會平順地沿著機翼的弧線流動，

But a wing’s curvature is not the cause of lift.

但升力並非機翼的弧度所造成。

In fact, a flat wing that’s tilted upwards can also create lift—

事實上，將平的機翼向前傾斜

也能創造出升力，

as long as the air bends around it,

只要空氣會因為它而彎曲，

contributing to and reinforcing the pressure difference.

造成和強化壓力差，即可。

Meanwhile, having a wing that’s too curved or steeply angled

而如果機翼的弧度太大或角度太陡，

can be disastrous:

可能會造成災難：

the airflow above may detach from the wing and become turbulent.

機翼上方的氣流可能會 與機翼分離，變成亂流。

This is probably what happened with Einstein’s wing design,

愛因斯坦設計的機翼 可能就有這個問題，

nicknamed “the cat’s back.”

他把這個設計暱稱為「貓背」。

By increasing the wing’s curvature,

之所以增加機翼的弧度，

Einstein thought it would generate more lift.

是因為愛因斯坦認為 這樣能產生更多升力。

But one test pilot reported that the plane wobbled

但一名測試飛行員回報說 飛機飛起來很不穩，

like “a pregnant duck” in flight.

就像是「懷孕的鴨子在飛」。

Our explanation is still a simplified description

我們的解釋其實也只是簡單描述

of this nuanced, complex process.

這個精密又複雜的過程。

Other factors, like the air that’s flowing meters beyond the wing’s surface—

其他因素，比如在距離機翼 數公尺外流動的空氣——

being swept up, then down—

先被捲向上，之後又下降——

as well as air vortices formed at the wing’s tips,

還有在機翼尖端附近形成的氣旋，

all influence lift.

都會影響升力。

And, while experts agree that the pressure difference generates lift,

且，雖然專家都認同 壓力差會產生升力，

their explanations for how can vary.

對於如何產生，卻眾說紛云。

Some might emphasize the air’s behavior at the wing’s surface,

有些人強調機翼表面空氣的變化，

others the upward force created as the air is deflected downwards.

其他人則著重空氣被迫轉向 向下時產生出的向上升力。

However, there's no controversy when it comes to the math.

但是，在數學上是沒有爭議的。

Engineers use a set of formulas called the Navier-Stokes equations

工程師們用一組方程式， 即「納維爾—史托克方程式」，

to precisely model air’s flow around a wing

來精確建立出機翼周圍的氣流模型，

and detail how lift is generated.

並詳細說明升力是如何產生的。

More than a century after Einstein’s foray into aeronautics,

在愛因斯坦初次涉足 航空學的一個世紀多之後，

lift retains its reputation as a confounding concept.

升力仍然保有 「困惑人的觀念」這名聲。

But when it feels like it’s all going to come crashing down, remember:

但在感覺好像要墜機時，別忘了：

it’s just the physics of fluid in motion.

這只是流體物理學在發揮作用而已。

This video was made possible with support from Marriott Hotels.

因為有萬豪酒店的支持， 這支影片才得以誕生。

With over 590 hotels and resorts across the globe,

在全球各地有超過 590 家 旅館和渡假飯店，

Marriott Hotels celebrates the curiosity that propels us to travel.

萬豪酒店讚頌的是 促使我們去旅行的好奇心。

Check out some of the exciting ways TED-Ed and Marriott are working together,

來看看 TED-Ed 和萬豪酒店 一些讓人興奮的合作方式，

and book your next journey at Marriott Hotels.

並為你的下一趟旅行， 在萬豪酒店訂房。