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为什么飞机会有后掠翼?

Why Do Backwards Wings Exist?

This episode of Real Engineering is brought to you by CuriosityStream,
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On December 12th, 1984, the United States Air Force and NASA
1984年12月12日 美国空军和NASA
began testing an unusual aircraft.
开始测试一架不同寻常的飞机
One that broke all aircraft design convention.
一架打破了所有设计常规的飞机
It’s wings pointed forward.
其机翼朝向前方
However this experimental aircraft dubbed the X-29,
然而这架被称为X-29的实验飞机
was not the first of its kind.
并不是首架这样的飞机
The German’s also experimented with the concept
德国人在二战后期就用Junker
in the late stages of World War 2 with the Junker JU-287,
JU-287尝试过这种概念
and it’s prototype airframes would eventually end up in the hands of the Soviets
而原型机最终却落入苏联人之手
who took and developed the design into
苏联人将这种设计用在了
the okb-1 ef 131 and OKB-1-140. [1]
OKB-1 ef 131和OKB-140的开发中[1]
All of these early iteration designs ran into the same problems.
所有这些早期的迭代设计都遇到同样的问题
This design was incredibly aerodynamically unstable.
这种设计在空气动力学上相当地不稳定
When the wing deflects the force of the oncoming wind
当机翼转向时 迎面风的力量
tends to make it deflect even more.[2]
往往会使机翼偏转得更厉害[2]
This is a rather obvious design flaw.
这是一个相当明显的设计缺陷
Intuitively you just now looking at it that something doesn’t quite make sense.
仅从直观上看 这些并没有特别意义
So why did Germany, The Soviet Union and the United States
那为何德国 苏联和美国
all see the design worthy of consideration?
都认为这种设计值得考虑呢?
To understand this, we first have to explore
为了理解这点 我们先要弄清楚
why wings are swept beyond a perfectly perpendicular angle in the first place.
为什么最初是呈钝角的掠翼
Looking at most aircraft developed during world war 2,
看看二战期间开发的大多数飞机
you can see that nearly all of them had straight wings.
你会发现几乎所有的飞机都是平直翼
The Spitfire, the mitzubishi A6M Zero and the P-51 Mustang.
诸如喷火式 三菱的零式A6M 野马P-51等战斗机
It was only during the later stages of the war,
直到战争后期
as more powerful engines came to the fore that other designs started to emerge,
随着更强的引擎涌现 才开始有不同的设计
and in one case the straight wing became a major design flaw
在一次事故中 平直翼成了主要的设计缺陷
that put the crew of the P-38 Lightning in serious danger.
将闪电P-38机组人员置于严重危险中
These problems arose directly as a consequence of how wings generate lift.
这些问题是机翼升力的产生方式直接导致的
An aerofoil is designed to make use of bernoulli’s principle,
机翼的设计利用了伯努利原理
where a low pressure zone is created on top of the wing as a result of airflow moving faster.[3]
机翼顶部的低压区是更快速的气流流动形成的[3]
People like to say this false , but it’s just one of the ways a wing generates lift,
人们会说这不符合实际 但这只是机翼产生升力的方式之一
there is a lot more to the story.
这里面的学问大着呢
Because this airflow actually speeds up as it crosses the wing,
因为气流穿过机翼时 实际上会加速
it can reach supersonic speeds long before the plane itself reaches supersonic speeds.[3]
机翼能比飞机本身更早达到超音速
This causes problems because supersonic flow means shock waves form,
这就是问题 因为超音速气流意味着有冲击波
which can disrupt normal airflow over the wing.
冲击波将破坏机翼上正常的气流
On November 4th 1941,
1941年11月4日
these problems resulted in the death of Ralph Virden,
这些问题导致了Ralph Virden
an expert test pilot,
一名专业试飞员
during a high speed test dive of the P-38 lightning.
在闪电P-38的高速俯冲飞行测试中的死亡
The causes of the crash were unclear at the time.
当时事故的原因不明
This version of the P-38 had been altered with superior servos for the control surfaces
这代P-38已改用优越的伺服控制面板
to help the pilot overcome aerodynamic stiffening,
可助飞行员克服飞行中空气动力加劲
where the force of the oncoming air at high speeds
即在高速的迎面空气阻力下
makes it difficult to move the control surfaces,
移动控制面板困难的问题
but the plane still entered an uncontrolled dive regardless of these measures.
虽采取了这些措施 飞机俯冲仍失控了
The engineers eventually discovered the airflow was separating from the surface of the wing
工程师最终发现 由于冲击波的形成
as a result of shockwave formation.
气流从机翼表面分离
This reduced the lift the wing could generate, while increasing the lift on the tail wing
这降低了机翼产生的升力 同时增加了
directly downstream of the flow separation.
尾翼直接下游气流分离的升力
This moved the centre of pressure
这让压力中心发生移位
and forced the plane to pitch downwards and gain even more speed,[4]
迫使飞机下倾并进一步加速
making it next to impossible to recover from.
这让飞机几乎不可能恢复正常
To solve this issue they incorporated a dive flap on the lower surface of the wing,
为解决这问题 工程师在机翼下面安装了俯冲襟翼
where airflow was not reaching supersonic speeds,
那里的气流没有达到超音速
which could be deployed during high speed dives to allow the wing
襟翼可在飞机高速俯冲时展开 以增加机翼升力
to increase lift and recover from the dive.
飞机就能从俯冲中恢复
As technology advanced however aeronautical engineers
然而随着技术进步 航空工程师们
started to see that straight wings were not suitable for transonic or supersonic speeds,
开始发现平直翼不适合跨音速或超音速飞行
and gradually started to adopt swept wings.
并逐渐开始采用后掠翼
The Germans confirmed the theory with high speed wind tunnel testing
1939年 德国人通过高速风洞试验
in 1939 by testing two wings,
[5]证实了这一理论 试验测试了两种机翼
a straight wing and a swept wing with a 45 degree sweep.
一个直翼与一个45度的后掠翼
Proving that a swept wing could delay the onset of supersonic flow AND reduce drag.
结果证明 后掠翼能延迟超音速气流发生 “同时”减少阻力
[5] They recognised that the swept wing would allow a plane to fly faster before shock waves formed
[5]他们发现早在让飞机更快的技术发明前很久
long before the technology that would enable them to fly faster was even invented.
后掠翼就可让飞机在冲击波形成前飞行更快
They used this knowledge to develop the Messerschmitt P.1101,
工程师利用这点开发出Me P.1101
a jet powered plane that could actually change it’s sweep angle before flight.
这种喷气式飞机实际上在飞行前就能改变掠角
But the end of the war came before the German’s could finish it and test the aircraft
但在德国人完成测试这种飞机前 战争就结束了
and with captured and developed into the bell X-5 by the United States
后被美国捕获并用于贝尔X-5的开发中
to understand the purpose of swept angle,
为了理解后掠翼的设计目的
let’s see how airflow evolves over this plane’s wing
我们要先了解随着机翼角度的变化
as the changes is swept angle, while compare flow over a straight wing,
与平直翼相比 后掠翼上的气流如何变化
Air flow over a wing perpendicular to the freestream air has one component,
机翼上与自由流垂直方向的气流有一个分量
the chordwise flow, [6] which is air that flows over the chord of the aerofoil.
即弦向流 [6]一种飞机弦上的气流
The chord is the imaginary line running from the leading edge to the trailing edge of an aerofoil.
弦是机翼从前缘到后缘的假想线
Chordwise flow does accelerate over the aerofoil, and thus contributes to lowering the speed
弦向流在机翼上确有加速 故有助于降低
at which supersonic flow begins.
超音速气流开始时的速度
Called the critical mach number.
被称为临界马赫数
Now let’s look at the flow components over the Bell X-5’s wing
我们看看贝尔X-5的机翼在最低掠角
at its lowest sweep angle, 20 degrees.
——20度时的气流组成
Here we can separate the airflow into two components.
在这我们可把气流分成两部分
[6]The chord wise flow, which is now offset at a 20 degree angle relative to the freestream,
弦向流 现在与自由流成20度偏移量
and the new second component the spanwise flow,
以及新的第二种气流——展向流
which flows along the length of the wing and does not accelerate
展向流沿着机翼伸展方向 无加速
and thus does not lower the critical mach number.
也不会降低临界马赫数
At lower speeds, where supersonic airflow is not a worry,
飞行速度较低时 超音速不成问题
you want as much of that airflow to be chordwise
我们希望尽可能多的气流是弦向的
and thus generate the necessary lift to fly.
这样才可产生必要的升力飞起来
However as the speed of the plane begins to climb,
不过随着飞机速度开始爬升
we are generating more than enough lift thanks to the increased air speed,
因空气速度的增加 飞机有了足够的升力
and thus can afford to convert some of that airflow into spanwise flow.
因此有部分气流就转换成展向流
We do this by increasing the sweep angle,
这通过增加后掠翼的角度即可实现
which the Bell X-5 could do in flight to a maximum sweep angle of 60 degrees.
贝尔X-5在飞行中后掠翼角度最大可达60度
[7] As the sweep angle increases a larger portion of that airflow is converted to spanwise flow,
随着掠角增加 大部分气流转换为展向流
which is great for increasing the top speed of an aircraft,
这对提高飞机的最高速度非常有利
but can cause some troublesome stall characteristics.
但也产生了麻烦的失速问题
Because a large volume of air is now originating at the root of the wing and travelling down
由于大量的空气会从机翼根部开始
to the tip of the wing, stall will begin at the tip of the wing and move towards the root.
向下流动至翼尖 失速就会从翼尖向机翼根部转移
This is a problem because our ailerons,
这就是问题 因为我们的副翼
the control surfaces that allow us to roll the plane,
和那个可以让飞机翻转的控制面板
are located on the outer wing.
位于外翼处
If stall occurs on the outer wing, we will lose roll control.
一旦外翼失速 我们就会失去侧滚控制
[8] A major problem for say at fighter jet attempting a high angle of attack maneuver
这是战斗机在保持完全受控状态下进行
while maintaining full control,
高攻角机动的主要问题
and this is one of the problems forward swept wings were trying to fix.
而这就是前掠翼试图解决的问题之一
This reverses the direction of the chordwise flow,
前掠翼逆转了弦向流的方向
so it originates at the wing tips and travels to the root of the wing.
气流从翼尖流向机翼根部
[9] Meaning stall occurs at the root of the wing first,
[9]这意味着失速首先发生在机翼根部
allowing us to maintain control of the plane for much longer.
让我们有更长的时间保持对飞机的控制
Not only that, but it reduced induced drag as a result of wing tip vortices.
此外 前掠翼还减少了因翼尖涡流引起的诱导阻力
Where high pressure air from the lower wing travels and mixes with low pressure air
从下机翼传来的高压气流会在翼尖与
on top of the wing at the wing tips.
机翼上方的低压气流混合
The Ju 287 was designed this way
JU287采用这样的设计
not to benefit from the superior aerodynamics characteristics,
并不是为了从出众的空气动力学特性中获益
but to move the wing box rear wards, which allowed the bomb bay to move forward closer
而是为了机翼箱能后移 炸弹舱前移
to the centre of gravity of the aircraft, which in turn allowed the plane in-carry
更靠近飞机的重心 从而使飞机装载更大的炸弹时
a larger bomb, while not increasing the trim drag to keep the plane balanced.
不增加配平阻力而保持飞机平衡
[10] But ultimately the materials of the time could not facilitate it.
但最终却因当时的材料不支持而无果
Under normal wing loading,
在正常的翼负荷下
the main force being exerted on the wing is upwards bending.
施加在机翼上的主要压力呈弯曲上行状
Where the force of lift pushes the wing upwards,
这样升力推动机翼向上
while the weight of the fuselage pushes downwards.
而机身重量却向下
To survive this we need to build an adequately strong and stiff wing.
要能飞行就需要打造一个足够强大而坚挺的翅膀
This is achieved through a beam called a spar
这样的翅膀通过一种被称为翼梁的横梁实现了
which runs the length of the wing.
翼梁与机翼同长
With a forward swept wing an additional stress is introduced,
在迎面而来的气流对机翼产生扭力时
where the force of oncoming air is attempting to twist the wing.
前掠翼就会产生附加应力
We can imagine this with a free body diagram with springs
可以想象成一个带弹簧的自由体图
representing the stiffness we need to incorporate into the wing.
这表示我们需要将刚度引入机翼中
[11] Here the kb is the spring stiffness that will be needed to resist bending,
[11]这里的kb是弹簧抵抗弯曲所需要的刚度
and kt is the spring stiffness needed to resist twisting.
kt则是弹簧抵抗扭力所需要的刚度
Creating a structural member that can act like this torsion spring over the entire wing
然而 要制作可以像这扭力弹簧一样作用于
however is no easy task and would require enough additional weight
整个机翼的部件并不容易 且所需足够的附加重量
to negate any positive attributes the forward swept wing would provide.
会抵消前掠翼所带来的积极特性
But that changed when advanced composite materials became available.
但随着先进复合材料的出现 情况就不一样了
Allowing planes like the X-29 and the Russian equivalent the SU-47 to be made.
诸如X-29和俄罗斯SU-47这样的飞机是能制造的
Both planes used carbon fiber reinforced plastics
两种飞机都使用了碳纤维增强塑料
laid up so the fibres would resist that twisting motion.
这样的纤维就能抵抗扭绞运动
I will focus on the X-29 from here,
这里我就重点讲X-29
as information on American technology is far more freely available.
因为关于美国技术相关的信息更容易获得
The X-29s primary structural member
X-29s主要用于抵抗扭力的
for resisting this twist was a closed box section, located here, [12]
结构件是一个封闭的箱体 在这个位置[12]
which was constructed of crisscrossed composite tape
它由纵横交错的复合胶带制成
that reached up to 156 layers deep.
深达156层
Essentially creating that spiral spring shape within the structural member,
这从根本上塑造了螺旋弹簧的结构成分
but with extremely stiff and lightweight composites.
却是极度刚硬而轻质的复合材料
Taming that twisting problem, and allowing the plane to fly successfully.
控制好了扭转问题 飞机就能成功飞行
Wind tunnel tests showed the forward swept design
风洞测试表明前掠翼设计
would provide a 20% gain in efficiency
与同类后掠翼飞机相比
compared to same plane with aft swept wings.[9]
能增加20%的效率[9]
This along with a supercritical wing design [13], which flattens the top edge of the aerofoil,
再加上超临界机翼设计[13] 该设计能压平机翼上边缘
to minimise the acceleration of the air over the top edge,
最大限度降低顶部边缘的空气加速度
while introducing a concave curve to the lower surface to increase lift,
同时在下表面设计一个凹曲面增加升力
allowed the X-29 to spend less fuel flying at a higher mach number.
这样X-29就能在更高的马赫数下耗费更少的燃料
Another drag reducing benefit of the forward swept wing was the shifting of centre of pressure
前掠翼的另一个降阻优势是压力中心向后方转移
rear wards.[14] Typically the lift generated by an aft swept wing needs to be counteracted
一般来说 后掠翼产生的升力需要尾翼来抵消
by a tail wing which generates downwards pressure to maintain pitch stability.
尾翼产生向下的压力来保持螺距稳定
This downwards pressure is wasted energy that contributes to drag.
这种向下的压力是损失的能量 会造成阻力
With a forward swept wing the centre of pressure is moved to the rear of the aircraft
前掠翼的压力中心后移至飞机尾部
behind the centre of gravity,
在重心之后
and thus to maintain static pitch control these pitch control surfaces,
为了保持静态俯仰控制 这些被称作鸭翼的
called canards, need to generate lift forward of the centre of gravity
俯仰控制面板 就需要有让前移的重心往上的升力
and thus contribute to useful lift.
这样才能产生有效的升力
This would seem like an obvious feature to incorporate into every aircraft,
这似乎是每架飞机的显著特征
but leads to instability that requires the control surfaces
但这也增加了不稳定性 需要不断调整
to constantly adjust to maintain a stable flight,
控制面板以维持稳定的飞行
and this was one of the massive challenges the designers faced.
这是设计师们面对的众多挑战之一
The X-29 was incredibly unstable, especially in pitch,
X-29非常不稳定 尤其在俯仰控制方面
even when compared to modern jet fighters.
甚至与现代喷气式战斗机相比也是如此
This means the flight control computers had to be constantly adjusting the control surfaces
这意味着飞行控制系统需要不断地调整控制面板
to maintain stable flight, about 40 times a second.
达到约每秒40次的频率才能保持飞机的平稳飞行
[15]To do this the X-29 had three flight control computers,
为了做到这点 X-29配备了三个飞行控制电脑
to provide redundancy if one failed.
冗余设计是为防万一有故障
As the plane would become essentially impossible to fly without the aid of a computer.
因为没有电脑的帮助 飞机几乎不可能飞行
Which made it even more worrying when all three shut down while preparing to take off.
更让人担心的是在准备起飞时 三个电脑全部关闭的情况
[16]This caused the plane to be grounded.
[16]这将导致飞机停飞
Delayed testing, which was due to accelerate with the arrival of a second X-29 fitted with
延迟测试源于第二架配备了降落伞系统的X-29的出现
a parachute system to allow high angle of attack maneuvers to be safely tested.
降落伞系统让高攻角机动安全测试得以实现
The spin parachute was installed to provide positive recovery from spins,
旋转降落伞的安装是为了确保飞机在旋转中能积极恢复
as spin-tunnel tests had indicated that the X-29A ailerons
旋转通道测试表明X-29A副翼
and rudder provided poor recovery from fully developed upright spins.
和方向舵在完全展开的垂直旋转中恢复较差
Eventually the problems were solved and high angle of attack testing resumed
就这样 这个问题终于得到了解决 完成了高攻角测试
and proved the X-29s capabilities, but the program ultimately
并证明了X-29s的实力 但该项目最后
ended on December 8th 1988,
还是在1988年12月8日终止了
almost four years to the day of it’s first flight.
距离首次飞行将近4年
In between that first and last flight
从第一次到最后一次飞行
the X-29 completed 242 flights
X-29共计完成了242次任务
with 179 combined flight hours.
联合飞行179小时
Providing valuable scientific data and design experience
为机身复合材料与计算机辅助飞行
in composite airframes and computer aided flight.
提供了有价值的科学数据和设计经验
Ultimately forwards swept wings weren’t incorporated into newer generations of planes,
前掠翼终究没有被应用于新一代的飞机中
as the benefits simply did not outweigh the cons.
因为这种飞机弊大于利
From the additional structural requirements, the poor pitch stability
由于额外的结构要求 俯仰稳定性也差
and perhaps most importantly it’s negative effects on stealth design [17],
而前掠翼最大的不足可能还是对飞机隐身设计有不利影响[17]
in the end and swept delta wings won out.
结果是三角翼大获全胜
This is just one of many unusual plane designs
这只是众多与众不同的飞机设计之一
that originated in world war 2 era Germany,
这些飞机起源于二战时期的德国
among other novel military vehicles.
还有其他新奇的军用车辆
You can learn more about these innovative machines
你可以在curiosity stream的这部名为
with this documentary titled “Hitler’s Miracle Machines” on curiosity stream.
“希特勒的奇迹武器”的纪录片来了解这些新颖装置的信息
You can watch them for free, by signing up to curiositystream using the code realengineering,
在curiositystream注册用《真实工程》的代码就可以免费观看
or using the link the description.
或者通过下方的链接进入
This will give you a month of completely free access
为你准备了为期一个月完全免费的时间
to over 2,400 documentaries and nonfiction
可观看超过2400部的纪录片和非虚构视频
titles from some of the world’s best filmmakers.
这些都是全球最好的电影制作人的作品
After that first free month you can continue your access for just two dollars ninety nine a month.
一个月的免费期之后 你可以每月2.99美元续订
As usual thanks for watching and thank you to all my Patreon supporters.
跟往常一样 感谢观看 感谢我所有Patreon的支持者
If you would like to see more from me, the links to my instagram, twitter,
如你想了解更多信息 下方是我的Instagram twitter
discord server and subreddit are below.
discord server和subreddit

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视频概述

带你进入新奇的机械世界,了解背后神奇的故事

听录译者

收集自网络

翻译译者

Anne

审核员

审核员XY

视频来源

https://www.youtube.com/watch?v=RN6vGxyMcVU

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