Icaristics:Flying in the lee 在背风处飞行

Cross Country 专栏文章
原作:Bruce Goldsmith  

Every pilot learns at paragliding school not to fly in the lee side. Flying in the lee is obviously dangerous because of lee side turbulence and rotor. This rotor can be violent leading to collapses and accidents. In the lee side even a reserve parachute may not work if the air is too turbulent.However, a meteorologist once told me that all thermals start in the lee side and if you fly in major competitions you’ll see top pilots flying in the lee on a regular basis. And what about protected flying sites such as Greolieres where you often fly the lee side? Why is it sometimes perfectly safe to fly in the lee and at other times extremely dangerous? And what exactly is conical hill convergence?None of these are easy questions to answer, but I hope that this article will give answers to at least some of these questions.

每一个滑翔学校都教育学生不要去背风面飞行--“背风面永远充满了危险。” 背风面存在着许多涡流和乱流,这些涡流会造成夹页和事故,在特别乱的地方,连你的备份救生伞都会“夹页”。但是,气象学者告诉我们,世界上所有的热气流都 是在背风面生成的。想想那些著名的背风场地,如我国的云南大理,事实如此。为什么某些时候我们可以在背风面安然自得,而另一些时候背风面却充满了危险?这 些都不是简单的问题,希望Bruce能作新长征路上的一盏明灯,

Every thermal starts in rotor 在涡流中诞生的热气流

Thermals start when the sun heats the air unevenly. If the wind blows constantly and smoothly over the ground, then the ground will heat the air in a similarly smooth and constant fashion, which is not ideal for thermal formation. What we need for thermals to form are differences in the air temperature. As soon as you introduce an obstruction into the airflow such as a house, a fence or a hill, you will get an area of turbulence behind the obstruction. Sometimes the obstruction allows the air to remain still for a time, protected from the wind, allowing the air a chance to warm up. Then the turbulence may separate the bubble of warm air from the ground releasing it into the wind above as a thermal. The same thing can happen on a small or large scale, so you may get thermals being kicked off behind a fence, a house or a hill as shown in the diagram. You can even get wind shadows or rotor behind a thermal itself, which can act as an obstruction to the prevailing wind.
热气流的诞 生归功于太阳对地表的不均匀加热。当懒洋洋太阳无聊地晒着大地万物时,空气被加热了,由于地表的颜色质地不同导致这个加热过程并不均匀。如果此 时有持续柔和的风来搅拌空气的话,会造成均匀加热的效果。风杀死了热气流,这可不是我们所需要的。当你将一所房屋,一棵树或一个山丘放置在风的前进道路 时,在这些障碍物的背面我们得到一块“乱流区”。 有时候障碍物会让空气静止,防止风的搅拌。当空气有机会得到足够的热量后,在“乱流”的作用下,热气泡离开地面射向无垠的蓝天。 这种效果可大可小, 涡流可以在篱笆后面产生,也可以在莽山的背后咆哮。你甚至可以在一个热气流的“影子”中感觉到“涡流”的存在,因为热气流团阻碍了风的前进。

So the lesson to be learnt is that rotor can be just as much a friend to the thermal pilot as a hazard.

Size Matters 尺寸很重要

It’s difficult to say when it is safe to fly in the lee side of an obstruction. The simple rule is - if in doubt, don’t! That is of course the ’cover your arse/limited liability’ answer that you can get from any paragliding school or textbook. However, this doesn’t really solve our problem... it’s just the safe answer. My own personal experience says that the single most important factor is the size of the hill. The bigger the hill or mountain, the more protection it will offer.
As an example, you can almost never fly in the lee side in the UK where the hills rarely reach more than 500m high. On the other hand, in Tenerife, nearly all the flying is done on the lee side of Mount Tiede which towers to over 3000m (e.g. the site of Taucho). You can even fly in the lee side in Tenerife when the prevailing wind is as much as 50km/h, however, you can expect strong lee side thermals and strong winds if you stray outside the protected area.
首先,你要知道,在背风面很难说那里是安全的。 最简单的规则是:“如果你怀疑那地方不安全,那你就永远不要去。” 靠这个答案,你能确保屁股安全落地,每一家滑翔伞学校都会这样回答你。但这种答案并不能解决我们的问题。以我个人的经验来说---尺寸很重要。越是大型的 山脉越安全。
举例来说,没人在英国或莽山后面的小沟里折腾。 这种的山脉也就500米的高,不足以提供保护。而在Tenerife,几乎所有的飞行场地都在背风面,这座山脉有3000米高,你甚至可以 在风速达到50公里时还安全的游弋,此时你可以感受强大的上升和……凛冽的飓风--如果你飞到保护区之外的话。在我国的云南大理也有同样的场地。

Wind Strength 风速

The lighter the wind, the less dangerous flying in the lee side is likely to be. Wind strength is extremely important when trying to fly safely in the lee. Flying in the lee in a 5km/h wind should pose very little problem, but if the wind is greater than 20km/h, then leeside flying is likely to be extremely hazardous and dangerous.

Solar Heating 阳光烹调涡流

Thermal heating of the lee side makes things a lot safer. This means that even if the air is turbulent on the lee side, it’s being heated by the sun and so the air will generally have an upwards motion. This is also the case in Tenerife where the site of Taucho faces southwest and into the afternoon sun. Other sites such as Greolieres and Monaco benefit from a similar thermal-powered protection.

Sea Breezes 海风
If the heating is combined with a sea breeze or a valley wind system, then the protection from the prevailing wind is even greater. The presence of the sea tends to lead to a larger scale air movement than simple rotor, so helping to make the air less turbulent.

Conical Hill Convergence 圆锥形山的收敛

When air flows round an isolated hill or mountain, the air divides on the upwind side of the hill and then flows together again on the downwind side. The area in front of the hill is therefore an area of divergence, and the area behind the hill is an area of convergence. Therefore, where the air is converging, it can produce lift, especially if there’s some thermic activity as well that helps produce some upward movement in the air when the air converges. If the air is not going up due to thermal activity, then it may be falling just as strongly as it could be rising.

Diagrams 1 and 2 show how conical hill convergence works. The diagrams show how you can get an area of lift downwind from a hill rather than an area of rotor, which is normally what you would expect, as shown in diagram 3. There are many different factors resulting in lift behind a hill, rather than rotor. Factors that help you get lift are:
1. The size of the hill, the bigger the better.
2. Heating from the sun on the downwind side of the hill.
3. The stability of the air. If the air is unstable, the air may flow over the top of the hill instead of round the side, meaning that it will come crashing down in an area of sink behind the hill. A layer of stable can help stop the air displacing vertically in front of the hill can help.
4. The exact shape of the hill.
1. 山的大小,越大越安全。
2. 阳光直射背风面。
3. 稳定的空气,如果空气不稳定,气流会从山顶而不是侧面吹向山后并砸落地面,这时背风面存在强烈的下降乱流。所以我们需要稳定的空气层阻止风在山前垂直运动和翻山而过。
4. 山的形状很重要。这就是为什么活佛Alex会在林州大沟里飞。

Summary 总结


The main factors to consider in flying in the lee are:

1. Wind strength 风的强度
2. Size of the obstruction 山的大小
3. Solar heating on the lee side 阳光直射加热背风面
4. Air stability 稳定的空气
5. Shape of the hill 山的形状
6. Sea breeze or valley wind considerations 海风或山谷风的作用

There can be no hard and fast rule as to whether if it's safe for flying in the lee, everyone must make their own decisions based on their personal skill level and their ability to cope with any turbulence they are likely to encounter.

One last consideration is that it’s often the edge of the lee side which is the most dangerous area. If you are completely in the lee, you may be fully protected - but if you’re at the edge of the protected area, then you may well encounter the maximum amount of turbulence. If you intend to fly in a protected area you need to go all the way into the lee. It’s often more dangerous to test out the waters by feeling around the edge of the protected area than by flying all the way into it.
最后要注意的一点,背风区的边缘往往是最危险的地方。如果你整个伞都在背风区内,那么你会得到很好的保护,如果你在边缘上晃悠,那么你会遭遇到该地区最强烈 的乱流。所以说,如果您决定飞背风坡,那您就得拿出刘胡兰董存瑞精神,直接杀将进去。人世间最危险的事情莫过于在背风区边上试水,所谓当断不断反受其乱。

One final word. Lee side flying is only for very experienced pilots, or for pilots under the instruction of a very experienced pilot flying in controlled conditions. Please take all possible precautions when considering flying in the lee.

作 者Bruce Goldsmith是一位天才飞行员,他是世界顶尖的竞赛飞行员并设计过许多伞翼。其实Bruce本身是一位专业工程师,也是Airwave品牌的创造者 之一。他设计的第一顶伞Black Magic于1989年问世,之后他设计了Taboo, Voodoo, Jive, Samba, Duet, Rave, Rave Race, Alto XM, Harmony, XMX, Fusion, XXX 和 Duplex。其中XXX曾经在 World Championships(世界锦标赛)法国站中获得一举囊括前三名,Bruce在次年获得了英国冠军。接下来的18个月中Bruce离开了 Airwave着手创建Ozone,作为Ozone的设计者,Bruce设计了一些列成功的伞翼,包括Electron, Proton, Cosmic Rider 和 Octane。1999年Bruce使用自己设计的Proton(质子)在PWC世界杯系列赛事中独占鳌头,最终获得总冠军。在2000年3月Bruce 离开了Ozone公司,回到了他的“故乡”--Airwave。十多年来,Burce一直在为Cross Country杂志撰写专栏Icaristics。Bruce也曾经与Barney Barnes合作拍摄过一部安全题材的滑翔伞电影"Instability"(不稳定性)。