[转帖]科学家发现鸟类不按严格的空气动力学规则飞行

地球全力

Birds ignore physics to fly in 3rd gear
Larry O';Hanlon
Discovery News
Monday, 23 July 2007


According to the laws of aerodynamics, birds should be able to fly faster than they do. So what';s holding them back? (Image: iStockphoto)
Birds aren';t the best flyers they could possibly be because they don';t follow the strict rules of aerodynamics, scientists say.
Instead, evolutionary baggage slows them down.
"Often people think that animals are optimised," says US bird flight researcher Bret Tobalske, a professor at the University of Portland. "[But] they evolved from ancestors that didn';t fly."
Birds are also constantly adapting to new challenges that are not necessarily flight-related.
As a result, they are stuck with some limitations that make them less-optimal flyers than human-designed aircraft, which can be made from scratch, based on pure aerodynamic theory.
The new study, by Swedish researchers, confirms this by tracking 138 species of birds using radar.
The birds flew at about 30-80 kilometres an hour and ranged from swans to geese, ducks, flamingoes, gulls, swifts, pelicans, crows, falcons, hawks and eagles.
The team then plotted the speeds of the different bird species against their body masses as well as their ';wing loading';, or the bird';s weight divided by the surface area of their wings.
If birds are as efficient as aeroplanes they should show no significant family differences in these flight characteristics.
But they do.
"Flight speeds among bird species scaled significantly differently with mass and wing loading than predicted from basic aerodynamic principles," report Professor Thomas Alerstam of Lund University and co-authors in the journal PLoS Biology.
Birds of a feather
Instead, the birds grouped on the graphs according to their family histories (or phylogenetic groups) rather than the laws of aerodynamics.
The closely related swan, geese and ducks were in one aerodynamic flock, for instance, while hawks eagles, ospreys and bee eaters were in another.
"hylogenetic group contributed in a highly significant way to explain the considerable variation in bird flight speeds that remained, even after the biometrical dimensions of the bird species had been taken into account," Alerstam reports.
This matches what would be expected from evolution, says Tobalske. Instead of making a perfect bird, evolution does the best it can from the animals that are around to evolve flight.
All change
What';s more, he says, the real world is always changing in unpredictable ways that require birds to adapt to keep pace or die out.
Few adaptations that help different birds survive also make them the best flyers.
It';s an example of what';s called the Red Queen Hypothesis, says Tobalske, referring to the character in Lewis Carroll';s Through the Looking Glass.
"The Red Queen is running just to stay in place," Tobalske says.
Evolution does the same with birds: keeping them in constant evolutionary motion but never allowing them to get any closer to aerodynamic perfection.

润土

广州弟兄发英文版文章是想说明自己是读过书的？

boy007

没时间学英文了。帮顶

地球全力

      蝙蝠空中悬停之谜揭开：空气泡提供动力
      http://www.sina.com.cn 2008年03月03日 10:11  新浪科技
      　　新浪科技讯
      北京时间3月3日消息，据国外媒体报道，根据目前的空气动力学原理，蝙蝠根本无法像蜜蜂一样在空中盘旋飞舞，但它们为什么却能够成功地做到这一点呢？瑞典的科学家们近日前称，他们最近通过实验最终揭开了这一谜底。
      　　在2月29日出版的最新一期《科学》杂志上，瑞典的科学家公布了他们的这项最新研究成果。为了研究蝙蝠何以能够在空中悬停，瑞典伦德大学生物学家安德斯-赫登斯特朗做了一个实验：将蝙蝠放在一个布满烟雾并装有特制摄像机的风道中。摄像机的拍摄结果表明，蝙蝠采用了一种搅动空气“泡”的方法，这种空气泡学名为“涡流”，它也是很多昆虫用以飞行的方法。赫登斯特朗说，他们研究了蝙蝠是如何用翅膀搅动周围气流的。研究表明，这种涡流为蝙蝠悬停提供了40%的动力。
      　　在研究蝙蝠的飞行行为时，赫登斯特朗和他的同事发现了蝙蝠在向下扇动两翼时，翅膀前端所产生的涡流。赫登斯特朗说，当蝙蝠向上扇动翅膀时，这种涡流就在翅膀周围旋转，紧紧地贴着蝙蝠翅膀，就像粘在上面一样。这种涡流如同气泡一样，通过减轻翅膀上方的空气压力，它给蝙蝠带来额外的向上托的动力。赫登斯特朗，向前飞行的速度可以产生浮力，但如果要悬停的话则全靠肌肉的力量。人们找不到比蝙蝠更大的更够悬停的动物，这是因为，它们如想悬停的话，翅膀要扇动得超乎想象得快。鸟类和吸蜜蝙蝠的飞行机制之所以会有所不同，可能是由于后者没有尾巴，因此无法利用鸟类的涡流模式进行飞行赫登斯特朗说，他希望将蝙蝠飞行的研究成果用于设计一种机器蝙蝠。
      　　赫登斯特朗的研究小组找到了帮助蝙蝠悬停在空中的涡流，但他们还有其它问题要研究。赫登斯特朗说，蝙蝠能够控制翅膀周围的涡流堪称神奇。现在他们想知道的是，蝙蝠是如何控制涡流的。考虑到昆虫的翅膀比较坚硬，而蝙蝠的翅膀非常柔韧，赫登斯特朗和他的研究小组认为，蝙蝠是通过在飞行过程中改变翅膀弯曲度来实现悬停的。虽然上述观点仍只是猜测，但赫登斯特朗打算用更快速度的胶片记录蝙蝠的悬停过程。最近一次实验所用的摄像机只有一秒十帧的速度，这比蝙蝠扇动翅膀的频率要低。
      赫登斯特朗说，如果幸运的话，他们希望能在蝙蝠每扇动一下翅膀时就拍下一张照片，他们最新的系统可以达到老系统20倍的速度。
      　　此前的研究表明，鸟类在飞行时两翼后侧分别产生的空气涡流会发生合并，形成单一的气流环，这样能够尽可能地减少飞行中产生的扰动和身体后方的拉力。而最新的研究表明，蝙蝠的飞行机制并非如此。蝙蝠膜状翼后方产生的涡流不会合并，两翼基本保持独立运行。尽管这样会减少空气动力作用的效率，但会给蝙蝠带来其他的好处--快速转弯。新的研究同时表明，在飞行速度较低时，蝙蝠翅膀向上扇动的过程中会产生很大的力。尽管过强的力对鸟类而言绝非好事(鸟类通过将翅端羽毛分开来刻意避免这一状况)，但对蝙蝠却有特殊的意义。赫登斯特朗表示，蝙蝠翅膀向下扇回时能够产生较大的升力，这一飞行方式很像大黄蜂。蝙蝠两翼会在扇动过程中产生弯曲，这就好比航海时水手利用风帆向预定的方向前进。(刘妍)

地球全力

对了，就是蝙蝠的这种，飞碟的主要低压区的气动结构就是一种“分离涡”！

duhui111

不可能严格的，没有这个基础的说。