Wednesday, May 28, 2008

Bumblebees can't fly?

Bumblebees are quite impressive insects: They make a nice humming sound, they cause a small stir when flying over patches of dusty ground, and with a mass of nearly a gram, they are heavy enough to bend clover flowers under their weight when landing to feed nectar.



And, of course, bumblebees cannot fly according to the laws of aerodynamics, or so goes the myth. This story, often invoked by people wanting to dismiss results of scientific reasoning, seems to go back to the 1930s, to students of Ludwig Prandtl, a pioneer of aerodynamics at the University of Göttingen in Germany.

As so often, there is some truth also to this myth: If one starts from the assumption that the uplift force comes about as for an airplane wing, this is fine to understand the flight of large birds. But this assumption works much worse for small birds and bats, and it fails for insects such as bumblebees. Obviously, bumblebees have found clever way to fly very different from that of airplanes.

But they don't defy the laws of aerodynamics anymore: The issue of Physical Review Letters of September 4, 2000, besides discussing Prospects of Detecting Baryon and Quark Superfluidity from Cooling Neutron Stars and Magnetic-Octupole Order in Neptunium Dioxide, had a paper by Z. Jane Wang with the unassuming title "Two Dimensional Mechanism for Insect Hovering". But as the abstract explains Resolved computation of two dimensional insect hovering shows for the first time that a two dimensional hovering motion can generate enough lift to support a typical insect weight. The computation reveals a two dimensional mechanism of creating a downward dipole jet of counterrotating vortices, which are formed from leading and trailing edge vortices. [...]". (Phys. Rev. Lett. 85 (2000) 2216-2219).


Snapshots of the vorticity field of the moving wings (black). The illustration, taken from Phys. Rev. Lett. 85 (2000) 2216-2219, shows the formation and dynamics of a "dipole jet" of vortices, which creates the uplift force that allows the insect to hover.


It seems that the rapid motion of insects wings creates tiny vortices in the air that keep them aloft. This is especially important when hovering around a clover bloom. Luckily, bumblebees can fly without reading the PRL.











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12 comments:

Alex said...

Honestly, I never believed that one.

Bee said...

Lovely photos! I haven't seen a single bee/bumblebee or even a wasp here this spring. Only flies.

Is that true the story of the bumbleebee that allegedly can't fly was 'invoked by people wanting to dismiss results of scientific reasoning'? Sounds quite absurd to me.

Uncle Al said...

Pigeons cannot Officially launch from the ground in still air. Their wings slap together at the topstroke. As they pull apart the partial vacuum helps lift the bird.

Do left and right shoes violate the Equivalence Principle? Given chemically identical, opposite parity mass distributions within theoretical expectations, sure,

http://arxiv.org/abs/0801.4148
http://www.scielo.br/scielo.php?pid=S0103-97332004000700014&script=sci_arttext

Somebody should look.

rillian said...

Wow, cool post. I guess the paired vortices carry enough momentum away to support the bee?

I have in fact heard the bumblebee held up as a reason to not respect science. I usually counter with the reliability of light switches.

I've also heard it in more scientific circles as shorthand for the uselessness of physics (from biologists for example). See also "Call me when you can solve the helium atom," which I think is a reference to the 3 body problem.

More cheerfully, it's been summerish weather here, and I did see quite a few bumblebees two days ago, gathering nectar from a rhododendron. They weren't quite heavy enough to bend those flowers, but they did take up a good deal of the space inside!

Kris Krogh said...

Thanks Stefan!

Maybe you have a special attraction to Bees? Stephen Dalton has some fine images of insects flying. My favorite.

Hi Uncle,

Some insects do something similar in normal flight. It's called "clap-fling."

Georg said...

Hello,
my impression is that the story of the
Bumblebees ist "popular" in anglosaxon
countries. I never heard/read that in Germany.
Today I watched a Xylocopa violacea
(Holzbiene) in our garden, which is about one inch long.
This "bee" is bigger than all bumblebees
we see here and is remarkable for its
high speed.
Georg

stefan said...

Dear all,


well, I didn't believe this myth either - but I remember I have read statements like "this is all just a theory, and, you know, these theories say that bumblebees can't fly" - though I do not remember whether this was in German or English. At least according to google, the myth exists also in German.

But I think it's cool that the explanation of bumblebee flight made it into PRL - it's such a beautiful "down to earth" stuff where really everyone can at least understand the question, though the answer may be complicated. I came across this more or less by chance just now, because of the bumblebee photos I took (without much purpose or planning) last week.

And somehow it's funny that this very same issue of the PRL contains two papers that started part of the stuff I've investigated in my thesis - so I have looked at that issue quite often, but never noted the bumblebee paper. Though, this may be not too surprising, given the title of the paper.


Hi uncle,

thanks for that info about pigeon flight! I have often wondered about the origin of this clapping sound when pigeons fly away, and thought that this sounds as if the wings clap together - but that didn't make sense to me...

Hi Rillian,

I guess the paired vortices carry enough momentum away to support the bee?

That's how I do understand this. The trick seems to be that the motion of the wings creates this localised vortex pair carrying air - and momentum - downwards. It's at this point that the viscosity of air comes into play, to ensure the formation of these vortices.

Best, Stefan

Phil Warnell said...

Hi Stefan,

Yes it was stated for many years that bees defy the laws of aerodynamics and yet that is to assume that bees studied law:-) With levity aside yet still observed, it would have been nice to have a quick scan of the related paper you pointed out and yet once again one is confronted with the dreaded old $25.00 fee. It did catch my interest, but not to that extent. I could buy almost a ¼ tank of gas for the same;-) As discussed before it truly would be nice if journals such as the physical review were to set up a more realistic subscription rate for individual private users as say around the fee charged for magazine subscriptions. Let’s face it the bulk of it comes from libraries and institutions and anything they would gather this way should be looked at as a bonus. Well I guess for now I’ll have to look to other sources to learn more about the birds and the bees:-)

Best,

Phil

michaeldcassidy said...

I heard about bumble bees defying the laws of aerodynamics when I read a book about Igor Sikorsky when I was about 12. Supposedly he had a plaque on his wall that read:

According to the laws of aerodynamics the bumblebee can not fly, but the bumblebee doesn't know the laws of aerodynamics so it flies.

The book said that he had it on his wall to silence doubters as he developed his helicopter,

I didn't think of the remark as anti-science as much as stating there was still much to learn.

Georg said...

Hello Stefan
I heard that bumblebee story for the first
time in 1991 from an british engineer.
He would not believe, that I did not know
this.
In the Net I found the story rather often.
Everything more recent than 1991 is
neither German nor Anglosaxon, it is just
myths distributed by the Internet, especially
if derived from Wiki.
Ask some German people of my age (60), whether
and when they came across this story.
Georg

stefan said...

i Phil,

it would have been nice to have a quick scan of the related paper you pointed out.

In this case, this is easy: You can find the PRL from 2000 as a PDF file and may more related papers at the the web site of Z. Jane Wang's research group of at Cornell University. Check out their list of publications!


Hi Michael,

thanks for the info about Igor Sikorsky. Using the reasoning that leads to the "conclusion" that bumblebees can't fly - would a helicopter be able to fly?


Hi Georg,

true, a google scan about anything related to "myth" or "fringe stuff" is for sure completely biased, so the German-language search results do not say much. I do not remember when and how I came across the story for the first time.

Anyway, thanks for sharing your experience!

Best, Stefan

Phil Warnell said...

Hi Stefan,

“In this case, this is easy: You can find the PRL from 2000 as a PDF file and may more related papers at the the web site of Z. Jane Wang's research group of at Cornell University. Check out their list of publications!”


Many thanks for now I am more enlightened on the matter. Now all I have to do is wait for that moment when it comes in handy. Perhaps it will be in response to someone’s smart ass remark that science still has no clue as to how a bee manages to fly and therefore should not be trusted or respected:-)

Best,

Phil