07.22Scientists Report: Bumblebees Can’t Fly
by Robert Novella
When I was in High School we had school meetings every Friday during which we would discuss current events, see movies, listen to speakers etc. One morning we listened to an accomplished disabled man who started his speech by describing how science has proven that bumblebees cannot fly. He continued by saying that the bumblebee did not know this, however, and therefore is able to fly. His point was, of course, that disabled people could transcend the limitations and expectations society places on them by hard work and perseverance. I wholeheartedly agree with this attitude but the bumblebee example he used is not only erroneous, it illustrates one of the pervasive myths in our culture that are accepted without much scrutiny and rarely questioned.
Often this bumblebee story is used to discredit scientific conclusions about the impossible nature of supernatural events or feats. For example, if someone claims to know a successful dowser and I describe that science and experiments repeatedly show this to be impossible, the advocate might say that his friend is like the bumblebee in that he doesn’t know that science says his feats are impossible. Or he might say, “What does science know, science also tells us that bumblebees can’t fly”.
This myth can lead the way to one of several different beliefs. One of these beliefs is that people can transcend their physical limitations and achieve the impossible regardless of what science tells us. The high school speaker mentioned above really seemed to believe this. Although he drew strength and inspiration from this it seems inevitable that unrealistic expectations would be created that can only lead to disappointment. Most importantly, one can derive from the bumblebee myth the pernicious belief that science is an unreliable and overrated enterprise. After all, bees obviously fly and anything that concludes otherwise is at best flawed and at worst a complete waste of time. One other possible interpretation of this myth could mistakenly support the belief that the power of the mind alone can overcome impossibilities, even if it’s a bee’s mind, I guess.
Some light has been shed on the origin of the bumblebee myth by author and aerodynamicist J.H. McMasters. (Zetie, ’96) He states that it all started in German technical universities in the 1930’s. Apparently, a famous and unnamed Swiss aerodynamics expert was having dinner with a biologist when the latter asked a question regarding the flying abilities of bees. A preliminary calculation showed that there was insufficient lift to allow bees to fly. Only about one third to one half of the required lift could be generated. The biologist started spreading the word about scientific “proof” that bees can’t fly and somehow the media got hold of the information. Today, decades later it is a ubiquitous myth that is rarely questioned and is often used to disparage science. The implication, of course, is that if “science” (as an abstract entity) claims that bumblebees cannot fly, when they clearly do fly, then “science” is bunk. The findings of science can therefore be comfortably disregarded as esoteric and irrelevant.
The issue seems cut and dried. Either science can account for the flight of bees, or it cannot. What’s the problem? The problem lies in the difference between insect flight and airplane flight. Conventional aerodynamics, the kind developed for airplanes and helicopters, focus on “steady state” situations. This refers to fixed wings or a rotating propeller. The motion of insect wings, in contrast, is very different, involving complicated 3D movements and rotations. Other differences were also ignored including the size of insects themselves. At the scale of bees the medium through which they fly (air) becomes more important for aerodynamics considerations. To bees, the air seems much more viscous than it does to us, almost like molasses. Aerodynamicists ignored all of these concerns primarily because it was not believed that insects could generate any exotic forms of lift. Charles Ellington, a zoologist from the University of Cambridge, has this to say: “Since the 1950s, we’ve been looking at insect flight with the wrong picture in mind.” (Brookes, 1997)
Modern jet planes flying at 1500 miles per hour rely on similar principles of aerodynamics as the Wright brother’s plane did at Kitty Hawk almost a century ago. The wings on both types of planes have a very special shape, called an airfoil, which is curved more on the top than the bottom. When an air stream meets the airfoil it splits into two separate air streams. Because the top air stream has a longer distance to travel (due to the curvature) it moves faster. Faster moving air exerts less pressure than slower moving air (by Bernoulli’s principle) and this is considered a key component to lift. It is this pressure difference that produces the lift necessary to keep airplanes in the sky. (To be fair, there is some disagreement about the relative contribution of the airfoil to overall lift compared to other lifting forces like the downward flow of air off the wing.) Think of sipping a soda using a straw. When you suck some of the air out of the straw you are lowering the air pressure in the straw which causes the higher pressure elsewhere to push the soda into your mouth.
When science “proved” that insects can’t fly the only thing it really proved was that insects with smooth and rigid wings could not glide (Zetie, ’96). Experiments have actually been carried out demonstrating that this is indeed true. Clearly, conventional aerodynamics was not formulated to account for small insects with a small wing size. Once this was recognized, however, research began uncovering startling new aerodynamic oddities of insect flight that produce previously unknown sources of lift.
One of the most significant discoveries involves the rotation of the wings during flight. Michael Dickinson of the University of California discovered in his studies of flying insects that their brains were inordinately concerned with the minutiae of wing rotations. Hoping to reveal new insights into aerodynamic lift, Dickinson took a close look at wing rotations and noticed that it primarily occurred at the end of each wing stroke. To further analyze his findings he created a scaled-up version of a fruit fly’s wings. To simulate the viscosity of air from the fly’s point of view Dickinson placed his robotic wings in mineral oil and flapped them slowly. Using sensors attached to his robotic wing he determined that by precisely timing the rotation of its wing, bees could generate 35% extra lift. What Dickinson and other scientists have discovered is that precise wing rotations at the end of a stroke causes the vortices of air on the wings to increase their speed thereby increasing lift.
A similar discovery by Charles Ellington (mentioned above) attributes previously unknown sources of lift to a phenomenon called delayed stall. This occurs when a wing is at a high angle of attack (close to vertical) and the airstream detaches from the top of the wing forming a leading edge vortex. This vortex causes a low-pressure region on the wing temporarily increasing lift. Visualize a paper airplane near the end of its flight with the tail end dipping down and its nose pointing close to vertical. There is a brief moment of added lift just before it lands. This is delayed stall (Brookes, 1997) (Velasco, 1999) and it is normally a transitory phenomenon because it is so unstable. Ellington, like Dickinson, also created a larger than life mechanical apparatus (this one called Flapper) to better visualize the phenomenon. His results astonished him. During each stroke of his over-sized robo-bug’s wing, a leading edge vortex formed but instead of quickly dissipating, it traveled along the outer edge of the wing for most of the entire stroke of the wing. This was never noticed earlier primarily because the vortex is so tiny. It took something the size of Flapper to make it noticeable.
It seems clear that science has not proven that bumblebees cannot fly. This myth began after attempting to use the equations of aerodynamics, which beautifully describe the flight of manmade objects, to describe the flight of nature’s best aerialists, the insects. But when the differences between planes and bees are not accounted for it leaves one with the feeling that either science knows nothing or there is something magical about insect flight. Neither is true.
1) K.P. Zetie; The Strange Case Of The Bumble Bee Which Flew. 1996. Winner of the Science in Print competition sponsored by the Institute of Phyics. http://www.iop.org/Physics/News/0012i.1
2) Brookes, Martin; On a Wing and a Vortex, New Scientist, 11 October 1997
3) Velasco, Juan; The Mystery of Insect Flight, New York Times, September 7, 1999, F5