Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics
The bumblebee flight myth: the idea that engineers have proved that bumblebees can’t fly, or that bumblebee flight is not consistent with known aerodynamics is extremely well known. The idea is that bumblebee’s wings are too small for the size of their bodies. Since they do fly, there is obviously something interesting going on – bumblebees can generate more lift from their wings (a higher lift coefficient) than conventional aerodynamic methods should allow. They must be using some unconventional aerodynamics, if we can exploit the same aerodynamics we might be able to shift more air with smaller wings or propellers. Smaller cooling fans in computers, for example.
The aerodynamics of bumblebee flight were solved in the early 2000s, and modern computational fluid dynamics can accurately model insect flapping flight (Young et al., 2009, Science). The key is unsteady aerodynamics and leading edge vortices. Insects like bumblebees flap their wings so that a leading-edge-vortex forms above the wing and is stably held there for the duration of the downstroke. The high speed rotating flow within a vortex generates really low pressure – this is why air gets sucked down the middle of a plughole vortex. Tea drinkers use leading edge vortices to mix milk and tea, forming the vortex over the back of the spoon by sweeping it across the teacup at an appropriate angle of attack relative to the tea. Skeeter, our dragonfly drone, makes use of those unconventional high lift aerodynamics to hover, and to cope with turbulence and gusty winds.
Where does the bumblebee flight myth come from? The meme that bumblebees can’t fly is extraordinarily widespread, and surprisingly old. A quick search of the internet finds a rather elegant paper by John McMaster, then Principal Engineer at Boeing (https://www.jstor.org/stable/27855657), that points to the meme being current in the 1930s amongst the world-leading German aerospace group led by Ludwig Prandtl, but concrete evidence of its origin is lacking.
I did some digging and I think I have found the source. During the 1st world war the first detailed analysis of animal flight by Reinhardt Demoll ‘Der Flug der Insekten und Vogel’, was published in 1918, and presented detailed measurements of the flight morphology, flight speeds and wingbeat frequencies of birds and insects. Demoll identified several different styles of flight f- kite-flight, rowing-flight, sailing-flight, gliding-flight, hovering. He suggested Kite-flight was the normal forwards flight mode of birds and aircraft, rather like the flight of a toy kite, and he suggested that kite-flight was very different from hovering (hubbewegung), which he suggested was the normal flight mode for insects. He even calculated how hovering differed from kite-flight:
If houseflies flew by kite-flight it would need wings with an area 42x larger per gram bodyweight than they actually have. He was clearly already aware that under conventional aerodynamics insect wings are too small to support their weight. However, Demoll was a biologist, not an engineer.
Demoll’s book was reviewed in Naturewischenschaften in 1919 by Prof. Dr. F. Stellwaag, Naturwischenschaften 1919. That review was picked up by Wilhelm Hoff, then head of the “Deutsche Versuchsanstalt fur Luftfahrt, DLV”, the German test establishment for Aerodynamics. Clearly Hoff was an aerodynamicist. He took issue with Demoll’s analysis, tabulated his data and calculated lift coefficients for various insects using the conventional aerodynamic formula and claimed that Demoll was wrong and conventional aerodynamics could explain insect flight, publishing his argument as a paper within the journal Naturwissenschaften. Demoll responded by using Hoff’s calculations to show that while birds fly at lift coefficients similar to aircraft, insects fly at much higher lift coefficients – too high for conventional aerodynamics.
The calculation is at the end of the first paragraph – for a pollenladen bee the Ca (lift coefficient) is 1.904. Demoll went on to point out the error – Hoff’s calculations were based on the flight speed of the animal, but the wings are moving much faster than that because they are flapping. He went on to suggest that insect wings are better compared to propellors rather than aircraft wings, which is slightly ironic since Hoff’s main engineering project at the time was developing a full-size test rig for aircraft propellors.
Since this academic disagreement, modern research using technology not available to Hoff or Demoll, such as aerodynamic modelling and high speed cameras, have provided further proof that bumblebees can indeed fly. Images like the one above show the elusive leading edge vortices (arrowed) over the wings of a bumblebee flying through smoke streams in the Oxford University Zoology Department’s Animal Flight windtunnel, which are responsible for the extra lift that these animals need to get them airborne. It’s a great story, and it’s a great example of how science, technology, and engineering all depend on each other as we try to improve our knowledge
This video also provides proof that bumblebees can fly really nicely…..