12 january 2023
Unlike wasps or hornets, bumblebees are widely seen as easy-going and peace-loving hymenoptera. This reputation is mostly down to the fact that they generally fly alone and are rarely aggressive – and not least because of their furriness and rounded form. But it is precisely the way they look that has led these colony-forming insects of the genus Bombus to be viewed with such suspicion by people who understand physics: their bodies are either too fat or their wings too small for them to fly, they say. So why can they do it anyway?
What has become known as the bumblebee paradox supposedly dates back to an event reported by a student of German physicist Ludwig Prandtl. As the story goes, in the early 1930s a biologist asked an expert in aerodynamics with whom he was dining why bumblebees were able to fly. The latter promptly took a serviette and scribbled down a calculation based on the weight and the wing area of a bumblebee: his conclusion was that it couldn’t generate enough lift to fly. If a bumblebee were to be as large as an aircraft and constructed like one, it would indeed be unable to fly. But it isn’t. This is because its wings, rather than being rigid aerofoils, consist instead of an extremely elastic and supple protein known as resilin. This can be stretched to three times its length without tearing, allowing bumblebees to move a relatively large mass of air using wings with a small surface area.
Rotational principle
Moreover, unlike those of an aircraft, the bumblebee’s wings are not rigidly fixed to its little body. These furry little creatures instead rotate their wings in a circular motion. And they do so up to 200 times per second(!) using a principle that is also employed by helicopters. This rotary motion gives rise to a vortex along the edge of the wings. British zoologist Charles Ellington successfully demonstrated this leading-edge vortex for the first time in 1996 by observing moths. This effect gives moths, bumblebees and other insects the lift they need to take off. Last but not least, bumblebee wings also have a special joint whose existence has not yet been definitively established in other bees and insect species. This allows them to bend their wings in flight in a way that enhances their aeronautical capabilities: when researchers immobilised this joint, the amount of weight the bumblebees could carry into the air was reduced by 8.5 percent. Without this limitation, however, the animals were easily capable of lifting twice their body weight in pollen into the air.
Above base camp
These chubby little insects are in all respects masters of flexible flight. They can fly not only forwards and sideways but even in reverse. And they can reach amazing altitudes into the bargain: bumblebees have been observed in flight on Mount Everest at an altitude of 5,600 metres. Theoretically they are even capable of flying right over the summit of the world’s highest mountain. In laboratory tests, bumblebees have flown in conditions which replicate the thin air found at an altitude of 9,000 metres.
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