How high can a bumblebee fly

Are bumblebees really too fat to fly?

The myth about the bumblebee, which shouldn't actually fly, but somehow does, goes back to the French entomologist Antoine Magnan. In 1934 he referred in his book "The Flight of the Insects" to the calculations of the mathematician André Sainte-Laguë. According to this, an airplane wing the size of a bumblebee wing cannot fly at bumblebee speed. The statement is true to this day. But bumblebee wings and airplane wings don't have much in common, except for one thing: how they generate lift.

A bumblebee is not an airplane

Lift results from an uneven flow around the wings, caused by the Bernoulli effect. The bionic scientist Albert Baars from the University of Bremen describes it as follows: "If there is less pressure on the upper side of a wing - regardless of whether it is an airplane or an insect - than on the underside, the wing can move upwards the angle to the flow and the geometry of the wing are chosen so that the air moves on a more curved path at the top than at the bottom. The greater this curvature, the lower the pressure on the top. "

Every fast driver knows this: when we drive around a tight, strongly curved bend, the centrifugal forces pull us strongly outwards - we hardly notice the forces in large, sweeping bends. "This is exactly what happens with the air flowing around an airfoil," continues Baars. "As the curvature increases, the air wants to move outwards. But there is already air there. So it continues to follow the curved path - which reduces the pressure and the wing is sucked upwards."

The secret of the bumblebee wings

Bumblebee wings are also curved: there is even a joint in their wings which they can use to further increase the curvature. But the main difference between the bumblebee wings and airplane wings is that they move. What with the insects looks like a simple up and down, in reality results in a self-contained movement. The bumblebee flaps its wings diagonally from back to front, with the wing tips always forming the foremost point; then the wings are rotated, the top and bottom are swapped, and the movement is returned. The secret of the bumblebee also lies in this complex process: the formation of a leading edge vortex. Charles Ellington from the University of Cambridge demonstrated this vortex in a moth that he examined in a wind tunnel in 1996.

"The leading edge vortex occurs when you tee off," says Baars. "It runs along the top of the wing, almost parallel to the leading edge." Leading edge vortices arise with every downward movement and generate strong lift: "The air on the upper side of the wing has to go around this vortex and thus follows a curved path again. This results in lower pressure - and thus additional lift," explains Baars.

Vortices that we have known for a long time

"When it comes to generating high lift, engineers are really keen on this vortex," continues Baars. "This is used in very small aircraft with flapping flight mode, but also in large aircraft with delta wings, such as the Concord or fighter jets. The vortex is not likely to be found in conventional aircraft." In contrast to those of bumblebees and fighter jets, the wing edges of passenger and transport aircraft are rounded. Leading edge vortices only arise on geometrically sharp edges if they are at a certain angle to the air flow. But they always bring with them risks: If the angle is unfavorable, the flow can stall.