Early efforts to understand insect flight got off to a bad start. Calculations performed over 80 years ago showed that the bumblebee’s flight was impossible because its wings are too small and its speed too slow to generate enough lift to keep its plump body aloft. The mistake in this line of reasoning was assuming that the aerodynamic principles of airplanes and birds apply to bees and flies, while insects actually use a completely unique flight strategy.
FT Muijres et al., Science 344, 172 (2014)
This computer model, based on video of real flies, shows that their wings move back and forth, not up and down.
The movement that insects use is more akin to swimming or treading water than what we usually think of as flying. Instead of flapping up and down like a bird, insects flap their wings back and forth. During the forward stroke, the wings are tilted at approximately 45° with respect to the horizontal, forcing the air down, which exerts an upward force or lift on the insect. During the reverse stroke, the wings are flipped to 135° so they continue to press on the air and generate lift.
For an airplane, a large wing pitch, or “angle of attack,” is catastrophic – too steep a nose causes it to suddenly lose all lift and fall rapidly, an event known as aerodynamic stall. The key insights that explain why insects don’t stand still came in the 1990s from robotic representations of insects, such as the Robofly from Dickinson’s lab [1] and the Flapper, developed by Charlie Ellington of Cambridge University in the UK [2]. These mechanical models were much larger than real insects, but the researchers used tricks to reproduce the insects’ flight conditions. For example, the Robofly was fueled with mineral oil, mimicking the sticky airflow experienced by a small bug. The teams also flapped the robotic wings more slowly, making it easier to visualize the fluid movement.
G Lauder, Nature 412, 688 (2001)
This is the robofly used by Dickinson and colleagues to reveal fluid forces on insect wings.
G Lauder, Nature 412, 688 (2001)
This is the robofly used by Dickinson and colleagues to reveal the fluid forces on insect wings.×
The robot experiments showed tiny whirlwinds, so-called leading-edge vortices, which create a negative pressure that “sucks” the wing upwards, avoiding a stall. In addition, the researchers were able to measure the forces acting on the wings, which helped uncover how insects are able to fly in different directions simply by changing their flapping behavior. “Robots play a huge role in studying the aerodynamics of flying insects,” says biologist Florian Muijres of Wageningen University in the Netherlands.
This early robotic work helped explain why insects adopted their unique flight strategy. A bird-like flutter—up and down at a low angle of attack—would only generate enough lift for an inch-sized insect if it flapped its wings extremely fast. However, bumblebees already beat 250 times per second. Mosquitoes flap 600 times per second. “Insects come up against a limit on how fast they can flap their wings,” says Dickinson. They compensate for this flapping limit by pitching their wings to a high angle of attack, which provides more lift than birds can generate. The downside of this high pitch is greater drag, or drag. In fact, insects get stuck with a lift-to-drag ratio of only about one, which is about 10 times less than birds and 100 times less than airplanes. “Insects are ridiculously inefficient when it comes to flying machines,” says Dickinson. “As a result, they just burn fuel, making them hungry all the time.”
G Lauder, Nature 412, 688 (2001)
A so-called leading-edge vortex forms as a result of airflow curling over the leading edge of an insect’s wing. The low pressure in the core of the vortex increases lift. (Red arrows indicate air being pushed down by wing motion.)
G Lauder, Nature 412, 688 (2001)
A so-called leading-edge vortex forms as a result of airflow curling over the leading edge of an insect’s wing. The low pressure in the core of the vortex increases lift. (Red arrows indicate air being pushed down by wing motion.)×
