Saturday, June 16, 2012

Fruit Flies

Time flies like an arrow, but fruit flies like a banana.

Those pesky rascals turn out to be tiny marvels of aerodynamic engineering. The flight regime they operate in is a somewhat difficult one, at a Reynolds number of around 200, which means, more or less, that inertial forces are 200 times larger than viscous forces. That sounds like a lot, but it isn’t. The kinds of aircraft that carry people typically operate with Reynolds numbers in the millions, and somewhere around Re = 40,000 there is a dramatic decrease in the lift to drag ratio for a smooth wing. That’s a likely reason why Drosophila’s wings, like those of other insects and small birds, are patterned rather than smooth.

Drosophila is elaborately instrumented for flight, with pride of place going to his remote optical sensors. These brick red eyes are large, with diameters of roughly 15% of his body length, or, in actual size, rather less than that of the period at the end of this sentence. A good aircraft also needs some air speed sensors, and the fruit fly antenna serves that purpose. Inertial navigation systems are required if you want to be up-to-date, and flies have been so equipped since the Middle Triassic, two hundred thirty million years or so ago, courtesy of modified wings called halteres.

With his flight equipment the fruit fly is phenomenally maneuverable. A 90 degree turn in flight is executed in 50 ms – a tribute to his flight control system and to his tiny moment of inertia. This sort of performance tends to make designers of unmanned aerial vehicles – often called drones – green with envy. The desire to design tiny UAV has in fact propelled a lot of research on bird and insect flight. Designing and building a robotic flyer with insect or bird like performance is still far beyond human technology, but plenty are trying. Some of the more impressive results come from Robert Wood’s lab at Harvard. Follow the link to see some of his amazing results and cunning methods of fabrication.

Nature’s fabrication methods are hard to match, though. The fruit fly, like the rest of us higher multicellular animalia, is a product of modular design. The master module maker, the homeobox, was first discovered in Drosophila melanogaster, and one of its central jobs is segment our embryonic forms into smaller pieces for further development. For some worms, most of those segments might look pretty similar except maybe for the head and tail, but for insects and people, the segmentation is more elaborately differentiated. For vertebrates like us, the individual vertebrae and their associated bony and neural structures are probably the most obvious traces of the original plan. For insects like the fruit fly, the remnants become mouth parts and body segments.

Our large aircraft achieve some performance metrics that outmatch anything in the animal kingdom. Those metrics are based mostly on the fact that heat engines, especially the jet turbine, can produce far higher power to weight ratios than anything in biology – mostly because they can operate at very high temperatures. Internal combustion engines don’t scale well to tiny sizes, because of the increasing problem of friction at smaller sizes and especially because tiny internal combustion engines lose heat too fast to be efficient. Electric motors do scale pretty well, but suffer from the problem that we have not yet figured out how to store electrical energy at high energy/weight ratios.