The Insect Plane

news and views Daedalus a b 10Ð6 The insect plane Aviation engineers look with envy on birds ) Atom 1 and especially insects. Their flapping Ð Vacancy s 10Ð7 wings lift and propel them far more Wire cross m c ( efficiently than the fixed wings of aircraft. V section V One reason is their ability to exploit the subtleties of stalling. Atom Ð8 Vacancy 10 If the angle of attack of a wing is increased, it ultimately stalls, with sudden, 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 disastrous loss of lift. No fixed-wing T (K) aircraft dare risk stalling. But an insect with oscillating wings can exploit an Figure 1 Superdiffusion in solid helium. a, Migration of crystal vacancies, and counterflow of atoms, intriguing loophole in the laws of around a stress-imposing wire in a solid crystal of helium, which allows the wire to move. b, Velocity aerodynamics. Accelerated at a high angle of the wire as a function of temperature as observed by Polturak et al.1, showing a peak at the of attack into the stalling regime, a wing temperature where ‘superdiffusion’ of atoms and vacancies occurs. This peak coincides with the takes a short while to stall. And until it temperature at which helium changes crystal structure, and is much larger than any such effect seen does, it generates enormous lift. By in metals (note that the ordinate scale is logarithmic). speeding into stall and out again at each flap, an insect wing develops amazingly softening of a particular kind of crystal defects. This feedback mechanism, and a high average lift. vibration, or mode in the phonon very high density of point defects to begin So Daedalus is inventing a non-steady- spectrum7. Polturak’s group thinks the same with, are crucial ingredients in producing a state aircraft wing. A conventional wing process causes the helium effect. The extra- mechanical instability sufficient to generate could never be made to flap, of course. But ordinary magnitude of the peak in Fig. 1b is melting: both are missing in rival theories of it might be covered with a flexible elastic associated with the very low formation ener- melting, of which there have been many over fabric, and this could be flapped by a gy for crystal point defects in helium (` 1 the years. system of rapid repeated inflation and meV) compared with b.c.c. metals (` 1 eV). Robert W. Cahn is in the Department of Materials collapse. It might even be made to flap This low formation energy for He suggests Science and Metallurgy, University of Cambridge, spontaneously in the slipstream, as a flag that both the concentration and the mobility Pembroke Street, Cambridge CB3 0DT, UK. does in the wind. But the ideal solution is of point defects, such as vacancies, might be e-mail: [email protected] simpler still. Instead of flapping the wing greatly enhanced near the peak. Computer 1. Berent, I. & Polturak, E. Phys. Rev. Lett. 81, 846–849 or its surface, Daedalus plans to flap the simulations by Polturak et al.3 suggest that (1998). airflow around it. 2. Berent, I. & Polturak, E. J. Low-Temp. Phys. 112, 337–354 the point defects responsible for the very fast (1998). Cunningly, he will generate this non- diffusion may in fact be a type of interstitial 3. Polturak, E. et al. Physica B (in the press). steady airflow from a non-steady structure in the crystal lattice rather than 4. Schuster, I. et al. Europhys. Lett. 33, 623–628 (1996). propulsive source, a pulse-jet of the type vacancies. 5. Guyer, R. A. in Encyclopedia of Physics 2nd edn (eds Lerner, used to power the old V1 missile. Its R. T. & Trigg, G. L.) 486–494 (VCH, New York, 1991). The acceleration of vacancy creep under a 6. Haskel, D. et al. Phys. Rev. B 47, 14032–14043 (1993). primitive motor drew in air through a one- small stress (also called superplasticity by 7. Petry, W. et al. Phys. Rev. B 43, 10933–10947 (1991). way valve, and mixed it with petrol vapour metallurgists) at a phase-transformation in its combustion chamber. When the temperature is a well-established fact, but in chamber was full, a spark ignited the metals the magnitude of such acceleration is mixture. The valve closed, directing the nothing like that observed in helium. Poltur- propulsive blast out through the tail-pipe. ak and colleagues2 have evidence that the The valve then opened and the cycle 3 presence of He enhances superdiffusion in errata In J. Bland-Hawthorn’s article “Clues repeated. So Daedalus’s new ‘pulse-wing’ 4 He by further reducing the point defect for- to galaxy formation” (Nature 400, 220–221; 1999) aerofoil has a leading edge enclosed in mation energy. So in isotopic mixtures, the the image in Fig. 1 was not produced by the cunningly shaped ducting, which acts as a magnitude of the diffusion peak at the tran- Virgo Consortium and was not published in long, thin pulse-jet combustion chamber sition temperature is even greater than that ref. 12 as stated. The image should have been stretching the length of the wing. shown in Fig. 1b. attributed to G. Kauffmann, J. M. Colberg, Each time the chamber fires, a sheet of The significance of these studies in heli- A. Diaferio & S. D. M. White, Mon. Not. R. hot gas blasts from the ducting, entraining um is the unprecedented degree of enhance- Astron. Soc. 303, 188–206 (1999), as part of the airflow round the wing. It speeds it up ment of point defect concentration and the GIF project (http://www.mpa- dramatically, and veers it upwards to put mobility at a phase transformation. These garching.mpg.de/~jgc/sim_gif.html). the wind into brief extreme stall, thus findings are also relevant to melting theory, creating a sudden pulse of enormous lift. in that a very high density of point defects The source of the cichlid images used in the The craft is both propelled and held aloft combined with a softened phonon mode graphic accompanying Tom Tregenza and Roger by repeated pulses. To minimize noise and can lead to melting, by reducing the shear K. Butlin’s “Speciation without isolation” (Nature vibration, Daedalus hopes to drive his resistance near the transition to the point 400, 311–312; 1999) should have been pulsed wing in a continuous, distributed where the solid becomes mechanically acknowledged as a photograph taken by manner. Each explosion will spread from 3 unstable. In their latest work , the authors Michael K. Oliver. The photograph of the species the wing root out to its tip, by which time suggest a feedback mechanism in which the concerned, Dimidiochromis compressiceps, and another explosion will be starting at the point defects in helium soften the phonon others can be viewed at The Cichlid Fishes of root. (vibrational) spectrum, and this in turn Lake Malawi, Africa http://www.connix.com/ David Jones enhances diffusion and creates more point ~mko/ NATURE | VOL 400 | 5 AUGUST 1999 | www.nature.com © 1999 Macmillan Magazines Ltd 513 news and views notoriously difficult to test. O’Connell and changes that we see in the hominin fossil Although most of the activity associated colleagues’ proposal may, or may not, fit with record a little less than 2 Myr ago. By playing with galaxy formation took place in the dis- the fossil and archaeological evidence, but down the role of meat eating at this stage tant past, there are good reasons for believ- possible tests include stable-isotope analyses in hominin evolution, O’Connell and col- ing that the outer reaches of galaxies are still to check for any dietary shift, and a search for leagues’ hypothesis raises the possibility that in formation, at least in backwaters like the residues of woodworking or starch grains on the increased body mass owing to foraging, Local Group6. Many astronomers are tanta- appropriate stone artefacts. Although the combined with access to a more reliable lized by this possibility, not just because it contemporary stone tools would not be source of meat, allowed the increase in may confirm our basic understanding of obviously effective for hunting, they would absolute and relative brain size that was the galaxy formation, but because of the chance have been adequate for preparing wooden next major innovation in human evolution. to identify truly ‘primordial’ structures in digging sticks. Homo ergaster’s body shape Bernard Wood and Alison Brooks are in the the near field. indicates that it is the first hominin to dis- Department of Anthropology, George Washington So where are these primordial building pense with a large digestive tract, so its diet University, Washington DC 20052, USA. blocks? Blitz et al.2 claim to have the answer. must have differed from that of its australo- e-mail: [email protected] More than one-third of the sky is peppered pithecine predecessors, who, apparently, had 1. O’Connell, J. F. et al. J. Hum. Evol. 36, 461–485 (1999). with compact clumps of neutral hydrogen a large gut like the living apes13. Moreover, H. 2. Ruvolo, M. Annu. Rev. Anthropol. 26, 515–540 (1997). whose motions depart fairly radically from 3. White, T. D. et al. Nature 371, 306–312 (1994). ergaster’s relatively small jaws and teeth are 4. Leakey, M. G. et al. Nature 376, 565–571 (1995). Galactic rotation. Discovered in 1963, these consistent with a diet requiring lower bite 5. Asfaw, B. et al. Science 284, 629–635 (1999). ‘high-velocity clouds’ (HVCs) have been the forces and less chewing.

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