AEROELASTICITY IN NATURE

While there are certainly examples of aerodynamics in nature, there are no rigid airfoils. And while there are examples of solids (aerospace structures) in nature, there are few, if any, rigid structures. On the other hand, nature is full of aeroelastic problems - with some very novel interfaces of fluid mechanics and solid mechanics, with dynamics as the adhesive.1 The flexibility of real world structures and the interaction with their environment still has many unanswered questions for us to investigate.

There are probably hundreds of references, but the ones pointed out here are those which stand out from the rest in this professor's opinion. Foremost among them is a now out-of-print book by Hertel2 translated from German in 1966. Structure-Form-Movement is unique, with many varied topics ranging from birds to insects to swimming creatures; from hummingbird explanations to feather structures; from flying fruits and seeds to comparison of a dolphin shape with a NACA 66-018 airfoil; and from snake swimming to flag flutter. This book is an excellent example of clear explanations and real communication - not always present in others - and for that reason alone may be worthy of detailed study. (Interestingly, in my own travels to many, many technical conferences, probably the most lucid presentations I have heard have been those presented by Prof. Gaul from Germany. His mixtures of English and German on slides and unique use of color and experiment have a clarity that is all too rare, much like the book by Hertel. A recent PhD graduate of AE here at AU showed similar clarity of detail not common among American students.)

Biomechanics by Alexander3 is a small treatise, again with very varied subject matter ranging from engineering aspects of cells and muscles to falling seeds, blood flow, and more. While not as aeroelastic-oriented as Hertel's book, there certainly are related principles in it, and its reading is bound to broaden the outlook of even the most experienced engineering student.

Two of the more interesting (and long investigated) areas of "Aeroelasticity in Nature" are Gray's Parodox and Korotkow (or Korotkoff) sounds. It would take volumes to just review the research done in these areas. [Several excerpts from the fundamental papers on Korotkow Sounds were attached as handouts (the typewritten one was prepared by a former student as a handout - much of it taken verbatim from work by Fung, and by Womersley)]. As most people know, the phenomenon is associated with compressing an artery in the arm with an inflatable cuff, then gradually releasing it until sounds associated with the bloodflow can be accurately distinguished. In this manner, some measure of blood "pressure" can be determined, but it is really not a pressure in the engineering sense. (In the AU Vet School, in a combined effort of our ME Dept. & the Vet School, actual blood pressures were measured [on the bull] during bull/cow mating. These actual pressures were found to be ten times higher than anyone thought possible in living humans or animals. The phenomena is obviously associated with cattle reproduction.)

One classic paper from 1935 is the original paper by Gray4 which led to Gray's Parodox (summarized in both Reference 2 and 3, and also in Reference 5). The perusal of this vintage paper of almost six decades ago should improve the engineer's perspective of "early" engineering and its rigor.

Korotkoff, a Russian, originally published his first work in 1905. An interesting poem, supposedly learned by medical students, conveys the concept as it relates to our "Aeroelasticity in Nature" notes:

Streamline flow is silent

Remember that, my boys,

But when the flow is turbulent

There's sure to be a noise.

 

So when the stethoscope picks up

A bruit, murmur, sigh,

Remember that it's turbulence

And you must figure why.

Concerning blood flow, the many amazing things about our blood system can hardly be explained by chance happenings, although you will find many references to the contrary. An excellent and concise description of these matters is given by a full professor of Theoretical and Applied Mechanics at the U. of Illinois at Urbana-Champaign, Marlyn E. Clark6. This reference describes the Circle of Willis, a number of things about the cardiovascular system, a number of flow patterns relating to valves, and other interesting phenomena. There is more about Prof. Clark and his blood flow work in a U. of Illinois Newsletter7.

Several other articles of interest are not as broad or as thoroughly done as Ref. 1 for example - since they are conference or journal papers - but they are still of interest and provide insight into the subject. See Blick8, and also Lissaman, Kuethe and Yates in an AIAA Student Journal issue entitled, "The Impact of Birds, Bees, Insects and Fish on Aerodynamics." 9

The origin of flight is a very interesting subject. Some speculate that flight just happened by chance, not only once with (1) birds, but differently three more times for (2) insects, (3) flying reptiles (now extinct), and (4) flying mammals (bats). Others claim that the absence of transitional forms leaves little evidence for such speculation. See Gish10 for a concise discussion of the subject (and 18 more references), especially with regard to the Archaeopteryx fossil find [Dr. Gish has a Ph.D. in Biochemistry]. In a very recent book by a law professor at Cal.-Berkley, there is considerably more discussion on this matter11.

Finally, in this review of some of the more interesting references, an AIAA conference paper by McMasters12 looks at natural and man-made low-speed flying devices, and a very recent book by Tennekes13 looks at "The Simple Science of Flight." In the latter, there are many numerical constants given for birds (wing area, aspect ratio, glide ratio, weight, speeds, wing span) as well as some data on popular airliners. Tennekes shows revealing diagrams which compare how forward motion is derived by the skater and flapping bird wings, both compared to force balances for simple aircraft flight. He also gives several profound plots (one a log-log plot of weight vs cruising speed of flys to 747's; another - rate of descent vs airspeed; another - wind & airspeed for butterflies, bees, sailplanes, etc.).

References 14 & 15, while not very aeroelastic-oriented, are natural phenomenon-oriented and are included here to "whet your interest." M. A. Cutchins, 1998 Handout in Aeroelasticity Classes

REFERENCES

1. Bisplinghoff, R.L. & Ashley, H., Principles of Aeroelasticity, Wiley, N.Y., 1962, p. vii.

2. Hertel, Heinrich, Structure-Form-Movement, Reinhold Publishing Co., N.Y., 1963 (translated into English in 1966) (Now out-of-print, I understand). QP301 .H413 c.2

3. Alexander, R. McNeill, Biomechanics, Chapman & Hall, London, 1975.

4. Gray, J., Studies in Animal Locomotion, "VI. The Propulsive Powers of the Dolphin," 1935, Journal of Experimental Biology, Vol 18, pps. 192-199. (Possibly Vol. 13)

5. Parry, D.A., "The Swimming of Whales & a Discussion of Gray's Parodox," Journal of Experimental Biology, Vol. 26, No.1, 1949, pps. 24-34.

6. Clark, M. E., "Our Amazing Circulatory System...By Chance or Creation?," ICR Technical Monograph No. 5, ICR, El Cajon, CA, 1975.

7. _______, "One Brain to Another," lead article in Engr. Outlook, Vol. 25, U. of Illinois at U-C, Sept. 1983.

8. Blick, E.F., "The Aerodynamics of Birds," (in Reference 9 below) [ Blick is a Professor of AE, ME & Nuclear Engr. at the University of Oklahoma.]

9. AIAA Student Journal (Articles by Blick, Lissaman, Kuethe, and Yates), Summer 1976.

10. Gish, D., "As a Transitional Form Archaeopteryx Won't Fly," Institute for Creation Research, El Cajon, CA, September 1989.

11. Johnson, P. E. [A Cal-Berkley law professor], Darwin on Trial, pps. 49, 78-79, 153, 166, 174.; Intervarsity Press, 1991.

12. McMasters, J.H., "An Analytical Survey of Low-Speed Flying Devices - Natural and Man-Made," AIAA/MIT/SSA 2nd International Symposium on the Technology & Science of Low Speed and Motorless Flight, Cambridge, Mass, Sept. 1974, AIAA Paper No. 74-1019.

13. Tennekes, H., The Simple Science of Flight, From Insects to Jumbo Jets, MIT Press, 1997.

14. Kane, T.R. & Scher, M.P., "A Dynamical Explanation of the Falling Cat Phenomenon," International Journal of Solids & Structures, Vol. 5, 1969, pps. 663-670.

15. Fairchild, J.E., Editorial; and "Boomerangs," Blick, E., both in Summer 1975 AIAA Student Journal entitled, "Applications of Aerospace for fun, games, ...," pps. 2-3 & 22-23, respectively.