This book is about things that spin in the air or in space.
Specifically it is about things that spin in the air and space that I find interesting. I am by training an aerospace engineer, but work as a planetary scientist. Indeed, as an aerospace engineering undergraduate, I regarded - unfairly in retrospect - my lectures in fluid mechanics and aerodynamics as only a necessary evil on the noble road to exploring space where such subjects tend not to apply. My main project during my 15-year career as engineer and scientist has been the Huygens probe. In early 2005 this probe descended through the atmosphere of Saturn s moon Titan, where as it turns out, these fields applied after all. In an attempt to gain familiarity with the dynamics of a slowly spinning vehicle like Huygens under its parachute, I began in 2002/03 some experiments with instrumented small-scale models. These little models recorded the swing and spin with small sensors, and provided me with insights I would not otherwise have gained, and not a little entertainment besides.
Some months after these experiments began, and as my instrumentation became more compact, I had the idea (while sitting on an airplane, appropriately enough) that the instrumentation was compact enough to install on a Frisbee without terribly altering its flight characteristics. I duly made such experiments, which introduced new challenges in attitude determination and range instrumentation. I found that there was in fact relatively little published work on the subject of Frisbee aerodynamics. I therefore had the opportunity to make some genuinely new observations, which have since been published in the academic literature.
I also observed that almost everyone I spoke to (mostly scientists and engineers, it must be conceded ...) thought that these experiments were rather cool. It might be interesting to assemble my experiments with the modest body of scientific work on the subject, although the Frisbee research would not be enough subject matter for a worthwhile book by itself.
However, these investigations reactivated latent interests of mine in many other areas. Like millions of other people, I have marveled at how a boomerang flies, or how a stone skips across the surface of a pond. And I realized that there was a common theme to these subjects - that of spinning flight - and the idea emerged of compiling a book with that theme. Thus motivated, I also began other experiments which are reported here.
Exploiting to the full one of the few privileges afforded to an author, I have been liberal in my interpretation of the theme. Although it was not in the project as originally conceived, I have interpreted "flight" to include space, thereby encompassing certain dynamical aspects of space probes, satellites, asteroids, and planets. I make no apologies for this - these cases are just as interesting as the more classically "aeronautical," and many are more so. Similarly, my coverage of spinning disk-wings such as Frisbees stretched a little to embrace radar early-warning aircraft with spinning disk antennae, and thence to include a few words and pictures on nonspinning disk-wings.
Where I have drawn a line - one must be drawn somewhere - i s before rotorcraft. Helicopters have fascinating aerodynamics and gyrodynamics, and are magnificent machines. There are also many excellent textbooks and more popular works that cover them in detail - I certainly have no significant insights of my own to add. The one exception is a class of rotorcraft wherein the whole body of the vehicle is spun up, in addition to the rotor. Again, the criterion for inclusion has been that I thought this was novel and interesting.
The book is not intended as a textbook, although students and researchers in various fields may find ideas for many outstanding investigations or problems, and I have tried to be rigorous in my use of terminology. Equations (simple ones) have been used in the text where they are the most succinct way of expressing something, but I have no wish to deter the casual reader.
In the hope that readers may be motivated to pursue investigations of their own, I have included appendices with some technical details of my own experiments and have been fairly rigorous in including references to papers. None of the bibliographies can claim to be complete, but should give ample starting material, and certainly are representative of what I have found to be the most recent, comprehensive, or useful papers on the various subjects. Although it did not exist 15 years ago, it barely needs stating now that the Internet is an enormous resource for information. A web search will rapidly bring far more material than is in these pages.
The book before you is a little larger than the original outline proposed to the publisher, and I thank them for their indulgence in accommodating the additional material. This is the first book I have written myself (I have had the good fortune to write a couple of previous books with some excellent and experienced co-authors) and so in this case all errors are entirely my own responsibility.
As this book was being completed, the Huygens probe ended its long journey successfully parachuting down to the surface of Titan. As I and my colleagues try and understand the probe s behavior and its environment, I have found the intellectual preparation deriving from my experiments to have been quite useful. To play is to learn.