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December 3, 2009

Book Review: An Engineering History of the ZRS-4/5 Fin Design

Filed under: Aeronautics,Lighter than Air — piolenc @ 6:52 pm

Review of: An Engineering History of the ZRS-4/5 Fin Design

by Jeffrey Cook

Reviewed by F. Marc de Piolenc

This review first appeared in Aerostation, Volume 21, Number 2, Summer 1998, which was incorporated in issue no. 36 of Gasbag, the journal of the [New Zealand] Lighter Than Air Institute.

Published by the Author:

Jeffrey Cook
1857 West Aspen Avenue
Gilbert, Arizona 85233

1997. 85 + vi pages, hardbound, 8 3/4 x 11 inches, summary, references, illustrations,4 appendices. Price is $25 US in the USA, Canada and Mexico, $30 US outside North America

This book is dedicated to the memory of Donald Woodward, who contributed to the author’s research and would have reviewed the resulting work for ABAC. Don was one of perhaps two or three people in the world qualified to pass on the validity of the author’s conclusions; this reviewer will have to be content with discussing presentation and plausibility.

The book is true to its title. It is a concise and (so far as this reviewer can determine) complete history of the design of the fins of the U.S. Navy rigid airships Akron and Macon. As such, it covers not just the procedures that were followed, but the key influences on the designers that led to the adoption of those procedures. As such, it can be read as a capsule history of an important phase of the design of light metal structures, as actualized
in (and to some degree actuated by) the design of those critical and ultimately fatal structures.

But it is unlikely that Cook, or anybody else, would have felt any compulsion to write this work if the structures in question had not been blamed for the loss of one of the ships. With the wreck of the Macon lost in deep water and beyond the reach of then-available salvage techniques (her remains were found in 1990), it was inevitable that there would be controversy concerning the exact mode of failure and the critical flaw that doomed the ship and the US Navy’s rigid airship program. It is into this critical-flaw controversy that Cook has bravely waded, reaching (and clearly and forcefully expressing) his own conclusions concerning the identity of the critical factor leading to the loss of the Macon.

Before discussing the substance of the book, a few words about format, typesetting and printing. The book is self-published and the author did the typesetting and page composition himself as well, so he is responsible for the degree to which these contribute to or detract from the organization of the book. The print quality is good, and the author’s two-column layout and generous leading make the text easy to follow. The 12-point serif type font used throughout is also legible and comes through very well in the impression, with no drop-outs, ligatures or smears.

The one truly deplorable feature of the book’s layout is that all illustrations are placed in a section of their own toward the end of the book. Having worked in desktop publishing for twelve years and encountered many of the same problems experienced by Cook, this reviewer cannot help sympathizing with this approach. It is in fact extremely difficult to arrange pictures and words together on a page in a way that is logical, consistent and visually pleasing. If the text is divided into columns, the difficulty increases. Constraining the illustrations to either column or page width has an unfortunate effect on the legibility, appearance and usefulness of many figures, while leaving them at their optimum sizes makes text flow awkward. Nevertheless it is always wrong to organize a book for the publisher’s convenience rather than the reader’s, and so one cannot help wishing that the author had made the (admittedly considerable) extra effort required to put the figures with the text that they support.

A spell-checker would have been helpful, as creative spelling (e.g. “beaurocracy” in a section heading, “Burghundy” and “Wiezen” on page 41) and typographical errors have escaped proofreading. That said, it must be noted that the quality of the photos is consistently excellent, a great deal of trouble having obviously been taken to make it so. The line drawings suffer from drop-outs in the fainter lines, but fewer than this reviewer would expect, considering the variety in the originals and the sometimes extreme reductions required to give illustrations of publishable size. Graphs, charts and tables are all clear.

The one significant and staggering omission from the illustrations is that there is no drawing of the aft hull and fin structure, which is the focus of the entire book! Assuming that there was a compelling reason for that omission, a description of the structure in question and of the frame numbering system would have been welcome. The one-page Introduction concisely exposes the purpose and organization of the book, but also self-deprecatingly (and inaccurately) describes it as “…written in a style which combines the abstract miasma of an academic thesis with the disjointed tedium of a government contract report, while cleverly omitting anything that might be considered even remotely interesting.” In fact the organization of the book is well thought out and the writer’s style is quite free of the disagreeable elements that he attributes to it.

Cook begins his story just after World War I, during the transition from externally-braced fins with nearly no inherent bending strength to cantilever and semi-cantilever fin designs. We are thus spared any attempt on the author’s part to summarize the entire history of rigid airship development to that point. Cook’s approach immerses us in the story at just that point at which the engineering issues that are the focus of the book came to the fore.

As the story opens, this reader at least is struck not only by the absence of knowledge of true fin loads possessed by the Zeppelin designers, but also by their nonchalance in the face of this ignorance. It does not accord with the popular image of systematic, penetrating German engineering research. Apparently, the Germans (at least the Friedrichshafen crowd; little is said about Schütte) were content with turning tests that gave gross hull bending moments, but no details of fin forces and their distribution. The result was fins that were understrength with respect to average pressures and dangerously weak with respect to the peripheral suction peaks that were left completely out of consideration. The short and unhappy careers of the ships ceded to the Allies as spoils of war are thus easily explained. It is clear that such success as the wartime Zeps had was due to alert aerology and skillful ship handling, rather than to design. In the immediate postwar years, with the German effort temporarily shut down, the scene shifts to the Allies, and particularly to Britain and the USA, newcomers to rigid airship design but with aggressive development programs staffed by competent researchers eager to know the truth. A pattern is soon established in which operational experience with one airship points out the need for revised load estimates. New tests are then ordered in support of the design of the next generation of ships. The test results are delayed (data gathering and data reduction were, by current standards, prohibitively slow and labor-intensive), arriving after the design is frozen based (necessarily) on guesses. The new guesses prove inadequate, and the cycle repeats. Thus Burgess’ 1920 average fin pressure estimate of 0.75 lb/ft2 gives way, after the R.38 (ZR-2) disaster and the C-7 turning tests, to 40% of dynamic pressure or 1.7 lb/ft2 on C-7, with peaks four times as great. Another consistent pattern is disregard of theoretical aerodynamic calculations if the results did not fit the designers’ current estimates. Clearly the empiricists had a stranglehold on airship development at that time, perhaps because aerodynamic theory, then undergoing rapid and successful development, still carried the taint of 19th century inadequacy. This is unfortunate, because in the early Twenties it appears that the theoreticians were well ahead of the empiricists. Cook notes that independent calculations made in Italy, Britain and the USA had all predicted turning loads much higher than those estimated for ZR-1 and ZR-2. These were simply ignored. The taint of theory seems to have also affected consideration of model-scale tests, however carefully and consistently executed. Thus Cdr. Fulton’s dismissal of the 1931 NACA tests on the 1/40 scale model, which clearly showed that the pressures and their distribution were very much different from what had been assumed. Fulton’s demand that the tests be done over effectively removed them from consideration. When the confirming test results were published in 1937, they were too late to contribute to anything but a post-mortem.

Despite the precariousness of contemporary knowledge of the atmosphere, its interaction with their fragile craft and the stresses evoked within the craft themselves, the LTA designers of the period (at least in the USA) appear to have projected such calm self-confidence that their work was treated by many laymen directly concerned with airship construction and operation as belonging to an established discipline like naval architecture, rather than one that was extending its reach furlongs beyond the boundaries of what was known. Thus we have the ludicrous spectacle of the Navy contracting officers, Goodyear-Zeppelin Corp. and the Bureau of Aeronautics wrangling over contractual penalties for a weight increase in the aft structure of the projected ships as if they were discussing a new class of destroyer escort rather than an experimental aircraft. This the author sets forth without comment, allowing it to tell its own story.

The specifics of the ZRS-4/5 fin design history yield fascinating tidbits. This reader knew that the fin design had been revised early in the design process, and had always assumed it was for aerodynamic reasons. After all, the new or Mark II fin design had a larger span and a higher aspect ratio. Clearly, this was a late manifestation of the early postwar dissatisfaction with the Zeppelins’ inadequate tail surfaces and consequent “hunting” motion. A higher aspect-ratio fin would give a steeper normal-force curve, hence stronger restoring forces. In fact (and regrettably, as it turns out) aerodynamics was not even considered in the change, whose purpose was to make the base of the lower fin visible from the control car! The change would both increase the steady fin forces and their moments (which the author discusses in detail) and make the fins more vulnerable to the gust transient loads (which are not discussed), and one of the widely-held beliefs considered by Cook is that the change caused the catastrophe by overloading Frame 17.5 (the first to fail in the Macon crash). It seems plausible—the new fins’ forces were taken by only two transverse frames, versus three in the original design. Without reinforcement, goes the argument, the remaining two frames would be overloaded. Cook dismisses this hypothesis with one little-known fact—that Frame 17.5 was in fact reinforced with about 170 pounds of additional material—and a simple and plausible argument namely that any engineer of even minimal competence would have altered the hull design to take the redistributed fin loads. The deep-frame-vs-cruciform controversy is also disposed of, the author stating (correctly) that there is nothing inherently bad or good about cruciform girders (advocated by classic Zeppelin designers) or the deep, inherently stiff frames used in ZRS-4/5, so long as the structural members are properly sized for the loads applied to them.

Fatigue in Frame 17.5 is next in line for consideration. Advocates of this hypothesis (perhaps influenced by Nevil Shute’s novel No Highway) point out that Akron and Macon had roughly the same number of flight hours when they crashed. For the coincidence in flight hours to be meaningful, however, the two ships would have had to have the same load histories, and Akron never had to endure the aerial whiplash over Texas that gave Macon‘s crew the first warning of her fins’—or rather the supporting frames’—inadequacy. Cook also points out that the only kind of fatigue that Macon‘s structure could have experienced is the “low cycle” variety, which came in for much scrutiny in the late Fifties after several airliner crashes. A key prerequisite for low-cyle fatigue is that the cyclic stresses approach the yield point of the material. That in itself would indicate an error in stress analysis or in the assumed aerodynamic loads, so that fatigue, even if it occurred, could not be the root cause of the frame’s failure. This leaves, of the four hypotheses considered, only the last, namely that incorrect assumptions concerning the magnitude and (especially) the distribution of aerodynamic loads were at fault. By thoroughly documenting both the assumptions that were made (including uniform areal load) and the experimental evidence that should have been allowed to alter this picture, the author makes a convincing case for the belief that the forward 1/3 of the fins were much more highly loaded than the designers expected. As most of the excess load would have been forward of the foremost attach point (at the infamous Frame 17.5),there would have been a torque on the attach point in addition to excessive bending, which also accounts for some observations by witnesses. Cook does not ignore the fact that the frame, and not the fin structure, actually failed. The fin should have failed first by design, and the fact that it did not is used to support the fatigue hypothesis. To counter this, Cook brings in the Special (Durand) Committee’s opinion that the stress analysis techniques used were not quite up to handling the complexities of the ZRS’ structures, the “energy” or “least work” method developed later indicating that Frame 17.5 should have been made stronger.

One striking omission from the treatment of fin design in the book is that absolutely nothing is said about the aerodynamic phenomena that give rise to the fin loads, and their peculiar distribution. For instance, the load peaks at leading and peripheral edges are conspicuous in the text and illustrations (and Cook attributes the loss of the Macon to ignorance of them), but leading-edge suction is never discussed. Likewise, the fact that gust loads on fins are much higher during gust entry than when a steady state is established is mentioned in connection with the Daniel Guggenheim Airship Institute’s tests, but the unsteady aerodynamic phenomena that give rise to the effect are not. Gust entry affects not just the total load on an aerofoil, but also its distribution, making unsteady aerodynamics highly relevant. But the author’s discussion is in terms of
an equivalent steady angle-of-attack, which may not be equivalent at all. Not that this consideration would threaten the author’s conclusion; it would in fact reinforce his thesis. It would, however, lessen the culpability of the Macon‘s contemporaries because unsteady phenomena were just beginning to be understood at that time. Unsteady lift might also account for the fact that Macon was de-finned by a gust generally thought not to have been especially severe. In unsteady flow, it is the rate of change of aerodynamic loading (the “steepness” of the gust onset) that matters, not absolute magnitude. The fascinating question of aerodynamically-induced distortion of a poorly-supported outer cover and its reciprocal effect on the fin loads is mentioned in connection with the Shenandoah‘s famous mast breakaway, in which the top fin also collapsed, but not considered further. Presumably, the Macon‘s outer cover was better-supported than that of the Shenandoah. The treatment is purely from the point of view of the structural engineer of that time, whose only reliable knowledge of the loads that he was meant to overcome was empirical, and usually reached him too late to do much good. The result is an implied rebuke to those responsible for designing ZRS-4 and ZRS-5, as if the author were saying “see, if I could figure this out, so could you.” In this, the author disagrees with Don Woodward, to whom the book is dedicated and who believed that fatigue—a phenomenon little known and poorly understood at the time—had played a critical role in the Macon fin failure.

Thorough without being pedantic, well presented and clearly reasoned, this book belongs on the shelf of anyone interested in rigids and their turbulent technical history.

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