In most fields of engineering, designers are not well served by textbooks that are mostly written by academics and tend to focus on research and development rather than the procedures whereby a new engineering artefact can be created. This is perhaps inevitable. Authors must, if they are true to themselves, write about what they know best, and career professors are more likely to have the motivation, time and opportunity to write textbooks than practising engineers. Senior engineering managers are often heard to grumble that starting engineers are ill-equipped for their profession, and about the length of time that it takes for a new engineer to become truly effective, but how is such a person to learn to be creative (for engineering is a creative profession) except by experience? With the benefit of a good mentor that process may be shortened, but such people do not always exist.
At first sight Wennerstrom may appear to be firmly in the R & D camp. In fact he opens by saying that his book is an attempt "to summarize the most important points which I have learned during a career devoted to advanced fan and compressor research and development." But note that the book carries the word design in its title, and that proves not misleading. He goes on to say "I have attempted to provide some guidance to the aerodynamic designer in most aspects of detail design
I have recommended practical and convenient design approaches
In my opinion, the simplest approach that can be logically defended is usually the best one unless a designer has a very good reason for escalating the level of complexity in any area." Few designers with any experience would disagree with the latter sentiment.
Wennerstrom spent thirty years working as a civilian with the USAF at Wright-Patterson Air Force Base, which is recognized as a world-class centre of aircraft engine development. Much of his work has found its way into production engines, both military and civil, as well as industrial and even consumer-oriented turbomachines.
He opens the book with a summary of just what high loading means in terms of axial fan and compressor design. Here he touches on some of the historical parameters and guidelines, but the treatment is brief and it is clear from the outset that he assumes some familiarity with the basics of his subject. This is not a book for the outright beginner, but for one who has at least some grounding in turbomachinery.
The actual design process is picked up after the preliminary design has been completed. The author is not concerned with deciding the number of stages, the work level per stage, or the approximate flow path. The book picks up at the point where detail design begins, and takes the reader through the design of each blade row. On the way he describes the empirical input required for detail design. Design has to start somewhere. The problem is too complicated and with too many dimensions to start from a performance specification and proceed directly to an optimum blade shape. Invariably one creates a trial design, analyses it, reviews the results of the analysis, makes changes and repeats the process. And so some empirical input is required just to get the whole process started.
Thereafter we proceed through various two-dimensional and three-dimensional flow field analysis techniques, taking up along the way the questions of how best to define the complex three-dimensional shape of an airfoil, and the most effective strategies to optimize a design using modern computer-based flow field analysis techniques. Although such methods have now reached a high level of sophistication they are not perfect, and the author is right to warn readers about their limits, and how one should tread very carefully when taking such methods outside of the current experience base. Other topics that feature include the supersonic flows and shock waves that figure large in very highly loaded blades, secondary flows, and boundary layer growth and development.
Aerodynamics is the principal subject area of this book, as it must be given the topic, but there are some important mechanical considerations also. These include the blade deflections caused by steady stress, and blade flutter or vibration. The former is important because rotor blades in service distort under stress and if this is not corrected for in the design process, can end up a different shape and with different aerodynamics from that which was intended. Forced vibration and flutter are life-limiting effects, and the prevention of high-cycle fatigue is a major aim of the structural designer. Various methods to predict and control it are reviewed.
The author finishes his work with a chapter of "Compressor Development Lessons Learned the Hard Way", which describes two case studies that went seriously wrong and analyses the technical reasons for this. Of course it is always easy to be wise after the event, and in retrospect one can see clearly how and where the designers attempted to over-reach themselves. Such lessons are always important. The questions of project control and management that allowed such things to happen would be at least as interesting and revealing, but are not discussed.
The main objection to this book is that at 100 pages it is too short and (no pun intended) too compressed. One feels that the author could write a lot more and in more detail without resorting to padding. Nevertheless, as a guide and inspiration to designers it is almost alone in its field.