Personal Aircraft Drag Reduction {Review}
Personal Aircraft Drag Reduction
(Published and sold by the author, 34795 Camino Capistrano, Capistrano Beach, California 92624, USA: 1995) 195pp, 195 illus., 26 tables, refs. Photocopy, 1/c, Velobound. US$25 postpaid in USA, $28 Canada, $33 Western Europe, $35 elsewhere.
Reviewed by F. Marc de Piolenc
This reviewer has spent a good deal of time searching for a better title for this book, since limiting its application to personal aircraft seems to deny its much wider usefulness. This compendium of aerodynamic design guidance (and rare, sometimes unique data) by a widely recognized authority in the field deserves a more sweeping title.
True, it does emphasize low-subsonic Mach numbers and low to medium Reynolds numbers, and discussion of powerplant installation drag is limited
to air-cooled piston engines, but the sophistication of some of the aerodynamic drag reduction techniques is well beyond the skill of most personal aircraft builders. This book is (with the exception of one acknowledged omission) a design manual for the ultimate low speed, high efficiency airplane, if the designer chooses to use it that way. It is in any case much more than a set of cookbook recipes for a cheap weekend flier. A military contractor working on a long endurance, high altitude reconnaissance airplane will find material useful to him, as will designers of pylon racers, CAFE competitors and others who are fighting installed-power restrictions or seeking the ultimate in aerodynamic performance.
The organization of the book reflects the author’s cognomen of “Mister Low Speed Aerodynamics,” his long-standing interest in laminar flow design and his determination to apply to flight articles concepts thought by many to be laboratory curiosities.
Beginning with a historical review of research aimed at enhancing external laminar flow, completed by an interpretation of trends in this field, the author goes on to offer examples of actual, flying aircraft incorporating drag reduction techniques that he advocates. There follows a section entitled “Drag Concepts” explaining the various tools available to the designer for measuring, comparing and estimating drag. While the rest of the book emphasizes reduction of parasite drag, this section necessarily considers the contribution of induced drag to the total drag of an airplane, both to give a complete picture and to give the reader an idea of the relative gains from reducing one or the other type of drag.
“Some Performance Considerations” defines a Universal Performance Function, considers the effect of drag reduction on the top speed of both propeller- and turbojet-driven airplanes and considers the “climb-out problem.”
This is followed by a compendium of laminar flow wing experimental data, prefaced with a discussion of the significance of Reynolds number and a comparison of theoretical airfoil drag prediction to wind tunnel results. This section covers only “natural” laminar flow, achieved by shaping the airfoil section for the longest possible “run” of favorable pressure gradient. The following, brief section covers all-laminar wings, where laminar flow is maintained artificially by suction. The author then briefly discusses the influence of three- dimensional effects on wing performance and the design of high lift devices. A wing optimization study then compares the minimum drag area of a reference aircraft with flaps and without flaps.
The section that follows contains data unique to this book. It concerns low drag bodies suitable for use as aircraft fuselages, and capable of achieving long runs of laminar flow. Again, theoretical considerations are followed by masses of experimental data on low-drag bodies culled from the author’s extensive work in this field. Surprisingly, the effect of body shapes on static aerodynamic stability is also considered in two subsections on body yawing and pitching moments. All-laminar bodies using suction to stabilize the laminar boundary layer are briefly considered. The section ends with a body drag optimization study based purely on natural laminar flow.
Tail surfaces are considered next, primarily from the point of view of minimizing control surface profile and parasite drag. Trim drag (the induced drag of the stabilizer or fin) is considered here only briefly. The thrust of the section is to guide the reader to a stabilizer design of minimum surface area and thickness.
The author then delves fearlessly into an area of aerodynamic design with a high voodoo content – component interference drag. Drawing on his own experience, Carmichael assigns it firmly to the parasite drag category, contradicting some eminent authorities. Having defined the source of the drag, he justifies his assertion by reference to experimental data and lists the many successful remedies applied. He also discusses one obscure and intriguing example of favorable wing-body interference.
The next section, on cooling drag, is perhaps the most frustrating to the reader. On the one hand, the author claims that a large portion (10%-30%) of engine power is consumed in keeping the engine cool, as against the 2-4% theoretically required, suggesting that great gains could be made in this area. He then disclaims experience in abating cooling drag and offers a brief discussion of inlet and exit design, a cursory discussion of internal flow losses, and eleven references. The author’s discussion centers on aircooled engines with conventional cooling arrangements, but some of the references include treatment of liquid-cooled engines and the use of exhaust-driven ejectors to induce cooling air flow.
“Practical Problems and Solutions” covers the problems that face constructors attemping to realize long runs of natural laminar flow in real aircraft. It is as satisfying, despite its brevity, as the preceding section was
frustrating. Here the author’s rare combination of theoretical understanding
and practical experience combine to make a rare treat for the reader. After the usual discouraging exposition of the influence of waviness on flow transition on a flat plate, he presents the results of flight experiments at Northrop that clearly showed that real flows over actual airfoils are less sensitive to waviness than flat-plate data would indicate. What is more, the trend with increasing Reynolds number shows allowable waviness decreasing less quickly than for flat plates. Both findings provide solid
encouragement to builders. More valuable and equally encouraging data on steps and gaps put natural laminar flow design squarely inside the realm of practical construction, even – with great care – on metal wings.
“Adverse Influences in Flight” discusses the influence of tractor propeller slipstream, noise and vibration, atmospheric turbulence, icing, rain and insect contamination on the performance of laminar light aircraft.
“Refinement through Flight Test” picks up where most design texts leave off. Answered here is the question of what to do when a new airplane’s performance does not meet expectations. As one might expect of a long-time soaring pilot, the flight test methods presented emphasize zero-propeller-thrust (engine running) glide tests and gliding tests with the cooling system sealed. Also covered are flow visualization tests using tufts or oil films. The references n this section are especially numerous, varied and useful. The techniques are well within the reach of serious and careful amateurs.
In “Conceptual Laminar Flow Design,” all the foregoing considerations are reviewed and their influences on design considered. Two aircraft – one actual (the Teal, designed by Carmichael’s mentor Ed Lesher) and one conceptual (Lars Gietz’s Vmax Probe, currently under construction) – are used for illustration. The author presents a full drag “buildup” (estimate) for Gietz’s machine, then compares it with actual figures from the Teal, taking into account differences in design and design goals.
The final section is not about laminar flow at all. It concerns Fabio Goldschmied’s ingenious and controversial proposals for large suction bodies that achieve staggeringly low drag with fully developed turbulent flow. On such bodies suction actually increases skin friction drag, but also
lowers the pressure drag of the body. (At higher suction flows, the “drag” becomes negative, i.e. the flow control system becomes a wake-ingesting propulsor.) The Griffith airfoils get a similar, illuminating discussion.
Summing up impressions of this book, disappointment in the cursory treatment of cooling drag is balanced by relief at the author’s reluctance to pontificate outside his avowed fields of competence. This book, plus selected references, should provide an alert designer having a basic knowledge of aerodynamics with a solid “core curriculum” in high efficiency, low speed airplane aerodynamic design. What is deeply regrettable is the crudeness of the book’s typesetting, page layout and printing. The text is output on a dot matrix printer in a two-pass, blocky sans serif face that is very difficult to read. The illustrations are pasted in, not scanned. The line drawings, most of them prepared or replotted by the author, are clear and well organized. Unfortunately, photos and heavy solids are not halftoned, making many photos unreadable (e.g. tuft and oil film flow visualization photos). The book is reproduced on a high speed electrostatic copier and velo-bound. One could reasonably ask for better quality than this, as desktop publishing equipment and software are now within easy reach of most. In particular, an adhesive binding that could open and lie flat would have been welcome, and would have added nothing to the cost of the book while enhancing its readability.
Addendum, September 1996: The author has made extensive revisions for the new edition of the book. The typeface has been changed to a more pleasing design (although output is still on an impact printer); key photos have been halftoned and are much clearer, and the new edition is perfect-bound with a wraparound cover. There is also new material on current low-drag aircraft projects.
Considered only as a compilation of research data, this book is a bargain when its price is compared with the cost of finding the data. It would be difficult to assign a dollar value to the author’s lucid exposition of theoretical concepts, his insights into the data, his well-organized and well-labeled plots and graphs, and his care in making the book “amateur- friendly” by avoiding the use of formulas. Fortunately, we don’t have to, as these features can be considered free supplements to an excellent compendium.
About the Author
Bruce H. Carmichael has earned the affectionate cognomen of “Mr. Low Speed Aerodynamics,” having made low Reynolds Number fluid flows the object of his life’s work. An aviation enthusiast since 1928, he earned his Bachelor of Science degree in Aeronautical Engineering at the University of Michigan in 1944, studying under Prof. Edgar Lesher. He worked for Chance Vought and Goodyear Aircraft as an Applied Aerodynamic Engineer. later he joined the late Dr. August Raspet’s team at Mississippi State College conducting flight research on boundary layer control, ontinuing that work under Dr. Werner Pfenninger at Northrop. Before retiring from North American Rockwell, he worked on low drag underwater vehicles with Dr. Max Kramer.
His 43-year career has been split between analytical and experimental work in both hydro- and aerodynamics. It included test programs in low-turbulence wind tunnels, in flight, in water tunnels, water basins, deep lakes and the ocean. The emphasis throughout was on laminar flow, both natural and suction-stabilized, and on the aerodynamics of the critical Reynolds Number regime.
Bruce Carmichael has lectured at Cal Tech, USC and MIT. He has been featured speaker at National Soaring Conventions, Experimental Aircraft Association conventions and Sailplane Homebuilders Association workshops. His work has been published in the IAS Journal, various NASA Contractor Reports, Northrop and Rockwell reports, Soaring, Technical Soaring, O.S.T.I.V publications, Sailplane Builder, National Free Flight Symposium journals, Sport Aviation, Kitplanes, Contact! magazine and the French magazine Experimental. He describes himself as a “ham-handed model airplane builder and sailplane pilot.”