by Dan Hampton
—Dan Hampton
Hanover, New Hampshire
March 2018
Acknowledgments
Once again, I owe my profound thanks to Peter Hubbard, executive editor at William Morrow/Harper-Collins, for his unquenchable belief in each of my books, and the immense amount of time, effort, exasperation, and perspiration he pours into each work. Without his expertise, rampant enthusiasm, and tolerant nature, none of these stories would be possible. I am grateful, though do not always say so loudly enough, to his associate editor, Nick Amphlett, and my publicist, Maria Silva, for their unstinting devotion to making these books a success. I can neither imagine nor wish for a more dedicated group of professionals than the entire HarperCollins/William Morrow team.
Special thanks to George Marrett, former test pilot and fellow author, for putting me in contact with the Chilstroms; Colonel John Scott Chilstrom, a brother pilot, was kind enough to arrange an introduction for me to his distinguished father, Colonel Ken Chilstrom, without whom this book would lack the rich detail and untold stories he provided.
My gratitude and admiration go out to the tireless professionals of our National Air and Space Museum. Namely, Dr. Alex Spencer, Curator of the Aeronautics Department, who took me to the museum’s amazing Suitland, Maryland, storage facility and permitted me to crawl around in our national treasures; Dr. Bob van der Linden, author and chairman of the Aeronautics department, who cheerfully tolerated my pesky questions and allowed me sift through reams of priceless original documents, transcripts, and reports; finally, my sincere respects to Dr. John Anderson, curator of aerodynamics, for his patience, boundless knowledge of all things aeronautical, and willingness to sacrifice his invaluable time. Dr. Anderson can count eight critically acclaimed books among his lengthy accomplishments, as well as membership in Who’s Who in America, a Glenn L. Martin Distinguished Professor for Education in Aerospace Engineering appointment, and Professor Emeritus status for the University of Maryland.
Dr. Mary Ruwell, archivist for the United States Air Force Academy, granted me access to her wonderful collection, and was tremendously helpful with the aviation history aspects of this book, as was Dr. John Terino of the USAF Air Command and Staff College, who is always willing to critique my writing and save me from public errors. Similarly, Mike Dugre of the USAF Historical Office, and Dr. Roger Launius of Launius Historical Services were instrumental in filling in the blanks for the NACA’s early history. Michael Lombardi, Boeing’s corporate historian, was kind enough to dig through his archives and send me the North American Aviation Test Reports filed by George Welch during the fall of 1947. Another fellow author, Lauren Kessler, now a professor at the University of Oregon, went beyond professional courtesy to personally answer my questions regarding Pancho Barnes.
Last, but never least, my parents; my wife, Beth; my children; and the rest of my family must be recognized for enduring the perils of living with an author. Uncooked dinners and unmade lunches, neglected projects, forgotten appointments, and dozens of other transgressions for which I alone am responsible, and for some reason they continue to forgive. In partial penance for this unsettled life—which none of you asked for—please accept my lasting apologies, love, and appreciation.
Appendix: Aerodynamics 101
Before venturing deeply into the history and mysteries of aviation, it would be advisable to review some basis concepts that, while fairly commonplace in today’s world, are fundamental to flight and were misunderstood for centuries. To begin with, an airfoil is the shape, seen in cross section, of a body that moves through a fluid. For our purpose this usually means a wing of some type, and the fluid is air, though the terms can and do apply elsewhere.
Nevertheless, it is this movement that produces an aerodynamic force as the fluid splits around the airfoil. We tend to consider fluid as being liquid, but it can be anything really, as long as its component molecules flow freely, such as air; or it can be something that assumes the shape of its container, like a gas or liquid. For our discussion we will use air, though visualizing water often aids in aerodynamic explanations as water can seen and air usually cannot. Aerodynamics, then, is a study of how bodies move through air; and air, according to NASA, is “a physical substance which has weight. It has molecules which are constantly moving. Air pressure is created by the molecules moving around. Air is a mixture of different gases; oxygen, carbon dioxide and nitrogen. All things that fly need air.”
So an airfoil can be your hand protruding from a car window, or a blade, feather, a wing—whatever is moving through the fluid. We will usually be discussing a wing, which is a specific airfoil, though keep in mind that propellers and turbines are also airfoils. For a symmetrical airfoil—one shaped equally on the top and bottom—the air would move across both surfaces at the same speed. The curve, or camber, of an asymmetrical airfoil is not the same on the top and bottom, and because mass (in this case airflow) must be conserved it must therefore be the same at any point along the airfoil’s cross section. In order to do this, the flow velocity over the top, cambered section is faster than that below. This results in less pressure being exerted downward as the higher-velocity molecules are dispersed, creating “thinner” air and less pressure. Slower air is “thicker” and produces greater pressure under the wing, which pushes upward and, meeting less resistance, correspondingly lifts the airfoil.
This relationship between velocity and pressure was well understood for centuries, at least as far as wind and water were concerned. Mariners applied it to the setting, or trim, of a sail (is really just a movable airfoil) for varying the pressure to move the ship that was attached to the sail. In 1738, Swiss mathematician Daniel Bernoulli articulated the principle in his Hydrodynamica, though its adaptation to the fledgling world of aviation would take a bit longer. Still, it is a simple notion. A strong pressure pushing upwards against a weaker pressure will continue to move in that direction and take the airfoil, or wing, with it.
Camber can be, and almost always is, designed into an airfoil to intentionally produce the pressure differential that creates lift. This curve, and its effects, can also be altered by the use of leading-edge and trailing-edge flaps, which give the wing different lifting qualities during specific flight areas, and by ailerons, which we discuss in detail later.
John Smeaton, an English civil engineer, discovered in 1759 that lift was greater when an airfoil was cambered rather than flat. This seems to have been largely forgotten and though the concept was grasped during the nineteenth century, it was not patented until 1884 by Horatio Phillips, who called it a “double surface airfoil.”
“The particles of air struck by the convex upper surface,” reads the patent text, “are deflected upward, thereby causing a partial vacuum over the greater portion of the upper surface. In this way a greater pressure than the atmospheric pressure is produced on the under surface of the blade.” This was a significant revelation, yet one that already intuitively occurred to several aviation pioneers. In any event, the properties were quickly grasped, improved, and incorporated into relatively efficient wings capable of producing lift, and it is this lift that enables flight. It does not, by itself, constitute flying, but without this force the other components of flight—weight, drag, thrust, and control—are generally impotent, at least as far as flight is concerned. However, the generation of lift alone does not mean the bird, insect, or aircraft will get off the ground; for that to happen the lift must be greater than the weight of whatever is being lifted. Weight is a gravitational force, a vector, so it must have both magnitude and a direction. Magnitude is a function of the combined mass from the pieces, parts, people, and the aircraft itself. Its direction is some component angle opposite the lift vector as gravity tries to pull the mass into the center of the earth. Bottom line: a wing must produce enough lift to overcome the associated weight in order to physically get itself and its attachments off the ground.
This is still not flying, but we are getting closer. Thrust is simply any
force that propels a craft through the air. A bird flaps its wings, getting the air moving over hundreds of tiny airfoils in the feathers, and thus producing lift. A glider wing can do this as long as it has forward velocity, but a glider has no way to produce thrust itself so man needed to artificially lift it. This began with the steam engine then continued with internal combustion, the rocket, and finally the jet.
Just as lift must be greater than the weight for a craft to become airborne, so must thrust exceed drag for the craft to move. Drag is the cumulative resistance of everything affecting the forward movement of a craft through the air. Primarily this is due to variations in air pressure and friction, acting locally all over a body. All the pieces, parts, and people on, or inside, the aircraft: everything generates drag of one type or another. For example, contact between air molecules and the surface of an aircraft causes a shear-stress, or friction, and this skin-friction is a major source of drag. Rubbing your hands together very fast illustrates the point; you feel heat, which is a result of friction.
Shape is also critical; visualize a torpedo moving through the water versus a block of wood, and the concept of form drag is clear. Pressure acting on a body and the velocity of the flow around it are not constant values; they are localized due to many factors, including the construction of the craft itself. Another type of drag is caused by interference: the physical joining of different parts of the aircraft, such as the fuselage meeting the wing. These have always been major design concerns, and refinement continues even today. Collectively, the combination of skin friction and form drag is termed parasite drag, as it is not caused by lift, but by the construction and design of the aircraft itself.
On the other hand, induced drag is a by-product of lift. Most of this is derived from revolving areas of flow, like smoke rings or miniature tornadoes, which are called vortices. These are created by the pressure differential that generates lift, and they spill over the wingtips. Now, the divergent airflow that permits lift also merges at some point downstream past the wing where the flow rejoins. The impact of both upper and lower flows crashing into each other produce vortices, which in turn create a disturbance and more drag. So the parasite and induced figures added together become total drag. Other types of drag, wave and compressibility drag specifically, occur as airflow becomes transonic and supersonic, but this will be discussed later.
However, the pictured airfoil is idealized. It is moving straight ahead, parallel to the horizon and directly into the relative wind. Real wings, especially those on highly maneuverable fighters, are very rarely level and all aircraft will vary the angle at which they strike the relative wind as they take off and land. This angular difference between the airflow and the wing is called the angle of incidence or, more commonly, the angle of attack. It is critical because this affects all the aerodynamic forces described above, and especially the amount of lift that is produced when the flow impacts the wing at angles other than zero. This calculation, vital to wing and aircraft performance, is expressed as a ratio of Lift over Drag, and reveals that lift increases as the angle of attack decreases. So the more directly a wing strikes air, then the greater pressure, and lift, will be generated because of it.
But what happens when the angle increases, as it does in slow-speed flight during landings, or during extreme maneuvering such as combat? Well, the wing continues to produce lift with varying degrees of efficiency until it reaches a critical angle of attack, the point where its maximum lift is produced. Visualize peeling an orange and feeling the skin finally ripping off in your fingers; this is what happens to the airflow when it finally separates from the top of the wing.
Beyond this point the wing stalls, just as a sail will flap ineffectively when pointed into the wind. When this occurs, the angle of attack must be decreased in order for air to move over the wing again and generate more lift than drag. This took some time to work out and did not fully mature, as with many other aerodynamic principles, until the advent of aerial combat during World War I.
Once aerodynamic forces were better understood, the next logical step was to account for each of these in aircraft development, and much of this effort toward refinement went into wing design. Benjamin Robins, an eighteenth-century English Quaker turned military engineer, noted that variously shaped airfoils produced different aerodynamic results. He was the first to develop the concept of a wing aspect ratio and relate it to wing design. This ratio is simply the wingspan squared divided by the planform area or, in the case of a rectangular wing, planform area. It revealed that in subsonic flight a rectangular wing, or one with a high aspect ratio, produces more lift than a stubby, low aspect wing, as there is more lifting surface available.
Of course, there are always aerodynamic trade-offs. High aspect wings produce more lift at the expense of structural stability; they are longer and protrude farther from the fuselage so in the early strut-and-wire days, extensive bracing was required. Longer wings also shift the pressure outboard, away from the fuselage, and as speed increases this affects controllability. Shorter, low aspect wings are much stronger, but do not generate lift to the same degree. The initial compromise was the development of biwing aircraft—the biplane. A pair of shorter wings permitted greater lift and increased strength, while keeping the aspect ratio fairly high. It is well to remember that comprehending, and putting into practice, these aerodynamic truths took years of trial and error, with spectacular failures and eventual successes.
The monoplane quickly replaced the biplane once technology and design evolved where a single, structurally strong, high aspect wing could be used in place of two wings and this resulted in much less drag, as there were no exposed struts and wires. Corresponding engine advancements generated higher power, and the increase in speed produced greater lift so smaller wings could be effectively used. These weighed less and generated less drag, so the more powerful engines now produced excess thrust. Wing design is a perfect example of the advantages for parallel development, in that the larger wing also permitted multiple gun mounts, with more interior space for ammunition storage and retractable landing gear—all made possible by aerodynamics.
Another component needs to be defined and, though not a force per se, control is essential for true flight; that is, the capability of an onboard pilot to physically and deliberately manipulate an aircraft through multiple dimensions. There are three primary axes by which this occurs. Pitch is along the lateral axis running from wingtip to wingtip, and the vertical up or down action is initiated by a stick or yoke in the cockpit. When a pilot pulls back, rectangular surfaces on the tail called elevators deflect upward into the airstream. The flow strikes the elevators and the resulting pressure pushes the tail down, which raises the nose of the aircraft. If the pilot pushes forward the opposite occurs; the elevators deflect down, which raises the tail and the nose falls.
Yaw is like swinging around a vertical pole. It is a rotation around the perpendicular axis and is controlled by a rudder that functions exactly like one on a boat. Installed vertically on the tail, it is moved left or right by pedals on the cockpit floor. Again, deflecting the airflow pushes, or yaws, the aircraft. Due to changes in lift, an aircraft cannot maintain level flight in a rudder-only type of turn, and yawing alone is a sloppy way to change direction, like skidding around a corner in a car, versus turning the wheel. To counter the loss of lift a pilot will roll the aircraft, using ailerons. These are smaller rectangular surfaces located on the trailing edge of the wings. Moving a stick or yoke left or right works the ailerons in tandem; one raises and one lowers, which decreases and increases the lift on both wingtips, respectively, and rolls the aircraft. It is through the combined use of all these controls that nonlinear oblique movements, just like a bird makes, are possible. Variations on these basic controls are as numerous as variations among aircraft themselves, and the flight control system of commercial airliner supports much different requirements than that of a jet fighter, yet they all work along the same principles.
So wi
th these basic aerodynamic forces in mind, how is an airplane put together to fly? The fuselage is the center of all this, the main body and principal structural component of an aircraft. From the French fuselé, or spindle, it was originally a girder and lattice structure filled with bracing and wires. Wood was the preferred material, since it could be readily cut, shaped, and molded, and the entire arrangement was covered with fabric, usually linen or cotton. Various coatings were employed for waterproofing, as the sky is often a wet place. Unfortunately for many early combat aviators, wood is easily shattered by force, or bullets, and coated with varnished, painted cotton it readily burned.
As engines became more powerful not only were more secure mountings needed, but also a framework that could withstand the additional stresses resulting from increased performance. The original solution was a monocoque, or single shell, fuselage that carries all the aerodynamic stress. A mold for each half of the fuselage was created, and thin strips of wood were layered at right angles and glued to one another. Once dry, both halves were glued together and the resulting structure was very light and extremely strong. This was time consuming and difficult to repair, especially battle damage, so as with most aviation issues a compromise was reached.
The solution was a synthesis of both two methods; semimonocoque or veneer, had a frame, spars, and cross bracing but was covered by wood panels rather than fabric. This type of fuselage would survive the strut-and-wire biplane era, progress into the all-metal aircraft age, and various derivatives are still used today. Aft of the fuselage is the tail assembly, or empennage. This vertical tail section contains a rudder and the horizontal tail with elevators. Structures vary, depending on the type of the aircraft and its purpose, but the function is the same. Construction of the empennage plays a crucial role in the plane’s balance, center of gravity, and aerodynamic performance. This was particularly true in the early days of supersonic flight test as the disturbed airflow trailing from the wings severely affected downstream airflow over the elevators, which resulted in deadly control issues.