The Airplane - Jay Spenser [60]
Pan Am’s Boeing 314s entered service in 1939 to inaugurate perhaps the most romantic and fondly remembered air travel of all time. Spanning 152 feet (46 meters) and weighing 41 tons (37 tonnes) at takeoff, Boeing 314A Clippers carried up to forty premium-fare passengers in overnight sleeping services and flew up to 3,500 miles (5,600 kilometers) nonstop.
An airplane’s empennage, or tail assembly, is the rear fuselage together with its protruding vertical and horizontal stabilizers with their control surfaces. Connected by hinges to the rear of the stabilizers, these control surfaces deflect in the slipstream to alter the airplane’s flight path. The one attached to the vertical stabilizer is the rudder, and the ones connected to the left and right horizontal stabilizers are the elevators.
Aviation decided quite early what an airplane’s rear end should be and do. There have been variations on the above theme, and many outright exceptions starting with the Wright Flyer, but the vast majority of airplanes ever built have had empennages designed to this formula.
Sir George Cayley gave us most of it right off the bat more than two centuries ago. Aviation’s Isaac Newton decided that an airplane’s back end should have both horizontal and vertical surfaces, not just horizontal ones like a bird. He also said that they should help the airplane fly smoothly through the air (stability) and adjust its course as need be (controllability).
Where did Cayley get these ideas? For stability, the feathers of an arrow were an obvious inspiration. Since prehistoric times, fletchers have known that stiff feathers at the rear help an arrow fly true. They have also known that these feathers must be properly aligned for stable and thus accurate flight. The French term empennage, which literally means “addition of quill feathers,” formalizes this association between arrows and aviation.
On the controllability front, Cayley drew inspiration from nature and human practice alike. Whales plunge deep by angling their great flukes downward and come up again by angling their horizontal tail upward. Fish too demonstrate a mastery of hydrodynamic control by aft-body deflection although their tailfins are vertical. Water being a dense medium, even the human act of swimming imparts intuitive insights into the cause-and-effect relationship between body motion and direction traveled.
The delightful Santos-Dumont Demoiselle of 1909 featured a Cayley-style cruciform tail.
National Air and Space Museum, Smithsonian Institution
Humans got the message early on. Boats equipped with steering oars or tillers at the rear date back more than two millennia. The idea that air too is a fluid medium came later, a product of Renaissance thinking and the scientific revolution. All of this made it conceptually easy for Cayley to translate hydrodynamic lessons to the emerging arena of aerodynamics. In fact, the idea of using rudders on airplanes leapt instantly to the minds of all early aerial experimenters. Reflecting this appropriation of maritime technology, the trailing control surface on an airplane’s vertical stabilizer is called the rudder to this day.
Cayley did not actually distinguish between stabilizers and control surfaces. Instead, he proposed a cruciform tail with fixed horizontal and vertical surfaces that served as both stabilizers and control surfaces. This was possible because he mounted the entire tail on a universal joint, allowing it to deflect left, right, up, down, or any combination thereof as a single integral unit.
This idea actually works, as Alberto Santos-Dumont demonstrated in his diminutive Demoiselle monoplane of 1909. The world’s first practical light airplane, the Demoiselle sported a Cayley-style movable tail. Since that