The Airplane - Jay Spenser [61]
Simple physics endorses Cayley’s thinking. Stabilizers, which guide the airflow, can be located anywhere on the airplane. The Wright flyers initially didn’t have vertical stabilizers; when they did, starting in 1905, the Wrights placed them at front and called them blinkers because they were reminiscent of the devices that let horses look only forward.
In fact, vertical and horizontal stabilizers actually work best at the rear, as do elevators and rudders. What’s more, placing these stabilizers and control surfaces as far aft as possible allows them to be smaller, lighter, and contribute less aerodynamic drag than if they were located elsewhere on the airframe. This enhanced efficiency is why most airplanes have these components at the back end. It’s also why they have their ailerons out near the tips of the wings instead of in at the roots. Intuitively we understand that the farther outboard we place these control surfaces, the more effectively they can bank the machine to the left or right.
What we’re sensing is the mechanical advantage provided by a longer moment arm. In physics, the principle of moments states that the farther from the center of rotation (which for airplanes is the center of gravity) that a given force is exerted, the greater its effect at the center. The principle of moments explains why crowbars work so well. This universal physical truth means that mounting an airplane’s stabilizers anywhere but at the aft end, which is as far as they can be from the center of gravity, will render them less effective.
Another reason to put them there is downstream drag, which enhances stability. Like a weathervane rotating to show wind direction, stabilizers that project into the slipstream tend to align the airplane with the airflow by slightly retarding the rear of the airframe relative to the rest of the machine. Feathers at the back of an arrow provide this same benefit.
It took a while for airplane builders to discover all this. During World War I, for example, the Red Baron’s infamous Fokker Dr. I triplane was a bear to fly because it had too short a fuselage and lacked a vertical stabilizer in front of its rudder. The Fokker firm addressed these failings in its next design, the D.VII, which displayed superb handling.
Companies were still learning lessons on this front on the eve of World War II, as Boeing’s experience with the Clipper shows. By then, however, it was strictly a matter of aerodynamic fine-tuning; nobody disagreed with the basic ideas of what an airplane’s tail should be and do.
Today, triple-tailed beauties such as the Boeing 314 or Lockheed Constellation are a thing of the past. Aerodynamic optimization has pushed the world to single vertical tails.
If all this was so intuitive, why did Wilbur and Orville Wright go such a different route? It all had to do with control in the air.
8 FLIGHT CONTROLS
THE CHARIOT’S REINS
They envisaged the flying machine…as a light and living structure, propelled and maneuvered about the sky as if it were a bird possessed.
—SIR CHARLES H. GIBBS-SMITH (1909–82) ON THE WRIGHT BROTHERS1
Wilbur Wright arrived in Le Havre in May 1908. Proceeding to the customs house, the forty-one-year-old retrieved a large crate containing a disassembled Wright Model A Flyer built in Dayton and shipped to France the previous year.
With the help of Léon Bollée, an amiable manufacturer of early automobiles, Wilbur took his airplane to Le Mans, some 130 miles (210 kilometers) west of Paris, and arranged to use its Hunaudières racecourse for a flying field. On opening the crate, however, he was distressed to find its contents in disarray, with many parts broken.
Living in Bollée’s factory, eating and sleeping beside his machine, Wilbur methodically repaired and assembled this Model A in preparation for the first public demonstration of a Wright airplane anywhere in Europe. He and Orville had left the bicycle business to manufacture