The Airplane - Jay Spenser [48]
There were two of these mechanical balances. Made out of old hacksaw blades and bicycle-spoke wire, they worked by comparing astonishingly small forces. One balance assessed an airfoil’s lift and the other its drag (the Wrights called the latter drift). From their results, the Wrights could also determine a given airfoil’s center of pressure, the point at which the lift is concentrated.
Wilbur locked his newly completed airfoil section in one of the balances at the angle to be tested. Lowering the balance into the wind tunnel, he secured it to the bottom and closed the glass top. At his nod, Orville turned on the fan and stood stock-still because walking around the room could affect the airflow, throwing off the delicate test results.
The tunnel thrummed as air washed at high speed across the sideways-mounted airfoil. Its reaction in the slipstream deflected the needle of a gauge. Peering through the glass, Wilbur wrote down the reading.
To learn what they needed to know, the Wrights had started with preliminary tests of a dizzying variety of airfoils. They assessed wings of rectangular, triangular, elliptical, and circular planform; wings with different cambers and aspect ratios; wings by themselves (monoplane) or in combination (biplane, triplane, or tandem like Langley’s aerodromes); and multiple wings with variations in the spacing between them to observe the effect of wing gap.
The brothers tested some two hundred airfoils over four weeks. Wilbur became so proficient at cutting and shaping airfoils that he could complete a new one in as little as fifteen minutes. Painstaking as it was to build up an entirely new body of knowledge, there was also the quiet thrill involved in exploring uncharted intellectual territory.
“We had taken up aeronautics merely as a sport,” Orville later wrote. “We reluctantly entered upon the scientific side of it. But we soon found the work so fascinating that we were drawn into it deeper and deeper.”10
Here was Kitty Hawk in microcosm. In fact, it was more productive than that because empirical results could be obtained without having to travel, construct full-scale gliders, or carry them again and again to the tops of sandy hills in long trudges. Better still, the brothers were spared nature’s capriciousness because the gusting, quartering, and faltering sea breeze made accurate test results hard to come by at Kill Devil Hill.
As fall turned to winter, the Wrights were focusing their attention on upward of fifty wing shapes that looked particularly promising. Each was tested at angles of attack ranging from zero to 45 degrees in 2.5-degree increments, first in one balance and then in the other. Calculations made according to simple equations allowed them to convert this raw lift and drag data to usable results. The tabulated aerodynamic performance of the different airfoils could then be plotted and compared.
As their knowledge grew, the brothers saw why their 1900 and 1901 gliders had performed poorly. Smeaton’s coefficient—a value named for British civil engineer and physicist John Smeaton, who in 1759 first measured the pressure exerted by moving air—was off by a considerable amount. As a result, Lilienthal’s tables of calculated airfoil lift also had been off.
Even so, the Wrights’ studies increased their admiration for Lilienthal, whose measurements of relative airfoil performance were surprisingly accurate for someone who tested with a whirling arm rather than a wind tunnel. By the same token, the Wrights found Samuel Langley’s comparative lift data so flawed as to be worthless.
As this intensive testing drew to a close, the Wrights settled on one airfoil in particular that showed the best overall performance. Called Wing #12, it was a narrow rectangle with a 6:1 aspect ratio (it was six times longer in span than in chord). This parabolic airfoil had a camber