Hiram Maxim also used a huge whirling arm to test airfoils. His whirling arm included elaborate instruments to measure lift, drag, and relative air velocity. Samuel Langley, the mathematician, astronomer, and secretary of the Smithsonian Institution, was another who experimented using a whirling arm before he built his aerodromes. His whirling arm was 60 feet (18 meters) in diameter and its 10-horsepower (7.5-kilowatt) engine could accelerate it to speeds of 100 miles per hour (161 kilometers per hour). But his results were thrown off by the winds and turbulence that the arm itself created.
People have learned to design wind tunnels that provide low turbulence levels in the ambient fluid – to give a chance to the boundary layer on the body of investigation to develop.