Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.

HISTORY OF THE HELICOPTER

Although first flight generally attributed to a fixed-wing aircraft, the helicopter actually represents the first style of flight envisioned by humans. The ancient Chinese developed a toy that rose upward when spun rapidly. As early as the mid-sixteenth century, the great Italian inventor Leonardo da Vinci had drawn a prototype for the machine that we now know as the helicopter.

Early in the twentieth century, a great deal of experimentation and revision was taking place with regard to helicopter flight. The well-known phrase “two steps forward and one step back” provided an apt descriptor for early flight development. Uneven lift, known as dissymmetry, caused the early helicopters to flip over and confounded the inventors until the creation of the swash-plate; this allowed the rotor blade angles to be changed so that lift would be equal on each side of the shaft.

On November 13, 1907, the French pioneer Paul Cornu made history by lifting a twin-rotor helicopter into the air for a few seconds without ground assistance. Several models followed without significance until in 1924 when another French pioneer, Etienne Oehmichen, became the first to fly a helicopter for one kilometer. It was a historic flight of 7 minutes and 40 seconds. By 1936, solutions have been found to many of the problems with helicopter flight.

With the introduction of the German Focke-Wulf Fw 61, the first practical helicopter became a reality.

Why is it important that lift be equal on both sides of the helicopter shaft?