The breakthrough came at the end of the nineteenth century when the internal combustion engine was invented and made it practical to develop full-sized helicopters with enough power. Then many other problems started to arise as the early pioneers started to design and test their helicopters.

Torque - the effect produced by a single rotor to force the fuselage to rotate in the opposite direction as the engine, had become the earliest first major issue of difficulties that needed to be solved.

The second problem related to the dissymmetry of lift - the action that often tends to cause the early single-rotor helicopters to flip over when translating from hovering to forward flight.

This problem confounded the early pioneers until the introduction of independent freedom of blade motions made possible by the invention of the swashplate.

The swashplate provides a means of varying the pitch of the blades in a cyclic fashion as they rotate around the central shaft. The provision of cyclic pitch control allowed the lift to be equalized on each side of the shaft and eliminate the tendency of the helicopter to tip over sideways.

There were however many other problems that needed to be worked out continually before the first practical helicopter was born.

On November 13, 1907, the French pioneer Paul Cornu lifted a twin-rotor helicopter into the air entirely without assistance from the ground for a few seconds.

Figure 6 An Early French Tandem Rotor Design

In about the same time, Henry Berliner created the first powered rotorcraft that successfully made a controlled flight. Berliner's helicopter only flew about 100 yards at an altitude of about 15 feet, but the flight was completely controlled by a pilot.

Figure 7 An Early Spanish Tandem Rotor Design