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Helicopters and gyroplanes both achieve lift through the use of airfoils, and, therefore, many of the basic aerodynamic principles governing the production of lift apply to both aircraft. These concepts are explained in depth in Chapter 2—General Aerodynamics, and con- stitute the foundation for discussing the aerodynamics of a gyroplane. 

## AUTOROTATION 

A fundamental difference between helicopters and gyroplanes is that in powered flight, a gyroplane rotor system operates in autorotation. This means the rotor spins freely as a result of air flowing up through the blades, rather than using engine power to turn the blades and draw air from above. [Figure 16-1] Forces are created during autorotation that keep the rotor blades turning, as well as creating lift to keep the air- craft aloft. Aerodynamically, the rotor system of a gyroplane in normal flight operates like a helicopter rotor during an engine-out forward autorotative descent. 

## VERTICAL AUTOROTATION 

During a vertical autorotation, two basic components contribute to the relative wind striking the rotor blades. [Figure 16-2] One component, the upward flow of air through the rotor system, remains relatively constant 

for a given flight condition. The other component is the rotational airflow, which is the wind velocity across the blades as they spin. This component varies signifi- cantly based upon how far from the rotor hub it is measured. For example, consider a rotor disc that is 25 feet in diameter operating at 300 r.p.m. At a point one foot outboard from the rotor hub, the blades are travel- ing in a circle with a circumference of 6.3 feet. This equates to 31.4 feet per second (f.p.s.), or a rotational blade speed of 21 m.p.h. At the blade tips, the circum- ference of the circle increases to 78.5 feet. At the same operating speed of 300 r.p.m., this creates a blade tip 

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