Rotorcraft Flying Handbook, FAA-H-8083-21
Rotorcraft Flying Handbook, FAA-H-8083-21
Rotorcraft Flying Handbook, FAA-H-8083-21
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Helicopters and gyroplanes both achieve lift through<br />
the use of airfoils, and, therefore, many of the basic<br />
aerodynamic principles governing the production of lift<br />
apply to both aircraft. These concepts are explained in<br />
depth in Chapter 2—General Aerodynamics, and constitute<br />
the foundation for discussing the aerodynamics<br />
of a gyroplane.<br />
AUTOROTATION<br />
A fundamental difference between helicopters and<br />
gyroplanes is that in powered flight, a gyroplane rotor<br />
system operates in autorotation. This means the rotor<br />
spins freely as a result of air flowing up through the<br />
blades, rather than using engine power to turn the<br />
blades and draw air from above. [Figure 16-1] Forces<br />
are created during autorotation that keep the rotor<br />
blades turning, as well as creating lift to keep the aircraft<br />
aloft. Aerodynamically, the rotor system of a<br />
gyroplane in normal flight operates like a helicopter<br />
rotor during an engine-out forward autorotative<br />
descent.<br />
VERTICAL AUTOROTATION<br />
During a vertical autorotation, two basic components<br />
contribute to the relative wind striking the rotor blades.<br />
[Figure 16-2] One component, the upward flow of air<br />
through the rotor system, remains relatively constant<br />
for a given flight condition. The other component is the<br />
rotational airflow, which is the wind velocity across the<br />
blades as they spin. This component varies significantly<br />
based upon how far from the rotor hub it is<br />
measured. For example, consider a rotor disc that is 25<br />
feet in diameter operating at 300 r.p.m. At a point one<br />
foot outboard from the rotor hub, the blades are traveling<br />
in a circle with a circumference of 6.3 feet. This<br />
equates to 31.4 feet per second (f.p.s.), or a rotational<br />
blade speed of <strong>21</strong> m.p.h. At the blade tips, the circumference<br />
of the circle increases to 78.5 feet. At the same<br />
operating speed of 300 r.p.m., this creates a blade tip<br />
Resultant Relative Wind<br />
Wind due to Blade Rotation<br />
Upward<br />
Airflow<br />
Figure 16-2. In a vertical autorotation, the wind from the<br />
rotation of the blade combines with the upward airflow to<br />
produce the resultant relative wind striking the airfoil.<br />
Relative Wind<br />
Direction of Flight<br />
Relative Wind<br />
Direction of Flight<br />
Figure 16-1. Airflow through the rotor system on a gyroplane is reversed from that on a powered helicopter. This airflow is the<br />
medium through which power is transferred from the gyroplane engine to the rotor system to keep it rotating.<br />
16-1
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