Figure 2-10

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HELICOPTER AERODYNAMICS WORKBOOK
CHAPTER 2
Figure 2-10
GEOMETRIC TWIST
Geometric twist is a blade design characteristic which improves helicopter performance by
making lift (and induced velocity) distribution along the blade more uniform. Consider an
untwisted blade. With rotational velocity being much greater at the tip than at the root, it follows
that AOA and lift will also be much greater at the tip. A blade with geometric twist has greater
pitch at the root than at the tip. A progressive reduction in AOA from root to tip corresponding
to an increase in rotational speed creates a balance of lift throughout the rotor disk. It also delays
the onset of retreating blade stall at high forward speed, due to reduced AOA. A high twist of 20
to 30 degrees is optimum for a hover, but creates severe vibrations at high speeds. No twist or
low twist angles reduces the vibration at high speed, but creates inefficient hover performance.
Blade designers generally use blade twist angles of 6-12 degrees as a compromise (figure 2-11).
Figure 2-11
FLAPPING
In order to maneuver the helicopter the rotor disk must be tilted. The rotor blades therefore
must be allowed some vertical movement. Vertical blade movement is termed flapping.
Flapping occurs for other reasons as well, which will be discussed later.