Figure 2-10

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.

LEAD AND LAG

Rotor blades also tend to move in the horizontal plane. The reason for this is angular

momentum. Physics tells us angular momentum must be conserved (MVR2=C). This concept is

well illustrated by a spinning ice skater who increases his/her spin rate by pulling the arms

toward his/her body (figure 2-12). The same sort of thing occurs while the rotors are turning. As

the blade flaps its center of mass moves with respect to the center of rotation. When the blade's

center of mass is closer to the center of rotation it will tend to lead (move faster). If the blade's

ROTOR BLADE AERODYNAMICS 2-9