Protech ZOOM 425 ccpm Specifications download pdf (Page 185)

This dissymmetry of lift is equalized in different ways by the flapping and seesaw rotor heads.

For a flapping head, the upward motion of the advancing blade decreases the relative speed of

the wind, and the downward motion of the retreating blade increases the relative speed of the

wind. This is similar to holding your hand at the window at 40-50 mph. If your hand is slightly

titled up at the leading edge, and you raise your hand, this decreases the apparent lifting pres-

sure, and lowering your hand increases the apparent lifting pressure.

The flapping head may also use a rotor head where the two blades feather together (are rigidly

connected on the feathering axis) and use blades where the center of pressure (center of lift) is

behind the feathering pivot point. This causes the two blades to naturally equalize lift as the

blade generating more lift will push its leading edge downward which also raises the leading

edge of the other blade.

For a seesaw head, the advancing blade will rise up due to the extra lift. This rising motion

causes the blade to feather and slightly decrease the angle of attack because it pivots around

the blade grip's control ball. The amount of feathering is dependent on the angle formed by a

line drawn between the blade grip control balls and a line perpendicular to the rotating axis of

the blades, and is called the delta-three angle.

The delta-three angle may also be used with the flapping head to induce the blade to feather

as it flaps up and down.

Note that this dissymmetry of lift is experienced by both the main and tail rotors on an R/C heli-

copter. The two rotor heads may use different methods of equalizing lift; for example, the ECO

8 uses a seesaw main rotor but uses a flapping tail rotor relying upon the delta-three angle to

equalize lift.

30.1.7. How gyros work

Many people seem to be confused about how gyros work. In order to understand how a gyro

works, it is necessary to first understand the relationship between the main rotor and the tail

rotor.

Most helicopters have a clockwise main rotor, so for this section, we will assume the main rotor

blades are spinning clockwise.

Also, some helicopter use a variable pitch tail rotor and some helicopters use a variable-speed

motor for the tail rotor, so this section will use the terminology "increase/decrease tail rotor

thrust" to accomodate both cases.

30.1.7.1. The functions of the tail rotor

The three basic function of the tail rotor are:

• Counter main rotor torque

• Turn (yaw) the helicopter

• Stabilize yaw

30.1.7.1.1. Counter main rotor torque

The first function provided by the tail rotor is to counter main rotor torque.

The motor in a helicopter spins the blades clockwise. But in order to spin the blades, the motor

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Protech ZOOM 425 ccpm Specifications Page 185