Understanding the mechanical gain of the tail and how it affects a gyro.

The mechanical gain of a helicopter tail is the combination of all factors that affect how the tail responds to a given correction requested by the gyro. Depending on the tail design the mechanical gain can vary significantly. With the more responsive tails (high mechanical gain) smaller corrections are needed from the gyro and as such the gyro gain will peak at a lower value. Equally, when the tail is not as responsive (lower mechanical gain) the gyro will need to request a much larger correction and a higher gyro gain is needed to achieve the same task. Neither excessively high nor excessively low mechanical gain is good for optimal tail performance. For example:

• Excessively high mechanical gain may be the reason for tail wag even when the gyro gain has been set to a very low value.
• Excessively low mechanical gain may be the reason of poor tail hold.

Below we list all factors that contribute to the overall mechanical gain of the tail. Some of these, such as the servo ball link distance and size of tail blades, can be modified in order to adjust the mechanical gain as needed.

Servo response Not all servos move the same amount for the same change of the control signal. The servo specification below show that the HS-5084MG requires 50% more change of the control signal in order to rotate the same amount as the S9257. As such the mechanical gain of the HS-5084MG servo is lower. If someone wanted to swap a S9257 to a HS-5084MG the gyro gain would need to be increased to compensate for the decrease of mechanical gain.

• The Futaba S9257 moves 45deg for 400 [uSec] change of the control signal.
• The Hitec HS-5084MG moves 45deg for 600 [uSec] change of the control signal.

Ball link distance from the servo centre The placement of the ball link on the servo arm has considerable effect to the mechanical gain. The example below assumes a servo rotation of +/- 45 deg using the standard holes on a Futaba servo arm.

• Ball link at the most inner hole (7.5mm from centre) the tail pitch push rod will travel 10.6mm.
• Ball link at the second hole (10.5mm from centre) the tail pitch push rod will travel 14.8mm (40% increase of mechanical gain from hole 1)
• Ball link at the third hole (13.5mm from centre) the tail pitch push rod will travel 19.1mm. (29% increase of mechanical gain from hole 2)
• Ball link at the fourth hole (16.5mm from centre) the tail pitch push rod will travel 23.3mm. (22% increase of mechanical gain from hole 3)

Tail mechanics The design of mechanical parts of the tail also have considerable effect to the mechanical gain. The length of various arms as well as the gear ratios affect the responsiveness of the tail and therefore the mechanical gain.

Blades The size and shape of the tail blades affect the rotor efficiency and therefore contribute to the overall mechanical gain of the tail. Larger blades produce more thrust therefore increasing the mechanical gain.

Main rotor RPM When the main rotor spins at faster speeds so does the tail rotor thus producing more thrust and increasing the mechanical gain. If you have two flight modes (Idle-1, Idle-2) with significantly different RPM you will most likely find the gyro gain will need to be different in order to compensate for the change in mechanical gain. Most modern radios can link the gyro gain to the Idle-Up switch so it can automatically change when you change flight modes.