How Gyro Gain Works

	How Gyro Gain/Mode works
Back to Main Page	Other gyro pages: - Tail Pitch direction - - Gyro & Revo Mix - - Common Confusions -

Gyro questions are one of the top five questions I get. And of the gyro questions most relate to a confusion of the gyro gain and switching between heading hold and non-heading hold.
Most heading hold gyros can be switched from HH (Heading Hold) to non-HH by your radio. As well as set the gain from the radio. Your receiver does not have a special plug to communicate to the gyro with. The only plugs are the standard servo plugs. So the gyro has to use these to receive the commands you give it. Ok, now we get a little technical here so stick with it and read through. Knowing the gyros side of the story will help you understand how your radio tells the gyro what to do. To start with you need to understand how a servo is told what position to go to. This is necessary because these same plugs are what the gyro is connected to. Each servo receives a signal that is between 1.0 millisecond and 2.0ms long. The duration of the pulse tells the servo what position to move to. 1.0ms would be full counter-clockwise*, 1.5ms would be center, and 2.0ms would be full clockwise. *Note: Some servos work backwards so 1.0ms would be full clockwise. So a signal from 1ms to 2ms tells the servo what position to move the servo arm. No digitial coded info here, just a simple analog pulse. So what does the gyro do with this, well it looks at the length of the pulse and for pulses from 1.0ms to 1.5ms it knows to set itself to non-HH mode*. For pulses above 1.5ms it sets itself to HH mode. The closer the signal is to the ends, 1ms or 2ms, the higher the gain. Half a millisecond represents the full range of gain. 1.95ms would represent 90% gain in HH mode. This comes from 0.90 times 0.5ms equals 0.45ms. Add that to 1.5ms, which is the center and you get 1.95ms. The same works for non-HH mode, 90% of 0.5ms is 0.45ms. Subtract that from 1.5ms and you get 1.05ms. This gives 1.05ms for 90% gain in non-HH mode and 1.95ms for 90% gain in HH mode.
*Note: some gyros work in reverse but either way, one half represents HH and the other non-HH.

Now for the radio end of it. Before radios came out with special gyro menus you would use the ATV settings in the radio to command the gyro. With the gain plug of the gyro plugged into channel 5 (typically called the 'gear' channel) you would go to the end point menu (aka ATV, EPA, Travel Adjust) and then set both sides (switch up and switch down). The 'gear' switch on the radio will switch the gyro from HH to non-HH. And the values you set for end points will tell the gyro how much gain. Now something that gets a little confusing is you would think 100% end point would be 100% gain, however you will notice that most radios have a range well beyond 100%. Some radios have a top end of 110%, 125%, 140%, or 150%. So to get 100% gain on the gyro you would have to max out the end point setting. Here is an example:

Switch position	End point (max 150%)	Signal sent to gyro	Gyro mode	Gain amount
Up	150%	2.0ms	heading hold	100%
Up	125%	1.916ms	heading hold	83%
Up	100%	1.833ms	heading hold	67%
Down	150%	1.0ms	non-HH	100%
Down	125%	1.083ms	non-HH	83%
Down	100%	1.167ms	non-HH	67%

So what about the radios with a special gyro menu, well lets look at an example using a radio such as the Futaba 8U Super. With this radio the gyro menu lets you set a value from 0% to 100%. This maybe a little confusing since now a range of 0% to 50% represents non-HH and above that represents HH mode. The closer to 50% the less gain the gyro will have. Note: that on this radio the end point and reverse setting for channel 5 are ignored when the gyro menu is active.

Switch position	End point (max 140%)	Gyro menu	Signal sent to gyro	Gyro mode	Gain amount
Up	not used	100%	2.0ms	heading hold	100%
Up	not used	92%	1.916ms	heading hold	83%
Up	not used	84%	1.833ms	heading hold	67%
Down	not used	0%	1.0ms	non-HH	100%
Down	not used	8%	1.083ms	non-HH	83%
Down	not used	16%	1.167ms	non-HH	67%

Note: Some brands use a range of -100% to +100% in the gyro menu as in the following chart.

Gyro menu	Signal sent to gyro	Gyro mode	Gain amount
+100	2.0ms	heading hold	100%
+83	1.916ms	heading hold	83%
+67	1.833ms	heading hold	67%
-100	1.0ms	non-HH	100%
-83	1.083ms	non-HH	83%
-67	1.167ms	non-HH	67%

One more example, the Futaba 9C has a range of NOR100 to AVC100. This is the same as the -100 to +100 example above.

Gyro menu	Signal sent to gyro	Gyro mode	Gain amount
AVC100	2.0ms	heading hold	100%
AVC83	1.916ms	heading hold	83%
AVC67	1.833ms	heading hold	67%
NOR100	1.0ms	non-HH	100%
NOR83	1.083ms	non-HH	83%
NOR67	1.167ms	non-HH	67%

So why did they make a gyro menu, what is special about it? The benefit is that you can select the switch you want to use. One of which is the flight mode switch. This is good because you can have the gyro gain and/or mode change automatically when you go from one flight mode to another. As an example you might want HH for normal and flight mode 1 but want non-HH when you flip the throttle hold for an autorotation. Another advantage is you can select HH or non-HH per switch position. With the old technique one side of the switch is always HH and the other is non-HH. With the gyro menu you can choose what mode you want for each position.

Revolution mixing:
What is it? This is a special rudder mix that compensates for the load changes as the main blades change pitch. So when you do a full power climbout your main blades are at 9 degrees pitch so there is more torque and you would need to give more tail pitch to compensate. And when you descend there is less torque and you need less tail pitch. What the revolution mix does is increase tail pitch as you move the throttle above half and decrease tail pitch as you go below half. This is a good thing to have when NOT in heading hold mode, but a very BAD thing to have if you are in heading hold. Look at the chart below:

Throttle stick	Revolution Mix	Signal to rudder channel (not gain)	What the gyro is told to do when in heading hold mode.
Full up	+20	1.6ms	Turn right
3/4 up	+10	1.55ms	Turn right slower
middle	0	1.5ms	Do not let the tail move
1/4 up	-10	1.45ms	Turn left slower
full down	-20	1.4ms	Turn left

In heading hold mode the gyro is watching the rudder channel (ch4) and as long as the rudder stick is centered (and zero sub-trim) then it sees a 1.5000000ms signal. This tells the gyro to not let the tail move. Any signal above or below that tells the gyro you want the tail to move in that direction. As you can see Revolution mix is a bad thing for a gyro in heading hold mode. You need to either turn off revo mix or set all values equal to zero. Make sure you turn it off in all modes, it would not be fun to flip to flight mode 1 and then have it active <grin> Note: In the chart I list 5 revo mix points but most radios only have 3 points. Some radios like the Futaba 6X only have two. Most radios have the revo mix changable by the flight mode switch. If you want to have the gyro set to work in non-HH in normal flight mode you can leave the revo mix active in this mode. Then set to all zeros for flight mode 1 and using the gyro menu set the gyro to HH mode. Now if you have the gyro mode set to a switch other then the flight mode then you will need to keep revo mix turned off unless your radio can have the revo mix assigned to the same switch.