Gyros and Revo-Mix
last updated 11-23-2003

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 Other gyro pages:
- Tail Pitch direction -
- Gyro gain -
- Common Confusions -
Here is how a common gyro like the Futaba gy401 gyro operates. It receives two signals from the receiver, one is to tell it (the gyro) how much sensitivity and whether to run in hh(Heading Hold) or non-hh. The second plug tells it to rotate the helicopter right or left. If you check out my <throttle mixing> page you can see diagrams of how the signals from the radio tell the servos what position to go to.
There are no specialty gyro connections on the receiver, the gyro uses the same communication as the servos do. The signal that the servos and gyro receives is a pulse that last anywhere from 1.0 milliseconds to 2.0 ms. When the gyro receives a signal of 1.5ms then it knows it is to do everything it can to not let the helicopter rotate. As the signal changes toward the 1.0ms or 2.0ms side then the gyro is told to rotate the helicopter at 'x' amount of degrees per second. The closer it is to 1ms or 2ms the faster it will rotate. BTW: from 1ms to 1.49 would tell the gyro to go left and from 1.51 to 2.0ms it says to go right. Now do keep in mind this maybe just the opposite depending on the servo you have. One brand will interpret the signal opposite of the other. This is why you notice that if you put a Futaba servo in place of a JR one then you have to reverse the setting in your radio. Okay now on to revolution mix, first we need to understand what it's purpose is. To do this lets go through an example of a helicopter setup with no revo-mix and a gyro that is operating in non-hh mode. The first step is to adjust the rudder linkage so that at center stick (hover point) and the rudder trim centered and zero sub-trim that the helicopter does not rotate right or left. This gets the tail blade pitch set to the right angle to compensate for the torque of the main blades. Now at this stage everything seems fine in hover however if you punch the collective to go up then you will see the nose rotate to the left (on a clockwise rotor). So why did the tail not remain straight? Lets look back at what the tails job is. At hover the blades have 5 degrees of pitch. As it pushes against the air molecules there is a certain amount of resistance. If the pitch of the tail blades were at zero degrees then as the main blades rotate clockwise the rest of the helicopter would go counter clockwise. To compensate the tail pitch is set to a certain amount of degrees to counteract the torque and keep the helicopter pointed in one direction. Now if the head speed or the pitch of the main blades changes then the tail pitch will also have to change to compensate for the new load. So if the main blades are at 5 degrees but now turning faster, then it is moving more air molecules. Or if the main blades are at the same speed but now at 9 degrees then again there are more air molecules to push out of the way. In either case the tail will need to change pitch to compensate.
So back to our example, non-hh gyro/rudder trim centered/no rudder sub-trim/hover at center stick/adjust control rod to keep the nose straight ahead. Now as you do a climb-out the blades pitch changes from 5 to 9 degrees however the tail pitch is still the same. The increased load causes the body of the helicopter to rotate to the left (opposite of the direction of the blades). And if you are hovering high and you make a gentle descent that means the main blades have less pitch yet the tail pitch is still the same so the nose will rotate right (with the direction of the blades). In this case the tail is overcompensating for the load. During the climbout it was under-compensating for the new load. They just didn't match up when the pitch was changed. So back before heading hold gyros or even gyros of any kind the helicopters had a lever that connected the collective to the rudder linkage. As more collective was added, it would increase the rudder pitch and at less collective it would take away tail pitch. Later radios came out that had a mix function in which this mechanical tail compensation could be done in the radio instead of with mechanical linkages. So as the collective stick moved, the built-in mix of the radio would tell the tail servo to also change. Helicopter radios came along with this mix included and was called revo-mix or revolution mixing. As gyros came along this mix was still necessary as there were no heading hold gyros back then. Gyros operating as non-hh work in a dampening effect mode. They only compensate for quick tail changes and have no knowledge of the overall direction of the helicopter. Later heading hold came along. HH gyros are standard gyros but have software (a program that tells the gyro what to do) to lock the heading of the helicopter.
In non-hh mode the gyro senses a direction of movement and issues a command to the rudder servo to go in the opposite direction. In HH mode it does the same but there is a software program running that at all times is keeping track of how many degrees of movement there is. If the helicopter moved 7 degrees to the left then the gyro tells the rudder servo to move to the right and as it does it monitors the rotation until it has moved 7 degrees to the right. Now you may not have caught what was just said, the gyro moves the servo one direction and keeps it there until the helicopter is back pointing where it needs to. What this says is that unlike non-hh mode there is no center position for the servo. It is all the time moving to whatever position it needs to keep the helicopter where you want it. So if the blade pitch changes or head speed changes then the gyro makes the appropriate change to the rudder servo. In non-hh mode a normal setup would mean that when your rudder stick is centered then the rudder servo arm is straight up (or 90 degrees to the rod going to the tail). This is the center position and at this position you have adjusted the length of the linkage so you have enough pitch in the tail blades to compensate for the torque at hover. However in HH mode if you return the rudder stick to center then the servo may not be straight up. This is one of the confusing things a lot of people ask about. It is however one way that you can tell whether the gyro is in HH or non-HH. If you move the rudder stick all the way to the right then back to center while you watch the rudder servo then the servo should return to the center (straight up) if in non-HH mode. If not then you know you are in HH mode. Now back to this center thing. Non-HH and HH interpret your rudder stick movements differently. In non-HH mode when you move the stick you are not really telling the gyro anything. It just passes the signal on through to the rudder servo. However in HH mode when you move the rudder stick you are telling the gyro what you want to do and the gyro tells the servo. In HH mode as you move the rudder stick you are actually telling the gyro to move a certain number of degrees per second. It then commands the rudder servo to move to what position it needs to match the rate of rotation you requested. When the stick is centered then the gyro is told to stop any rotational movement. It is important in this mode that you do not have any revo-mix or rudder trim. A HH gyro is looking for a center signal of 1.5ms. If you did have revo-mix in then as you moved the collective up it would make the rudder channel signal go to the right and now it is no longer sending a 1.5ms signal. So the gyro thinks you want the helicopter to rotate. That is why you have to turn off the revo-mix.