I am new to heli's and to Gyros. I have a strange occurance I think and wanted to ask if it is normal. I have a LMH 120 with a cirrus piezo gyro. when the gyro is plugged into the rudder servo it limits the amout of rotation on the servo arm by about the last 1/3 of the tx stick. In other words, when you go to move the stick to the right or to the left it will stop moving at about 2/3 of the full stick motion. It is not binding and is not the servo because is I bypass the gyro the servo works fine and moves full throw to the right and left. Anyone ever heard of this or know what may be causing it????
Thanks

Two quick questions:

-Is the servo reaching full throws with the gyro plugged in?

-Is this a normal/rate gyro and not heading hold?


Assuming normal/rate operation…
If the servo reaches full throw but the transmitter stick is not yet at full throw, I think you are ok. If, on the other hand, plugging the gyro in actually reduces the servo throw, you might have a problem.

This was a real head scratcher for me as well, even a year after I had been using my gyro (a Futaba 401). My situation, which I think might be what you are talking about, was that the servo throw was the same with or without the gyro in the loop, but with the gyro the servo reached full throw when the transmitter stick was only at say 2/3 throw. I incorrectly adjusted my radio’s travel volumes in order to make the stick movement match the servo movement. I have since set the travel volumes back to 100%, recreating the situation described above, and the gyro is operating as it was intended.

An easy way to think about what the gyro is trying to accomplish would go like this:

<Gyro Thinking> The radio user (you) is requesting a large yaw rate (say 2/3rds transmitter stick)
<Gyro Thinking> My piezo sensor says the helicopter is turning very slowly or not at all (it is stationary on the bench)
<Gyro Thinking> I will command the servo to full deflection in order to achieve the large requested yaw rate as quickly as possible
<Gyro Thinking> When the yaw rate begins to match the requested yaw rate, I will reduce the servo deflection to match that request. (say 2/3rds servo deflection, now matching the 2/3rds stick deflection, and also matching the current yaw rate of 2/3rds max pirouette)

Following this logic, a piezo gyro operating in normal/rate mode would command the servo to full deflection for any stick deflection past a certain point, ie: 2/3. Moving the transmitter stick further will obviously have no effect since the servo is already at full deflection. While actually flying, the full servo deflection will obviously get the helicopter turning pretty quickly. Once the helicopter is actually turning near the rate you request by pushing the transmitter stick, the gyro backs the servo down to a more reasonable level in an effort to maintain that rate.

I have asked some gyro guys about their logic and most seem to use an optimized PID type of control theory. This is the same sort of control theory that will floor the accelerator pedal on your car when you hit “resume” on the cruise control and are at a significantly lower speed than the cruise control was set to. As the car approaches the correct speed, the PID controller will begin to reduce the throttle in an effort to not overshoot the requested speed. Just like in your helicopter, the controller puts the servo / accelerator pedal at or close to maximum even though you only requested say 60% of your cars top speed.


Of course, heading hold works on a somewhat different theory. More on that if you want it. I hope this helps clear things up more than it muddies the water

For what it is worth,
Spiro