We use gyros to help us maintain the desired heading and/or pirouette rate of the helicopter in the hover, in forward flight, in backward fight, while climbing and descending and while performing any, and all, aerobatics we can think of while doing so.
Types of gyros:
There are two types currently in use. Discounting the antiquated mechanical units (these still can be very effective for initial training up to and including basic forward flight aerobatics), most units today use a non-mechanical (a less desireable piezo crystal or the much better SMM device) yaw detector. The big difference between the two current types is in the way they work. This is what will differentiate the yaw rate (YR) unit and the heading hold (HH) unit. Most HH units can also be operated in the YR mode. You can get some gyros that operate strictly in the YR mode, others in both YR and HH, some at only one gain setting and others that can be set to select different gain and rate settings from switches on the radio transmitter.
Yaw Rate Mode:
This mode operates as a yaw damper. It will try to keep the heli from yawing under any condition, be it from an external force such as the relative wind, or from a command from the transmitter. For example, if the heli is in a hover and a cross wind component would happen to blow through for 5 seconds. This wind is at a strength that would normally cause the heli to yaw clockwise at a rate of 50 degree per second. The gyro gain (combined with the heli's tail control system) may be set to prevent yaw demands of only 49 degree per second. This would mean that (if one did not give any counter tail rotor command) the heli would rotate into the wind at a rate of one degree per second (50-49). As the gust lasted for 5 seconds, the heli would have changed headings by 5 degrees. This would be easily handled by even the most novice of pilots by just a small nudge on the T/R control stick to move it back the 5 degrees.
In fact, many pilots would not even bother to make such a small correction. On the other hand, an FAI contest pilot would definitely do so, because the judges would have picked up that small deviation in heading and deducted a point or so. The main advantage of this type of gyro is that it slows down the yaw rates to a level that is easily managed by the human "brain to thumb" feedback system. The thumb will still have to make the correction
The Heading Hold Mode:
This mode uses a more complex piece of computer software in the gyro that does not just resist changes in heading, but calculates how much heading change happened during the detection process, and sends a command directly to the servo to correct that exact amount of deviation. The faster a gyro can feed these correction commands to the servo, the faster the servo can accelerate to full power to make these corrections and the better will be the gyro/servo system. This is why digital servos work best in this application. They will "ramp-up" to full speed and power at least 5 times faster then even the fastest analogue servos. Consequently, the best gyro manufacturers always recommend their gyro be used with just such a servo. In fact there are some gyros that are designed to only work with such a servo. If one would take a lower end HH gyro and connect it to one of these faster digital servos (e.g. a Futaba S-9254) he would discover a large improvement in the system's capability to lock on heading through hard maneuvers.
The speed at which the gyro processes information and feeds commands to these servos also makes a difference in the all out performance of the system. This is why the current Futaba GY611 system, with its' special servo is arquably the best in holding the tail through hard, high speed sideways & backwards 3D aerobatics. It processes everything at a much higher rate then the GY240 or GY401/9254. This is also why the GY401/9254 combo will be better at this then would lesser gyros, with even a fast analogue servo.
I am talking here about the relative holding power of these various combinations through some very demanding 3D maneuvers. A reasonably good pilot will be able to get even these more basic combinations to do most all of these maneuvers except for the most demanding high speed backwards stuff. They are all more then adequate for most sports pilots.
Of note, is that the software in an HH gyro is also in direct control of the rate at which the heli will yaw. This is dierectly related to the position of the pilot's rudder stick and the rudder ATV settings he has his TX set to. And the gyro will do its' best to maintain that same rate throughout the maneuver. This is why some gyro/servo combinations can maintain the same piro rate during a fast forward or backwards high speed pass while that rate will change with lesser gyro/servo combinations. The higher the frame rate these can work together at, the better.
Difference in setting them up:
Because the TR and the HH gyros work from differing parameters, they set up differently. This has caused a lot of confusion for those with gyros that can be switched to operate in the HH mode and the YR mode.
The YR gyro, is strictly a damping device, any tail commands from the radio (TX/RX) to the system (gyro/servo) are commands that go directly to the tail rotor servo. Therefore rudder ATV settings do change the amount that the servo travels, the rudder trims do change the center position of the rudder servo, the revo-mixing functions do mix different t/r servo positions to collective positions. This is why, if you give full left or right stick the servo will move fully to the left or right, and as soon as you release the stick it will return to the trimmed center position.
On the other hand, the HH gyro is an active part of the system that uses its own computer to control the t/r servo directly. The radio talks to the gyro and the gyro talks to the servo. This is very different then the operation of the YR unit. Consequently, the ATV function in the radio DOES NOT change the throw of the t/r servo and the trim will not set the center travel of the servo. The gyro software will do this. The more basic units will have the amount of servo throw pre-set and one will not be able to change it. So if there is any binding in the system at either end of the control travel, or if the travel does not take full use of the pitch range available from the tail rotor system, then one must use longer or shorter servo arm lengths to set for the optimum amount of control throw. On the more expensive units you will find more adjustment available from the gyro. In this case there may be adjustments on the gyro that may allow you to limit or extend the t/r servo travel.
When using a gyro in the HH mode, one must always remember that any command for tail rotor servo movement, at the TX (ATV, trim, sub-trim, revo-mixing) will be interpreted by the gyro as a request for a heading change in that direction, at a specific rate. This rate will be directly related to the amplitude of the command. The larger the amplitude, the faster the heading change. e.g. a small trim command (low amplitude) to the left will ask for a slow counterclockwise pirouette while a full stick deflection to the right (high amplitude) would ask for a very fast clockwise pirouette.
Considering the preceding, the best way to set up a HH gyro is to follow these steps.
On the bench:
1 - Insure all T/R trim (trims and sub trims) functions in the TX are defaulted to zero.
2 - Insure that the revo-mixing function is inhibited or zeroed.
3 - Turn on the TX and radio/gyro in the helicopter
4 - Insure the gyro is selected to be working in the YR mode
5 - Attach the servo arm to the servo so that it is 90 degrees to the control rod
6 - Adjust the mechanical t/r control linkage rod to a length that will give some right rudder (opposite for helis with counter-clockwise rotating main rotors) as per the helicopters manual. If this is not given, set for about 15 degrees (this would leave about a 3/8" space between the tips of the two t/r blades when they are folded back)
7 - Switch the gyro into the HH mode. If the t/r servo moves use rudder trim on the TX to stop it.
8 - Move stick full left and rotate t/r to insure no binding. Do the same to the right. If there is, adjust the gyro (NOT the TX ATV's) or shorten the servo arm length just enough to keep from doing so.
9 - Set the gains for about 50% in each mode.
10 - Check that the Tail Rotor commands and gyro direction will relate to a yaw in the proper direction.
TIP: Turn the tail rotor so that the blades are vertical. When you apply left stick, the trailing edge of the lower rotor blade should move to the left. If it moves to the right, reverse the T/R servo. Note which way the tail rotor servo now moves. Then, while observing the tail rotor servo, yaw the heli to the right. The t/r servo should correct by rotating in the direction it does when you give a left command on the rudder stick. If it doesn't, then reverse the gyro.
Flight set up:
1 - Turn radio on in HH mode and wait for the gyro to initialize.
2 - Start heli and be sure gyro is in HH mode. If the servo is drifting trim so it stops and move stick to bring servo to neutral.
3 - Bring heli into a hover and adjust trim to stop any drifting.
4 - Set heli down and switch into YR mode
5 - Hover the heli and note which way it drifts. Land the heli and MECHANICALLY adjust the length of the t/r control rod to trim for no drift in this mode. DO NOT use the trim in the radio TX.
6 - Hover again to confirm no drifting in YR mode. Repeat 5 and 6 until there is no drifting.
Now you should be able to switch between YR and HH and see no change in heading and no drift in either mode.
Fly the heli in the idle-up/flight mode that you have set with the highest head speed. Now adjust the YR gain and the HH gain settings in the TX for the highest gain possible without the tail wagging (If your gyro has gain and delay settings you can combine these for best feel) and the least "bounce" when stopping a quick tail rotor command. If the tail starts to wag, just switch to an idle-up/flight mode that has a lower speed and land to readjust the gain.
Now do a number of hovers while lowering of the gain a bit at a time until the tail feels a bit mushy, then bump it up about 10% or until it hold through all maneuvers. This ill probably a lot lower then the max allowed before tail wag, but will allow for a lot less unnecessary servo workings so that it will last a lot longer and result in less drain on your flight pack.
Setting the Pirouette rate with a HH/YR gyro:
These will be adjusted in HH mode by adjusting the rudder ATV (end point) settings. These numbers tell the gyro what rates you want as maximum. e.g. If your ATV settings of 100% results in a piro-rate of 200 degrees per second, then atv's of 80 will give 160/sec and 120 will give 240/sec. Now you will have to accept whatever rate results when in the YR mode.
Setting the Pirouette rate with a YR ONLY gyro:
Because this is a yaw damping device, it also damps the commands from the TX stick to the servo. So if you cannot get as fast a piro-rate as you wish, with max rudder ATV, you can use a longer arm on this type of gyro (DO NOT do so with a HH device as it can result in binding) to increase the rate. This may well cause binding on the bench, but as soon as you lift the skids off of the ground, the yaw rate gyro gain comes into effect and takes away a good deal of that servo throw.
I hope this will help you understand the workings of our modern day gyros and will be of assistance in setting up whatever system you are using. Remember that the tail rotor and its' mechanical control components are all part of total system. The smoother the controls operate, the more efficient and the more powerful the tail rotor is, the better it will all work as a unit.