Servos and gyros and rotors, oh my!

Proper tail rotor setup is one of the most frequent topics on the h-list. It was complex enough when everyone was using honest-to-god gyroscopes, (little metal flywheels in little plastic boxes!). When heading hold gyros came out, the issue got twice as confusing.

And it's no wonder. Now you have to set up your gyro twice, once in heading hold mode and once in standard mode. And the setup for heading hold mode is dramatically different from everything you might have learned about standard gyros. Egads! What's a chopper mechanic to do?

don't panic

Gyro setup isn't that difficult, if you follow the instructions. However, instructions aren't famous for their universal comprehensibility and crystal clarity.

For most satisfactions, one is frequently wished to locate instructions proofreader, and with him replace one who can assemble sentences of your native country's most pleasing grammatical correctness.

If ya know what I mean...

I've created this web page to address some of the most common pitfalls of gyro setup. Though gyros come in varying shapes and sizes, and they all have their own quirks about them, some things are true for all of them. If you follow the instructions here, you'll at least be flying. The various tips and tweaks for your particular gyro are beyond the scope of this document.

First, know this: When the gyro is in heading hold mode, the rudder servo does what the gyro tells it to do, not what the transmitter tells it to do. Most notably, the transmitter features that once controlled servo travel (adjustable travel volume (ATV), dual rates, etc), won't do squat. Why?

This has probably led to more botched heading hold gyro setups than any other factor. The instructions that came with the first heading hold gyro - the CSM 360 - were actually quite good, but I quickly lost count of the number of people who read their instructions, filtered everything through what they knew of standard gyros, set things up creatively, and then complained that their highly sought-after whiz-bang super gyro wasn't any better than their old 153BB. Don't get caught in this trap. Trust me. Heading hold gyros are weird. Follow these instructions to the letter.

Step One: Electrical Connections

1. Plug the gyro's "rudder input" lead to the receiver's rudder socket.
2. If the gyro has a remote gain lead, connect it to an unused channel on your receiver. For the simplest setup, this should be a channel with a two-position switch. If you wanna get creative, more power to ya, but you're on your own.
3. If the gyro's sensor is packaged separately from the gyro's "brain," connect the sensor to the brain in accordance with your gyro's instructions.

Step Two: Transmitter Setup

1. Set the rudder channel's ATVs to 100%
2. Set the gyro channel's ATVs to 50%
3. Set your rudder dual rates to 100% (high) and 50% (low).
4. Adjust the rudder trim so that the rudder servo remains motionless while the gyro is in standard mode.

We'll refine these later. First, make sure everything is set correctly so far. Make sure that the rudder servo moves from side to side when you wiggle the rudder stick. Next, figure out which gyro gain switch position is normal and which is heading hold. In normal mode, the gyro will always center itself when you center the rudder stick. In heading hold mode, the servo will act a bit more strangely, lagging behind the rudder stick movements, not centering itself, etc. Why?

You may end up adjusting the rudder trim on your first flight. Set it to wherever it needs to be to keep the heli from yawing. After this the rudder servo will probably wander to one side of the other while the heli is sitting on the bench in heading hold mode. Why?

Step Three: Gyro Direction

Make sure that your gyro isn't reversed. Push the rudder stick to the left, and watch which way the tail rotor servo moves. Return the rudder stick to center. Pick up the helicopter, and hold it with a finger or thumb atop the rudder servo horn. Yaw the helicopter to the right, as violently as you can (within reason of course). You should feel the rudder servo moving just as it did when you commanded left rudder. If it moves the opposite direction instead, do not fly until you have reversed the gyro and completed this test successfully.

Step Four: Mechanical Setup

Let's get the mechanical setup right. We'll do this in heading hold mode. If the mechanical setup is correct in heading hold mode, it will also be correct in standard mode. The reverse is not true - if you set everything up nicely in standard mode, it might be terribly wrong when you switch to heading hold mode. Why?

Some gyros have adjustable servo travel in heading hold mode, and some do not. No matter. Leave it where the factory set it, and we'll get back to it later if you want. For now, let's keep things simple.

When the gyro is in heading hold mode, the gyro has built-in travel limits. The servo may move all the way to these limits even with your ATVs and dual rates turned all the way down. We're going to set up the tail rotor control system within the parameters of this built-in travel limit.

For starters, connect the rudder pushrod to the middle of the rudder servo arm - maybe 10mm away from the center of the servo's output shaft.

Put the gyro in HH mode, put the rudder to full left, hold it there, and observe the travel of the tail rotor pitch slider. Put the rudder to full right, hold it there, observe. If it's not hitting the limits on either side, move the rudder pushrod ball further out along the servo arm, or use a longer servo arm if you run out of room. If it's binding on both sides, move the ball closer to the output shaft, or use a shorter servo arm.

If it's hitting on one side but not the other, adjust the linkage length until it's equal on each side - either binding equally hard, or leaving an equal amount of unused travel.

Err on the side of caution - it should almost-but-absolutely-not-quite-bind on either side when you do this test. If moving the ball between two adjacent position on the servo arm means binding on both sides or leaving a sliver of unused travel on both sides, opt for the sliver of unused travel.

Oh, and with some helis (Futura SE, for one), you have a couple of different place to attach the rudder pushrod to the tail rotor bellcrank - these will have the same sort of effect as changing the rudder servo arm length, so experiment a bit.

When you're done, you'll be able to move the rudder stick to either side, hold it there until the servo stops moving, and you'll observe that you'll have no binding on either side, and maybe just a sliver of unused tail rotor pitch travel remaining.

Once you get this set up, don't touch the mechanical setup again. People often suggest messing with the mechanial setup to tune out a slight bobble that might happen when switching between standard and heading hold modes. I've heard some gyro manuals even suggest it. I think that's a terrible idea. Don't. Why?"

Step Five: Tail Rotor Compensation

When in heading hold mode, you want no tail rotor compensation mixing going on (this is also commonly known as revolution (or revo) mixing). Turn off the mixes, or set them to 0% everywhere, depending on your radio.

When in standard mode, you want tail rotor compensation mix percentages similar to those used with your previous gyro.

Due to the tremendous variety of radios out there today, I can't tell you how to make this happen. For this step, you're on your own. Just trust me when I tell you that your tail rotor compensation mix must be doing nothing while you're in heading hold mode. Why?

Step Six: Set the gyro gain

For this step, you need to fly the helicopter. Set the gyro to heading hold mode, disable your revo mixing, and hover.

Rule 1: If the gyro wags, turn the gain down until it stops
Rule 2: If the gyro doesn't wag, turn the gain up until it starts to wag, then refer to rule 1.

Set the gyro to standard mode, with some revo mix if you like, and repeat.

Step Seven: Fine Tune the Rudder Trim

Skip this step if you have a Futaba 401, 502, 601, or anything more recent. You can probably skip this step if you have a Futaba GY501 too, but I'm not 100% certain.

For this step, you need to fly the helicopter. Set the gyro to heading hold mode, disable any revo mixing and lift into a hover. If the helicopter holds its heading, you're done. Go to the next step. If the helicopter drifts, adjust the rudder trim until the helicopter does not yaw. I like to point the helicopter directly away from myself and center the rudder. If the tail rotor wanders to one side or the other, add some trim and re-center it. If the tail rotor stays behind the chassis for 20 seconds or so, you're done.

You will probably never need to adjust the rudder trim again. If you do, you'll probably find that you soon need to readjust it back to where you just put it.

Step Eight: Adjust the Tail Rotor Mixing

You may wish to set up a tail rotor mixing curve (or two or three) to make the heli easier to fly when the gyro is in standard mode. Or you may not. I almost never use standard mode these days, so I've taken to leaving all tail rotor mixing off, and just flying the tail myself (call it the "mental mixer") when I'm in standard mode.

This part works just like with regular gyros. Fly around, hover, climb, descent, and fiddle with the mixing percentages until the heli quits changing direction on you.

Just don't touch the rudder trim. That will mess up your heading hold performance. If you find yourself wanting to change the rudder trim, change the mix percentages instead. It will have the same effect, but without interfering with heading hold performance.

Step Nine: Adjust the Pirouette Rate

I suggest making these adjustments in heading hold mode. But that might just be because I fly in heading hold mode almost all the time. Maybe you should do this in whichever mode you plan to fly in most often.

First, use the dual rates to get a couple of pirouette rates you're happy with.

For a long time, I used a "low" rate for normal flying, and a "high" rate for blindingly fast pirouetttes, just to spice up the occasional stall turn. Lately I've been using a "low" rate that's slow enough that I can hold full rudder and do pirouetting circuits - "high" rate is the normal-flying rate I described above, just fast enough for a clean 540.

Pick rates that suit you. Fiddle with them all you want later. It's just numbers, you can change them all you want!

Most modern heading hold gyros will have equal pirouette rates in either direction. But, if not.... use the rudder channel ATVs get the pirouette rate equal on both sides. Select your low dual rate, and hold full left rudder. Then hold full right rudder. If one direction seemed to pirouette faster than the other, adjust your ATVs until both directions feel the same. You may end up with something like 105% in one direction and 95% in the other direction. No worries. If it feels the same in flight, then go with it. Don't worry too much about the numbers.

Step Ten: Fly

Do all the stuff you normally do.

Step Eleven: Tinker

If you want a faster or slower pirouette rate, adjust the rudder channel ATVs or dual rates.

If you get unwanted yawing in standard mode, adjust the tail rotor compensation mixing, just like you with with a regular standard gyro.

Do not change the rudder pushrod length. You took great pains to set this precisely (step four). If you change it, you'll just end up binding the linkages in one direction, and losing tail rotor authority in the other direction.

Do not change the rudder trim to "fix" a problem in standard mode. When you switch to heading hold mode, that trim change will manifest itself in reverse. Set your trim in heading hold mode, and make standard mode adjustments with your tail rotor compensation mixing (aka revo mixing).

Step Twelve: Get Serious (only if you want to)

Some gyros (most notably the CSM 360 and 540) work better with longer servo arm. If you put on a longer arm, you'll need to turn down the gyro's travel limiter - this is not the same as the transmitter's travel limiters (ATVs).

Some gyros have fun parameters like "lookahead" and "tracking" and "input delay" to play with. These are beyond the scope of this document. Why?

Some gyros work better with some mounting tapes or foams. Some don't seem to care much. Experiment all you want. Use the factory-supplied material as a baseline.

A few words about drift

But let's make sure we're talking about the same kind of drift. You may (probably will) need to add a touch of rudder trim in order to get the heli to maintain a perfect hover with no manual rudder corrections. That's normal. Set your trim resolution to the finest setting possible just to be sure.

Drift means different things to different people, but the drift problem that indicates a bad HH gyro goes like this: You hover for 30 seconds and dial in the rudder trim for a perfect tail lock. You fly around for a couple minutes and find that you need to add a little bit of trim to get that lock back. Then, you fly around for a couple minutes and find that you need to add a little bit MORE trim to get that lock again. And so on. And then for your next flight you find you need to undo all that trim at the start of the flight, and add it back in over the course of the flight, just like last time. This is bad, and this indicates a defective gyro (or a telebee (cheap shot but how could I pass it up?)).

If you can make the gyro hold center for a whole flight just by making a small trim change at the start of a flight, then you've got a good gyor - leave the trim in that precise spot and don't let yourself touch the rudder trim again!

But, if you have to constantly add trim during a flight, you got a bad one. Send it back and get a good one. You'll be glad you did.

I had a CSM 360 that drifted. I sent it back and got one that's perfect. If you're needing to add rudder trim over the course of the flight it's possible you just got a bad one.

If that's the case, I'd send it back.

Why?

Why don't the transmitters travel limiters and dual rates work like they used to?

In the old days of proportional-feedback (aka "standard") gyros, the rudder travel limiters and dual rates worked like they did for every other channel - they set the limits of the servo's travel range. With heading hold gyros, the transmitter doesn't tell the servo where to go. The transmitter just tells the gyro how fast you want the heli to yaw. The gyro then decides where to put the servo. In order heading hold gyros to work their magic, the gyro needs to work the servo for you. So the gyro needs to know what the servo's limits are.

Why does the servo behave so oddly when it's in heading hold mode?

It's trying to control the helicopter's yaw, but since it's actions don't do anything hen the heli is on the bench, it gets confused and just tries harder - hence the tail rotor pitch slider wanders off to one extreme or the other. If you pick up the heli and yaw it like the gyro is trying to do, you can center the slider without touching the transmitter. It's kinda neat, try it some time.

Why doesn't it work if you set things up in standard mode first?

See the note above about the travel limits. If you have the gyro in standard mode, the travel limits you see might be affected by the transmitters ATV and dual rate settings. You want to get those out of the picture while you set things up, so switch to heading hold mode for the travel and linkage-length adjustments.

Why is it that, in heading hold mode, when I trim the rudder so that the servo holds still on the ground, the heli yaws in a hover? And when I trim the rudder so that the heli remains motionless in a hover, the rudder servo creeps off to one side when the helicopter is on the ground?

I honestly don't know. But I have observed this with every heading hold gyro I've seen so far.

Why am I so certain that the tail rotor mixing must be disabled when the gyro is in heading hold mode?

Because I've used three different heading-hold gyros for extensive 3D flying in two different helicopters, and I've found this to be the case. The gyro is capable of adjusting itself to to what revo mixing does. It notices that you're holding center stick and the helicopter is yawing, so it adds some 'trim' for you. This happens faster than you can see it, so basically what you have is built-in revo mixing that tunes itself automagically.

Why not play with the mechanical setup to get rid of a yaw twitch that happens when switching between standard and heading hold modes?

Because the mechanical setup must allow the tail rotor pitch slider to get full travel in order to make the fullest possible use of the available tail rotor thrust. The steps above will show you how to get full travel without binding. If, after you've set up the mechanical linkages for full travel without binding, you adjust the length of the tail rotor pushrod, guess what happens? You'll now be getting binding in one direction, and in the other direction you'll no longer getting full tail rotor thrust.

Not having full thrust will give you poorer pirouette stops and poorer holding power in sideways flight.

Allowing the linkages to bind stands a very good chance of burning out the servo prematurely. This was less of an issue in the days of standard-mode gyros, because the servo would center itself if you let go of the stick, and would generally avoid full travel in flight. With heading hold gyros, the servo is apt to wander off to one side if you let go of the rudder control while the heli is on the ground. It will stay that way indefinitely. If the gyro's travel adjustments allow the linkages to bind, the servo will work itself to death.

Why not adjust the revo mixing while in HH mode?

Because the gyro doesn't know revo mixing from rudder stick movements. It will interpret the revo mix as a command to yaw. This will cause your heli to yaw one direction or the other when you vary the collective. If you want the heli to stay on the same heading while the rudder is centered (trust me - you do), you need to leave the revo mixing inhibited, or set to zero at all points.

Why no suggestions about the more exotic parameters of the newer gyros?

Because I haven't had enough time to experiment with them yet - I'm not quite sure what they do or how they work. Rest assured, when I know, I'll add more to this page.