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Modern RC Helicopter Gyros and Tail Setups – What You Need to Know.

A short guide on the history, types and setup of gyros on helicopters.  I can't remember where I found this article, but will gladly give credit to the original auther if anyone can let me know.

I found this article some time ago, and unfortunately can't remember where I got it from.  If you are the author or can tell me who is, I will gladly give them credit.  It is a brilliant article for mechanically setting up the tail servo on your helicopter.

 

A brief history....

Many years ago, model helicopter gyros contained a motor and a spinning device, a bit like the old gyroscope that many of us (of a certain age!) used to have as a toy when we were kids. When you pulled the string, it would spin very fast, and would always stand up vertically on its point, no matter what inclination the supporting surface was at.

This was the basic mechanics of the early gyros (one very well known and top RC helicopter flyer by the name of Bob Johnson would call them a “Man’s” gyro!), and has all but disappeared from the hobby now, so that’s about all you need to know about the past. Nowadays, we have solid-state gyros known as Heading Hold Gyros (HH), with no moving parts inside, just some fancy electronic jiggery-pokery. Not all gyros are HH, but they are used pretty extensively by 30 size and above RC helicopter pilots for both IC (engine powered) and electric powered helicopters. They are cheap (in relative terms), accurate, reliable and user-friendly these days, to such an extent that they easily mask a poor helicopter tail setup, which means that in order to get the most out of your helicopter tail control system, there is a need to understand just how the gyro operates in both HH mode and ‘Normal’ (or AKA ‘Rate’) mode and how it harmonises with the rudder servo, and, I will attempt to explain all the average club pilot needs to know here without getting lost in excessive detail.

I will also use the term ‘Normal Mode’ rather than ‘Rate Mode’ as I shall later be using ‘rate’ to explain something different from gyro mode. I will also make no apologies for not understanding or attempting to describe exactly how the HH gyro electronics work on the inside of the little box – my aim here is to explain how it operates and how to get the best performance out of them.

Heading Hold vs Normal Mode

Basically, the gyro acts like a big damper, which operates around the tail servo’s natural or centre hovering point. So, when you give the helicopter a load of collective pitch input from the transmitter, the gyro tells the tail servo to counteract the torque generated from the main blades during the manoeuvre. Furthermore, when you give the helicopter a tail input, the gyro tells the tail servo to move either left or right in a controlled manner. In effect, the gyro is a middle-man between your transmitter and the desired tail servo output.

Now the big difference between HH mode and Normal mode is that in HH mode, the gyro knows which way the helicopter is pointing at all times. So, if the helicopter is in a steady hover in a cross-wind, then the gyro will attempt to hold that heading (the direction that the helicopter is pointing) regardless of what external gusts of wind are encountered. In HH mode, the helicopter tail will not turn unless you ask it to i.e. you enter a rudder stick input from the transmitter.

In Normal mode, the helicopter will slowly weather-cock into the wind, as the gyro doesn’t know which way it is pointing on the planet. It is simply compensating (quite crudely) for the changes in torque and cyclic commands from the main rotor blades. In Normal mode, the helicopter will always drift into wind, just like a weather-vane.

Understanding this difference between the HH and Normal modes is important for the next bit. In order for a standard clockwise rotating helicopter to hold a steady hover, the tail blades need a few degrees of right-turn pitch to counteract the natural torque generated on the fuselage from the main blades. Let’s say for arguments sake that this pitch value is 7º right rudder, just to hold the hover at a given head speed – see Figure 1. This tail pitch slider position must be considered to be the helicopter’s natural (or neutral) point, and it’s important to mechanically adjust the tail control system so that it hovers ‘hands-off’ in Normal mode.

figure 1
Figure 1

Why the Need for Mechanical Centering?

Just imagine that wherever you have set your tail pitch slider on the bench as being the centre point of travel is 25% wrong, say to the left for argument’s sake. Now assume that the full range of servo travel is 100 points left and 100 points right from the centre position. So, to hold that helicopter in a steady hover, you’ll need 25 points of right rudder to over-come the poor mechanical setup. In Normal mode, this means actually holding the stick over, or adding a load of rudder trim. But, in HH mode, the gyro will automatically do it for you, and will simply hold the helicopter on its current heading by ‘hook or by crook’.

“Great”, I hear you cry, but errrrm no, it’s not that simple. Now that you are holding 25 points of right rudder in just to keep the helicopter straight, then you’ve only got 75 points left to go to the right before you hit the end-stop. Similarly, you’ve now got 125 points to go to the left before you hit the stop the other way (see Figure 2). So, you’ve got a tail system that is hugely out-of-balance. This out-of-balance setup is going to happen regardless of which gyro mode you’re in, but the difference is that in Normal mode, you’ll know about it because you’ll be holding in a load of right rudder just to keep the hover, but in HH mode, you won’t know about it, because the gyro will be holding the tail pitch control across for you, thereby still inducing the out-of-centre problem.

Figure 2
Figure 2

In both the cases illustrated above, the servo left and right travel limits are fixed and limited to points set by the gyro (more about that later), and it cannot move any further than these limits, so the effectiveness of travel to the right and to the left are now massively out-of-balance due to the mechanical setup being wrong. In order for the gyro to work at its optimum in HH mode, you therefore first need to find the neutral point for the machine i.e. the natural tail pitch slider centre point. This is done by the following process, with reference to Figure 3.

  1. Set up the helicopter and tail as per the instruction book – they’re pretty good these days so it won’t be too far out.
  2. Set the gyro to Normal mode and hover the helicopter into wind (a perfectly calm day is best for doing this).
  3. Use the rudder trim to get the helicopter to maintain a steady hover ‘hands-off’. Remember, you are in Normal mode at the moment, so the rudder trim will adjust the centre point of the tail system.
  4. Land the helicopter and stop the engine. Put it on the bench, and take a note of how much trim you’ve added to keep the steady hover.
  5. 5. You can now mechanically adjust either the position of the tail servo on the tail boom, or the length of the tail push rod to get that tail pitch slider in the new centre position with the transmitter trim set back at zero.
  6. Hover it again and repeat the process until it hovers ‘bang-on’ in Normal mode with NO trim either way.

figure 3
Figure 3

 

Figure 3a shows the 90º setup following the instructions in the assembly manual, as indicated in stage 1 of the above process. Figure 3b shows an example servo position after you have adjusted the rudder trim in Normal gyro mode following a hover, by using the trim slider (i.e. stage 3 above). Figure 3c shows the neutral servo position found at stages 5 and 6 above, now mechanically established by moving the tail servo along the boom a little, and also returning the rudder trim back to centre, and thereby reestablishing the 90º setup. Some helicopters do not have a boom-mounted tail servo, so this affect has to be achieved by adjusting the length of the tail pushrod, as has been mentioned already. For those that do have a boom-mounted tail servo, take note of where dimensions w, x, y and z apply in each of Figures 3 a, b and c to achieve the perfect z and y positions.

You’ve now found the neutral point for your particular helicopter. One word of warning here – your tail servo to push rod linkage should be as close to 90º as you can possibly get it, with NO use of transmitter sub-trims or main trim. This is easily achieved with most modern servos as they have an odd number (rather than an even number) of splines on the servo output arm, which means that you can rotate the servo arm on the servo output shaft through all it’s available positions until you find the one that gives you 90º exactly.

So, by now you should have a helicopter that hovers ‘hands-off’ in Normal mode with a servo-to-pushrod angle of 90º exactly. This is the mechanical setup for the helicopter, and now that this is right, the electronics can operate much more efficiently around their centre position.

Transmitter End Points

There is a lot of misunderstanding about transmitter end points on tail setups, so let’s be clear about where they are, and what they do.

Your gyro probably has its own end-point control. Some have one control that affects both the left and right sides together, where as others (usually the better, more expensive ones) have separate left and right controls. It is the gyro that controls how far the servo travels, not the transmitter. By adjusting the end points on the gyro, you should set the servo travel so that the tail pitch slider is just short of binding against the tail mechanics at each end of its full travel in both directions.

Your transmitter also has end-point settings, so if the gyro controls how far the servo travels, then what do the end-points on the transmitter do? Well, they affect how fast the helicopter rotates, or pirouettes. Time for more simple theory……

Remember, in HH mode, the gyro always knows which way the helicopter is pointing, and will maintain that heading to its best ability regardless of what external forces are acting on the machine. So in other words, the tail will not turn unless you tell it to from the transmitter. But, unlike in Normal mode, when you give a stick input to the rudder from the transmitter, you are asking the gyro to rotate the helicopter at a given rate.

What do I mean by “rate”? This is the rate (speed) of rotation i.e. one full rotation (360º) in one second for example. Two full turns (720º) in one second would be twice as fast and half a turn (or one full turn in two seconds) would be twice as slow. Now, the clever bit about HH gyros is that they will attempt to maintain this rate of rotation regardless of any external forces such as gusts of wind, so will constantly monitor the helicopter’s heading and adjust the tail servo accordingly to achieve this. This the basis of why you must never have any rudder trim applied in HH mode on a helicopter – because the trim is affectively holding the stick over a little bit for you, and is therefore asking the gyro to rotate the helicopter constantly, albeit at a slow rate (usually!).

Doing a pirouette on a calm day in a steady hover is easy – bang the rudder stick full over and the helicopter spins around at a nice constant rate. Now try doing it in HH mode while you’re in fast forward flight (see Figure 4). What happens? The first half of the pirouette is quite slow, and the second half is much faster. Why? Because in forward flight, when you push the rudder stick across, you are asking the tail to over-take the canopy, which requires a lot of (if not maximum) tail power, so in an attempt to maintain that constant rate of rotation, the gyro pushes the servo to its maximum travel position. For an instant, the helicopter is travelling backwards (when half way through the pirouette), and from then on the canopy is over-taking the tail for the second half of the manoeuvre, which requires very little tail effort due to the natural weather-cocking affect, so the gyro automatically backs off the servo arm position, as it’s still trying to maintain that constant rate of rotation. It may even send the servo the opposite way completely in an attempt to slow down the rate of rotation during the second half of the pirouette. This is the HH gyro working hard to maintain that desired rate of rotation.

Again, I’ll repeat what I’ve already said, it’s the end-points on the transmitter that affect how FAST the pirouette rate is, and the end-points of the gyro that affect how FAR the servo travels. The amount of travel visible on the tail pitch slider is NO indication of how fast the helicopter will pirouette. An easy way to see this is to do a full stick pirouette in a steady hover – the helicopter spins around fast. Now try doing the same using only half the rudder stick input as before, and see what happens – the helicopter spins around at half of the speed, because the speed of pirouette is being controlled by the amount of transmitter-to-gyro stick input, not by the gyro-to-servo mechanical movement limits.

One word of warning here – a good friend of mine recently wrecked an Xcell Stratos 90 size helicopter when the tail servo died. There was a top-of-the-range Futaba gyro system installed in that helicopter. Why did the tail servo die? Because the pirouette rate was set so high (I’ve never seen a 90 size helicopter pirouette that fast!), that when he let go of the stick at the end of pirouette moves, the gyro is shouting “STOP NOW!” to the tail servo. Being such a fast tail servo, it duly responded, placing massive mechanical loads on the servo in a very short space of time, which eventually ripped the servo’s mechanical gears to pieces. So, be careful with just how fast you set that pirouette rate. It’s not the spinning around that causes problems, it’s the stopping that’s hard on your tail servo.

Figure 4
Figure 4

Gain

What does the gain do on a gyro? In both HH and Normal modes, the gain controls how sensitive the gyro is to changes in heading. If it’s too high, then the tail will wag backwards and forwards quite quickly, as the gyro will be over-compensating against its own servo corrections. If the gain is too low, then the tail will be sluggish, may wag very slowly, and will drift badly when a large amount of collective pitch is applied – i.e. it will not be able to compensate for the increased applied torque because it’s simply not sensitive enough.

Most modern HH gyros work best at around 30-50% gain values, but this is not necessarily the value that you see on your transmitter screen. You might be used to seeing 60 or 70%. This is because you are seeing 60% of 140 (all the channels on most transmitters work at 280 point resolution – 140 points each way from centre), not 60% of 100. 60% of 140 = 42%, so your actual gyro gain is 42%, not 60%. This is easy to see on gyros such as the Futaba 611 and 601, where the ‘real’ gain value is shown on the gyro amplifier, and will always be different from the setting on the transmitter.

Anyway, as a rule-of-thumb, start with a transmitter gain setting of around 50%, test hover and if all is ok, increase it in 5% intervals until the tail starts to wag from side to side. Then, back it off by 5%, and that’s a good place to start your gain setting. When you do a full pitch climb-out, you should expect to see little or no tail drift as the main blades impart their torque on the airframe.

A very slow side-to-side tail wag is an indication of a gyro gain that is too low, and a fast tail wag indicates a gain that is set too high. An extremely fast wag is probably nothing to do with the gyro at all and an indication that something mechanical like a shaft somewhere is bent, or a bearing is duff.

Fine Tuning

Those RC helicopter fliers lucky enough to own a JR Vibe 90 (the author included at the time of writing this) will be aware that Curtis Youngblood recommends that you set the servo arm not at 90º to the tail push rod, but one or two splines off-centre on the servo output shaft, towards the front of the model, in an attempt to give an even better ‘balanced’ feel to the rudder control. Why? Well it comes back to the torque generated by the main blades – remember that we need a few degrees of right tail blade pitch just to keep the helicopter hovering on one fixed heading. Now because the tail is producing a turning effort just to keep the helicopter straight, this suggests that it will take even more mechanical effort to turn the helicopter against the torque of the main blades, and therefore less mechanical effort to rotate the helicopter with the torque from the head.

Off-setting the servo output ball gives you more effective push rod travel in one direction than the other, so setting the helicopter up in this way gives the tail system more travel in the direction that rotates the helicopter against the torque (i.e. where it is needed) and less travel in the direction that rotates the helicopter with the torque affect (i.e. where it isn’t needed so much).

This is a nice touch to fine-tune a helicopter tail setup, but care should be taken when working out which way to offset the servo arm, as helicopters vary considerably in their design, and what works for the JR Vibe, may well be different for a TT Raptor or a Hirobo Sceadu. The theory is still the same, but I cannot guarantee that you should move the servo arm one spline forwards – it might need to go backwards on any other helicopter. This of course, seemingly goes against all that was said earlier with reference to Figure 2 and the mechanical centre (or neutral point) of the helicopter, but this is a tip for advancing your understanding and operational performance of the tail setup once all the basics are in place.

The End, or is it?

Well, I hope this has been useful, in an attempt to explain in lay-man’s terms how heading hold gyros work, and how to set them up. There is much info on the web about specific gyro makes and models, and how to best set the gain and end points for particular servos and gyro combinations, but the crux of a good gyro setup is in the mechanics, not the electronics. As you might have realised from reading this, I believe strongly that good mechanical setup is at the heart of a well behaved RC helicopter, but then I am a mechanical engineer, not an electrical engineer, so I am somewhat biased! Electronics are great for fine-tuning, but are useless for fundamental accuracy.

One final word of advice – make sure your tail servo-to-blade mechanical mechanism is ‘silky’ smooth. It should take almost no effort at all to adjust the pitch on the tail blades on the bench, by hand, from the ball joint that connects onto the servo. Make sure there’s no binding or roughness in the system at all. This simple check gives the tail system a good fighting chance of working well.


Written By: interhost
Date Posted: 11/27/2007
Number of Views: 3369


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