by Ben Minor, Team Futaba Helicopter
February 19, 2009
For all the wonderful information the internet is able to provide today’s modelers, one of the downsides to some of the information is that setups are often copied for equipment from one person’s model to another without regard for differences in model type, pilot preferences, or flying style. When this happens under the topic of gyros, the result is sometimes gyro performance that is sub-par, and the pilot is often at a loss for what to change to improve the performance of the gyro and, therefore, the way their model performs. This article will attempt to offer a sound basis for proper gyro installation, setup, and tuning in order to maximize the performance of Futaba gyros.
Perhaps more than any other single component in a model, the gyro’s sensor is sensitive to vibration. This point is quite obviously more of an issue in fuel powered models than in electrics.
The first thing that should not be assumed is that the manufacturer’s provision or location for the gyro sensor is ideal. I know of at least one company that seeks to mount the sensor in probably the single worst position on the model. Sensors need to be mounted on the most rigid point possible in the model and as far away as possible from direct exposure to engine vibration. Sensor placements far out on a flexing radio or battery tray or close to the engine mount or engine driveline are to be avoided. When the mounting surface for the sensor flexes in flight, you can expect to see tail jerking and jumping at certain times in flight or simply an inability to run a sufficient amount of gain without causing a tail oscillation or “wag”.
Sensor exposure to excessive high frequency vibration from the drivetrain or engine will absolutely cause unwanted drift in the gyro. Futaba gyros are sold with a very effective sensor mounting tape. The main draw back to the use of this tape is that it is poorly resistant to any fuel residue. Use caution when the tape and sensor are exposed to fuel residue, and replace the tape often in these installations. The tape is excellent in its ability to effectively block transmission of vibration to the sensor. As good as the stock tape is, it is not always the best tape for every application.
For this reason I suggest that if you’re having issues with drifting in the model that you first seek to find the cause of the excess vibration and then if the problem persists that you experiment with other types and combinations of servo tape. It is sometimes useful to create a tape sandwich which consists of a layer of tape, a small plate of metal or coated plywood, then another layer of tape, followed by mounting to the frame’s surface.
Do not be afraid to experiment with sensor location in other places on the model beyond where the manufacturer suggests. The best location is the one that keeps drift to a very minimum or eliminates it entirely and which allows for a gain to be dialed to a useable level without tail wag.
The gyro can only work as well as the model allows it to. This first means that fuel powered models must be tuned well and running smoothly. The gyro sensor sees not only torque shift in the yaw axis but also any vibration in the system. If the engine is not running well, the gyro will not work well either.
Next, the tail rotor linkages must be silky smooth. This is no place for sticky or worn ball linkages, draggy pushrod supports, and tail rotor hub thrust races that are not lubricated or are otherwise in need of replacement. The gyro, by way of the tail rotor servo, must be able to input tiny corrections into the tail rotor system without resistance. If it cannot, the tail can be expected to appear jumpy or otherwise erratic in flight.
Futaba gyros are remarkably tolerant of servo arm choice, but this fact does not mean that the choice of the arm’s radius should be without care. You are looking for a servo arm size that will throw the tail rotor control linkage the majority of its available distance or travel with the gyro’s limits set at or around 100%. Too small a servo wheel results in the need for excessive gain to get the tail to hold and thus overworks the servo. Too large a wheel tends to reduce the amount of useable electronic gain to the point the tail will not hold well either.
Small electric models aside, a good starting point for tail rotor servo arm radius is 12-15mm. On small electrics, you will still want to choose an arm size that allows the gyro’s limits to be at or around 100% with full travel of the tail rotor linkage. Some models have better tail rotor control systems than others. The more ideal design centers the tail rotor linkage on the typical hovering or flying tail rotor pitch for the model. This arrangement balances the available remaining servo and pushrod travel on either side of that neutral point. Other model designs fly with the tail rotor linkage off center. In the case of these models, I would suggest that you set up the control travel so that the “short” side of the travel range occurs around a limit of about 100% and simply set the limit for the other, longer side of the travel to the same number. The excess available travel beyond the set limit on “long” side of neutral is of no consequence. Limit values should always be set to similar if not identical values.
Without exception, the proper setting of gain seems to create more confusion than any one other part of gyro set up. The reason this occurs is because most users are under the impression that one and only one gain setting is appropriate for a given gyro. This trend is particularly prevalent for the GY601 and GY611.
For optimum gyro performance, gain should always be set and optimized for a given model and given flying style. There is no question that the use of excessive gain with heading lock gyros has a negative effect on tail rotor servo longevity. There is furthermore no question that no specific gain value represents the limit above which the servo can be expected to fail in short order.
Consider a typical 3D model with a reasonably smooth running engine. In this application, the tail may hold fairly well with a gain reading in the upper 30’s. At this gain setting, the tail rotor servo may get just a bit warm. Next consider a model set up the exact same way but which has a poorly tuned engine. In this case, the tail rotor servo gets much hotter in flight due to the gyro sensor’s picking up of the excess vibration in the model and then funneling it to the servo. This model may experience much poorer servo longevity than the smooth running model. Indeed, because the model in the first example is running so well, it is entirely reasonable to run more gain with creating adverse amount of heat in the tail rotor servo.
As a final example of how much the gain setting can very from model to model, I have a GY611 and BLS251 brushless tail rotor servo mounted in competition helicopter. This model runs glass smooth in hover. In this particular case, I am running the gyro gain in hover at 95%. I run 55% in forward flight. In neither case does the servo ever get more than warm. I am not presenting these values as a basis for you to set the gain in your model. I just want you to understand how much the numbers can safely vary from one example to another. The more gain we can safely run in these gyros, the better the tail will hold a given position the more consistently the model will pirouette.
The most common cause of premature tail rotor servo failure is motor failure from excess heat. Since each model and flying style differs, we must set the gain of the gyro first at a sensible starting point and then increase it carefully while monitoring the tail rotor servo’s temperature. At no point should the servo be hot after a flight. Warm is OK, but it should not be hot. If the servo is just barely warm after a flight and you happen to feel as though the gyro could be doing a better job, then by all means increase the gain in 2-5% increments. Watch the servo temperature and observe how the model flies. It is fairly easy to arrive at a happy medium between the point where the gyro really is working well and still under the point where the servo gets hot to the touch.
Recently Futaba released their line of brushless tail rotor servos. I cannot overstate how good these servos are not only in terms of longevity but also in terms of flight performance. With these servos, it is possible to increase gain by 10% to as much as 20% over where you were flying with their coreless predecessors, yet the servos stay remarkable cool in flight. You’ll be nothing less than stunned at how much better the gyro works with these new brushless servos.
Did you know your gyro system can “talk”?
Well, not really talk, but you would be surprised at how much the gyro and tail rotor servo convey to you in flight if you understand their language.
If your tail is drifting slowly in flight, the gyro is telling you that there is likely some source of excess high frequency vibration in the model. We touched on this point earlier.
If your tail is jumping about in hover and just will not sit still, then your gyro may be upset over the way the engine is tuned or is “seeing” some sort of bind in the tail rotor control system.
Finally and perhaps the most important of all, no servo lasts forever, even those of the highest quality. Tail rotor servos rarely just up and quit. The signs of impending servo failure can include nothing more than erratic, jerky behavior in flight, sometimes lasting only a moment two, or something as simple as a wag that occurs at a gain setting that formerly created no such issue. Your servo will provide you with a few of these little warnings before it fails completely, so please land if you observe any such abnormal behavior. With the release of the new brushless tail rotor servos, we can expect to see far fewer servo failures due to the dramatic increases in servo motor longevity that these servos provide.