Gyros 101: Make your model fly better
This article is for folks who’d like to know how gyros work and what they’re used for. I cover most of the important aspects of gyros without getting too technical.
What is a gyro? Quite simply, it is a device that can sense rotation. It’s handy for changing aircraft and helicopter flight and response characteristics, since the model has to rotate when it’s disturbed by wind gusts or maneuvering. We can use a gyro to reduce aircraft rotations and “smooth things out.”
Gyros and models. As far as I know, the first use made of gyros on RC models was in helicopters, so I’ll use the heli as an example (fixed-wing pilots, stay with me; there’s more).
Anyone who has flown a heli knows that they can be flown without a gyrobut only with great difficulty. You find this out in a big hurry when you try to take off with the gyro switch set to “low gain” mode! The heli’s fuselage tends to yaw back and forth whenever there’s any disturbance, whether because of a control input, a change in engine setting, or a gust of wind.
The differences between flying an RC heli and an RC airplane are especially evident when hovering and at low airspeeds; an airplane’s forward motion ensures a steady airflow over the tail surfaces to stabilize things. Some hells don’t even have tail surfaces! When the fuselage yaws right or left, there’s little to resist the rotation. Engineers call this situa tion “lightly damped.” This is where a gyro can be used to improve things.
If we install a gyro so that its “sense axis” is parallel with the main rotor shaft, it will respond to yaw rotation. The gyro uses the sensed rotation to generate a signal that is mixed with the pilot’s rudder servo commands coming from the transmitter to add damping to the fuselage motion. This damping tends to slow the rotation of the helicopter’s fuselage. A sketch of the connections in a typical gyro setup is shown in Figure 1.
If we increase throttle or a gust of wind makes the fuselage swing clockwise in the yaw axis, we want the gyro to command the tail rotor (the rudder channel) to move in the opposite direction and slow the fuselage motion. This is what an ordinary rate gyro does: it simply helps to damp out unwanted swings in the heli’s movement.
It’s important to know that a rate gyro does not help to keep the fuselage pointing in a constant direction. On a helicopter, even with the best rate gyro, if you hold full rudder, the fuselage will turn in circles at a steady speed. All the gyro does is prevent the circling heli from spinning faster and faster as long as you held the rudder command.
To envision how a rate gyro works, sit on a chair that will spin, and spin as fast as you can while holding large pieces of cardboard broadside to the wind. The cardboard damps out rotation; the larger the cardboard’s area, the greater the damping. Increasing the size of the cardboard pieces is similar to increasing the gain (or sensitivity) on your gyro.
TYPES OF GYRO
Mechanical gyros. When the gyro was first introduced, there was only one type: the mechanical rate gyro-mechanical because it has a spinning flywheel (or two) mounted on a pivot. The flywheel is spun by a small electric motor to serve as a gyroscope. I won’t go into gyroscopic theory here, but the spinning flywheel’s axis will try to rotate whenever the gyro case is rotated. The flywheel usually has a set of centering springs, and the rotation against those springs is sensed electronically and is turned into a corrective signal. This signal is fed into a servo and used to make our models fly better. If you’d like to learn more about gyros on the Internet, go to www.lance.co.uk/w3mh/articles/html /csm7 8.htm.
Mechanical gyros are straightforward, work fine and are still sold, but they have a few shortcomings. Because the flywheel is spun by a motor, there’s a constant current drain, so a larger battery pack is often called for. Also, its bearings, pivots and other moving parts can wear out, and that can cause slop, reduced sensitivity, and eventually failure.
Piezoelectric gyros. About 20 years ago, Watson Industries introduced a rate gyro that had a piezoelectric drive and sensing mechanism. Part of the word-“piezo”-is derived from a Greek word meaning stress or applied force. In piezoelectric materials, an applied force will generate a voltage, and conversely, you can also apply a voltage to drive them. An example of these materials may be found in the common gas lighter systems that produce a spark when you “click” on their trigger. The Watson’s piezo element was cut from quartz crystal so that when it was driven, it would produce a signal that was proportional to the rate at which the gyro was rotated. The rotating motor and flywheels were eliminated.
The use of piezoelectric crystals for rate gyros was revolutionary; the new system was more sensitive than a mechanical gyro and showed itself to be more robust during encounters with the ground. Unfortunately, the Watson stabilizer never really gained popularity; it cost a lot to produce, and the modeler had to solder a wire into the servo amp to make it operate, but it was really way ahead of its time.
After a few years, piezoelectric materials were reintroduced into gyros-and with great results. Now, most of the major RC system manufacturers sell piezo gyros that offer great performance at a reasonable cost. Improvements in electronics have allowed these gyros to be made even smaller than the mechanical units. They also have much less current drain, since a motor is no longer needed to spin the flywheel. Because they don’t use much battery power, you can get by with smaller battery packs and so reduce your model’s overall weight. Sensitivity to motion is enhanced because there are no rotating parts with bearings to wear out. The piezo gyros are the way to go.
Wing gyros. These are intended for fixed-wing airplanes; they smooth things out in the roll axis. They’re simply rate gyros that have two servo outputs rather than one and are intended for aircraft that have two independent aileron servos. These are mainly used to stabilize the motion of scale models and any model that has to land in wind when the wings might bounce back and forth. I’ve also used a wing gyro in a competition glider to smooth flight performance during landings.
Heading-hold (HH) gyros. The newest gyros provide not only the damping control discussed earlier but also the “stiffness” needed to keep an aircraft at a certain attitude; in the case of a helicopter, they keep the fuselage pointing in a constant direction.
The analogy of the HH gyro is to sit on a pivoting chair and to tie a spring to the ground. The spring causes the chair to always return to the same heading: the stronger the spring, the quicker it returns to that heading. They also provide damping like conventional rate gyros to prevent your heli from “overshooting” the desired position and oscillating back and forth.
HH gyros are very popular with heli pilots, as they essentially allow them to ignore the tail rotor while hovering. More experienced pilots like the way the HH gyro holds a particular heading both in hover and when doing aerobatics in crosswinds, backward flight and in 3D flying.
HH gyros are more expensive than other types, but their performance makes them worth the extra cost. In the RC heli world, they are fast becoming the norm, both with beginners and master 3D fliers.
INSTALLATION AND SETUP
To help ensure that your gyro is not subjected to excessive vibration, you should plan its installation carefully. I make a balsa box and line it with foam rubber so that the gyro case fits snugly inside. This helps to protect it from the engine “noise” and vibration that can cause performance problems. If you have a gyro with more than one piece, the sensing portion needs the most protection. The other parts can be mounted more firmly, perhaps with double-sided adhesive foam tape.
The gyro’s sense axis is usually marked on its case with a circular arrow (see Figure 1). If it isn’t marked, it’s generally perpendicular to any circular parts of the sensor case. Double-check to ensure you installed the gyro as you want it, or you’ll see some strange aircraft behavior!
Gyros sense rotation, so it isn’t too important where we mount them, as long as the sense axis is pointed the right way. Remember that when a body rotates, every part of that body rotates the same amount. In a helicopter, we could mount the gyro at the very front of the cockpit, in the rear near the rotor shaft, or even in the back of the fuselage near the tail rotor! The only things to worry about are ensuring the proper temperature, sense-axis direction, adequate mounting space and low vibration levels.
Having installed the gyro system, just plug the servo into it, and connect the gyro to the receiver. Depending on the brand and type of gyro, you’ll have one or two connectors to plug into your receiver. If there’s a second connector, it will be used to remotely control gyro sensitivity or gain from the transmitter. Wing gyros have a third connector for the second servo. Rudder gyros, often used to smooth takeoffs in tail-dragger and scale airplanes, will not have this third connector.
After you’ve connected the gyro to your airborne gear, you must check some very important things:
Power up both the transmitter and the airborne system.
Allow the gyro about 5 to 10 seconds to warm up and settle into its normal operating mode.
I’ll assume that you installed the gyro to damp out vertical rotations (yaw or rudder direction). When you quickly move the tail to the right (the nose moves left), the gyro should command the rudder/tail-rotor servo to move in a way that makes the tail move left. If you damp wing roll, you should check for aileron servo motion opposing the roll you input by rotating the wings.
Depending on how the radio is installed, it is possible to have gyro response reversed, and if you do, you’ll have to reverse the gyro’s polarity by flipping a switch on its case. If your gyro does not have a polarity-reversing switch, you’ll need to rotate it so that the sense axis is 180 degrees from where it was before. Note that you cannot reverse the servo’s direction to get the proper response you want.
Gyros and fixed-wing aircraft. Scale models, in particular, can benefit from having a wing and/or rudder gyro, especially when the model is not very stable or is very sensitive. You can put a rate gyro in a model to better manage rudder input, and it will let you take off without needing as much yaw correction as you usually need.
As previously noted, you can also install a wing gyro with its sense axis along the fuselage centerline so that it will damp rolling motion and will give opposing aileron input whenever a wingtip drops. This is a nice feature, but it isn’t very helpful if you’re trying to do snap rolls!
If you fly competitively, before you install a gyro in your model, be sure it’s allowed. Some scale events allow gyros only on rudder.
Gyro sensitivity. Once you’ve verified that the gyro’s polarity is correct, you need to set its gain, or sensitivity. Some gyros have one or two trimmer potentiometers that can be adjusted with a small screwdriver. How much gain you have is important: too much gain and the model will start wagging, or “hunting,”-not a good thing. A gain that’s set too low is like not having a gyro at all. It’s best to start with low gain, increase it until the model starts to wag back and forth, and then back off the gain a little. If you have a gyro with two gain settings, set one to be nearly off so that you can use it if you’ve set the other setting too high!
How much gain (sensitivity) you’ll need will depend on a bunch of things, including the model’s dynamics, servo travel and servo speed. You can take the plane off and try the high gain setting: if the model’s wings start to “wag,” you’ve set the gain too high. Switch to the low gain setting and adjust the gyro until you get good holding power.
Important: the faster the servo is, the better the gyro works to stop unwanted motion. Also, you should minimize slop or free play in any of the linkages.
Heli pilots usually want two gain settings: a lot for hover, and none for aerobatics. Fixed-wing pilots may want high gain for takeoff and landing and intermediate or low gain for regular flight and none for aerobatics.
For aerobatics, turn off the gyro, or the gyro will try to “fight” with you. For this, be sure to pick a gyro that has gain settings that can be remotely adjusted by a separate channel on your transmitter. Some of the smaller, inexpensive gyros don’t have this remote sensitivityadjustment capability.
If you have a computer radio capable of selecting a variety of “flight conditions,” you can set up a condition so that when you move any of the control sticks past a certain position (such as half or three quarters of stick travel), the gyro is automatically turned off.Then you don’t ever have to worry about switching the gyro on or off! Helicopter pilots use this method quite a bit in their different modes of flight. When flying, you must know which gyro gain you’re using. Make sure you understand which switch controls gyro gain and which position is which. Write it down on your field box to make sure you don’t forget.
This article has only touched the surface of the vast world of gyros. I hope you’ve learned enough to be able to use them in your models.
Copyright Air Age Publishing Oct 2000
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