Gyros 101: Make your model fly better

Model Airplane News, Oct 2000 by Edberg, Don

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.