Don clears up some fog over GYROS. I'm not talking the Greek Food!!!

There are two main types of gyros, rate gyros and attitude gyros.

From : Don Stackhouse

The ones they use for the really sophisticated autopilots in full-scale aircraft are attitude gyros. These sense the "attitude" (i.e.: position, relative to the pitch, roll and/or yaw axis) of the airplane, and try to hold it at some particular setting. For example, if the autopilot is set for level flight, and a gust drops one wing into a 10 degree bank, the gyro will trigger a response from the autopilot that will roll the wings back to level flight. It will then trigger additional changes to bring the airplane back to its original course.

The "rate" gyros we typically use in models (and also in the simple "wing leveler" gyros used in some general aviation aircraft, more on those in a moment) don't sense position, they sense the "rate of change" of a position. For example, if you install one in the airplane so that it senses changes in bank angle, any time the bank angle starts to change, the gyro will send commands to the aileron servo to try to stop that motion. If we have a situation where a momentary gust might make the bank angle in a non-gyro airplane change by about 20 degrees, a gyro might be able to stop the rolling motion before the bank angle had changed more than a few degrees. The plane would still respond to the gust, and still require some correction from the pilot to get the wings level again, but the amount of the disturbance would be reduced.

Rate gyros act like added dynamic stability (i.e.: the ability to damp out oscillations). For example, a rate gyro set to control pitch with the elevators would have the same effect as making the horizontal stabilizer bigger, or increasing the horizontal tail's moment arm. This can be very beneficial on airplanes that tend to have inherently poor dynamic stability, such as flying wings tend to have in pitch. If your airplane tends to porpoise a lot, a rate gyro in pitch could make a big improvement.

In the case of a "wing leveler" autopilot used in some general aviation planes, the gyro is actually mounted to sense heading change (yaw), but tied into the ailerons (roll). Whenever the plane tries to change heading, this will show up as a yawing motion and the gyro will change the bank angle as required till the heading change stops. This will bring the wings completely back to level, but it will not turn the airplane back to its original course - to do that will require some action from the pilot.

The old fashioned rate gyros used a flywheel on an electric motor, with the whole assembly mounted on a single-axis gimbal and held in the center position by a spring. Whenever the plane started to move, the gyroscopic effects of the flywheel (the engineering term is "precession") caused it to try to tilt 90 degrees from that direction (if you try to tilt a gyroscope to the side, it will respond by trying to tilt forwards or backwards). The spring would resist this motion. The rate of the motion determined the precession force and therefore how far the gyro would tilt in its gimbal against the restraining force of the spring. A sensor would measure this tilt in the gimbal and send that to the autopilot as a measure of the rate of rotation of the airplane around that control axis.

An attitude gyro has gimbals that allow the gyro to rotate freely about two axes (typically pitch and roll, the motor shaft itself makes the gyro free to rotate in the yaw direction), with no restraining effect from centering springs. If the airplane tilts around the gyro in pitch or yaw, the gyro remains stationary relative to the earth, and the change in the position of the airplane relative to the gyro's fixed attitude is sensed and sent to the autopilot. The friction in the gimbal pivots is extremely important, since these could cause precession and a drift in the gyro's neutral point. The need for perfectly friction-free motion of some very complex gimbals, and for absolutely force-free sensing of the gyro position makes these types of gyros extremely delicate and expensive. Modern attitude gyros often use other means, such a laser beam circling inside of a fiber-optic ring, to accomplish the same thing. Still very complex and expensive, but more friction-free and accurate than even the best mechanical pivots.

Our piezo gyros are based on a principle inspired by the common housefly. Ever wonder how a housefly can do all those wild maneuvers without getting dizzy? They need gyros to keep themselves oriented. The rear pair of wings has evolved into just a pair of tiny stalks with knobs on their ends, that vibrate rapidly like tiny pendulums. Hang a weight on the end of a string, hold the other end of the string in your hand and set the weight swinging. Now turn your body around to a different heading. Note how the pendulum continues to swing in the same direction as the one you started from. The housefly uses the same phenomenon to sense changes in its attitude from the direction of vibration of the modified aft wings. If you snipped off those little stalks (SHHH, don't tell the animal rights people!), the housefly would become completely disoriented. (Just take my word for it, no need to go out and torture some innocent and unsuspecting housefly.)

Our piezo gyros use a small crystal stalk, set in motion by an alternating electrical signal, and using the electrical feedback caused within the crystal itself by its own vibrations to sense its motion, in the same manner as the housefly's aft wings.

The housefly can also get attitude information from this (not just rate) by keeping track of each tiny measured rate change, adding up the calculated motions that correspond to each of these rates and their durations, and subtracting those from the original position to find the current position. It requires very accurate measurements of even the tiniest movements, since missing or miscalculating any of them will result in an accumulating error, but it is possible. Your brain uses the same basic idea to keep track of your own position and balance from the tiny motions of the fluid in the semicircular canals of your inner ears. Notice how these form three loops each, oriented 90 degrees from each other? Pitch, roll and yaw. Unfortunately, fluid is sensitive to accelerations and the resulting sloshing, which is why flying objects such as the housefly need to use a different approach.

Of course all of that is way more trouble than it's worth for our model airplane applications, which is why our piezo gyros merely sense rate, not attitude.

Does it work well enough to keep say my electric zagi flying in the direction I want it too as if it were on rails, so to speak?

That depends. Properly set, it will resist any attempt by the airplane to change direction in that particular control axis by itself (i.e.: a roll gyro will resist roll motions, but will not normally have any effect on pitch or yaw). The key word here is "resist". The airplane will feel the initial disturbance from a gust, thermal, etc., and start to move in response. The gyro will sense this motion and move the controls to make the motion stop. It can't do this instantly, so the airplane will change attitude a little, and a rate gyro like we use in our models will not bring it back to the original attitude. If you set it to sense yaw but control roll, it will level the wings but it will not return the airplane to its original course. You need an attitude gyro to do that, and attitude gyros are far more complex, expensive and usually much heavier.

Will it counter the wind blowing it off-course well enough if it is set correctly?

The airplane and the gyro do not know anything about the plane's course over the ground. The gyro (assuming you're using it for roll stability augmentation) will resist changes in bank angle, not eliminating them entirely, but keeping them to a minimum. You will still need to make some minor corrections to keep the wings perfectly level, and to keep the airplane on your desired course, including correcting for the fact that the gust that tried to change your plane's bank angle also probably altered the wind drift between it and the ground.

Don Stackhouse
DJ Aerotech