Text 3 B. What a directional gyro is
The vertical gyro is designed for people who want to know which way is up or down. But there are times when gyros are needed to tell direction.
It turns out that if you mount a two-degree-of-freedom gyro with the spin axis horizontal and with the output axis vertical (or in alignment with the azimuth axis) you get a stabilized pointing line in space. When used in this configuration, the unit is called a directional gyro. In this application, the directional gyro performs almost the same functions as magnetic compass. A compass points North during straight and level flight, as the pendulum which points down during straight and level flight. At other times, however, the magnetic compass cannot be trusted for accurate readings, and the directional gyro must be used.
A directional gyro is subject to the same type of errors as is the vertical gyro. If the gyro spin axis originally pointed North, it would apparently drift away from that heading due to earth rotation, as the vertical component of earth rate effects the performance of the directional gyro since the output axis is vertical and is equal to earth’s rate times sine latitude. This means that at the equator (latitude equals 0 deg) the vertical component of earths rate is 0, while at the North Pole ( latitude equals 90 deg), the vertical component of earth rate is 15 deg/hr. At New York City the latitude is about 41 deg and the vertical component of earths rate is about 9.85deg/hr. Therefore, if in New York City we originally lined up the spin axis of a directional gyro with North, the unit will have appeared to drift away from North by about 9.85deg at the end of 1 hr.
There is another type of apparent drift which effects the directional gyro. North defines the direction on the earth that we must face to point toward the North Pole. If you travel along a meridian or longitude line, you travel North or South. All the meridian lines converge at the North and South Poles. This means that if airplane flew along the path defined by a space stabilized directional gyro, it would be flying straight in space. This could not be the same as flying North, since flying North is a curved line in space. This effect is sometimes called apparent drift due to meridian convergence. It is also called North streaming error – and the amount of error depends upon speed and latitude.
Questions to the text:
1. What are the reasons for the apparent drift in directional gyros?
2. What errors is the directional gyro subjected to?
3. What is called North Streaming error?
Text 3 C. Flux Valve
Read the text and then explain what a flux valve is.
The directional gyro has the same random drift problems common to the vertical gyro. Even originally pointed North, random torques due to pickoff friction, mechanical unbalance, etc., would cause the gyro to precess in an unpredictable fashion.
Therefore, we must have a control system which would keep the spin axis of the directional gyro always pointing North. Why not use a compass as a reference, as we used the pendulum for the vertical gyro? Basically, that is what is done, except that is difficult to hook up a position pickoff to compass needle. The forces available are so small that most any measuring device foul up compass performance. Instead, we hang a flux valve in the earth’s magnetic field. A flux valve is much like a synchro. In the synchro, a particular set of voltages is set upon the three stator winding – depending upon the orientation of the rotor with respect to the stator. In the flux valve, a particular set of voltages are set up in the three stator windings – depending upon the orientation of the flux valve with respect to magnetic North. In the synchro, the voltages are induced from the rotor. In the flux valve, the voltage is induced by the earth’s magnetic field. There is nothing in the flux valve equivalent to the rotor of a synchro.
EXERCISES
EX.1. Translate the following word combinations without consulting the dictionary.
Straight and level flight; meridian lines converge at North and South; curve in space; vertical component of earths rate equals earth’s rate times sine latitude; drift relative to meridian line; flux valves sense earths magnetic field.
EX.2. Define the following words:
1) artificial horizon;
2) figure of merit;
3) follow up system.
EX.3. Questions and problems.
- What are two characteristics of gyro motors?
- Explain the nature of apparent drift in directional gyros.
- Explain why directional gyro can be compared with magnetic compass.
- Why cant a compass be used as a heading reference?
- What is the effect of the horizontal component of earth‘s rate on a directional gyro?
- What are the uses for vertical gyro?