It is important to monitor vacuum pressure during flight, because the attitude and heading indicators may not provide reliable information when suction pressure is low. Most modern airplanes use engine-mounted vacuum pumps. The turn coordinator is the more recent development. This flux causes current to flow in the three pickup coils, Figure 11. Any spinning object exhibits gyroscopic properties. In September 1928 he first tested some revolutionary new instruments, items that would allow improved situational awareness for the pilot and safe passage through clouds. Protruding from this hub are a number of carbon or aluminum plates, or vanes. The turn-and-slip indicator is incapable of tumbling off its rotational axis because of the restraining springs. Because of precession caused by friction, the heading indicator creeps or drifts from its set position. One additional tool that can be added to the aircraft is a yaw string. For bank, it would work very similarly. The turn coordinator can be used to establish and maintain a standard-rate turn by aligning the wing of the miniature aircraft with the turn index. Remote indicating compasses were developed to compensate for the errors and limitations of the older type of heading indicators. Rigidity in space refers to the principle that a gyroscope remains in a fixed position in the plane in which it is spinning. Most aircraft have at least two sources of power to ensure at least one source of bank information is available if one power source fails. In a skid, the rate of turn is too great for the angle of bank, and the ball moves to the outside of the turn. The instrument gives an instantaneous indication of even the smallest changes in attitude. We know that it will. During preflight, ensure that the inclinometer is full of fluid and has no air bubbles. If that gyro tips over like so, then we know that we would see the little wings of the airplane rotate then in the same direction. Aircraft Owners & Pilots Association Find it free on the store. Some airplanes do the reverse, providing pressure to the gyros on the panel; this is called a pneumatic-pressure system. This bar is fixed to the gyro and remains in a horizontal plane as the aircraft is pitched or banked about its lateral or longitudinal axis, indicating the attitude of the aircraft relative to the true horizon. 2022 Aircraft Owners and Pilots Association. The current in each of the three pickup coils changes with the heading of the aircraft, Figure 12. Since the wheels are rotating in a clockwise direction when viewed from the right side of the bicycle, if a rider leans to the left, a force is applied to the top of the wheel to the left. So, this gyro once spinning inside the instrument remains, once again, rigid in space. As mentioned earlier, the lines of flux in the Earths magnetic field have two basic characteristics: a magnet aligns with them, and an electrical current is induced, or generated, in any wire crossed by them. For rigidity in space, we have a little mini gyro here that were zoomed in on. A right deflection indicates a clockwise error of the compass card; a left deflection indicates a counterclockwise error. [Figure 2] Precession can cause a freely spinning gyro to become displaced from its intended plane of rotation through bearing friction, etc. The gimbal in the turn coordinator is canted; therefore, its gyro can sense both rate of roll and rate of turn. The relationship of the miniature aircraft to the horizon bar should be used for an indication of the direction of bank. As they slow, they become more susceptible to deflection from the plane of rotation. Instrument pilots must become familiar with the characteristics of the equipment in their aircraft. The amount of vacuum or pressure required for instrument operation varies, but is usually between 4.5 "Hg and 5.5 "Hg. One source of vacuum for the gyros is a vane-type engine-driven pump that is mounted on the accessory case of the engine. So last up is the attitude indicator. Those two advances were the precursors to the modern gyroscopic attitude indicator and directional gyro, the two main devices in the instrument pilot's arsenal. Next up is the turn coordinator. Continue searching. Every pilot should be able to interpret the banking scale illustrated in Figure 7. Coordinated flight is maintained by keeping the ball centered. This capability is the result of the development of the Attitude and Heading Reference System (AHRS). If inadequate right rudder is applied in a right turn, a slip results. The wheel is spinning and that means that this force is applied around a different plane of rotation. The answer is no, it will not topple over. A heading indicator displays headings based on a 360 azimuth, with the final zero omitted. The artificial horizon, for example, contains the gyro wheel spinning on the vertical axis. Some heading indicators referred to as horizontal situation indicators (HSI) receive a magnetic north reference from a magnetic slaving transmitter and generally need no adjustment. Notice on the ground how the instruments respond those indicating movement about the yaw axis should move freely during taxi, and the AI should show any changes in pitch, such as you might have traversing the potholes in front of the FBO. There are a number of designs of the remote indicating compass; therefore, only the basic features of the system are covered here. [Figure 12] The pictorial navigation indicator is commonly referred to as an HSI. The wheel or rotor is free to rotate in any plane in relation to the base and is balanced so that, with the gyro wheel at rest, it remains in the position in which it is placed. Too much right rudder causes the aircraft to skid through the turn. There are two fundamental properties of gyroscopic action: rigidity in space and precession. When rolling into or out of a turn, the miniature aircraft banks in the direction the aircraft is rolled. The green pointer is driven by the ADF. A single gimbal limits the planes in which the gyro can tilt, and a spring works to maintain a center position. When the airplane begins to turn, the compass card on the front will begin to turn only when the gyro reacts to the yawing of the airplane during the turn. This principle allows the gyro to determine a rate of turn by sensing the amount of pressure created by a change in direction. First and foremost, the turn coordinator utilizes this concept of precession inside of gyroscopes in order to indicate our rate of turn. The magnetic slaving transmitter is called a magnetometer. One good example of this is the RMI in Figure 13. If the bearings are worn, dirty, or improperly lubricated, the drift may be excessive. Even the most mundane trainers come with a standardized package of attitude indicator (AI), heading indicator (HI), turn coordinator (TC), or, in its place, a turn and bank (TB). The attitude indicator is reliable and the most realistic flight instrument on the instrument panel. A standard-rate turn is defined as a turn rate of 3 per second. While taxiing, the instrument should indicate turns in the correct direction, and precession should be normal. Today were going to talk a little bit about the gyroscopic instruments. There is a filter inside the cabin that removes particulates from the air going into the gyros. The relationship of the miniature aircraft to the horizon bar is the same as the relationship of the real aircraft to the actual horizon. Aircraft with only D.C. electrical systems can use A.C. instruments via installation of a solid-state D.C. to A.C. inverter, which changes 14 or 28 volts D.C. into three-phase 115-volt, 400-Hz A.C. As I bank to the left or bank to the right, the gyro wont move, it will just be me and the airplane and the whole case of the instrument that will move. When the system is in free gyro mode, the compass card may be adjusted by depressing the appropriate heading-drive button. Whenever the aircraft is in a turn and the card rotates, the slaving meter shows a full deflection to one side or the other. General aviation has been using gyro instruments long enough to have the bugs pretty well worked out. Its gimbal allows only one axis of freedom (vertical) and connects the mount to the card on the instrument's face through bevel gears. The flux valve is a small, segmented ring, like the one in Figure 10, made of soft iron that readily accepts lines of magnetic flux. The magnetic slaving transmitter is connected electrically to the HSI. We know normally, if I were to keep the wheel upright like this, and let go, obviously it will fall over like so. A yaw string is simply a string or piece of yarn attached to the center of the wind screen. Some other models move in the opposite direction from that in which the aircraft is actually banked. An adjustment knob is provided with which the pilot may move the miniature aircraft up or down to align the miniature aircraft with the horizon bar to suit the pilots line of vision. These weights move the instrument face about 3 degrees per minute. Now if we turn this wheel into a gyroscope, so we spin it, and now I let go, will it topple over? A coil wound around the iron spacer in the center of the frame has 400 Hz alternating current (AC) flowing through it. If you hear one of the gyros whining over the sound of the engine, it's a good bet the instrument will not be long for this world. Cross-checking the instruments routinely is a good habit to develop. Precession can also create some minor errors in some instruments. Heading indicators that do not have this automatic northseeking capability are called free gyros and require periodic adjustment. The adjustment knob is used to align the heading indicator with the magnetic compass, Figure 9. When the roll stabilizes, it indicates rate of turn. Well really what its measuring is a turning force. So, in both cases, its the same concept. A wheel or rotor designed and mounted to utilize these properties is called a gyroscope. What will happen then is, as we pitch up our airplane, the tail will go down, the nose would go up, and now I would just see this gyro and I could measure the angular difference from the case of the instrument to this gyro. Copyright 2022 AeroGuard Flight Training Center. In some aircraft, all the gyros are vacuum, pressure, or electrically operated. Electronic flight displays have replaced free-spinning gyros with solid-state laser systems that are capable of flight at any attitude without tumbling. Along the periphery of the gyro disc are small, cup-like cutouts. Driven by signals from a flux valve, the compass card in this RMI indicates the heading of the aircraft opposite the upper center index mark. Since the AI and HI are usually powered by the same pneumatic system, it's prudent to have another gyro instrument independent from them. What were going to do is do this. The operation of the heading indicator depends upon the principle of rigidity in space. Also be alert for proper suction indications for the pneumatic instruments; the gauge should be in the green arc not far above idle speed and most definitely by the run-up rpm. If I dont have the wheel spinning, which means its not acting as a gyroscope right now, and I let go, this will fall over. The force actually acts 90 in the direction of rotation, which has the effect of applying a force to the front of the tire, causing the bicycle to move to the left. So, the gyroscope will simply hold it level to the table. The heading information is derived from a magnetometer that senses the earths lines of magnetic flux. So inside of a heading indicator, the gyro is not mounted sideways, like what we saw at the beginning of the video but is instead mounted vertically like this. Several flight instruments utilize the properties of a gyroscope for their operation. To correct for a skid, increase the bank and/or decrease the rate of turn. The heading indicator is fundamentally a mechanical instrument designed to facilitate the use of the magnetic compass. To correct for these conditions, and improve the quality of the turn, remember to step on the ball. Varying the angle of bank can also help restore coordinated flight from a slip or skid. Using the example of the bicycle, precession acts on the wheels in order to allow the bicycle to turn. In fact, the attitude indicator contains a set of weights intended to drive the instrument toward level flight by sensing gravity. It is important to check the indications frequently (approximately every 15 minutes) and reset the heading indicator to align it with the magnetic compass when required. Precession of a gyroscope resulting from an applied deflective force, Figure 4. The attitude indicator also utilizes the principle of rigidity in space, but it works a little bit differently. hank Jimmy Doolittle. There are two general types of mountings; the type used depends upon which property of the gyro is utilized. A typical vacuum system consists of an engine-driven vacuum pump, relief valve, air filter, gauge, and tubing necessary to complete the connections. So, the concept of whats happening inside of the heading indicator is, as we make turns in the airplane, the gyro is constantly pointing in the same direction. So how we can imagine this is, if I was flying along and the gyro was here in the airplane like this, and lets say we pitch up, the gyro will remain rigid in space. For example, 6 represents 060, while 21 indicates 210. The ball should also be resting at its lowest point. Thats pretty easy to explain. By evacuating the instrument chamber with help of the vacuum pump, ambient-pressure air comes rushing in, propelling the gyro wheel. to take in the event of an instrument failure, Direct current (D.C.) electrical instruments are available in 14- or 28-volt models, depending upon the electrical system in the aircraft, A.C. is used to operate some attitude gyros and autopilots. Today we take a full gyro panel for granted. Inside each of the gyro devices is a spinning wheel or disc. If aileron and rudder are coordinated during a turn, the ball remains centered in the tube. One basic gyroscopic concept is precession any force applied to the gyro will result in movement of the gyro wheel 90 degrees out of phase. When the vacuum pressure drops below the normal operating range, the gyroscopic instruments may become unstable and inaccurate. So how does it do that? Because of the way the gyro is mounted, the turn-and-slip indicator shows only the rate of turn in degrees per second. So hopefully you have a little bit better understanding as to the principles of a gyroscope and how each of these three instruments utilize those principles to display the information that they do. This information is then processed and sent out to the PFD to generate the heading display. [Figure 4]. Its gyro wheel spins on the horizontal axis, but the pivot is oriented transversely, parallel to the wing span. The gyro in the turn-and-slip indicator rotates in the vertical plane corresponding to the aircrafts longitudinal axis.