LESSONS · AIRCRAFT SYSTEMS · 15 MINUTES · AUSTRALIA AND NZ
Gyroscopic instruments.
Three common flight instruments use spinning gyros: the attitude indicator, the heading indicator and the turn coordinator. A spinning rotor does two useful things. It holds its plane in space, and it answers a push a quarter turn later.
These are the three most common gyro instruments. They work on two properties of a spinning rotor.
Several flight instruments utilize the properties of a gyroscope for their operation. The most common instruments containing gyroscopes are the turn coordinator, heading indicator, and the attitude indicator.
Precession: push a spinning rotor and it does move, but not where you pushed.
PREDICT FIRST
A rotor spins steadily on its axle. You push the top of its rim along the line of the axle. Which part of the rim moves the way you pushed?
TRY IT
Spin it, then push it
Each push presses the top of the rim along the axle, the way the arrow points, for one second. The dashed ring is where the rotor started. The rates are illustrative; the proportions follow PHAK.
Axle turns at
15°/s
Last push moved it
none yet
Spinning. Push the top of the rim and watch where the rotor moves.
Push harder and the axle turns faster. Spin it faster and the same push turns it less. Instrument gyros are heavy for their size and spin fast on low-friction bearings. A faster rotor is steadier in its plane.
The rate at which the gyro precesses is inversely proportional to the speed of the rotor and proportional to the deflective force.
Two important design characteristics of an instrument gyro are great weight for its size, or high density, and rotation at high speed with low friction bearings.
Its gyro spins in a horizontal plane, and the horizon bar is fixed to the gyro. When you bank or pitch, the gyro and its bar stay level and the aircraft rotates around them. The miniature aircraft sits against that bar the way your aircraft sits against the real horizon.
The gyro spins in the horizontal plane and resists deflection of the rotational path. Since the gyro relies on rigidity in space, the aircraft actually rotates around the spinning gyro.
The horizon bar represents the true horizon. 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.
Its rotor spins in a vertical plane and carries the compass card, so the card holds its direction in space while the aircraft turns around it. The forces that make the magnetic compass hard to read do not affect it.
The rotor turns in a vertical plane and fixed to the rotor is a compass card. Since the rotor remains rigid in space, the points on the card hold the same position in space relative to the vertical plane of the gyro. The aircraft actually rotates around the rotating gyro, not the other way around.
You set the heading indicator to the compass, then fly straight and level for 30 minutes. What do you expect to see?
So you reset it to the compass, but only when the compass is steady: straight and level, at a steady speed. In a turn the compass has turning errors, and past about 18° of bank it is erratic. In straight and level flight, if the card is swinging, use the average between the swings.
Adjust the heading indicator to the magnetic compass heading when the aircraft is straight and level at a constant speed to avoid compass errors.
Because of this constraint, the compass only indicates correctly if the card is horizontal. Once tilted out of the horizontal plane, it will be affected by the vertical component of the Earth’s field
This jewel-and-pivot type mounting allows the float freedom to rotate and tilt up to approximately 18° angle of bank. At steeper bank angles, the compass indications are erratic and unpredictable.
Its gyro is canted, mounted at an angle, so it feels roll as well as turn. While you roll into a turn the miniature aircraft leans further, then settles on the rate of turn once the roll stops. The second mark on each side is a standard-rate turn, 3° per second. It shows rate and direction of turn, not bank angle.
The turn coordinator is mounted at an angle, or canted, so it can initially show roll rate. When the roll stabilizes, it indicates rate of turn.
A standard-rate turn is defined as a turn rate of 3° per second. The turn coordinator indicates only the rate and direction of turn; it does not display a specific angle of bank.
In the simulation's example aircraft at 100 kt, about 15° of bank gives close to a standard-rate turn. Marchman makes the same point for a small trainer.
for a small general aviation trainer airplane, a coordinated two‑minute turn does occur at about a 15 degree bank angle.
In some aircraft a vacuum pump spins the attitude and heading gyros, while the turn coordinator runs on electrical power.
In other aircraft, vacuum or pressure systems provide the power for the heading and attitude indicators, while the electrical system provides the power for the turn coordinator.
The vacuum pump fails. In an aircraft with an electric turn coordinator, which instrument still shows your turn reliably?
TRY IT
Fly on the gyros
Bank to turn and watch the turn coordinator lead while you roll. Fly on 30 minutes, then set the heading indicator to the compass, once with wings level and once in a turn. Then fail the vacuum pump. The example heading indicator drifts 10° an hour. The vacuum failure and the compass errors in a turn are illustrative, and so is the turn coordinator's response to roll. Our sources do not say how an attitude indicator recovers once suction returns, so the simulation keeps its error until you press Reset.
AttitudeHeadingTurn coordinator (second mark: standard rate, 3° per second)
Heading: the aircraft and two instruments
Actual heading
090°
Heading indicator
090°
Magnetic compass
090°
Rate of turn
0.0°/s
Straight and level at a steady speed: the compass is settled, so this is the time to set the heading indicator.
That is why the suction gauge matters: when suction is low, the attitude and heading indicators may not be reliable. Most aircraft have a second power source, so at least one source of bank information survives a failure.
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.