Gyroscopic Instruments & Principles

• Ms. Kaye Repatacodo is the creator of the information

Gyroscopic Instruments

• Gyroscopic flight instruments are used in most general aviation and older commercial aircraft.
• Examples include:
  • Attitude indicators
  • Heading indicators
  • Turn coordinators (turn and slip indicator)
• Gyroscopes are typically electrically or vacuum driven.
• They utilize gyroscopic principles to display the aircraft's attitude.
• Modern installations use laser gyros instead of mechanical gyroscopes.

Gyroscopic Principles

• Rigidity in Space is the primary characteristic of a gyro that makes it suitable for use in attitude instruments.
• Precession is a secondary gyroscopic principle that must be understood and compensated for.

Rigidity in Space

• The primary trait of a spinning gyro rotor is rigidity in space, also known as gyroscopic inertia
• Newton's First Law: a body in motion tends to move in a constant speed and direction unless acted upon by an external force
• The spinning rotor maintains a constant attitude in space as long as no external forces act to change its motion.
• Gyroscopic aircraft instruments use heavy materials in rotors designed to spin at 10,000 to 15,000 RPM.s

Rigidity in Space and Attitude/Heading Indicators

• Gyros are unchanging reference points: rotors maintain a constant position relative to the horizon or direction once spinning
• A universally mounted gyro's rotor stays put, even when surrounding frames or gimbals move.
• Aircraft rotate around the gyro, not changing the rotor's position.
• Aircraft attitude or heading compared to enable the instruments to display attitude or direction.

Precession

• Precession: Tilting or turning of the rotor axis due to external forces
• A static gyro rotor moves in the direction of force if the deflective force is applied.
• The force is applied to a spinning rotor rim causes movement as if applied 90𝆩 around the rim in the direction of rotation.
• This turning movement, known as precession, places the rotor in a new plane of rotation parallel to the force

Attitude Indicator Function

• Attitude Indicator utilizes a miniature aircraft and horizon bar to depict the aircraft's attitude.
• The relationship between the miniature aircraft and the horizon bar mirrors the relationship between the actual aircraft and the real horizon.
• The instrument provides instantaneous indications of even minimal changes in attitude.

Attitude Indicator Gyroscope

• The attitude indicator's gyro is mounted in a horizontal plane.
• Its operation is dependent on rigidity in space.
• The horizon bar represents the true horizon.
• It is fixed to the gyro.
• It remains in a horizontal plane as the aircraft is pitched or banked, therefore it displays the aircraft's attitude relative to the true horizon.

Attitude Indicator Adjustment

• An adjustment knob moves the miniature aircraft up or down.
• This aligns it with the horizon bar to suit the pilot's line of vision.
• Wings should overlap the horizon bar during straight-and-level cruising flight.

Pitch and Bank Limits

• The pitch and bank limits depend on the instrument's make and model.
• Banking plane limits are usually 100𝆩 to 110𝆩
• Pitch limits are usually 60𝆩 to 70𝆩.
• Exceeding these limits causes tumbling, incorrect indications until realignment.
• Modern attitude indicators may not tumble.

Attitude Representation

• The attitude indicator's representation corresponds to the aircraft's relation to the real horizon.

Heading Indicator

• Mechanical instrument that assists use of magnetic compass.
• Not affected by magnetic compass interpretation forces.

Heading Indicator Operation

• Depends on rigidity in space.
• The rotor turns in a vertical plane.
• A compass card is fixed to the rotor.
• Points on the card hold in same position relative to gyroscope.

Heading Indicator Gyro

• The aircraft rotates around the rotating gyro, not vice versa.
• The instrument case revolves around the gyro's vertical axis with aircraft.
• The card gives clear heading indication.

Turn Indicators

• Turn Coordinator is mounted at an angle, can initially how roll rate and indicates turn rate once roll stabilizes
• Both provide turn direction and coordination quality
• Serves as back-up bank info when attitude indicator fails
• Aircraft use 2 types of turn indicators: Turn-and-slip Indicators and Turn Coordinators.
• Because of its gyro mounting, turn-and-slip the indicators only the rate of turn in degrees per second.

Turn-And-Slip Indicator

• The gyro rotates in the vertical plane along the aircraft's longitudinal axis.
• A single gimbal limits the planes in which the gyro can tilt.
• A spring maintains a center position.
• a yawning force caused by precession causes the gyro to tilt left or right, as viewed from the pilot seat.
• Turn-and-slip indicator uses a turn needle or pointer, to show direction and rate of turn.
• Incapable of "tumbling" because of restraining springs.
• Applying extreme force to gyro displaces it from normal plane of rotation, rendering it invalid.

Turn coordinator

• The Gimbal is canted in the turn coordinator
• It can therefore sense the rate of roll and turn
• When rolling into or out of a turn, the miniature aircraft banks in the direction.
• A rapid roll rate causes the miniature aircraft to bank more steeply than a slow roll rate.

Turn Coordinator Details

• Figures show turn coordinator.
• Two marks on sides (left and right) used on the face for reference.
• The first mark references a wings level zero turn rate.
• The second mark indicates a standard rate of turn.
• A standard-rate turn is defined as 3 degrees per second.
• A slip results when inadequate right rudder applied in right turn.
• Skid results is when too much rudder applied.
• Centering ball gives coordinated turn.
• Turn coordinator only indicates rate and direction (but not angle of bank)
• Rely on precessions
• Gimbal rotation
• Horizontal gyro
• Canted gyro
• Turn-and-slip indicator
• Standard rate turn index
• Inclonometer.

Pitot Pressure Measurement

• Pitot pressure is measured using a pitot tube or pressure head.
• A pitot tube is an open tube facing forward along the aircraft's axis.
• The measured pressure combines static pressure and pressure from the aircraft's forward speed.
• Pitot tubes are strategically positioned to minimize errors caused by airflow.

Pitot Tube Maintenance

• Covering the pitot tubes when the aircraft is parked for extended periods reduces the chance of blockage or contamination.
• Electrical heating reduces contamination from moisture and prevents blockage by ice.

Pitot System Redundancy

• Commercial aircraft have at least two independent pitot systems.
• This provides redundancy in case of system failure.

Air Data Computer (ADC)

• Modern aircraft have an ADC.
• The computer uses pitot-static system and temperature sensors for information.
• It determines:
  • Indicated Airspeed
  • Mach Number
  • True Airspeed
  • Altitude
  • Vertical Speed
  • Outside Air Temperature (OAT)
  • Total Air Temperature (TAT).
• These data feed to aircraft systems, mostly the Electronic Flight Instrument System.