Gyroscopic Instruments

Introduction

Gyroscopes can be very perplexing objects because they move in peculiar ways and even seem to defy gravity. These special properties make ­gyroscopes extremely important in everything from your bicycle to the advanced navigation system on the space shuttle. A typical airplane uses about a dozen gyroscopes in everything from its compass to its autopilot. The Russian Mir space station used 11 gyroscopes to keep its orientation to the sun, and the Hubble Space Telescope has a batch of navigational gyros as well. Gyroscopic effects are also central to things like yo-yos and Frisbees!

Objective

• What is a Gyroscope?
  • An apparatus composed of a wheel which spins inside of a frame and causes the balancing of the frame in any direction or position.
  • Basically, any spinning disc takes on the properties of a gyroscope.
    • This includes the spinning propeller in front of our airplane!

Gyroscopic Principles

What properties do spinning discs (gyroscopes) have?

Rigidity in Space

• Principle that a gyroscope remains in a fixed position in the plane in which it is spinning
• By mounting this wheel, or gyroscope, on a set of Gimbal rings, the gyro is able to rotate freely in any direction.
  • If the gimbal rings are tilted, twisted, or otherwise moved, the gyro remains in the plane in which it was originally.
  • Think of the gyro as being aligned with the horizon, and the airplane rotates around it.
• Stability increases if the rotor has great mass and speed
  • Approximately 15,000 rpm for the attitude indicator and 10,000 rpm for the heading indicator

Gyroscopic Precession

• Whenever a force attempts to tilt the plane of rotation, the force is applied 90 degrees ahead of, and in the direction of rotation
  • Inversely proportional to the speed of the rotor and proportional to the deflective force

Problems with using Gyroscopes

Precession

• Unavoidable precession is caused by aircraft maneuvering and by the internal friction of attitude and directional gyros
• Causes slow "drifting" and thus erroneous reading

Tumbling

• When deflective forces are too strong or are applied very rapidly, most older gyro rotors topple over rather than merely precess

Gyroscopic Instrument System

Power Sources

Vacuum System

• Runs the Attitude Indicator and Heading Indicator
  • Engine Driven Pump creates suction through system
  • Air sucked through system is diverted over "buckets" in gyroscope walls to turn gyros
  • Semi-frictionless design

Electrical System

• The turn coordinator uses an electrical gyro so that in the event of a vacuum system failure, the pilot still has one working gyroscopic instrument

Gyroscopic Instruments

Attitude Indicator

• Provides an artificial horizon (not AOA!) to the pilot to display information about both pitch and bank
• Gyroscope has two gimballs that the aircraft can rotate about for pitch and bank
• 10,20,30,60,90 degree markings for bank
• Pitch angle is indicated by a series of lines, each representing 5° or 10° of pitch
• Pilot can set where the miniature airplane meets the horizon before takeoff

Errors

• Turn Error
  • During a normal coordinated turn, centrifugal force causes the gyro to precess toward the inside of the turn.
  • This precession increases as the bank steepens; therefore, it is greatest during the actual turn
  • Error disappears as the aircraft rolls out at the end of a 180 degrees turn at a normal rollout rate.
• Acceleration Error
  • As the aircraft accelerates, gyro precession causes the horizon bar to move down, indicating a slight pitch up attitude.
• Deceleration Error
  • Deceleration causes the horizon bar to move up, indicating a false pitch down attitude

Heading Indicator

• There are a number of errors when using the magnetic compass. The gyroscopic heading indicator makes it easy to turn to headings.
• Unfortunately, the heading indicator does not seek magnetic north by itself
• Vertically mounted gyroscope with one gimbal gives us heading information
  • We set where the gyroscope considers north with the knob and it tells us heading as we rotate around it.

Errors

• Drift
  • Because the earth rotates and because of small accumulated errors caused by friction and imperfect balancing of the gyro, the Heading Indicator will drift over time
  • Must be set every 15 min

Turn Coordinator

• Uses an electric gyroscope to give pilot information about rate of turn and rate of roll
  • Tells us direction and how quickly we are rolling initially
  • Then tells us rate of turn, or how many degrees per second
• Markings at "Standard Rate Turn", which the airplane will turn 360 degrees in 2 minutes
• Gyroscope mounted diagonally, balanced by a spring, and works by precession to sense bank angle
• Inclinometer (ball)
  • Separate instrument used to measure quality of turn
  • Ball on the inside of turn indicates a slipping turn
  • Ball on the outside of turn indicates a skidding turn

Errors

• Human Factors - The turn coordinator does not provide any information about pitch or rate of climb
  • Therefore, just because the turn coordinator shows a level plane, you cannot assume you are level with the ground