Gyroscope Free Vs Tied
Explanation of Free vs Tied gyroscopes
🧭 1️⃣ What Makes a Gyro “Free” or “Tied”
A gyroscope has a spinning rotor that resists changes to its orientation due to rigidity in space. How free it is to move depends on how its gimbals are connected to the aircraft.
| Type | Meaning | Example Use |
|---|---|---|
| Free Gyro | Moves freely in all 3 axes (not referenced to the aircraft) | INS, IRS, Gyro Compass |
| Tied Gyro | Restricted by reference forces (gravity, Earth’s rotation, accelerometers) | Attitude Indicator, Directional Gyro |
⚙️ 2️⃣ Free Gyro — “Completely Free in Space”
Characteristics:
- Has 3 degrees of freedom.
- The spin axis maintains a fixed direction in space (rigidity in space).
- It doesn’t sense or care what the aircraft does — it stays aligned with the stars, so to speak.
- Used where an independent, stable reference is needed.
Advantages:
- Provides a true inertial reference (not tied to the aircraft or Earth).
- Core of Inertial Navigation Systems (INS) and Inertial Reference Systems (IRS).
Disadvantages:
- Subject to drift (caused by bearing friction, Earth rotation, transport rate).
- Requires complex correction systems (Schuler tuning, platform leveling, etc.).
Example:
- INS platform gyros that keep their spin axes fixed in space so the system can measure aircraft movement relative to the Earth.
⚙️ 3️⃣ Tied Gyro — “Connected to an External Reference”
Characteristics:
Has 2 or fewer degrees of freedom.
“Tied” to a reference source:
- Gravity (via pendulum, erection system, or accelerometer)
- Magnetic north (via flux valve)
- Aircraft axes (via torque motors or erection systems)
The system continually corrects the gyro’s orientation to align it with the chosen reference.
Examples:
| Instrument | Reference | How It’s “Tied” |
|---|---|---|
| Attitude Indicator | Gravity | Uses erection system (pendulum or air jets) to keep gyro vertical |
| Directional Gyro (DG) | Magnetic North | Flux valve senses direction, torque motors keep gyro aligned |
| Turn Coordinator | Aircraft rate (yaw/roll) | Gyro is physically limited to sense rate about one axis |
Advantages:
- Constantly corrected → less drift over time.
- More stable in normal operation for pilot reference.
Disadvantages:
- Loses independence — relies on external sensors (can fail or give false reference).
- Limited in degrees of freedom (can’t remain fixed in space like a free gyro).
⚙️ 4️⃣ Simple Summary
| Feature | Free Gyro | Tied Gyro |
|---|---|---|
| Degrees of Freedom | 3 | 2 or less |
| Reference | None (space-fixed) | External (gravity, magnetic, etc.) |
| Used In | INS, gyro compass | Attitude & Directional gyros |
| Corrected For Drift? | No (needs computation) | Yes (by erection system or sensors) |
| Example | INS platform gyro | Artificial horizon, DG |
✈️ 5️⃣ Real-World Analogy
- Free Gyro → A spinning top floating in space — it stays pointing in one direction no matter what.
- Tied Gyro → A spinning top that’s nudged or “steered” to keep pointing down toward the ground or north.