GyroscopesNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 9
Rigidity of a gyroscope depends on:
All 9 questions — Gyroscopes
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. Rigidity of a gyroscope depends on:
- A.weight, force applied and speed of rotation
- B.rate of precession and the force applied
- C.weight, rate of precession and speed of rotation
- D.mass, radius of gyration and speed of rotation✓
Why: Rigidity depends on Moment of Inertia (= f(mass, radius²)) and RPM. Radius of gyration is the effective radius at which the mass operates — together with mass, it defines moment of inertia. The three factors are mass, radius of gyration (effective radius), and speed of rotation (RPM). See Section 2 . — Rigidity = f(Mass, Radius, RPM). "Weight" ≈ mass but in a technical context, weight is force (mass × g) — mass is the correct term. Always use "mass" not "weight" in these questions.
Q2. A force is applied to deflect a gyroscope. If the rpm of the gyro is then doubled, the precession rate will:
- A.remain as before
- B.increase
- C.decrease✓
- D.cease altogether
Why: Precession rate ∝ Torque / (MI × RPM). If RPM doubles (with the same applied torque and same moment of inertia), the denominator doubles → precession rate halves → decreases. See Section 5 . — Higher RPM = more rigid = harder to precess = lower precession rate for same torque. This is the fundamental trade-off in gyro design.
Q3. In gyroscopic theory the term 'topple' is defined as:
- A.real wander only, in the horizontal plane
- B.real wander only, in the vertical plane
- C.wander, real or apparent, in the vertical plane✓
- D.wander, real or apparent, in the horizontal plane
Why: Topple is wander (whether real or apparent) in the vertical plane. It applies to both real wander (manufacturing imperfections) and apparent wander (earth rate, transport wander). Drift is wander in the horizontal plane. See Section 6 . — Topple = vertical plane (both real and apparent). Drift = horizontal plane (both real and apparent). These definitions apply regardless of whether the wander is real or apparent.
Q4. A force applied to the spinning axis of a rotor is precessed:
- A.through 90° in the direction of spin of the rotor✓
- B.through 90° in the direction of spin of the rotor in the northern hemisphere and through 90° in the opposite direction in the southern hemisphere
- C.through 270° in the direction of spin of the rotor
- D.at a rate proportional to the speed of rotation of the gyro
Why: Precession always acts 90° ahead (in the direction of spin) of the applied force — this is a universal rule of gyroscopic physics, independent of hemisphere. See Section 4 . — 90° in direction of spin — always. This is a physical law (angular momentum). No hemispheric variation. 270° in direction of spin = 90° against = wrong direction.
Q5. Real wander of a gyro can be caused by:
- A.asymmetrical friction at the spinning axis✓
- B.rotation of the earth
- C.increasing the rpm of the rotor
- D.moving the gyro north or south of its present position
Why: Real wander is caused by manufacturing imperfections — including uneven/asymmetrical friction at the spinning axis (rotor bearings), gimbal friction, imbalance in rotor mass, and unbalanced gimbals. These produce torques that cause the gyro axis to move with respect to inertial space. See Section 7.1 . — Real wander = manufacturing defects = internal causes. Apparent wander = external/geometric causes = earth rotation + transport. The distinction is always tested.
Q6. A gyro with only one degree of freedom is known as a:
- A.tied gyro
- B.earth gyro
- C.space gyro
- D.rate gyro✓
Why: A rate gyro has one gimbal and one degree of freedom. It measures angular rate (°/second). Examples: turn rate indicator, yaw dampers. Displacement gyros have two gimbals and two degrees of freedom. See Section 10 . — 1 DoF = rate gyro (1 gimbal). 2 DoF = displacement gyro (2 gimbals). The function (measuring rate vs. angle) is directly tied to the number of degrees of freedom.
Q7. A perfectly balanced space gyro at the equator has its spin axis aligned with true north. After 6 hours the axis will be aligned with:
- A.true east direction
- B.true west direction
- C.true north direction✓
- D.true south direction
Why: At the equator, Earth Rate = 15 × sin(0°) = 0°/hour. The gyro experiences no earth rate drift. Furthermore, a space gyro at the equator with its axis pointing true north (east-west horizontal axis) — the earth's rotation is perpendicular to the gyro axis, so the gyro axis continues to point to the same fixed star, which happens to remain aligned with true north at the equator for this particular axis orientation (along the equatorial plane). The gyro axis stays fixed in space = true north. See Section 8 . — Earth Rate = 15 × sin(lat). At equator, sin(0) = 0 → zero drift → gyro stays aligned…
Q8. The main advantage of electric gyros are:
- A.light weight, high rpm, constant speed, inexpensive
- B.high rpm, only require low voltage DC, constant speed, sealed casing
- C.high rpm, high moment of inertia, rapid build-up of speed, constant RPM✓
- D.sealed casing, constant speed, high precession rate, low cost
Why: Electric gyros can spin faster (higher RPM), achieve this speed more rapidly, and maintain it more accurately — giving higher moment of inertia and therefore greater rigidity. These are the primary advantages over suction-driven gyros. See Section 12.2 . — Electric gyro advantages: fast spin-up, high RPM, consistent RPM, high rigidity. Disadvantages: heavier, more expensive, needs electrical power. Suction: immune to electrical failure, but vulnerable to contamination and altitude.
Q9. Apparent wander of a gyro can be caused by:
- A.rotation of the earth✓
- B.clear air turbulence
- C.gimbal friction
- D.external torque
Why: Apparent wander occurs because of changes in the observer's frame of reference. The two causes are earth rate (due to earth's rotation) and transport wander (due to eastward/westward flight). Rotation of the earth is directly responsible for earth rate. See Section 7.2 . — Apparent wander: earth rotation (earth rate) + east/west flight (transport wander). Real wander: manufacturing defects. "Rotation of the earth" is always the correct answer for apparent wander cause.