Flight Instructors Training Procedures _revised AIC_x

4. THE AIRCRAFT AS A GYROSCOPE
i. The A gyro is the aircraft rolling plane.
ii. The B gyro is the aircraft pitching plane.
iii. The C gyro is the aircraft yawing plane.
5. MOMENTS OF INERTIA
An aircraft yawing may be likened to a gyroscope in the yawing plane.
If a rolling velocity in the same direction as the direction of yaw is applied to the aircraft, the
rolling force will be precessed to give a nose up pitch.
This pitch up is a moment of inertia.
6. AUTOROTATION AND GYROSCOPIC PRECESSION
The value of the C (yawing gyro) is higher than that of A (rolling gyro), since C relates to the
distribution of mass around a normal axis, and thus includes the mass of both the wings and
the fuselage. A (rolling gyro) relates to the distribution of mass around the longitudinal axis. As
the mass of the fuselage is close to the longitudinal axis, its effect is not great.
When yaw and roll are to the same side, as in autorotation the inertial pitching movement will
be nose up, due to the value of C being higher than that of A. The angle of attack will tend to
increase, thereby keeping the aircraft in autorotation, and when all the forces and moments
acting on the aircraft reach a state of equilibrium, the aircraft settles into a steady spin.
7. THE BALANCE OF FORCES
Describe the balance of forces in a steady spin.
8. YAWING MOMENTS
Without a yawing movement there will be no pitch, thus anything which increases the yaw is
PRO-SPIN and anything which reduces the yaw is ANTI-SPIN and will assists in the recovery
from the spin.
The aircraft will be subject to yawing moments of two types:
i. Aerodynamic:
a. Autorotation .................................................................................... Pro-Spin.
b. Applied rudder ................................................................................ Pro-Spin.
c. Weather cocking – damping of yaw by rudder and fuselage ......... Anti-Spin.
ii. Inertial yawing moments:
a. B-gyro plus roll ................................................................................ Anti-Spin.
b. A-gyro plus pitch.............................................................................. Pro-Spin.
This is the basis of the B ratio.
A
The larger this ratio is, the stronger the anti-spin moments will be, resulting in the
aircraft being reluctant to spin and also recovering more easily.
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