Principles of Flight – DGCA CPL Ground School (India)

Establish the physical foundation: the atmosphere, fluid properties, and the basic laws governing airflow around a body — the bedrock of every subsequent aerodynamic concept.

Quantify the forces acting on an aircraft in flight. Understand how lift and drag coefficients behave with changing AoA, speed, and configuration, and how the four forces balance in each flight phase.

Understand the aerodynamic stall in depth — its causes, symptoms, types, and recovery — plus spin aerodynamics and the role of boundary layer control, all to DGCA CPL examination depth.

Master all three axes of stability and control — longitudinal, lateral, and directional — covering both static and dynamic stability, control surface aerodynamics, and the DGCA-required stability criteria for commercial aircraft.

Address the aerodynamics of high subsonic, transonic, and supersonic flight — critical for CPL candidates aiming at airline careers — plus the V-n diagram and structural/aerodynamic limits of the flight envelope.

Consolidate all five modules through integrated problem-solving sessions, past-paper analysis, full-syllabus mock tests, and a final review of commonly examined topics — structured to mirror the actual DGCA CPL computer-based test format.

Gyroscopic Effect
A rotating propeller has the properties of a gyroscope - rigidity in space and precession. The
characteristic which produces “gyroscopic effect” is precession. Gyroscopic precession is the
reaction that occurs when a force is applied to the rim of a rotating disc. When a force is
applied to the rim of a propeller, the reaction occurs 90° ahead in the direction of rotation and
in the same direction as the applied force. As the aircraft is pitched up or down or yawed left
or right, a force is applied to the rim of the spinning propeller disc.
Note: Gyroscopic effect only occurs when the aircraft pitches and/or yaws.
For example, if an aircraft with a clockwise rotating propeller is pitched nose-up, imagine that
a forward force has been applied to the bottom of the propeller disc. The force will “emerge”
at 90° in the direction of rotation, i.e. a right yawing moment. Gyroscopic effect can be easily
determined when the point of application of the imagined forward force on the propeller disc
is considered.
Pitch down - forward force on the top, force emerges 90° clockwise, left yaw.
Left yaw - forward force on the right, force emerges 90° clockwise, pitch up.
Right yaw - forward force on the left, force emerges 90° clockwise, pitch down.
Gyroscopic effect will be cancelled if the propellers are contra-rotating. Spiral Slipstream Effect
As the propeller rotates it produces a backward flow of air, or slipstream, which rotates around
the aircraft, as illustrated in Figure 16.20. This spiral slipstream causes a change in airflow
around the fin (vertical stabilizer). Due to the direction of propeller rotation (clockwise) the
spiral slipstream meets the fin at an angle from the left, producing a sideways force on the fin
to the right.
Spiral slipstream effect gives the aircraft a yawing moment to the left.
The amount of rotation given to the air will depend on the throttle and RPM setting. Spiral
slipstream effect can be reduced by:
• the use of contra or counter-rotating propellers.
• a small fixed tab on the rudder.
• the engine thrust line inclined slightly to the right.
• offsetting the fin slightly.

Asymmetric Blade Effect
In general, the propeller shaft will be inclined upwards from the direction of flight due to the
angle of attack of the aircraft. This gives the down-going propeller blade a greater effective
angle of attack than the up-going blade. The down-going (right) blade will generate more
thrust. The difference in thrust on the two sides of the propeller disc will give a yawing
moment to the left with a clockwise rotating propeller in a nose-up attitude.
Asymmetric blade effect will be greatest at full power and low airspeed (high angle of attack).
Effect of Atmospheric Conditions
Changes of atmospheric pressure or temperature will cause a change of air density. This will
affect:
• the power produced by the engine at a given throttle position.
• the resistance to rotation of the propeller (its drag).
An increase in air density will increase both the engine power and the propeller drag. The
change in engine power is more significant than the change in propeller drag.
Engine and Propeller Combined
If the combined effect of an engine and propeller is being considered, it is the engine power
change which will determine the result. For an engine driving a fixed pitch propeller:
• if density increases, RPM will increase.
• if density decreases, RPM will decrease.
Engine Alone
If the shaft power required to drive the propeller is being considered, then it is only the propeller
torque which needs to be taken into account. To maintain the RPM of a fixed pitch propeller:
• if density increases, power required will increase.
• if density decreases, power required will decrease.