RPL Exam Aerodynamics: Complete Study Guide for CASA Theory Questions

Aerodynamics questions appear throughout the RPL theory exam — not just in the aerodynamics section, but embedded in performance, human factors, and meteorology questions too. Mastering aerodynamics gives you a foundation that pays off across multiple sections. This guide covers every aerodynamics concept that CASA tests.

The four forces of flight

Every aircraft in steady level flight has four forces acting on it in balance:

• Lift — acts perpendicular to the relative airflow, generated by the wings
• Weight — acts vertically downward through the centre of gravity
• Thrust — acts forward along the flight path, produced by the engine and propeller
• Drag — acts opposite to the direction of motion, resisting movement through the air

In steady level flight: Lift = Weight, and Thrust = Drag. In a climb: Thrust > Drag (net forward force). In a descent: the component of weight along the flight path exceeds thrust.

How wings generate lift

The aerofoil shape of a wing causes air flowing over the upper surface to travel faster than air under the lower surface. By Bernoulli's principle, faster airflow equals lower pressure. This pressure differential produces lift.

Lift is also produced by angle of attack — the angle between the chord line and the relative airflow. Increasing angle of attack increases lift up to a point, after which the wing stalls.

Factors that affect lift

• Air density — lower density (high altitude, high temperature) produces less lift for the same speed
• Wing area — larger wings produce more lift
• Airspeed — lift increases with the square of airspeed (double the speed = four times the lift)
• Angle of attack — increases lift until the critical angle is reached
• Wing camber and shape — a more curved wing produces more lift at low speeds (flaps increase camber)

Drag

Two main types of drag are tested in the CASA RPL exam. Understanding how each behaves with changes in airspeed is essential for answering performance and flight planning questions correctly.

Parasite drag

Parasite drag (also called profile drag) is caused by the shape of the aircraft moving through the air — the fuselage, undercarriage, antennas, and any surface exposed to the airflow. Parasite drag increases with airspeed: the faster the aircraft moves, the greater the parasite drag.

Induced drag

Induced drag is a byproduct of lift generation. A high angle of attack produces high lift but also high induced drag. Induced drag decreases as airspeed increases — at higher speeds, less angle of attack is needed to maintain level flight, so induced drag falls.

Total drag = parasite drag + induced drag. There is a speed — V_MD (minimum drag speed) — where total drag is at its lowest. This speed is important for achieving maximum range and minimum fuel burn.

Angle of attack and the stall

The critical angle of attack is approximately 16° for a basic low-speed aerofoil — this is the specific value to remember for the CASA exam. When this angle is exceeded, airflow separates from the upper wing surface and the wing stalls. A stall occurs at a specific angle of attack, not at a specific airspeed.

Key stall facts

• The stall always occurs at the same critical angle of attack, regardless of speed, weight, or attitude
• Stall speed increases with weight — a heavier aircraft needs a higher speed to generate enough lift
• Stall speed increases in a turn because load factor increases
• Stall indicated airspeed stays approximately the same regardless of altitude, but stall true airspeed increases with altitude
• Stall recovery: reduce angle of attack (apply forward pressure on the controls) and add power

Stall warning devices

Most light aircraft have a stall warning horn that activates 5–10 kt above the stall speed. Buffeting from disturbed airflow over the wings can also precede the stall. Never ignore a stall warning in training — treat it as the stall itself.

Load factor

Load factor (n) is the ratio of lift to weight: n = Lift ÷ Weight. In straight and level flight, load factor = 1g. Load factor increases in turns and during pull-up manoeuvres, which directly affects stall speed.

Load factor in turns

In a banked turn, the wing must produce enough lift to support the aircraft's weight and provide centripetal force. This requires a higher load factor as bank angle increases:

• 30° bank: load factor = 1.15g
• 45° bank: load factor = 1.41g
• 60° bank: load factor = 2.0g
• 75° bank: load factor = 3.86g

Since stall speed increases with the square root of the load factor, at 60° bank (2g), stall speed = stall speed × √2. If the straight-and-level stall speed is 50 kt, the stall speed in a 60° bank = 50 × 1.41 = approximately 71 kt.

V_A — manoeuvring speed

V_A is the design manoeuvring speed. Below V_A, full control deflection in one axis will not overstress the aircraft — the wing will stall before structural limits are exceeded. Above V_A, avoid full or abrupt control inputs. Importantly, V_A decreases as aircraft weight decreases, so the value published in the Pilot's Operating Handbook applies only at maximum weight.

Stability

Aircraft stability describes how an aircraft responds to disturbances from its trimmed flight condition. CASA tests all three axes of stability for the RPL exam.

Longitudinal stability (pitch)

A statically stable aircraft returns toward its original attitude after a pitch disturbance. The horizontal tailplane (stabiliser) provides this by generating a restoring pitching moment. The centre of gravity (CG) position is critical: a CG too far forward makes the aircraft very stable but increases stick forces and trim drag; a CG too far aft results in neutral or negative stability, which is dangerous.

Lateral stability (roll)

Dihedral angle — where the wings are angled upward from root to tip — provides lateral stability. If a wing drops, the aircraft sideslips toward the lower wing. The lower wing then develops a higher effective angle of attack and generates more lift, restoring level flight automatically.

Directional stability (yaw)

The vertical fin and rudder act as a weathervane, keeping the nose aligned with the relative airflow. This prevents persistent yaw deviations and is what allows the aircraft to self-correct after a yaw disturbance.

Propeller effects

CASA tests several propeller-induced effects, particularly those that are most pronounced during take-off. All of the following effects apply to aircraft with clockwise-rotating propellers as viewed from behind — which includes most Australian training aircraft.

Torque reaction

By Newton's third law, if the propeller spins clockwise (viewed from behind), the aircraft tends to roll left. This is counteracted by aileron input and, in some aircraft, by design features built into the airframe.

Asymmetric blade effect (P-factor)

At high angles of attack during a climb, the descending propeller blade has a higher angle of attack than the ascending blade, producing more thrust on the right side of the disc. This causes a yaw to the left. The effect is most pronounced at high power, low airspeed, and a high nose attitude — typically during climb-out.

Slipstream effect

The propeller slipstream spirals around the fuselage and strikes the left side of the vertical fin, causing a yaw to the left. This is often corrected by rudder trim or a small offset in the fin angle built into the aircraft during manufacture.

Gyroscopic precession

The rotating propeller acts as a gyroscope. Any force applied to it produces a precessing effect 90° ahead in the direction of rotation. This is most noticeable when the nose is moved rapidly in pitch or yaw — for example, during a tail-wheel aircraft's take-off roll when the tail is raised.

Flaps and slats

High-lift devices are commonly tested in the CASA RPL exam, both in the aerodynamics section and in the context of circuit and landing performance.

• Flaps — increase both lift and drag. Full flap gives maximum drag and is best for landing (steeper approach, lower speed). Partial flap (e.g., 10–20°) increases lift with less drag penalty and may be used for take-off on some aircraft.
• Leading edge slats — re-energise the airflow over the upper wing surface, allowing higher angles of attack before the stall. This effectively lowers the stall speed.

A key exam point: extending flaps lowers the stall speed and changes the aircraft's trim. Retracting flaps at low altitude after take-off must be done carefully — sudden retraction causes a momentary sink as lift is reduced.

Ground effect

When an aircraft flies within approximately one wingspan's height of the ground, the ground interferes with the formation of wingtip vortices. This reduces induced drag — the aircraft seems to "float" and is reluctant to touch down. Ground effect is beneficial during landing as it cushions the touchdown, but it can be dangerous during take-off.

During take-off, an aircraft may become airborne in ground effect before it has sufficient speed to sustain flight out of ground effect. If the pilot attempts to climb out of ground effect prematurely, the aircraft may settle back or stall. Always ensure the aircraft has reached the correct climb speed before attempting to leave the ground effect region.

Common aerodynamics exam traps

The following are frequently tested points where students lose marks. Review each one carefully before sitting the exam.

• A stall occurs at the critical angle of attack (approximately 16° for a basic low-speed aerofoil), not at a specific speed — but stall speed does change with weight and load factor
• At 60° bank, stall speed increases by 41% (a factor of √2 = 1.41) — memorise this figure
• Induced drag decreases with increasing speed — this is the opposite behaviour to parasite drag
• V_A is not a fixed speed — it reduces when the aircraft is at a lighter weight
• Ground effect reduces induced drag specifically, not total drag equally — take care with how exam questions phrase this distinction
• Torque reaction, P-factor, and slipstream effect all produce a left-yaw tendency in most Australian training aircraft (with clockwise-rotating propellers as viewed from behind)

Practise before the exam

Aerodynamics questions in the RPL exam frequently use scenario-based framing — for example: "An aircraft is in a 45° banked turn at 100 kt. If the pilot maintains altitude, what happens to the load factor compared to level flight?" This format rewards application of knowledge, not just memorisation. Practise aerodynamics questions on The Pilot Exam to get comfortable with how CASA frames these questions in the two-hour exam.

Related Resources

• Aerodynamics Study Guide
• Stalls & Spins Guide
• All RPL Exam Topics