Aerodynamics: Complete RPL & PPL Study Guide

The four forces of flight — lift, weight, thrust, and drag — act on an aeroplane at all times during flight. Understanding how these forces interact, and how the aerofoil generates lift, is fundamental to controlled flight and is a core topic in both the RPL and PPL CASA exams.

Angle of attack and angle of incidence

The angle of attack is the angle between the chord line of a wing and the relative airflow. This is distinct from the angle of incidence, which is the angle between the chord line and the longitudinal axis of the aeroplane. These two terms are frequently confused in the CASA exam — angle of attack changes constantly in flight, while angle of incidence is fixed by the aircraft's design.

The effect of airspeed on lift and drag

At a constant angle of attack, both lift and drag will increase if airspeed is increased. Controlling the angle of attack and airspeed is fundamental to maintaining balanced flight and achieving desired performance.

Drag is the aerodynamic force that opposes an aircraft's motion through the air. It acts rearward, parallel and opposite to the relative airflow. For the purpose of aerodynamic study, total drag is divided into two main categories: parasite drag and induced drag.

Parasite drag

Parasite drag is all drag that is not associated with the production of lift. It increases with the square of the airspeed — double your speed and parasite drag quadruples. Parasite drag has three components:

• Form drag (pressure drag): Caused by the shape of the aircraft and the pressure differential between the front and rear surfaces of objects exposed to the airflow. Streamlining significantly reduces form drag. A flat plate produces far more form drag than a streamlined shape of the same frontal area.
• Skin friction drag: Caused by the friction between the air molecules and the aircraft's surface within the boundary layer. Smooth, clean surfaces reduce skin friction drag. Dirt, insects, rivets, and rough paint all increase it.
• Interference drag: Caused where airflows from different parts of the aircraft meet and interact, creating turbulence — for example, where the wing meets the fuselage. Fairings and fillets are used to reduce interference drag.

Induced drag

Induced drag is the drag directly associated with the production of lift. It is caused by the pressure differential between the upper and lower wing surfaces. High pressure air beneath the wing spills around the wingtip to the low pressure area on top, creating wingtip vortices. These vortices cause a downward component to the airflow over the wing called downwash, which tilts the total aerodynamic reaction rearward. The rearward component of this tilted force is induced drag.

Induced drag is greatest at high angles of attack and low airspeeds — such as during slow flight, climbing, or turning. It varies inversely with the square of the airspeed, meaning as speed decreases, induced drag increases dramatically. Induced drag is also greater with increased weight, at higher load factors (e.g., in turns), and with lower aspect ratio wings. High aspect ratio wings (long and narrow) produce less induced drag because the wingtip vortices affect a smaller proportion of the total span.

Total drag and the drag curve

Total drag is the sum of parasite drag and induced drag. When plotted against airspeed, the total drag curve is a U-shaped (or bucket-shaped) curve. Understanding this curve is frequently tested in the CASA exam.

• At low speeds, induced drag dominates and total drag is high.
• At high speeds, parasite drag dominates and total drag is high.
• At one particular speed, the two curves cross and total drag is at its minimum. This is the minimum drag speed (V_MD).

Minimum drag speed and best L/D ratio

The speed at which total drag is least is called V_MD (minimum drag speed). At this speed, the aircraft achieves its best lift-to-drag ratio (best L/D). This is a critically important speed because:

• It gives the best glide range in still air (maximum distance per unit of altitude lost) — the speed you fly in a forced landing after engine failure.
• It gives the maximum endurance for a jet aircraft (though for piston-engine aircraft, maximum endurance occurs at a lower speed, where power required is minimum).
• It is the speed for maximum range in a jet aircraft.

For a typical training aircraft, the best L/D ratio might be in the order of 8:1 to 10:1, meaning that for every 8,000 to 10,000 ft travelled forward, the aircraft descends 1,000 ft in a glide at V_MD. You must know your aircraft's best glide speed and fly it accurately during engine failure situations — too fast or too slow will reduce the glide range.

It is important to note that V_MD is an indicated airspeed (IAS) and remains essentially constant regardless of altitude, provided weight does not change. However, if weight increases, V_MD increases, and if weight decreases, V_MD decreases.

In a climb or descent, the balance of forces acting on the aircraft changes significantly compared to level flight. Understanding how excess power, glide ratio, and factors such as weight, wind, and flap affect performance is essential for safe flight — and is frequently tested in the CASA exam.

Climbing Flight

In a climb, the engine must produce enough thrust to overcome drag and a component of the aircraft's weight that acts rearward along the flight path. The ability to climb depends on excess power — the power available from the engine beyond what is required to maintain level flight at that speed.

Rate of Climb vs Angle of Climb: V_Y vs V_X

Two key climb speeds determine how an aircraft gains altitude, and choosing the correct one depends on the situation.

• Best rate of climb speed (V_Y) gives the greatest gain in altitude per unit of time (feet per minute). This is achieved at the speed where there is the greatest excess power available. V_Y is used for a normal climb when no obstacle clearance is required, as it gets you to your desired altitude in the shortest time.
• Best angle of climb speed (V_X) gives the greatest gain in altitude per unit of horizontal distance. This is achieved at the speed where there is the greatest excess thrust. V_X is always lower than V_Y, and is used when obstacle clearance is required after take-off — it produces the steepest flight path over the ground.

As altitude increases, both V_X and V_Y converge and the rate of climb decreases. At the aircraft's absolute ceiling, V_X equals V_Y, the rate of climb is zero, and only one speed can sustain level flight.

Factors Affecting Climb Performance

Several factors can reduce an aircraft's ability to climb. Understanding each one will help you anticipate performance limitations, particularly during the critical climb-out phase after take-off.

• Weight: Increased weight increases drag and the rearward weight component in the climb, reducing both rate and angle of climb. V_Y and V_X both increase slightly with increased weight.
• Power (density altitude): Reduced power available — for example, at high density altitude, high temperature, or high elevation — directly reduces excess power and thrust, degrading climb performance significantly. Engine power output decreases with increasing altitude.
• Airspeed: Flying at the correct speed (V_X or V_Y as appropriate) is essential. Flying too fast or too slow reduces climb performance.
• Flap: Extending flap increases drag substantially. Even though some flap settings may slightly increase lift, the increased drag reduces climb performance. Flaps should normally be retracted for the climb, unless the Pilot's Operating Handbook (POH) specifies otherwise for short-field take-off.
• Wind: A headwind improves the angle of climb over the ground — producing a steeper climb path relative to terrain — but does not affect the rate of climb. A tailwind worsens the angle of climb over the ground. Wind has no effect on rate of climb because rate of climb is measured through the air mass.
• Bank angle: Any bank increases the load factor and therefore increases the stall speed and induced drag. Even gentle turns in the climb reduce climb performance. Avoid steep turns at low altitude during the climb-out.
• Altitude: As altitude increases, air density decreases, reducing both engine power output (for normally aspirated engines) and propeller efficiency. Climb performance steadily deteriorates with altitude.

Descending and Glide Performance

In a power-off glide, the aircraft is sustained by converting potential energy (altitude) into kinetic energy (airspeed). The forces acting in a glide are lift, drag, and weight — thrust is zero. The component of weight acting forward along the flight path replaces thrust to overcome drag.

Best Glide Speed

The best glide speed corresponds to the speed for the best lift-to-drag ratio (the minimum drag speed). At this speed, the aircraft will cover the greatest horizontal distance for altitude lost in still air. This is the speed that must be adopted immediately in the event of an engine failure — it is published in the POH for each aircraft type.

Flying faster or slower than best glide speed will reduce the glide range. The best glide speed gives a fixed glide ratio in still air — for example, if the lift-to-drag ratio is 9:1, the aircraft glides 9 units forward for every 1 unit of altitude lost.

Effect of Wind on Glide Path

Wind affects how far an aircraft travels over the ground during a glide, but does not change the aircraft's performance through the air mass. This distinction is frequently tested in the CASA exam.

• A headwind reduces the glide range over the ground — the aircraft covers less ground distance for the same altitude lost. In a headwind, a slightly higher speed than best glide speed may be used to optimise ground distance, as per POH guidance.
• A tailwind increases the glide range over the ground — the aircraft covers more ground distance for the same altitude lost.
• Wind does not change the glide ratio through the air mass, only the distance covered over the ground.

Effect of Flaps on Approach and Descent

Extending flap increases both lift and drag, but drag increases proportionally more than lift — particularly at larger flap settings. The overall lift-to-drag ratio decreases. Despite this, flap is an essential tool for managing the approach and landing.

• Flap allows a steeper descent path at a lower airspeed without increasing the rate of descent excessively, which is very useful on approach.
• With flap extended, the aircraft can adopt a more nose-down attitude on approach, improving the pilot's forward visibility over the nose — a significant safety benefit.
• Because flap reduces the stall speed, approach speed can be reduced, leading to shorter landing distances.
• Full flap gives the steepest approach angle. If flap is not available, the approach must be made at a higher speed with a flatter descent path, requiring more runway.

Understanding how to manage glide speed and flap selection during a descent is a core skill for all pilots. For more on how lift and drag interact with aircraft performance, see our aerodynamics study guide.

An aircraft stalls when it exceeds its critical angle of attack (approximately 16° for most light aircraft aerofoils). A stall is not about airspeed — it can occur at any airspeed, any attitude, and any power setting if the critical AoA is exceeded.

Key factors affecting stall speed (IAS):

• Weight: Higher weight = higher stall speed
• Load factor (turns): Stall speed increases by √(load factor). At 60° bank (2G), stall speed increases ~41%
• Flap: Extending flap decreases stall speed
• Power: Increased power decreases stall speed
• Frost/ice: Increases stall speed and degrades stall warning
• Wind shear: May cause stall at higher IAS than expected

Stall recovery: Lower the nose (reduce AoA), apply full power, level wings with rudder (not ailerons), recover with minimum altitude loss.

Spin vs spiral dive: A spin involves a stalled wing with low, constant airspeed. A spiral dive is NOT stalled — airspeed is high and rapidly increasing. Check the ASI to differentiate. Spin recovery uses PARE (Power idle, Ailerons neutral, Rudder opposite, Elevator forward). Spiral dive recovery: reduce power, level wings, ease out of dive.

For the complete guide including detailed recovery procedures, worked stall speed examples, and spin characteristics, see the dedicated Stalls, Spins, and Recovery Procedures study guide.

In a level turn, the aircraft must produce more lift than in straight and level flight. The lift vector is tilted by banking the aircraft, so a horizontal component of lift provides the centripetal force to turn the aircraft, while the vertical component continues to support the aircraft's weight. Understanding how bank angle affects load factor and stall speed is essential for safe flight — and is frequently tested in the CASA exam.

Balanced Turns

For a turn to be balanced (no slip or skid), the correct combination of bank angle and back pressure must be applied, and the balance ball must remain centred. Because the vertical component of lift must still equal the aircraft's weight throughout the turn, the total lift produced must increase as bank angle increases. This increased lift demand is expressed as the load factor.

Load Factor and G-Force

Load factor is the ratio of the total lift produced to the aircraft's weight. In straight and level flight, the load factor is 1G. In a banked level turn, the load factor increases according to the formula:

Load Factor = 1 / cos(bank angle)

This is a secant function — it increases gradually at shallow bank angles but rises very steeply as bank angle increases beyond about 45°. It is not an exponential increase, but rather a trigonometric (secant) relationship. Key values to remember:

• 30° bank: Load factor = 1 / cos(30°) = 1 / 0.866 ≈ 1.15G
• 45° bank: Load factor = 1 / cos(45°) = 1 / 0.707 ≈ 1.41G
• 60° bank: Load factor = 1 / cos(60°) = 1 / 0.5 = 2.0G
• 75° bank: Load factor ≈ 3.86G

Notice how the load factor roughly doubles between 45° and 60° bank — the increase is non-linear and accelerates rapidly at steeper bank angles. At 60° bank, the wings must produce twice the aircraft's weight in lift to maintain altitude.

Stall Speed in Turns

Because a higher load factor requires more lift, and more lift at any given speed requires a higher angle of attack, the stall speed increases in a turn. The stall speed in a turn is calculated by:

V_s(turn) = V_S × √(load factor)

Where V_S is the level flight (1G) stall speed. Using a level flight stall speed (V_S1) of 48 kt as a worked example:

• At 30° bank: Load factor = 1.15G. Stall speed = 48 × √1.15 = 48 × 1.073 ≈ 51 kt
• At 60° bank: Load factor = 2.0G. Stall speed = 48 × √2.0 = 48 × 1.414 ≈ 68 kt

At 60° bank, the stall speed has increased by approximately 41% above the level flight stall speed. This is a very significant increase and brings the stall speed much closer to normal approach and manoeuvring speeds.

Dangers of Steep Turns at Low Altitude

Steep turns at low altitude are extremely dangerous for several reasons. At low altitude and low airspeed — such as in the circuit — the margin between flying speed and stall speed may become dangerously small, leaving insufficient height for recovery if a stall occurs.

• Increased stall speed: As shown above, the stall speed rises significantly in steep turns. A stall at low altitude leaves insufficient height for recovery.
• Altitude loss: If the pilot does not add sufficient back pressure and power to maintain altitude in a steep turn, the aircraft will descend rapidly. At low altitude, this can result in ground contact.
• Stall/spin risk: If the aircraft stalls in a steep turn — particularly in an uncoordinated turn with excessive rudder — a spin may develop. A spin entry at low altitude is almost always unrecoverable before ground impact.
• Reduced excess power: In a steep turn, the increased drag from the higher lift requirement and increased induced drag may exceed the available power, making it impossible to maintain altitude.

Steep Turns During a Glide (Engine Failure)

During a glide with the engine failed, steep turns are particularly hazardous. Without power available to compensate for the increased drag, the rate of descent increases significantly and glide range is reduced considerably. A 60° bank turn in a glide will result in a dramatic loss of altitude and distance capability.

• There is no power available to compensate for the increased drag and maintain airspeed — rate of descent increases significantly.
• The increased load factor raises the stall speed, reducing the safety margin above the stall.
• The increased drag reduces glide range considerably.
• During a forced landing, turns should be planned carefully and kept to moderate bank angles — ideally no more than 30° to 45° — to conserve altitude and maintain a safe margin above the stall.

Turns Shortly After Take-off

Pilots must avoid steep turns shortly after take-off. During the initial climb-out, the aircraft is at a relatively low airspeed and low altitude with a high power setting. The margin above the stall speed is already reduced compared to cruise flight, and a steep turn in this phase of flight will significantly increase the stall speed, increase drag (reducing or eliminating the ability to climb), and leave no altitude for recovery if a stall or spin occurs.

Good airmanship dictates that turns after take-off should be gentle — no more than approximately 15° to 20° of bank — until a safe altitude and speed are established. If a turn-back to the runway is considered after engine failure on take-off, pilots should be aware that the steep turn required will dramatically increase stall speed and altitude loss. In most light aircraft, a turn-back from low altitude is not survivable, and a landing ahead — or slightly to the side — is the safer option.

During take-off and landing, aircraft operate close to the runway and enter ground effect. This phenomenon causes an effective increase in lift and reduction in induced drag, which can prolong the float and cause the aircraft to balloon on approach.

Wake turbulence awareness during take-off

To avoid wake turbulence when taking off behind a large departing aircraft, you should ensure lift-off is made well before the larger aircraft's lift-off point, then climb above their flight path.

Wheelbarrowing during the landing roll

If the aircraft wheelbarrows during the landing roll, the first action should be to relax forward pressure on the control wheel.

Related Resources

• Stalls, Spins & Recovery
• Aircraft Performance
• RPL Aerodynamics Blog