Principles of flight

Connect airflow, lift, drag, stability and manoeuvres in principles of flight. Check worked examples by drawing forces and stating what stays constant.

Principles of flight explains how airflow produces forces and moments, and how an aeroplane responds. Before choosing an equation, draw the forces and state what stays constant. The equation comes next.

This guide is for Airline Transport Pilot Licence aeroplane, or ATPL(A), study. Its map groups European Union Aviation Safety Agency (EASA) Subject 081 in AMC1 FCL.310; FCL.515(b); FCL.615(b); FCL.835(d), October 2026 consolidation. Helicopter principles are a separate subject. This is an independent study guide, not an exhaustive syllabus or official endorsement.

A topic map for principles of flight

The separate EASA Private Pilot Licence aeroplane (PPL(A)) syllabus, AMC1 FCL.210; FCL.215, section 5.1, includes airflow, lift, drag, stability, control and flight mechanics. These are useful PPL foundations, not a claim that its depth matches ATPL study. UK candidates should check current UK Civil Aviation Authority (CAA) requirements through their training organisation rather than assume an EU reference defines their examination.

A recommended study sequence

Start with force directions and Newton’s laws. Lift is perpendicular to the relative airflow and drag is parallel to it; weight acts towards the Earth’s centre. Keep those directions separate from aircraft body axes.

Next, connect angle of attack to lift coefficient and stall, then examine how drag changes with lift and speed. Add configuration and propeller effects before studying stability, control and trim.

Use that foundation for climbs, glides and turns. Finish by extending the airflow model to compressibility and high-speed effects. This is our editorial sequence, not an official requirement. Return to the force drawing whenever the answer seems to depend on a slogan such as “more lift means climb”.

Four ways to check the assumptions

1. Write the constants beside the lift equation

The relationship is L = ½ρV²SCₗ: lift depends on air density ρ, true airspeed V, reference wing area S and lift coefficient Cₗ. The NASA Glenn lift equation collects the effects of shape and flow conditions into that coefficient.

In an invented model, lift is 4,000 N at 50 m/s. Doubling speed to 100 m/s gives 16,000 N only if density, area and lift coefficient stay constant. The ratio is (100/50)² = 4.

That is not a prediction for an aeroplane maintaining straight-and-level flight at the same weight. In that case the required total lift stays approximately equal to weight, so the lift coefficient must reduce as speed increases. The equation describes the relationship; the flight condition tells you which variables can change.

2. Separate constant speed from constant velocity

An aeroplane in a steady turn changes direction even if its speed remains constant. Velocity includes direction, so there is acceleration towards the centre of the turn.

For a simplified coordinated level turn at 30° bank, assume lift supplies the vertical support and horizontal turning force. Vertical balance gives L cos 30° = W, so L/W = 1.155, approximately. The horizontal lift component supplies the turning acceleration. There is no contradiction between constant speed and non-zero net force.

This model assumes no significant vertical thrust component and a steady level path. It is not an aircraft operating instruction. The jointly produced manufacturer/ICAO AUPRTA, revision 3, section 6.4.5 explains the load-factor relationship.

3. Ask what the drag curve holds constant

In a simple subsonic, fixed-configuration model at constant density and weight, straight-and-level flight requires approximately constant lift. With an idealised parabolic drag polar, parasite drag varies as V² and induced drag as 1/V².

Suppose each component is 100 N at a reference speed. At twice that speed the model gives 400 N parasite drag and 25 N induced drag, or 425 N total. It does not give half the total drag just because one component falls.

Keep the assumptions beside the curve. A change of lift, configuration, density or compressibility can change the comparison. The speed-squared relationship also assumes the relevant parasite-drag coefficient remains constant. The UK MOD’s AP3456 Volume 1, chapter 1-5 distinguishes drag and power required.

4. Read the vertical axis before choosing a minimum

A drag-force graph and a power-required graph answer different questions. Power required to overcome drag is P = DV, so a low force at a higher speed does not automatically mean lower power.

For the same idealised model, let x be speed divided by minimum-drag speed. Normalised drag is x² + x⁻², with its minimum at x = 1. Normalised power is x³ + x⁻¹, whose minimum is at x ≈ 0.760. The minimum-power speed is lower in this model. These are mathematical ratios, not recommended speeds; actual aircraft data and propulsion efficiency need their own context.

Check your understanding

Does constant speed prove that forces balance? No. A changing direction means changing velocity, as in the level turn.

Must lift exceed weight in a steady climb? No. With thrust aligned with a straight, steady flight path at angle γ, resolving forces gives L = W cos γ and T = D + W sin γ. For a positive climb angle, lift is less than weight in this model. Other thrust directions require their own resolution.

Can you find minimum power by selecting the bottom of a drag-force curve? No. First multiply drag by speed, or use the correctly labelled power-required curve.