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PPL principles of flight course: 12 illustrated chapters

The PPL principles of flight course follows the syllabus in 12 chapters: 86 key points explained in a few paragraphs, often with a diagram, what to remember and, for many, the classic exam trap. The first chapter, « Air and Airflow », can be read in full on this page.

Read chapter 1The 419 questions of this subject

Contents

Each chapter explains, point by point, what the questions of the bank ask, with a « Remember » box and often the classic exam trap. The first chapter is free, below and in the app; the next ones are for PPL subscribers.

  1. 1.Air and Airflow

    Free

    Composition and density of air, the Venturi effect, Bernoulli and the origin of lift.

    7 key points · 14 questions of the bank

  2. 2.The Aerofoil and the Forces

    Subscribers

    Aerofoil vocabulary, angle of attack, pitch attitude and flight path angle, total aerodynamic force, centre of pressure and aerodynamic centre, the four forces.

    7 key points · 14 questions of the bank

  3. 3.Lift, drag and coefficients

    Subscribers

    The Rz and Rx formulae, the effect of speed, density and angle of attack, CL and CD.

    7 key points · 28 questions of the bank

  4. 4.Drag, glide ratio and ground effect

    Subscribers

    The types of drag, wingtip vortices and wake turbulence, lift-to-drag ratio and the polar curve, ground effect and the second regime (back of the power curve).

    10 key points · 21 questions of the bank

  5. 5.Climb, descent and landing

    Subscribers

    Equilibrium in climb and descent, gradient and vertical speed, Vx and Vy, flare, gusts and wind gradient.

    7 key points · 18 questions of the bank

  6. 6.The turn and load factor

    Subscribers

    Tilting of lift, n = 1/cos φ, power in a turn, turn radius and rate, pull-up.

    9 key points · 75 questions of the bank

  7. 7.The stall

    Subscribers

    Critical angle of attack, warning signs, stall speed and its factors, calculation in a turn, dynamic stall, spin, slow flight.

    8 key points · 175 questions of the bank

  8. 8.Spin and spiral dive

    Subscribers

    Asymmetric flight, autorotation, causes of and recovery from a spin, recognising and recovering from a spiral dive.

    7 key points · 22 questions of the bank

  9. 9.Flaps and slats

    Subscribers

    Role of high-lift devices, types of flaps and slats, effects on lift, drag and pitch, VFE and VFO.

    5 key points · 20 questions of the bank

  10. 10.Stability and balance

    Subscribers

    The three axes, stability and manoeuvrability, longitudinal stability and the aerodynamic centre, balance (centre of gravity), directional and lateral stability.

    7 key points · 18 questions of the bank

  11. 11.Controls, trim tab and engine effects

    Subscribers

    Control surfaces and their effectiveness, ailerons and adverse yaw, trim tab, propeller effects about the three axes.

    6 key points · 11 questions of the bank

  12. 12.Flight envelope and limiting speeds

    Subscribers

    Characteristic speeds, airspeed indicator arcs, limit load factors, Va, turbulence and the limits of the flight envelope.

    6 key points · 6 questions of the bank

Chapter 1: Air and Airflow

Composition and density of air, the Venturi effect, Bernoulli and the origin of lift.

Air, a fluid that has mass

← Faites glisser le schéma →

Composition of air and standard density

© ForAllPilots

The aeroplane relies on the air to support it: so we need to know this fluid.

  • dry air contains about 78% nitrogen, 21% oxygen and 1% other gases (argon, carbon dioxide...);
  • its density ρ ("rho"), often just called density in exam questions, is 1.225 kg/m³ at sea level in the standard atmosphere.

ρ decreases as altitude increases, as temperature increases and, to a lesser extent, as humidity increases. Less air "to work with" means less lift and less drag at the same speed.

Remember

Nitrogen 78%, oxygen 21%. ρ0 = 1.225 kg/m³ (kilograms per cubic metre).

Exam trap

The most abundant gas is nitrogen, not oxygen. Also watch the unit: kg/m³, not g/m³ or m³/kg.

What the air exerts on a body

← Faites glisser le schéma →

Pressure and friction on a body placed in an airflow

© ForAllPilots

When a body moves through the air, every point on its surface is subject to two actions:

  • pressure, perpendicular to the surface;
  • friction, tangential to the surface, due to the viscosity of the air.

The sum of all these small forces forms the total aerodynamic force. It is then resolved into lift and drag.

Remember

At every point of the surface: pressure + friction.

The Venturi effect

← Faites glisser le schéma →

Venturi tube: at the throat, maximum speed and minimum pressure

© ForAllPilots

In a tube, air does not build up: the same flow rate passes through every section (S × V = constant). Where the tube narrows, the air must therefore accelerate.

This acceleration is accompanied by a drop in static pressure. At the throat (the narrowest point):

  • speed is at its maximum;
  • pressure is at its minimum.

This is the principle of the carburettor and of many pressure sources.

Remember

Venturi: cross-section ↓ → speed ↑ → pressure ↓.

Exam trap

Answers such as "maximum pressure and speed" are wrong: speed and pressure always vary in opposite directions.

Bernoulli's theorem

Along a streamline (smooth flow, at light-aviation speeds), the sum of the static pressure and the dynamic pressure remains constant:

Ps + ½ ρ V² = constant

  • if speed increases, static pressure decreases;
  • if speed decreases, static pressure increases.

It is the same idea as the Venturi effect, expressed in terms of energy: what the air gains in speed, it loses in pressure.

Remember

Bernoulli: Ps + ½ ρ V² = constant. Speed ↓ → static pressure ↑.

Airflow around an aerofoil

← Faites glisser le schéma →

Streamlines squeezed together over the upper surface, spread apart under the lower surface

© ForAllPilots

A wing aerofoil placed at a small angle of attack alters the airflow:

  • on the upper surface (upper surface, *extrados*), the streamlines squeeze together: the air accelerates and the pressure drops (suction);
  • on the lower surface (lower surface, *intrados*), the air is slightly slowed down: the pressure increases a little (overpressure).

Lift comes mainly from this pressure difference between the lower and upper surfaces.

Remember

The air moves faster over the upper surface than over the lower surface.

Exam trap

The flow is not "laminar above, turbulent below": the correct idea is simply "faster over the upper surface".

What supports the wing: the top or the bottom?

← Faites glisser le schéma →

Schematic distribution of pressures around an aerofoil

© ForAllPilots

Adding up the pressure forces, we find that:

  • the suction on the upper surface provides about 3/4 of the lift;
  • the overpressure under the lower surface provides only about 1/4.

The wing is therefore sucked up from above more than it is pushed up from below. Practical consequence: an upper surface that is dirty, iced or covered with frost severely disrupts lift.

Remember

Upper surface (suction) ≈ 3/4; lower surface (overpressure) ≈ 1/4.

Exam trap

Questions like to swap the words around: the suction is on the upper surface, the overpressure under the lower surface.

The other view: action and reaction

← Faites glisser le schéma →

The wing deflects the air downwards, the air pushes it upwards

© ForAllPilots

Newton's third law gives a complementary explanation: the wing deflects downwards a large mass of air (deflection). In return, the air exerts on the wing an upward reaction: lift.

Bernoulli (pressure difference) and Newton (deflection of the air) describe the same phenomenon from two angles. Neither the weight of the aeroplane, nor engine thrust, nor drag creates lift.

Remember

Lift = pressure difference between upper and lower surfaces = reaction to the downward deflection of the air.

The course goes on with chapter 2, « The Aerofoil and the Forces ». It is read in the Course tab of the app with a PPL subscription, and each chapter ends with the questions of the bank about it. See pricing

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