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Lesson 1 · Principles of Flight

Lift & the Four Forces

The four forces, how a wing makes lift, and static, dynamic and total pressure — EASA PPL Principles of Flight (081)

~20 min SEP · VFR EASA Part-FCL

1 — The Four Forces

Everything in this subject hangs off four forces. In flight an aeroplane is acted on by lift, weight, thrust and drag, and the way they balance decides whether it climbs, descends, accelerates or holds steady.

  • Weight is gravity, acting straight down through the centre of gravity (CG).
  • Lift is the aerodynamic force that holds the aeroplane up, acting (by convention) perpendicular to the relative airflow, through the wing’s centre of pressure (CP).
  • Thrust is the pull (or push) of the propeller, acting roughly forward.
  • Drag is the air resistance to motion, acting straight back along the flight path.
The four forces in straight-and-level, steady flight
Lift Weight Thrust Drag Lift = Weight Thrust = Drag

In steady, straight-and-level flight the forces cancel in pairs: lift balances weight, and thrust balances drag. When all four are in equilibrium the aeroplane flies straight at a constant speed and height — with no acceleration.

Steady flight means balance
In straight-and-level, unaccelerated flight the forces cancel in pairs: lift = weight and thrust = drag. If a pair stops balancing, the aeroplane accelerates in that direction — more lift than weight and it climbs; more drag than thrust and it slows. This single idea reappears in every later lesson.

2 — How a Wing Makes Lift

Lift is the force worth understanding first, because the whole subject builds on it. There are two classic explanations, and an exam may lean on either — but they are two descriptions of one piece of physics, not rivals.

Two views of how a wing makes lift — switch between them
Relative airflow Downwash −−−− +++ Lower pressure (faster flow) Higher pressure (slower flow) Lift

Newton’s view: the wing deflects a mass of air downwards (the downwash). By the third law, that downward push on the air produces an equal and opposite upward reaction on the wing — lift.

Bernoulli’s view: the air speeds up over the more curved upper surface, so its static pressure falls; the slower air underneath stays at higher pressure. The pressure difference, acting over the wing area, is lift.

Both are the same physics described two ways. The air is turned downwards (Newton) and that turning goes hand in hand with faster, lower-pressure flow over the top (Bernoulli). Neither alone is “the” cause — together they give lift.

Newton looks at momentum: the wing turns a large mass of air downwards (the downwash). Pushing that air down requires the wing to push on it, and the air pushes back up by an equal and opposite reaction — that reaction is lift.

Bernoulli looks at pressure: the air accelerates over the more curved upper surface, so its static pressure falls, while the slower air beneath stays at higher pressure. That pressure difference over the wing area is lift. Both views agree, and both require the wing to meet the air at an angle of attack — the star of Lesson 2.

3 — Static, Dynamic and Total Pressure

Bernoulli’s idea rests on splitting the air’s pressure into two parts whose sum stays constant along a streamline:

Static, dynamic and total pressure
Total pressure slow fast Dynamic pressure Static pressure
Dynamic pressure q = ½ ρ V²
Static pressure
the undisturbed ambient pressure of the air
Dynamic pressure
the pressure of motion — grows with the square of speed
Total pressure
static + dynamic — what the pitot tube senses (pitot pressure)

Along a streamline the total pressure stays constant: where the air speeds up (over the top of the wing), dynamic pressure rises and static pressure falls — and that drop in pressure on top is what makes lift. The airspeed indicator reads speed from dynamic pressure; because q depends on density, that reading is indicated airspeed (IAS), not true airspeed.

  • Static pressure is the ordinary ambient pressure of the air.
  • Dynamic pressure is the pressure of motion — and it grows with the square of the speed: q = ½ ρ V².
  • Total (pitot) pressure is the two added together.
Dynamic pressureq = ½ ρ V²  ·  (ρ = air density, V = true airspeed)

Why your ASI shows IAS, not TAS
The airspeed indicator measures dynamic pressure. Because q depends on air density, the needle reads indicated airspeed (IAS) — and at altitude, where the air is thin, the true airspeed is higher than indicated. That is the exact density story from Flight Performance & Planning · ISA & Density Altitude, seen from the aerodynamics side.

4 — The Aerofoil and the Relative Airflow

A wing’s cross-section is an aerofoil. A few terms recur constantly:

  • The chord line joins the leading edge to the trailing edge.
  • The relative airflow (RAF) is the airflow as the aeroplane meets it — always parallel to, and opposite to, the flight path.
  • The angle of attack (α) is the angle between the chord line and the relative airflow. (Lesson 2 makes this the master variable.)

The wing’s total reaction to the air is one resultant aerodynamic force acting at the centre of pressure. We always split it into two components, defined relative to the airflow: lift at right angles to the RAF, and drag along it.

Lift is defined against the airflow, not the ground
Because lift is perpendicular to the relative airflow (not to the horizon), in a climb or a banked turn the lift vector tilts with the aeroplane. That is why a steep turn needs more total lift to keep enough of it pointing up — a thread we pick up in stability, loads and flight mechanics later in the subject.

5 — Why This Matters to the Pilot

You never fly the formulas, but you fly their consequences. Lift must equal weight to hold height, and you have only two handles on lift: how fast you go (dynamic pressure) and the angle of attack you set. Speed up and you can fly at a smaller angle of attack; slow down and you must raise the angle of attack to keep lift equal to weight — until you run out of angle and the wing stalls (Lesson 4).

The mission link
Every handling skill on your PPL — holding altitude, trimming, turning, recovering from a stall — is this lift-versus-weight balance managed in real time. Get the four forces clear now and the rest of Principles of Flight is variations on the theme.

All numbers here are illustrative; your aircraft’s real figures and limitations come from its POH/AFM.

Knowledge Check

Question 1
In straight-and-level, unaccelerated flight, how do the four forces relate?
Question 2
Newton's explanation of lift is best summarised as:
Question 3
Dynamic pressure is given by q = ½ ρ V². If true airspeed doubles, dynamic pressure becomes:
Question 4
The angle of attack is the angle between:
Question 5
By convention, lift acts:

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