The Four Forces and Equilibrium

British Young Pilots · Principles of Flight · Module POF-1.1

Content:

Four forces, two tug-of-wars

Every aeroplane in flight is in a giant tug-of-war. Four invisible players pull on it all the time:

  • Thrust pulls it forward: the engine and propeller
  • Drag pulls it backward: the air pushing against it
  • Lift pulls it up: made by the wings
  • Weight pulls it down: gravity

Source: Pooleys Vol 4 (10th ed.) p.3–4.

The four forces

Every aeroplane in flight is pulled and pushed by four forces:

  • Lift holds it up: made by air flowing over the wings
  • Weight pulls it down: gravity acting on the aeroplane's mass, through the centre of gravity (CG)
  • Thrust drives it forward: from the engine and propeller
  • Drag holds it back: the air resisting the aeroplane's motion
LIFT WEIGHT THRUST DRAG

Notice the vertical arrows are drawn much longer than the horizontal ones. That's deliberate (see "How big are they?"). Source: Pooleys Vol 4 (10th ed.) p.3–4.

Both matches a draw = keep cruising

Two tug-of-war matches happen at once: the up-team (lift) against the down-team (weight), and the forward-team (thrust) against the backward-team (drag). When both matches are a draw, nobody wins, and the aeroplane just keeps cruising: same speed, same direction, until something changes a pull.

Source: Pooleys Vol 4 (10th ed.) p.4.

Balanced pairs = equilibrium

In steady straight and level flight, lift balances weight and thrust balances drag. With both pairs balanced the total (resultant) force is zero, so the aeroplane neither speeds up nor slows down: it is in equilibrium, and keeps the same speed and direction until the pilot or a gust changes something. That is Newton's First Law of Motion working in front of you.

Source: Pooleys Vol 4 (10th ed.) p.4.

How big are they?

The pairs balance, but the pairs are not the same size as each other. In the cruise, a training aeroplane's lift (and weight) is roughly ten times its drag (and thrust), a lift/drag (L/D) ratio of about 10:1. The engine only has to match the drag, not carry the whole aeroplane.

STRETCHIf thrust only needs to match drag, and drag is a tenth of the weight, why can a glider stay up with no engine at all?

Source: Pooleys Vol 4 (10th ed.) p.4.

Technical: mass, weight and velocity

Mass is not weight. Mass is the amount of matter the aeroplane contains. Weight is a force: the product of mass and gravitational force ('g'), acting through the centre of gravity towards the centre of the Earth. Somewhere gravity differs, the mass is unchanged while the weight changes.

Velocity is a vector. In equilibrium the aeroplane continues at a particular velocity: the same speed and the same direction. Changing either one requires a resultant force. That is why even a constant-speed turn needs a force acting on the aeroplane.

The 10:1 consequence. If lift is about ten times drag in the cruise, the engine only has to supply roughly a tenth of the aeroplane's weight as thrust. The wings carry the aeroplane; the engine pays the drag bill.

Source: Pooleys Vol 4 (10th ed.) p.3–4 and p.7. Lift gets the full treatment in module POF-2.5 (the lift equation).

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Label the forces

Tap a force name, then tap the arrow it belongs to.

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