Week 1: The Aeroplane and the Four Forces

British Young Pilots · Wings · Week 1

Content:

👋 Welcome

Welcome to Wings

Today we meet the aeroplane: what it is made of, what its controls are called, and the four invisible forces that fight over it every second it flies.

Part 1 of 3 · Module FT-1.1 · Flying Training

Aeroplane Structure

Full module (its own cards, game and quiz): FT-1.1 Aeroplane Structure

One body, everything attached

The basic training aeroplane is a fuselage (the body) carrying the wings, the tail, the wheels and an engine. The propeller generates thrust to pull it through the air, and the airflow over the wings generates lift. Take the thrust away and it can still fly: a gliding descent needs no engine at all.

The tail section (empennage) sits well behind the fuselage and provides a balancing, stabilising force. It consists of the vertical stabiliser (fin) and the horizontal stabiliser (tailplane).

FOUNDATIONSThrow a dart backwards and it flips round: the flights drag the tail behind. An aeroplane's tail steadies it the same way.

The cockpit usually seats two abreast, and the pilot-in-command sits on the left.

Source: Pooleys Vol 1 (15th ed.) p.1.

The controls that fly it

The main controls are the flight controls and the throttle (right hand: forward opens for more power, back closes for less). The flight controls move hinged surfaces that change the airflow around the wings and tail:

  • Elevator, on the tailplane's trailing edge: pitches the nose up and down (column fore and aft)
  • Ailerons, on the outer trailing edge of each wing: roll the aeroplane (column sideways)
  • Rudder, on the fin's trailing edge: yaws the nose left and right (rudder pedals)
  • Flaps (inner trailing edge) and the elevator trim tab

A control column, a control wheel (yoke) or a side-stick all do exactly the same job.

Source: Pooleys Vol 1 (15th ed.) p.2–3.

Wheels, engine, propeller

Most modern trainers have a tricycle undercarriage: two main wheels plus a nosewheel, with the nosewheel steered by the rudder pedals on the ground. Some aircraft carry a tailwheel instead. The brakes act on the main wheels.

The typical trainer has a piston engine on Avgas driving a fixed-pitch propeller; power reads as RPM on the tachometer.

STRETCHWhy might a taildragger be harder to steer on the ground than a tricycle type? Think about where the weight sits relative to the main wheels.

Source: Pooleys Vol 1 (15th ed.) p.4.

Technical: design variations and FADEC

Same principles, different shapes. Wings mount high, low or mid on the fuselage; the tailplane may ride high on the fin (a T-tail); some designs merge tailplane and elevator into one all-flying surface, the stabilator; a few use a butterfly (V) tail.

FADEC. Some modern trainers use Full Authority Digital Engine Control: an Engine Control Unit adjusts manifold pressure, injected fuel and propeller speed to deliver the selected power. Usually paired with a variable-pitch propeller, two independent ECUs for redundancy, and often Jet A1 fuel rather than Avgas.

Source: Pooleys Vol 1 (15th ed.) p.3–4.

Part 2 of 3 · Module POF-1.1 · Principles of Flight

The Four Forces and Equilibrium

Full module (its own cards, game and quiz): POF-1.1 The Four Forces and Equilibrium

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).

Part 3 of 3 · Module POF-1.2 · Principles of Flight

Weight and the Centre of Gravity

Full module (its own cards, game and quiz): POF-1.2 Weight and the Centre of Gravity

Weight acts through one point

Gravity pulls everything vertically towards the centre of the Earth. The aeroplane's mass times that gravitational force is its weight, and in flying we always mean the all-up weight: the whole loaded aeroplane, people, fuel and bags included.

All of it can be treated as one force acting through one point: the centre of gravity (CG), the aeroplane's point of balance. Hang the aeroplane from a rope at the CG and it would sit level.

CG WEIGHT

The quartered circle is the centroid symbol, which is how the CG is marked on diagrams. Source: Pooleys Vol 4 (10th ed.) p.7.

Two limits: how much, and where

How much: maximum weights such as the Maximum Take-Off Mass (MTOM) are set for every aircraft, because of the structural strength of its components and what the design is required to do.

Where: the CG's position affects the aeroplane's stability and performance, so it must stay within the manufacturer's limits at all stages of flight. You can be under MTOM and still dangerously out of balance; they are separate checks.

Source: Pooleys Vol 4 (10th ed.) p.7.

The CG moves, and the weight shrinks

The CG sits wherever the load puts it, so it moves when the load moves: passengers shifting seats, fuel transferring between tanks. Meanwhile the all-up weight normally decreases through the flight as fuel burns off. That's why weight and balance are checked before flight; exceeding any limit at any point compromises safety.

FOUNDATIONSBalance a ruler on your finger, then slide a coin along it, and your finger has to chase the balance point. Loading an aeroplane is the same game.

STRETCHWhere would you put the fuel tanks so the CG barely moves as fuel burns away?

Source: Pooleys Vol 4 (10th ed.) p.7–8.

Wing loading

How hard are the wings working? In straight and level flight the wings support the whole weight, and wing loading spreads that over the wing area:

Wing loading = weight of the aeroplane ÷ wing area

Example (our own figures): 900 kg on 12 m² of wing → 900 ÷ 12 = 75 kg/m².

FOUNDATIONSImagine the aeroplane's weight shared out evenly across every square of the wings, like students spreading out evenly across a trampoline.

Source: Pooleys Vol 4 (10th ed.) p.8 (formula; example figures are ours).

🏁 Wrap up

Next week

Reading the instruments, what each control really does, and your first landing on the simulator.

Tap a card to flip it.

Games in this lesson

One game per module. Pick one; the chips remember which is open.