Module 8.3 — Theory of flight: forces, glide, turns, load factor and lift augmentation

Scope

Sub-module 8.3 covers the relationship between lift, weight, thrust and drag; glide ratio; steady-state flight and performance; the theory of the turn; load factor (stall, flight envelope, structural limits) and lift augmentation. Level 1 for A and B3, level 2 for B1, B2 and B2L.

The four forces

Flight condition	Force balance
Straight and level, constant speed	Lift = Weight; Thrust = Drag
Accelerating in level flight	Thrust > Drag
Steady climb	Thrust = Drag + W sin γ (γ = climb angle); Lift = W cos γ, so lift is slightly less than weight
Steady descent / glide	A component of weight along the flight path replaces thrust
Steady level turn	Lift > Weight (vertical component of lift = W)

Couples and the tailplane

Lift (through the centre of pressure) and weight (through the CG) are rarely in line; nor are thrust and drag. Designers usually arrange the lift/weight couple to be nose-down and the thrust/drag couple to be nose-up, so that loss of thrust gives a nose-down pitch (towards a safe gliding attitude). Any remaining imbalance is trimmed by a (normally downward) tailplane load.

Glide

In a steady glide with no thrust, the forces are lift, drag and weight. The glide angle depends only on the lift/drag ratio:

• Glide ratio = horizontal distance ÷ height lost = L ÷ D.
• Best glide range is achieved at the angle of attack (and the associated speed) for L/D max.
• Weight does not change the best glide angle; a heavier aircraft must fly faster to keep the same angle of attack, so it covers the same distance but arrives sooner.
• A headwind reduces glide range over the ground; a tailwind increases it.

Worked example — glide

An aircraft with L/D = 12 glides from 3 000 ft (≈ 914 m). Still-air range = 12 × 914 m ≈ 11 km (≈ 5.9 NM).