The four forces of flight

Four forces act on the airplane

Definition

In flight, four forces act on the airplane: lift, weight, thrust, and drag. Lift acts perpendicular to the relative wind; weight acts straight down at the center of gravity; thrust pulls the airplane forward; drag resists its motion through the air.

Explanation

Lift opposes weight, and thrust opposes drag. Every phase of flight is these two pairs shifting balance.

Figure 3-1. The four forces. — Figure 3-1 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 74)

Why it matters

The four forces are the foundation for everything else in this module — stalls, climbs, glides, and load factor are all just the balance of these forces changing.

From the Right Seat

I have students point to each force on the airplane and name what it opposes before we ever talk about maneuvers. Get the four arrows solid first.

A DPE might ask

Name the four forces and what each one opposes.
Which direction does lift act — relative to the wing or the horizon?

PHAK ch. 4 — Principles of flight

In steady flight they balance

Definition

In steady, unaccelerated flight the forces are in equilibrium: lift equals weight and thrust equals drag. The opposing pairs cancel, so the airplane holds a constant speed and altitude.

Explanation

Nothing is 'winning.' Change one force — add power, raise the nose — and the balance is upset until the airplane settles at a new speed or altitude.

Why it matters

Understanding equilibrium is what lets you predict what the airplane will do when you move a control or the throttle — it's chasing a new balance.

Common error

Students often think lift must be greater than weight to stay level or to climb. In steady level flight lift equals weight — and you climb by adding power (excess thrust), not by making more lift than weight.

In steady, level, unaccelerated flight, how does lift compare to weight?

CFI Tip

Purists' note for the checkride: in a steady climb, lift is slightly LESS than weight — thrust supports part of the weight and weight tilts aft along the flight path. You climb because of excess power, not extra lift.

A DPE might ask

In level unaccelerated flight, how do lift and weight compare?
What actually makes the airplane climb?

PHAK ch. 4 — Forces in equilibrium