Practice test
Unit 3 · Aerodynamics · Lesson 1 of 7

The four forces

7 min readLesson 10 of 59

By the end of this lesson you'll be able to:
  • Name the four forces acting on an airplane in flight
  • Explain how the forces balance in straight-and-level, unaccelerated flight
  • Describe what changes in a climb and a descent

Four forces act on an airplane in flight: lift, weight, thrust and drag.

  • Lift acts perpendicular to the relative wind, mostly upward. The wings make most of it.
  • Weight pulls straight down toward the center of the earth, acting through the center of gravity (CG).
  • Thrust pulls the airplane forward. The propeller makes it.
  • Drag resists the airplane's motion through the air, acting rearward, parallel to the relative wind.

Balance in steady flight

In straight-and-level, unaccelerated flight (constant speed, constant altitude), the forces are in equilibrium: lift equals weight and thrust equals drag. The airplane isn't "pushing harder" than it needs to; it's simply in balance.

The important word is unaccelerated. When the forces are out of balance, the airplane accelerates: it speeds up, slows down, climbs, descends or turns until a new balance is reached.

Climbs and descents

In a steady climb, part of the weight acts backward along the flight path, so the airplane needs extra thrust to hold its airspeed. That's why you add full power to climb in most trainers. In a steady descent, part of the weight acts forward, which is why the airplane speeds up if you lower the nose without reducing power.

Pitch and power. A useful way to think about it: in most phases of flight, pitch controls airspeed and power controls altitude (climb or descent rate). They work together, so changing one usually means adjusting the other.

Key takeaways

  • The four forces are lift, weight, thrust, drag.
  • In straight-and-level, unaccelerated flight: lift = weight and thrust = drag.
  • Unbalanced forces mean the airplane is accelerating (speed, direction or altitude changing).

What the examiner may ask

Practice answering out loud before you open each answer.

What are the four forces and how are they balanced in cruise?
Lift, weight, thrust and drag. In straight-and-level unaccelerated flight, lift equals weight and thrust equals drag.
What controls airspeed and what controls altitude?
In general, pitch controls airspeed and power controls altitude, and you coordinate both.
What happens if you add power in level flight without changing pitch?
The airplane accelerates and tends to climb until a new balance is reached.

Common mistakes

  • Thinking lift is greater than weight in a steady climb (in a steady climb, lift is actually slightly less than weight).
  • Chasing altitude with pitch alone during approaches.
  • Forgetting that unbalanced forces mean acceleration.

Read it in the FAA sources

PHAK · Chapter 4Principles of FlightOpens the PHAK at this chapter ↗PHAK · Chapter 5Aerodynamics of FlightOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

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