Module MOD-03 · 9 min · ACS PA.VII · ACS PA.I.G

The Four Forces, Lift and Drag

← Aerodynamics and Principles of Flightdraft — pending CFI review

Why this matters in flight: Every maneuver you fly is a deliberate change to the balance of lift, weight, thrust, and drag. Understanding how lift is created and how drag changes with speed explains why the airplane climbs, descends, and performs the way it does.
The four forces of flightA side view of an airplane. Lift acts upward and weight acts downward through the center of gravity; thrust acts forward and drag acts rearward. In steady, unaccelerated flight the opposing forces are in balance.CGLIFTWEIGHTTHRUSTDRAG
DRAFT schematic — pending CFI review. Not to scale; not an FAA-approved figure.

Four forces act on an airplane: lift opposes weight and thrust opposes drag. In steady straight-and- level flight they are balanced, so the airplane holds its altitude and airspeed; change one and the airplane climbs, descends, speeds up, or slows down until balance is restored. Lift comes from the wing turning the air, and it grows with angle of attack — the angle between the wing’s chord line and the relative wind — as well as with airspeed, air density, and wing area. Both Bernoulli’s pressure difference and Newton’s downward deflection of air contribute. Drag comes in two flavors that trade off with speed: induced drag is a by-product of lift and is greatest at low speed and high angle of attack, while parasite drag comes from pushing the airframe through the air and grows with the square of airspeed. Total drag, and thus best efficiency, is lowest where the two are equal.

Key terms

Angle of attack
The angle between the wing chord line and the relative wind.
Induced drag
Drag produced as a by-product of lift; greatest at low airspeed.
Parasite drag
Drag from moving the airframe through the air; grows with airspeed squared.

Summary

Four balanced forces govern steady flight; lift rises with angle of attack, airspeed, density, and area; and induced drag (low speed) trades off against parasite drag (high speed), with minimum total drag where they meet.

Quick check ▾

One question on what you just read.

Question 1 of 1

Objective mastery: 15%

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In steady, straight-and-level, unaccelerated flight, which relationship between the forces is true?

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Sources

Every claim traces to a source — paraphrased knowledge elements pointing at the governing FAA publication; not yet verified against a retrieved source.

  • PHAK Ch. 5 — Pilot’s Handbook of Aeronautical Knowledge unverified
  • PHAK Ch. 5 — Pilot’s Handbook of Aeronautical Knowledge unverified
  • PHAK Ch. 5 — Pilot’s Handbook of Aeronautical Knowledge unverified

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