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Key Takeaways

  • •In unaccelerated flight the four forces are in equilibrium: Lift = Weight and Thrust = Drag.
  • •Lift acts perpendicular to the relative wind; weight acts straight down through the CG.
  • •Climb needs excess thrust (Thrust > Drag); in a constant-speed turn Lift > Weight.
  • •Lift increases with airspeed², wing area, air density, and AoA up to the critical AoA.

The Four Forces of Flight

PHAK Ch 5

In unaccelerated flight, the four forces are in equilibrium. How they interact is the foundation of all flight.

Lift

Upward force from airflow over the wings. Acts perpendicular to the relative wind.

  • Generated by pressure differential (lower pressure on top, higher on bottom)
  • Increases with airspeed², wing area, air density, and angle of attack (up to critical AoA)
  • Acts perpendicular to the relative wind, NOT straight up
  • In a turn, lift tilts — vertical component decreases, requiring back pressure or more speed

Weight

Gravity pulling the aircraft toward the center of the Earth. Acts through the center of gravity (CG).

  • Always acts straight down toward the center of the Earth
  • Includes aircraft empty weight + fuel + passengers + cargo
  • Changes during flight as fuel burns off
  • CG position affects stability and controllability

Thrust

Forward force from the engine-driven propeller. Acts parallel to the direction of flight.

  • Produced by the propeller converting engine power to forward force
  • In level cruise: Thrust = Drag
  • Increasing thrust (adding power) accelerates the aircraft or allows climb
  • Propeller efficiency decreases at very high and very low speeds

Drag

Rearward force opposing the aircraft's motion through the air. Acts parallel to and opposite the relative wind.

  • Parasite drag: increases with airspeed² (form, skin friction, interference)
  • Induced drag: byproduct of lift, decreases with airspeed (highest at slow speeds)
  • Total drag = parasite + induced; minimum at L/D max speed
  • L/D max speed = best glide speed (Vg) — refer to your POH for the specific value

Force Relationships in Flight

PHAK Ch 5
Straight & Level (unaccelerated)
Lift = Weight, Thrust = Drag
Climb (constant speed)
Lift = Weight, Thrust > Drag (excess thrust)
Descent (constant speed)
Lift = Weight, Drag > Thrust
Constant-speed turn
Lift > Weight (vertical component = weight), Thrust = Drag
Acceleration (level)
Lift = Weight, Thrust > Drag