Manfolk.net - Lesson 2.1 – The Four Forces of Flight

Lesson 2.1 – The Four Forces of Flight

Every airplane in flight is acted upon by four forces. Understanding these forces is your bridge to controlling these forces.

Relationship of Forces Acting on an Aircraft

(figure 1)

Force Vectors During a Stabilized Climb (figure 2)

The Four Forces of Flight

Lift The upward force created by the wings. The shape of the wing (airfoil) and the angle at which it meets the air allow it to generate lift. Lift opposes weight.
Weight The force of gravity pulling the airplane toward the Earth. Weight always acts straight down through the airplane’s center of gravity.
Thrust The forward force produced by the propeller. Thrust opposes drag.
Drag The rearward force that resists the airplane’s movement through the air. Drag opposes thrust.

Straight-and-Level Unaccelerated Flight

In steady, straight-and-level flight (constant altitude and airspeed):

Lift = Weight
Thrust = Drag

The four forces are in balance. The airplane is not speeding up, slowing down, climbing, or descending.

Primary FAA Reference:

FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 5 – Aerodynamics of Flight https://www.faa.gov/sites/faa.gov/files/07_phak_ch5_0.pdf

Force Vectors During a Turn (figure 3)

Forces in a Banked Turn

When you transition the aircraft from straight-and-level flight into a coordinated banked turn, the relationship between the forces shifts dynamically:

The Lift Vector Splits: Total lift no longer points straight up; it tilts toward the inside of the turn. This divides your lift into two separate forces:
1. Vertical Component of Lift: Continues to act straight up to oppose gravity and weight.
2. Horizontal Component of Lift (Centripetal Force): The actual force that pulls the airplane around the radius of the turn, overcoming centrifugal inertia.
The Altitude Loss Danger: Because total lift is split, the vertical vector shortens and is no longer strong enough to counteract the aircraft's weight.
The Correction: To maintain altitude in a bank, the pilot must increase back-elevator pressure. This increases the Angle of Attack (AoA), boosting total lift so that the vertical component once again equals the aircraft's weight.

Check-ride Alert: Maintaining altitude in a turn requires an increase in total lift, which directly increases the load factor (G-load) on the aircraft. At a 60-degree bank, the required lift doubles (\(2.0\ Gs\)), meaning the wings must support twice the static weight of the aircraft.

Always remember: a steeper bank angle requires a higher angle of attack to maintain vertical equilibrium, which significantly increases your stall speed!

Climbing Flight

When the airplane is in a steady climb, the forces are no longer simply vertical and horizontal.

Important points:

The nose is pointed higher than the actual climb path.
Pitch Attitude is the angle between the airplane’s longitudinal axis and the horizon.
Climb Angle (Flight Path Angle) is the angle between the airplane’s actual path through the air and the horizon.
Angle of Attack is the angle between the wing’s chord line and the relative wind.

Typical Cessna 172 normal climb (Vy):

Pitch Attitude ≈ 8–10° nose-up
Climb Angle ≈ 5–7°
Angle of Attack ≈ 3–5°

Key relationship (no wind):

Pitch Attitude ≈ Climb Angle + Angle of Attack

To maintain your airspeed, you must also add power as you enter your climb.

Wind and Rate of Climb

A steady wind does not affect the airplane’s rate of climb. Rate of climb depends on the airplane’s performance relative to the airmass (excess power, weight, and density altitude). A steady wind only changes groundspeed and the airplane’s path over the ground. The Vertical Speed Indicator will show the same rate of climb whether you have a headwind, tailwind, or no wind.

Why This Matters

The wings generate lift primarily through a combination of angle of attack and airspeed.
Raising the nose increases the angle of attack. If the angle of attack becomes too high, the wing will stall.
Understanding the difference between where the nose is pointed and where the airplane is actually going prevents many common student errors.

Important Note

In a climb:

In a steady, constant-airspeed climb, Lift is actually slightly less than Weight. Because the flight path tilts upward, the Thrust Vector also tilts upward, acting as a partial vertical force that helps support a portion of the aircraft's weight. The wings actually do less work to support the aircraft than they do in level flight. This is a question you will likely be asked. Perhaps make a "Checkride Alert" in your notebook.

Key Definitions to Master:

**Pitch Attitude:

The angle between the airplane’s longitudinal axis and the horizon.

**Climb Angle (Flight Path Angle):

The angle between the airplane’s actual path through the air and the horizon.

**Angle of Attack (AoA):

The angle between the wing’s chord line and the relative wind. **Important Note:**

The nose is always pointed higher than the actual climb path.