Aircraft Forces

Simulation

Steady climb and descent

Flight-path angle +8°

Model assumptions

Straight, constant-speed flight
Thrust aligned with the flight path
C_D · Coefficient of drag

Force balance

Steady, constant-speed flight

Perpendicular to the flight path

Lift balances the normal weight component

ΣF⊥ = L − W cos γ = 0 L = W cos γ

L = 1.00 W × cos(8°) = 0.99 W

Along the flight path

Excess thrust balances the aft weight component

ΣF∥ = T − D − W sin γ = 0 T = D + W sin γ

T = 0.25 W + 1.00 W × sin(8°) = 0.39 W

Reference

What changes with the angle?

Level flight · γ = 0°L = W and T = D.

Climb · γ > 0°Lift becomes slightly smaller than weight, while required thrust increases.

Descent · γ < 0°The forward weight component reduces the thrust required for constant speed.

The diagram assumes equilibrium with no acceleration and ignores thrust-line inclination, turns, wind and changes in drag with lift or airspeed. For illustration, D = 0.25 W.

Aircraft silhouette: Majo statt Senf, adapted under CC BY-SA 4.0.