The four forces

Four forces act on an aircraft in flight: lift (up, perpendicular to the relative airflow), weight (down), thrust (forward) and drag (rearward).

In steady straight-and-level unaccelerated flight the aircraft is in balance:

• Lift = weight (the vertical pair).
• Thrust = drag (the horizontal pair).

Any inequality is an acceleration: more lift than weight climbs, more thrust than drag speeds up.

Climb and glide tilt the picture. The forces line up with the flight path, not the ground:

• Climb: required thrust is greater than drag, because a component of weight now acts rearward along the path. Rate of climb depends on excess thrust/power, not on extra lift (lift is actually a little less than weight in a climb).
• Glide (no thrust): a forward component of weight acts along the descending path and balances drag.

Lift equation: L = CL · ½ρV² · S (CL = lift coefficient, set by angle of attack; ρ = air density; V = airspeed; S = wing area). Lift rises with the square of airspeed (double the speed, four times the lift) and with angle of attack through CL, up to the stalling angle.

Note: the lift equation (the V² law) and the climb/glide force-vector split are basic aerodynamics; TP 1102 gives the four forces, their directions and the level-flight balance, but not these formulas.

Trap: thinking a climb comes from extra lift. It comes from extra thrust/power; lift still roughly equals weight.

Practice