Four forces in equilibrium
Four forces in flight:
| Force | Direction | Acts through |
|---|---|---|
| Lift | Up, perpendicular to relative airflow | Centre of pressure (CP) |
| Weight | Down | Centre of gravity (CG) |
| Thrust | Forward | n/a |
| Drag | Rearward | n/a |
- Lift is perpendicular to relative airflow, not necessarily to the ground.
Steady straight-and-level unaccelerated flight (balance):
- Lift = weight (vertical pair).
- Thrust = drag (horizontal pair).
- General: upward components = downward; forward = backward.
- Steady climb at constant airspeed: part of thrust acts upward, so lift is less than weight (PHAK 5-2).
Inequality = acceleration:
- Lift > weight: climbs.
- Thrust > drag: speeds up.
- Power cut below cruise: thrust < drag, decelerates; drag falls as speed drops, settling where thrust = drag.
- Takeoff roll: lift below weight, builds proportionately with forward speed. Airborne when lift reaches weight (TP 1102).
Equilibrium = no acceleration. Newton's First Law: body at rest stays at rest, body in motion stays in straight-line motion, unless a force acts.
| Case | Equilibrium? |
|---|---|
| Parked aircraft | Yes |
| Straight-and-level, constant airspeed | Yes |
| Straight climb or descent, constant airspeed | Yes |
| Turn at constant height and airspeed | No: always accelerating toward turn centre |
Newton's Second Law: F = ma (constant mass, PHAK ch.4). Unbalanced force accelerates aircraft in its direction.
Trap: L = W and T = D hold only in unaccelerated straight-and-level flight, not always.
References
- TP 1102 (Flight Training Manual)