Forces in a turn
Banked lift splits in two, at right angles (PHAK):
| Component | Direction | Role |
|---|---|---|
| Vertical | Up | Still opposes weight |
| Horizontal | Toward centre of turn | Centripetal force: actually turns the aircraft |
Turning:
- Not steered like a boat or car.
- Unbanked: no sideways force, so straight path.
- Banked: turns, unless slipping to the inside of the turn.
- Rudder does not turn the aircraft. Only aligns nose and tail with relative wind, correcting yaw from rolling in. Turning force is entirely the horizontal lift component.
- Centrifugal force: equal-and-opposite reaction to the turn, acting outward against the horizontal lift component. Not a true force; a body tending to continue in a straight line (TP 1102).
Holding altitude in a bank:
- Part of lift points sideways, so vertical component drops below unbanked value.
- Needs back pressure (more AOA); more AOA means more induced drag, so more power.
- Both scale with bank angle.
Slip vs skid, and the ball:
| Turn | Cause | Force balance | Aircraft moves | Ball rolls to | Fix |
|---|---|---|---|---|---|
| Slipping | Bank too steep for rate of turn | Horizontal lift component exceeds centrifugal force | Toward inside | Inside of turn | Decrease bank, increase rate, or both |
| Skidding | Rate of turn too great for bank | Centrifugal force exceeds horizontal lift component | Toward outside | Outside of turn | Decrease rate, increase bank, or both |
- Ball (TP 1102, lines 2328 and 5381-5389; IFH ch.5 for too-steep/too-shallow wording): too steep for rate of turn rolls it inside; too shallow for rate of turn rolls it outside.
Trap: crediting rudder with turning the aircraft, or forgetting that holding altitude in a bank needs back pressure and power, not bank angle alone.
References
- PHAK ch.5 Aerodynamics of Flight
- TP 1102 (Flight Training Manual), Turn-and-Bank Indicator, ball side (lines 2328, 5381-5389)
- FAA-H-8083-15 IFH ch.5, Turn Indicators (ball in a too-steep or too-shallow bank)