Drag & lift/drag ratio
Drag: air's resistance to aircraft motion. Caused by friction (air sticking to surface) and viscosity (air sticking to itself) (PHAK). Two parts: parasite drag and induced drag.
| Parasite drag | Induced drag | |
|---|---|---|
| Cause | Aircraft pushing through air | Price of making lift |
| Speed trend | Rises with square of speed | Inversely with square of airspeed: falls as speed rises (PHAK) |
| Dominates (parasite) / largest (induced) at | High speed | Low speed / high angle of attack |
Parasite drag parts:
| Part | What it is | Cut by / raised by |
|---|---|---|
| Form drag | Shape resisting air (cowlings, antennas, any unstreamlined component) | Cut by streamlining (TP 1102) |
| Skin friction drag | Air clinging to surface | Raised by ice, dirt, insects, rough finish. Cut by flush rivets, clean waxed glossy finish (TP 1102, PHAK) |
| Interference drag | Eddies and turbulence where two airstreams meet; worst where surfaces meet at a sharp angle (e.g. wing root) | Cut by fairings, spacing components apart (PHAK) |
- Profile drag = form drag + skin friction (TP 1102, PHAK).
- Skin friction layers: air touching aircraft nearly motionless; each layer above moves a little faster until free-stream speed.
Induced drag sequence (TP 1102, PHAK):
- Higher pressure under wing spills around tip into lower pressure above.
- Spill rolls up into wingtip vortices.
- Vortices leave downwash behind.
- Downwash tips lift vector backward as angle of attack rises. Backward tilt = induced drag.
Total drag and L/D: parasite + induced drag gives a U-shaped total-drag curve.
Bottom of the U = minimum drag = best lift/drag ratio (L/Dmax), CL / CD:
- Higher L/D = more efficient airfoil.
- Low angle of attack: CD low, barely moves.
- Near the top: small angle-of-attack changes swing drag a lot (PHAK).
Speeds:
- L/Dmax speed close to Vy (best rate-of-climb speed), not identical (standard aerodynamics).
- Minimum-drag speed (L/Dmax) and minimum-power speed differ (TP 1102, PHAK).
Trap: mixing up which dominates (parasite wins at high speed, induced at low), and minimum-power speed is not L/Dmax speed (different points on the power curve).
References
- TP 1102 (Flight Training Manual)
- FAA-H-8083-25 PHAK ch.5 Aerodynamics of Flight