🪂 APPI Aerodynamics — Summary for Training 

1. Reaction Force & Lift Creation

• Lift originates from Newton’s 3rd law: air particles are deflected downward, producing an opposite upward reaction force. 

• A flat plate creates large vortices (high drag), while an airfoil produces smooth airflow with higher lift and lower resistance. Both upper and lower surfaces contribute to lift.

2. Airflow Behavior

Laminar vs Turbulent Flow

• Laminar flow = smooth, parallel streamlines.

• Turbulent flow = chaotic, swirling motion.

• When flow is steady (unchanging), particle paths are streamlines. 

Venturi Effect

• When a stream tube narrows, speed increases and pressure decreases.

• Example: wind accelerates where a valley narrows. 

Static vs Dynamic Pressure

• Static pressure: pressure of still air.

• Dynamic pressure: added pressure caused by motion.

• Faster airflow → higher dynamic pressure, lower static pressure. 

Bernoulli Principle

• Static + Dynamic = Constant Total Pressure along a streamline.

• Increasing airspeed over a surface reduces pressure.

• Demonstration: blowing between two papers pulls them together.

Stagnation Point

• Point where airflow stops before splitting.

• On paragliders, the leading edge intake sits at the stagnation point → internal pressure rises with speed. 

3. Lift, Drag & Profile Behavior

Attached Flow vs Turbulence

• At low airspeed, a wing cannot deflect air → no lift.

• At appropriate speed, airflow separates at the trailing edge producing an attached vortex and lift.

Types of Drag

• Parasitic Drag: form drag + skin friction. Increases with speed.

• Induced Drag: caused by wingtip vortices; higher at low speed & high angle of attack. Decreases with speed. 

Total Drag

• Minimum total drag occurs at a specific speed → best glide performance.

4. Pressure Distribution & Angle of Attack

• Upper surface airflow is faster → low pressure, producing ~⅔ of total lift.

• Lower surface pressure adds remaining ⅓.

• Angle of attack (AoA) = chord line vs airflow direction.

• Even at 0° AoA, asymmetric airfoils generate lift.

• Critical AoA (~15–20°) → airflow detaches → stall. 

Manufacturers set wing tips to stall first for safer behavior. 

5. 3D Wing Aerodynamics

• Real wings have finite span → air leaks from bottom to top at tips, reducing lift and creating tip vortices.

• This increases induced drag, especially on low-aspect-ratio wings (common in paragliding).

• High-aspect-ratio wings = less induced drag but more demanding.

6. Lift & Drag Formulas

Lift and drag depend on:

• Air density (ρ)

• Velocity (v²)

• Surface area (S)

• Coefficients (Cz, Cx) linked to wing shape

Lift and drag grow with the square of speed.
At high altitude (lower density), more airspeed is needed to create the same lift. 

7. Glide Performance

Glide Ratio

Defined as:
Horizontal speed / Vertical speed = Lift / Drag
A ratio 6 wing travels 6 km horizontally for 1 km of descent in still air. 

Example glide angles (approx.):

• Ratio 6 → ~9.5° glide angle.

Braking increases AoA → increases drag more than lift → reduces glide ratio and slows flight. Excess can cause stall.

Polar Curve

Shows:

• Trim speed

• Best glide

• Minimum sink

• Stall point
  Used to analyze wing performance. 

8. Turning Aerodynamics

How a Turn Begins

• Pulling one brake → more lift & drag on that side → glider rolls and turns.

• Turning creates a horizontal component of aerodynamic forces.

• Wing speeds up automatically to maintain lift in the turn. 

Centrifugal Force & Load Factor

• In turns, pilot feels increased “G‑force.”

• Apparent weight = real weight + centrifugal force.

• Higher bank angle → higher load factor. 

9. Stability

A glider’s reaction to disturbances:

• Stable → returns to normal (desired).

• Indifferent → stays in disturbed state (e.g., twist).

• Unstable → divergence increases (dangerous).

Paragliders gain stability from:

• Low center of gravity

• Proper wing loading

• Airfoil design

Caution near the ground:
Pendulum movements after abrupt maneuvers increase sink and collapse risk.