Aerofoil Aerodynamics

A basic understanding of the physics of flight will give you clarity and confidence, allowing you to progress faster. You will also save yourself from experimenting with suboptimal or dangerous practices.

The fundamental principle that causes an aerofoil to produce lift is a pressure differential between the upper and lower surfaces. The convex form of the top surface forces air moving over aerofoil to accelerate. According to the Bernoulli's principle, this acceleration results in a reduction in pressure, which causes lift. This holds true while the air flow over the top and lower surfaces are lamina.

The reason that the wing is tapered at the rear is such that the two separate lamina flows can combine without turbulence, as this turbulence would reduce efficiency, or even cause the wing to stall.

The reason that the front of the wing is fatter than the centre cross-section is that the asymmetry improves the efficiency of the convergence of air flow at the rear of the wing, ensuring lamina flow at high air speed.

What is an Aerofoil?

An aerofoil is the cross-sectional shape of a wing or propeller blade. It is designed with a curved upper surface and a flatter lower surface to produce an aerodynamic lifting force as it moves through the air.

How Aerofoils Generate Lift

Aerofoils generate lift through a combination of Bernoulli's principle and Newton's laws:

1. As air flows over the curved upper surface of the aerofoil, it speeds up and the pressure decreases, creating an area of low pressure above the wing.
2. Simultaneously, the air flowing under the flatter bottom surface is slower, creating an area of higher pressure below the wing.
3. This pressure difference results in an upward force on the wing, which is lift. The lift force acts perpendicular to the oncoming airflow.
4. According to Newton's third law, as the aerofoil pushes the air down, the air pushes the wing up with an equal and opposite reaction force, contributing to lift.
5. The airflow over the top of the wing also has to travel a greater distance, which requires it to move faster to keep up with the air underneath, further reducing the pressure on top via Bernoulli's principle.

Angle of Attack

The angle of attack is the angle between the oncoming air and the chord line of the aerofoil. A higher angle of attack deflects the air downward more sharply, increasing lift up to a point. If the angle is too high, the airflow separates from the top of the wing and stall occurs, dramatically reducing lift.

That is to say that if the wing is pitched toward the air flow, the angle of attack is reduced. Conversely, if the wing is pitched away from the air flow, the angle of attack is increased. Exceeding the critical angle of attack, both positive or negative, will result in a stall. With a paraglider or speedwing, these conditions induce either a parachutal stall, or a frontal collapse.

Aerofoil Shape

An asymmetric aerofoil shape helps generate lift even at small angles of attack:

• The rounded leading edge is insensitive to angle of attack changes
• The elongated shape moves the maximum thickness point backward to prevent flow separation
• Camber (curvature) increases lift and improves stall characteristics

Thin Aerofoil Theory

Thin aerofoil theory relates the lift generated to the aerofoil shape and angle of attack, using simplifying assumptions of inviscid, incompressible flow. It models the aerofoil as a vortex sheet and uses the Kutta–Joukowski theorem to calculate lift.

In summary, aerofoils generate lift through a combination of pressure differences arising from their shape, and the deflection of air resulting in a reaction force. Their curved upper surface and angle of attack accelerate airflow to create low pressure, while the flatter underside maintains higher pressure. This pressure difference, along with the downward deflection of the airstream, produces the lift force that allows aeroplanes to fly. The specific aerofoil shape is carefully designed to optimize lift for different flight conditions.

Reflex

Reflex airfoils are a special type of airfoil design characterized by an upward curve or "reflex" near the trailing edge. This upward curve helps to stabilize the airfoil and provides several unique aerodynamic properties.

Characteristics of Reflex Airfoils

• Moment Coefficient (Cm): The upward curve of the trailing edge allows the moment coefficient (Cm) to be adjusted to nearly any desired value. Starting from a symmetrical airfoil, bending the trailing edge smoothly upwards by 5-10° can significantly change the Cm.
• Stability: The reflex shape helps to automatically increase the angle of attack if it decreases, keeping the airfoil in a neutral, stable position relative to the load. This provides substantially increased stability at high speeds compared to classic non-reflex airfoils.
• Lift and Drag: The longitudinal stability provided by the reflex comes at the cost of a lower lift coefficient. Careful design of the wing shape and airfoil thickness is needed to maintain a good glide ratio. Reflex airfoils also tend to have higher drag than non-reflex airfoils.

In summary, reflex airfoils provide an important tool for aircraft designers to tune the stability and moment characteristics, especially for tailless and powered aircraft, at the expense of some lift and drag performance. Careful design is needed to find the right balance of characteristics for each application.

Stall

What is an Aerofoil Stall?

An aerofoil stall occurs when the angle of attack of an aerofoil exceeds its critical angle, causing a sudden decrease in lift. The airflow over the top surface of the wing separates from the surface, becoming turbulent and unsteady. This results in a dramatic loss of lift, increase in drag, and change in pitching moment.

Critical Angle of Attack

The angle of attack is the angle between the oncoming air (relative wind) and the chord line of the aerofoil. As this angle increases, the lift produced by the aerofoil also increases, up to a certain point called the critical angle of attack. This is typically around 15-20 degrees for most aerofoils, but can vary based on the aerofoil shape.

Boundary Layer Separation

At angles of attack below the critical angle, the airflow remains smoothly attached to the aerofoil surface. However, as the critical angle is exceeded, the boundary layer separates from the top surface of the wing, starting near the trailing edge. This separated flow region grows larger as angle of attack increases further.

Stall Progression

The exact way a stall progresses depends on the aerofoil shape:

• Thick aerofoils (>14% thickness) tend to stall gradually from the trailing edge. Lift decreases smoothly and stall is gentle.
• Moderately thick aerofoils (6-14%) often have an abrupt leading edge stall with a sharp loss of lift.
• Thin aerofoils (<6%) stall from the leading edge via a growing separated flow bubble. Lift loss is smooth but pitching moment changes significantly.

A speedwing will exhibit the stall of a thick aerofoil, however, due to the high wing-load, stalls are abrupt and violent.

A paraglider will stall gradually and comparatively slowly, whereas a speedwing will offer little warning. On a speedwing, the time between stall-point and full-stall is very short, especially at high wing-loads. The first sign is that the glider begins to vibrate, and wing-tips nudge backwards as the glider enters the stall.

Consequences of a Stall

When an aerofoil stalls, several things happen:

• Lift decreases substantially, resulting in loss of height.
• Drag increases significantly, due to the turbulent separated flow.
• The wing may roll to one side if the stall is asymmetric. This must be countered immediately with weight-shift, to maintain a safe heading.
• The effectiveness of brake input is greatly reduced due to the separated flow, increasing the importance of controlled weight-shift.

Stall Recovery

To recover from a stall, the angle of attack must be reduced below the critical angle. The recovery procedure is:

1. Reduce angle of attack by releasing any break or riser input.
2. Maintain heading with centred and neutral weight-shift. In the case of an asymmetric stall, weight-shift away from the stalled side is crucial.
3. In the case of an asymmetric stall, a spin is often induced. This must be handled by following the glider with your shoulders to avoid a twist.
4. Wait for the glider to produce lift by allowing it to take speed.

In summary, an aerofoil stall is a serious in-flight condition caused by exceeding the critical angle of attack. It results in a large loss of lift, increase in drag, and potential loss of control. Pilots must act promptly to reduce the angle of attack and restore smooth airflow over the wings in order to recover from a stall. Understanding the aerodynamics of stalls is crucial for safe flying.

In addition, SIV training with a paraglider is a safe way to gain experience with stalls. I highly recommend this for any pilot serious about maximising their safety while flying any type of paraglider, mini-wing, or speedwing. This process will train you with proper reflexes, giving you the ability to recover from such scenarios. Positive reflexes are particularly important while flying speedwings, as these in-flight conditions are more violent at high wing-loads, and you will typically have less distance from terrain while speedflying. One might say that SIV training is more important than carrying a reserve while speedflying, as a preventative measure, rather than a reactive measure.

Of course, I still recommend that you always fly with a helmet, reserve, back protector, and airbag.