Module 8: Basic Aerodynamics

Module 8: Basic Aerodynamics — B3 Category Study Material

1. Module Overview

Module 8, Basic Aerodynamics, is a fundamental subject for the EASA Part-66 B3 (Helicopter) licence. It provides the essential physical and aerodynamic principles required to understand the behaviour of both aeroplanes and helicopters. For the B3 category, a significant portion of this module focuses on rotorcraft aerodynamics, which differ considerably from fixed-wing principles.

The module is divided into two main areas:

• 8.1 Physics of the Atmosphere: Covers the International Standard Atmosphere (ISA), air density, pressure, temperature, and humidity, and their effect on aerodynamic forces.
• 8.2 Aerodynamics: Covers the theory of flight for both aeroplanes and helicopters, including airflow around bodies, lift generation, drag, forces in various flight conditions, stability, and high-speed aerodynamics.

For the B3 licence, the depth of knowledge required is at Level 3 (detailed theory) for helicopter-specific topics and Level 2 (general knowledge) for general aeroplane aerodynamics. This study material synthesises the core knowledge required to meet these syllabus objectives.

---

2. Key Concepts Explained in Detail

2.1 The Atmosphere and Airspeed

International Standard Atmosphere (ISA)

The performance of an aircraft is directly influenced by the state of the atmosphere it flies through. To provide a common reference for performance calculations, instrument calibration, and engineering design, the International Standard Atmosphere (ISA) is defined. It assumes:

• Sea level pressure: 1013.25 hPa (hectopascals) or 1013.25 mb.
• Sea level temperature: 15 °C (288.15 K).
• Temperature lapse rate: 1.98 °C per 1,000 ft (approximately 2 °C) up to the tropopause at 36,090 ft (11,000 m), where the temperature becomes constant at -56.5 °C.

Air Density and Its Effects

Air density (ρ) is the mass of air per unit volume (kg/m³). It is a critical parameter in aerodynamic force calculations. Density decreases with increasing altitude and temperature, and increases with decreasing temperature and increasing pressure.

The dynamic pressure (q) is the kinetic energy of the airflow and is defined by the formula:

q = ½ ρ V²

Where:

• q = Dynamic pressure (Pa or N/m²)
• ρ = Air density (kg/m³)
• V = True airspeed (m/s)

This relationship is fundamental. The lift and drag forces generated by a wing or rotor blade are directly proportional to dynamic pressure.

Airspeed Definitions

• Indicated Airspeed (IAS): The speed read directly from the airspeed indicator. It is a function of the dynamic pressure (q) acting on the pitot-static system.
• True Airspeed (TAS): The actual physical speed of the aircraft relative to the surrounding air.
• Relationship between IAS and TAS: The airspeed indicator is calibrated to read correctly at sea level in ISA conditions. At altitude, where density is lower, the aircraft must fly faster (higher TAS) to generate the same dynamic pressure (q) and thus the same IAS. Therefore, TAS > IAS at altitude. For a constant IAS, the dynamic pressure (q) remains constant, regardless of altitude.

Exam Focus Point: If a pilot maintains a constant IAS while climbing, the TAS increases. If a pilot maintains a constant TAS while climbing, the dynamic pressure (q) decreases.

---

2.2 Fixed-Wing Aerodynamics (Aeroplane)

Forces in a Steady Climb

In a steady climb, the aeroplane is in equilibrium, meaning the sum of all forces is zero. The forces are resolved into components parallel and perpendicular to the flight path.

• Parallel to the flight path: Thrust (T) must balance Drag (D) plus the component of Weight (W) acting along the flight path (W sin γ). Therefore, T = D + W sin γ. This means Thrust > Drag in a climb.
• Perpendicular to the flight path: Lift (L) must balance the component of Weight acting perpendicular to the flight path (W cos γ). Therefore, L = W cos γ. This means Lift < Weight in a climb.

The Effect of Power Changes

Consider an aeroplane in steady, straight-and-level flight. If the pilot reduces power without adjusting the elevator:

1. Thrust decreases, causing an imbalance of forces along the flight path.
2. The aeroplane begins to decelerate.
3. This deceleration causes the relative airflow to come from slightly below the wing's chord line, which increases the angle of attack (AoA).
4. However, the dynamic pressure (q) decreases as the airspeed decays. The reduction in dynamic pressure has a greater effect on lift than the small increase in AoA, so the total lift decreases.
5. The load factor (n = L/W) decreases below 1, and the aeroplane will enter a descent.

Angle of Attack and Lift

The angle of attack is the angle between the wing chord line and the relative airflow. As AoA increases, lift increases proportionally (up to a point) because the airflow is deflected more downwards. This continues until the critical angle of attack (stalling angle) is reached. Beyond this angle, the airflow separates from the upper surface, causing a rapid loss of lift and an increase in drag (stall).

---

2.3 Helicopter Aerodynamics

Hover and Induced Flow

In a hover, the rotor system accelerates air downwards. This downward velocity of air through the rotor disc is called induced flow (or downwash). This induced flow is a key factor in rotor blade aerodynamics.

The airflow relative to a rotor blade is a combination of:

• The rotational relative airflow (due to the blade's rotation).
• The induced flow (downwards).

The induced flow is added vectorially to the rotational relative airflow. This has the effect of reducing the resultant relative airflow angle, which in turn decreases the effective angle of attack of the blade. To compensate for this loss of effective AoA and maintain a constant thrust, the pilot must increase collective pitch.

Transverse Flow Effect

During a hover, the airflow through the rear portion of the rotor disc is not purely vertical. It has a downward and outward component. This is because the induced flow velocity is not uniform across the disc. The rear of the disc experiences a different inflow angle than the front. This results in a reduced lift on the rear portion of the disc, creating a roll moment (typically to the right for a main rotor rotating anti-clockwise when viewed from above). To counteract this drift, the pilot applies left cyclic to tilt the tip-path plane to the left.

Ground Effect

When a helicopter hovers close to the ground (within approximately one rotor diameter), the ground restricts the downward flow of air. This restriction reduces the induced flow velocity through the rotor disc. The benefits are:

• Reduced induced drag on the rotor blades.
• Improved rotor efficiency.
• The helicopter can hover at a lower power setting than it could out of ground effect (OGE).

When the pilot raises the collective in ground effect (IGE), the increased blade pitch (AoA) combined with the already reduced induced flow results in a higher effective angle of attack, generating more lift for a given power.

Rotor Blade Twist

The rotational speed of a rotor blade increases from the root to the tip. Therefore, the rotational relative airflow is much faster at the tip than at the root. Without any design changes, the tip would generate a disproportionately large amount of lift and stall first, while the root would be inefficient.

To solve this, rotor blades are twisted along their span. The blade is designed with a negative twist (washout), where the geometric angle of attack is higher at the root and lower at the tip. This equalises the effective angle of attack along the blade span, producing a more uniform lift distribution and delaying the onset of tip stall.

Blade Tip Design

The tip of a rotor blade is a region of high aerodynamic activity. The pressure difference between the upper and lower surfaces causes air to flow around the tip, creating a tip vortex. This vortex is a source of induced drag and noise. Modern blade tips are often swept or upward-curled to reduce the strength of these tip vortices, which decreases induced drag and improves overall rotor performance.

Blade Coning

When the rotor is turning at normal operating RPM, the blades are subjected to two main forces:

• Centrifugal force (CF): Pulls the blades outward, away from the rotor hub.
• Aerodynamic lift (L): Acts perpendicular to the blade span, pushing the blades upwards.

These two forces reach an equilibrium, causing the blades to rise into a slight upward angle, known as the coning angle. The coning angle is a normal and necessary condition for stable rotor operation.

Dissymmetry of Lift and Retreating Blade Stall

In forward flight, the relative airflow over the advancing blade is the sum of the rotational velocity and the forward airspeed. Over the retreating blade, it is the difference between the rotational velocity and the forward airspeed. This creates a dissymmetry of lift, with the advancing blade producing more lift.

To balance this, the rotor system relies on flapping. The advancing blade flaps up, which reduces its effective AoA and lift. The retreating blade flaps down, which increases its effective AoA and lift. However, as forward speed increases, the retreating blade's effective AoA must increase further to compensate. Eventually, the retreating blade reaches its critical angle of attack and stalls. This is retreating blade stall, which limits the maximum forward speed of a helicopter.

Autorotation

Autorotation is the condition of powered-off descent where the rotor is driven by aerodynamic forces rather than engine power. The helicopter descends, causing a resultant relative airflow that is upward through the rotor disc.

The rotor disc is divided into three regions:

1. Stall Region (Inner): Near the blade root, the airflow is stalled due to high local AoA.
2. Driving Region (Middle): The upward airflow produces a resultant force that has a forward component, which drives the rotor and sustains rotor RPM.
3. Driven Region (Outer): The outer portion of the blade produces a resultant force with a backward component, which consumes energy.

In a steady autorotation, the energy produced by the driving region balances the energy consumed by the driven region and the stall region, maintaining a constant rotor RPM.

Effect of Collective Increase on Rotor RPM

If the pilot increases the collective pitch without adjusting the throttle, the blade pitch (AoA) increases. This increases the aerodynamic drag on the rotor blades. If the engine cannot maintain the rotor speed against this increased drag, the rotor RPM will decay. A lower RPM reduces the tip speed. On the retreating blade, the already high AoA is further increased by the collective input, worsening the retreating blade stall condition.

---

3. Important Formulas and Relationships

---

4. Common Relationships Between Concepts

• Density ↔ Altitude ↔ Temperature: As altitude increases, density decreases. As temperature decreases, density increases.
• Dynamic Pressure ↔ IAS: IAS is a direct measure of dynamic pressure. A constant IAS means a constant dynamic pressure.
• TAS ↔ Density ↔ IAS: For a constant IAS, TAS must increase as density decreases (with altitude).
• Induced Flow ↔ Effective AoA ↔ Collective: An increase in induced flow (downwash) decreases the effective AoA, requiring an increase in collective pitch to maintain thrust.
• Ground Effect ↔ Induced Flow ↔ Power: Ground effect reduces induced flow, which reduces induced drag and the power required to hover.
• Rotor RPM ↔ Blade Drag ↔ Collective: An increase in collective increases blade drag, which, if not compensated by throttle, leads to a decay in rotor RPM.
• Forward Speed ↔ Retreating Blade Stall: The need to balance lift between advancing and retreating blades leads to an increasing AoA on the retreating blade, eventually causing a stall at high forward speeds.

---

5. Typical Exam Focus Points

For the EASA Part-66 B3 Module 8 exam, candidates should focus on the following areas:

1. Atmosphere and Airspeed: Be able to explain the relationship between IAS, TAS, density, and dynamic pressure. Understand the conditions of the ISA.
2. Forces in Flight: Be able to resolve forces in a steady climb and understand why Thrust > Drag and Lift < Weight.
3. Helicopter Hover: Understand the concepts of induced flow, transverse flow effect, and ground effect. Know the corrective actions (cyclic and collective inputs) required.
4. Rotor Blade Design: Understand the purpose of blade twist, tip shapes, and the phenomenon of blade coning.
5. Autorotation: Be able to describe the three regions of the rotor disc and the direction of airflow during a steady autorotation.
6. Retreating Blade Stall: Understand the cause, effect, and consequences of retreating blade stall.
7. Effect of Power Changes: Be able to predict the immediate aerodynamic consequences of a power change on an aeroplane, including the effect on AoA and load factor.