Unit 6: CONCEPT OF ROTORCRAFT

1. Classification of Rotorcraft

A rotorcraft or rotary-wing aircraft is a heavier-than-air flying machine that uses lift generated by wings, called rotor blades, that revolve around a mast. Several rotor blades mounted on a single mast are referred to as a rotor.

Rotorcraft are classified based on how their main rotor system is powered and how they generate thrust for forward flight.

• Helicopter: The main rotor(s) are powered by an engine throughout the entire flight. The rotor provides both lift and thrust. This allows helicopters to perform Vertical Take-Off and Landing (VTOL), hover, and fly forwards, backward, and sideways.
• Gyroplane (or Autogyro): The main rotor is unpowered and rotates due to the aerodynamic forces produced by the air moving up through the rotor (a process called autorotation). Thrust for forward flight is provided by a separate, engine-powered propeller, typically mounted on the rear of the aircraft. Gyroplanes cannot hover (without a strong headwind) and require a short runway for takeoff.
• Compound Rotorcraft: A hybrid aircraft that incorporates features of both a helicopter and a fixed-wing aircraft.
  • Compound Helicopter: Has a main rotor that provides lift at low speeds, but also features wings and a separate propulsion system (propellers or jets) for thrust. At high speeds, the wings provide significant lift, "unloading" the rotor and allowing for higher speeds than a conventional helicopter.
  • Convertiplane (Tiltrotor/Tiltwing): An aircraft that can transition between helicopter mode and fixed-wing mode. It takes off and lands vertically like a helicopter, then its rotors (and sometimes wings) tilt forward to act as propellers for high-speed, efficient forward flight. The Bell V-22 Osprey is a prime example.
• Cyclocopter (or Cyclogyro): A conceptual type of rotorcraft that uses cycloidal rotors, which consist of multiple blades revolving around a horizontal axis. The pitch of the blades is varied continuously during rotation to generate lift and thrust. This design is highly complex and remains largely experimental.

2. Components of Rotorcraft

While designs vary, the following are the primary components, with a focus on the conventional helicopter configuration:

• Main Rotor System: The primary lift-generating component.
  • Mast: The cylindrical shaft to which the rotor hub is attached. It is driven by the transmission.
  • Hub: The central assembly at the top of the mast to which the rotor blades are attached. It allows the blades to change their pitch.
  • Rotor Blades: The airfoils that generate lift when they rotate.
• Tail Rotor System: A smaller rotor mounted on the tail boom to counteract the torque effect of the main rotor and provide yaw control.
• Fuselage: The main body of the aircraft.
  • Cockpit: Contains the flight controls, instruments, and seating for the pilot(s).
  • Cabin: The area for passengers or cargo.
  • Tail Boom: The structural part of the fuselage that extends to the rear to hold the tail rotor.
• Powerplant (Engine): Provides the power to turn the rotors.
  • Piston Engine: Common in smaller, simpler helicopters.
  • Turboshaft Engine: A gas turbine engine optimized to produce shaft power rather than jet thrust. Used in virtually all modern medium and large helicopters due to its high power-to-weight ratio.
• Transmission System: A system of gearboxes that reduces the high RPM from the engine to the optimal, much lower RPM required by the main and tail rotors.
• Landing Gear: Supports the aircraft on the ground. Can be skids (common on lighter helicopters) or wheels (often retractable on larger models).
• Flight Controls:
  • Collective Pitch Control (Collective): A lever that changes the pitch angle of all main rotor blades simultaneously, controlling total lift and thus altitude.
  • Cyclic Pitch Control (Cyclic): A stick that tilts the main rotor disc by changing the pitch of blades cyclically, controlling the direction of flight (forward, backward, left, right).
  • Anti-Torque Pedals: Foot pedals that control the pitch of the tail rotor blades, controlling yaw (the direction the nose is pointing).

3. Gyroplane

A gyroplane, also known as an autogyro or gyrocopter, is a type of rotorcraft that uses an unpowered, autorotating main rotor to develop lift and an engine-powered propeller to provide thrust.

• Principle of Operation:
  1. Takeoff: The aircraft accelerates along a runway, pushed by its propeller. The forward motion forces air up through the unpowered main rotor disc.
  2. Autorotation: This upward airflow causes the blades to spin. As the rotational speed increases, the blades generate lift.
  3. Flight: Once sufficient lift is generated, the gyroplane takes off. In flight, it remains in a constant state of autorotation, with the propeller providing thrust and the rotor providing lift.
• Key Characteristics:
  • Cannot Hover: A gyroplane cannot hover in still air because it requires a constant flow of air up through the rotor disc to keep it spinning. This airflow is only created by the aircraft's forward motion or a very strong headwind.
  • STOL Capability: They are known for Short Take-Off and Landing (STOL). They can land with almost zero forward roll.
  • Simplicity and Safety: Mechanically simpler and less expensive than helicopters. They are also considered very safe; in case of an engine failure, the main rotor continues to spin, allowing for a controlled descent and landing.

4. Stability Concept of Gyroplane

Gyroplanes are known for their significant inherent stability, especially compared to helicopters.

• Pendulum Stability: The center of gravity (CG) of the aircraft is located below the pivot point of the rotor hub (the center of lift). This creates a pendulum effect, where if the fuselage is displaced by a gust, gravity will tend to pull it back to a level attitude.
• Rotor Disc Damping: The autorotating rotor disc acts like a large gyroscope. Due to gyroscopic precession, it resists any changes to its plane of rotation, thereby damping out external disturbances and contributing to a stable flight path.
• Lack of Torque Reaction: Since the engine does not drive the main rotor, there is no torque reaction trying to spin the fuselage. This eliminates the need for a complex anti-torque system and removes a major source of instability and control complexity found in single-rotor helicopters.

5. Flying Concept of Compound Helicopter

A compound helicopter is a hybrid aircraft designed to overcome the speed limitations of conventional helicopters.

• Concept: It combines the vertical flight capabilities of a helicopter with the high-speed efficiency of a fixed-wing aircraft.
• How it Works:
  • Low-Speed Flight & Hover: At low speeds, it operates like a conventional helicopter. The engine-driven main rotor provides nearly all the lift and control.
  • High-Speed Flight: As the aircraft gains speed, two things happen:
    1. Wings Generate Lift: Small, stubby wings begin to generate a significant portion of the total lift. This "unloads" the main rotor, meaning the rotor doesn't have to produce as much lift.
    2. Separate Thrust System: A dedicated propulsion system (e.g., a pusher propeller or jet engine) takes over the role of providing forward thrust. The main rotor is then only needed for lift and control, not propulsion.
• Advantages:
  • Higher Speed: By offloading the rotor and using a dedicated thruster, compound helicopters can break through the aerodynamic limitations (like retreating blade stall) that cap the top speed of conventional helicopters.
  • Greater Range and Efficiency: At high speeds, the wings and thruster are more efficient than a rotor for providing lift and thrust, leading to better fuel economy and range.

6. Helicopter and Convertiplane

While both are advanced rotorcraft, their method of achieving high-speed flight differs fundamentally.

7. Methods of Varying Lift

The lift produced by a rotor blade is determined by the formula: L = ½ * ρ * V² * A * C_L
Where:

• ρ (rho) = Air density
• V = Airflow velocity over the blade
• A = Blade surface area
• C_L = Coefficient of Lift (which is primarily a function of the Angle of Attack (AoA))

In a rotorcraft, a pilot primarily varies lift by changing two factors:

1. Rotor RPM: The rotational speed of the blades (V). The engine's governor system works to keep this constant during normal flight, so it is not the pilot's primary means of instantaneous lift control.
2. Blade Pitch Angle: The angle between the chord line of the blade and the plane of rotation. Changing the pitch angle directly changes the blade's Angle of Attack (AoA), which in turn changes the Coefficient of Lift (C_L). This is the primary method for controlling lift.

8. Collective Pitch

Collective pitch is the mechanism for changing the pitch angle of all main rotor blades simultaneously and by the same amount.

• Control: The pilot uses the collective lever, located to the left of their seat.
• Action:
  • Raising the collective: Increases the pitch angle of all blades. This increases the overall Angle of Attack, generating more lift and causing the helicopter to climb or ascend. This also increases drag, requiring more engine power.
  • Lowering the collective: Decreases the pitch angle of all blades. This decreases the overall Angle of Attack, reducing lift and causing the helicopter to descend.
• Function: The collective is the primary control for managing altitude and vertical speed.

9. Swash Plate System

The swash plate is a brilliant mechanical device that translates the stationary flight controls from the cockpit into inputs for the rotating main rotor blades. It consists of two main parts connected by a bearing.

• Stationary Swashplate (Lower): This plate does not rotate. It is connected via pushrods to the pilot's cyclic and collective controls. It can be moved up and down and tilted in any direction.
• Rotating Swashplate (Upper): This plate is mounted on the stationary plate via a large bearing. It rotates with the main rotor mast and is connected to each individual rotor blade's pitch horn via pitch links.

How it works:

1. Collective Input: When the pilot raises the collective lever, a mechanism pushes the entire swashplate assembly (both stationary and rotating parts) vertically up the mast. This pushes all the pitch links up by the same amount, increasing the pitch of all blades equally.
2. Cyclic Input: When the pilot moves the cyclic stick, the stationary swashplate tilts in the corresponding direction. This tilt is transferred to the rotating swashplate. As the rotating plate spins in its tilted orientation, it moves the pitch links up and down. This causes the pitch of each blade to change cyclically throughout its 360° rotation—increasing at one point and decreasing at the opposite point. This unequal lift causes the entire rotor disc to tilt, producing a horizontal thrust component that moves the helicopter in the direction of the tilt.

10. Rotor Disc Incident with Flow

• Rotor Disc: The circular area swept by the main rotor blades. For aerodynamic analysis, the complex system of rotating blades can be simplified to a single "actuator disc."
• Hover: In a hover, air is drawn from above the helicopter and accelerated downwards through the rotor disc. This downward flow is called induced flow or downwash. The rotor disc is parallel to the ground.
• Forward Flight (Translational Flight): When the helicopter moves forward, the rotor disc is tilted forward. The airflow experienced by a blade is now a combination of three components:
  1. Rotational Flow: The airflow created by the blade's own rotation.
  2. Induced Flow: The downward flow of air through the disc.
  3. Translational Flow: The horizontal airflow from the helicopter's forward movement.
• The vector sum of these flows is the resultant relative wind, which determines the angle of attack and lift at any given point on the blade. This complex interaction is why the advancing blade (moving into the wind) and retreating blade (moving away from the wind) experience different airspeeds and require cyclic pitch changes to maintain balanced lift.

11. Torque Reaction and Hovering Turn

• Torque Reaction: An application of Newton's Third Law of Motion. As the engine and transmission apply a torque to turn the main rotor in one direction (e.g., counter-clockwise), the main rotor applies an equal and opposite torque to the fuselage, causing it to want to spin in the opposite direction (e.g., clockwise).
• Anti-Torque Systems: To fly straight, this torque reaction must be countered. Common methods include:
  • Tail Rotor: A variable-pitch propeller on the tail boom that produces a sideways thrust to counteract the main rotor torque. This is the most common system.
  • Coaxial Rotors: Two main rotors on the same mast spinning in opposite directions, canceling out each other's torque.
  • Tandem Rotors: Two main rotors, one fore and one aft, spinning in opposite directions (e.g., CH-47 Chinook).
  • NOTAR (NO TAil Rotor): Uses a ducted fan to create a low-pressure air circulation around the tail boom (the Coandă effect) and a direct jet thruster for yaw control.
• Hovering Turn (Yaw Control): The pilot controls the direction the helicopter's nose is pointing (yaw) using the anti-torque pedals.
  • Pressing one pedal increases the pitch of the tail rotor blades, creating more sideways thrust. This overcomes the main rotor torque and yaws the nose in the direction of the pedal press.
  • Pressing the other pedal decreases the tail rotor pitch, reducing its thrust. The main rotor torque then becomes dominant, yawing the nose in the opposite direction.

12. Translational Lift – Steady Flight

Translational lift is the additional lift obtained when a helicopter transitions from a hover to forward flight, resulting from the rotor system becoming more efficient.

• Mechanism:
  • In a hover, the rotor is drawing air down through itself, and this downwash creates turbulent vortices at the blade tips. The rotor system is essentially re-ingesting its own turbulent air, which is aerodynamically inefficient.
  • As the helicopter begins to move forward (typically at about 16-24 knots), the rotor moves into clean, undisturbed air. The horizontal flow of air "blows away" the rotor's own downwash and vortices.
• Effects:
  • The rotor operates in less turbulent air, allowing the blades to produce more lift for the same power setting.
  • The airflow becomes more horizontal across the disc, reducing induced drag.
  • The helicopter will feel a "bump" or a tendency to climb as it passes through this speed range. The pilot must lower the collective slightly to maintain a constant altitude during the transition from hover to forward flight.

13. Hovering and Autorotation

Hovering

A maneuver where the helicopter is maintained in a motionless state over a fixed point at a constant altitude.

• Aerodynamics: A state of equilibrium where lift produced by the main rotor exactly equals the aircraft's gross weight, and thrust from the tail rotor exactly equals the main rotor's torque.
• Power Requirement: Hovering is one of the most power-intensive phases of flight because there is no translational lift, and the rotor must work entirely on its own to generate lift by accelerating a mass of air downwards.
• Hover In-Ground Effect (IGE): When hovering close to a surface (usually within one rotor diameter), the downwash is restricted by the ground. This creates a cushion of higher-pressure air beneath the rotor, which reduces the velocity of the induced flow. This makes the rotor more efficient, requiring less power to hover.
• Hover Out-of-Ground Effect (OGE): When hovering high enough that the ground has no influence on the rotor downwash. This requires more power than an IGE hover because there is no air cushion to assist. An aircraft's OGE hover ceiling is always lower than its IGE ceiling.

Autorotation

The state of flight where the main rotor is turned by the action of air moving up through the rotor rather than by engine power. It is the primary emergency procedure for engine failure.

• Purpose: Allows a pilot to make a controlled landing after the engine has quit.
• Mechanism:
  1. Entry: Upon engine failure, the pilot must immediately lower the collective. This reduces the pitch (and thus drag) on the blades, preventing them from slowing down rapidly. A freewheeling unit disengages the engine from the rotor, allowing the rotor to spin freely.
  2. Descent: The helicopter descends. This descent creates an upward flow of air through the rotor disc.
  3. Driving the Rotor: The blade's airfoil is angled such that this upward airflow creates an aerodynamic force that has a component pushing the blade forward in the direction of rotation. This keeps the rotor spinning at a safe operating RPM. The pilot uses the collective to control this RPM.
  4. Flare and Landing: Nearing the ground, the pilot performs a "flare" by pulling back on the cyclic. This converts forward speed into additional lift, slowing the descent rate. Just before touchdown, the pilot raises the collective. This uses the stored rotational inertia in the blades to create a final burst of lift to cushion the landing.