Principles of Flight — PPL(H)

A rotor blade is an aerofoil rotating at high speed. Each blade generates lift by the same mechanism as a fixed wing — the difference is that the "airspeed" seen by the blade is the combination of its rotational speed and the aircraft's forward speed (or descent speed in autorotation).

• Blade pitch (collective): angle of the blade chord relative to the plane of rotation — analogous to angle of attack.
• Tip speed: the outermost part of the blade moves fastest — generates the most lift.
• Root: inner section moves slower, generates less lift. Root cutout is unproductive.
• Disc area: the swept area of the rotor. Larger disc = more efficient at low speed (lower disc loading).

When a helicopter moves forward, the advancing blade (moving in the direction of flight plus rotation) has a higher relative airspeed than the retreating blade (moving rearward against the direction of flight). If uncorrected, this would roll the helicopter toward the retreating blade.

• Advancing blade: high airspeed → high lift. Would produce excessive lift on one side.
• Retreating blade: low airspeed → lower lift. Would produce deficient lift on the other side.
• Correction — blade flapping: advancing blade flaps up (reducing pitch/AoA), retreating blade flaps down (increasing pitch/AoA). Lift is equalised automatically.
• Fully articulated rotors: dedicated flapping hinges allow this. Teetering rotors (R22/R44) achieve this through the teetering action.

As forward speed increases, the retreating blade's relative airspeed decreases. To compensate for dissymmetry of lift, the retreating blade pitch is automatically increased. At high forward speed, the retreating blade reaches its critical angle of attack and stalls.

• Symptoms: vibration, pitching up, rolling toward the retreating blade (usually left on most helicopters).
• Causes: high forward airspeed, high altitude (less dense air), high aircraft weight, turbulence.
• Recovery: reduce collective, reduce airspeed, reduce bank angle.
• Never exceed Vne (Velocity Never Exceed) — this is set to prevent retreating blade stall.

The main rotor is a large gyroscope. Like all gyroscopes, it exhibits precession — an applied force is felt 90° later in the direction of rotation. On most Western helicopters, the main rotor turns anti-clockwise when viewed from above, so precession acts 90° ahead in the rotation.

• Torque reaction: main rotor turns anti-clockwise (above) → fuselage wants to turn clockwise (right yaw) → tail rotor provides left thrust to counteract.
• Gyroscopic precession: a force applied at the front of the disc is felt at the left side; a force at the left is felt at the rear.
• This is why cyclic inputs require slight correction — the rotor precesses the effect 90° ahead of where you pushed.
• On the R22/R44, the rotor turns anti-clockwise (viewed from above) — UK/US/European convention.

Ground resonance is a mechanical oscillation that can destroy a helicopter in seconds. It occurs when the natural frequency of the landing gear oscillation aligns with the frequency of rotor blade lead/lag movement, creating a feedback loop.

• Occurs only on the ground when blades can lead and lag.
• Symptoms: increasing rocking oscillation of the airframe.
• Recovery: immediately get airborne if rotor speed is adequate, OR close throttle and lower collective rapidly if take-off is not possible.
• R22/R44 semi-rigid rotors have no lead/lag hinges — not susceptible to ground resonance.
• Ground resonance is a risk on fully articulated helicopters.