Principles of Flight (Helicopter)
Core framework
Helicopter flight theory explains how the rotor produces lift and thrust. Understand the aerofoil, angle of attack, induced drag, and the four forces (lift, weight, thrust, drag). Know how rotor systems generate lift through blade flapping, feathering and lead-lag.
Concepts to master
Translational lift (effective translational lift as speed increases), dissymmetry of lift and how flapping/blade feathering compensate, retreating blade stall, and autorotation. Understand ground effect (IGE) and its loss OGE. Know the vortex ring state and how to recover. Understand torque effect and anti-torque.
Traps
- Confusing angle of attack (blade to relative airflow) with pitch angle (blade to rotor plane).
- Forgetting that dissymmetry of lift is compensated by flapping, not by changing rotor RPM.
- Mixing retreating blade stall (high speed, high load) with vortex ring state (low speed, high descent rate).
- Assuming translational lift only occurs at high speed - it begins as soon as airflow through the disc becomes effective.
Rule of thumb
"More collective = more pitch = more lift, but also more torque and more induced drag." Manage power and RPM together.
Worked example
In a steep descent at low airspeed, the helicopter enters vortex ring state - the rotor recirculates its own downwash. Recovery: reduce collective, gain forward speed, and fly out of the condition.