FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A pilot is transitioning through effective translational lift during takeoff in a single main rotor helicopter with a counterclockwise-turning rotor (viewed from above). The pilot notices the helicopter tends to pitch and roll slightly as the airflow through the rear of the rotor disk changes relative to the front. What phenomenon is this?
- ADynamic rollover
- BRetreating blade stall
- CSettling with power
- DTransverse flow effect
Show answer & explanationAnswer & explanation
Correct answer: D. Transverse flow effect
Transverse flow effect occurs during the transition to forward flight when the rear portion of the rotor disk develops a higher angle of attack (and more lift) than the front, because the front disk's air has already been accelerated downward while the rear disk meets relatively undisturbed air; this asymmetry causes vibration, a slight pitch-up, and often a roll tendency.
Why the other options are wrong
- A. Dynamic rollover involves a pivoting skid on the ground, not airflow differences in flight.
- B. Retreating blade stall occurs at high forward airspeeds, not during the low-speed transition.
- C. Settling with power involves a descending helicopter in its own downwash, not forward transition.
Transverse Flow Effect
An aerodynamic asymmetry between the front and rear of the rotor disk during low-speed forward flight transition, where the rear disk produces more lift than the front, causing vibration and pitch/roll tendencies.
- Occurs during transition to forward flight (~10-20 kts)
- Rear rotor disk has greater induced flow lag, producing more lift
- Often causes a slight pitch-up and low-frequency vibration
Memory trick: Transverse flow: front is 'flowed out,' rear is fresh — imbalance shakes the disk.