Aerodynamic factors on takeoff every helicopter pilot should know

Gyroscopic precession

The spinning rotor disk of a helicopter is essentially a gyroscope, one of the principles of a gyro is precession. Gyroscopic precession works like this, any force that is applied is felt 90 degrees later in the direction of rotation. In a counterclockwise rotor system, a force pushing up at the 3 O'clock position would be seen at the 12 O'clock position of the rotor disk. This principle is important as we go on to learn about the different aerodynamics that effect the rotor system on any given phase of flight.

Dissymmetry of lift

Dissymmetry of lift is defined in the Helicopter Flying Handbook chapter 2 "Dissymmetry of lift is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half”. To compensate for this, helicopter rotor systems allow the blades to flap and feather. The advancing blade flaps up to reduce the AOA and the retreating blade flaps down to increase the AOA. This flapping and feathering motion allows the blades to equalize lift across the rotor disk, articulated rotor systems have a flapping hinge that allows the individual blades to flap up and down. Semi-rigid rotor systems use a teetering hinge, this allows the rotor disk to flap up and down as a unit.

Translational lift

Translational lift is defined in the Helicopter Flying Handbook chapter 2 "Improved rotor efficiency resulting from directional flight is called translational lift". As the helicopter gains forward airspeed the rotor system becomes more efficient with each knot of air impacting the blades (either forward movement of the aircraft or surface wind). As the helicopter moves forward, vortices created by the helicopter are left behind. Effective Translational Lift (ETL) is when the helicopter completely outruns these vortices, this usually happens around 16-24 knots (in zero wind)

Transverse flow effect

As the helicopter begins to move forward the rotor disk gains translational lift, but at some point, half of the front disk is operating in completely undisturbed air while the aft portion of the rotor disk is still operating in disturbed air. This difference causes unequal lift between the fore and aft portion of the rotor disk. To compensate for this, the forward portion of the disk flaps up to reduce the AOA while the aft portion of the rotor disk flaps down. Due to procession, this flap down is felt at the 3 O’clock position. This results in the helicopter rolling to the right at approximately 20 knots. Transverse flow effect happens between 12-15 knots, many pilots confuse the vibrations of transverse flow effect with the vibrations of ETL. To counteract transverse flow effect the pilot must apply left cyclic.

Translational Thrust

Anyone who has ever flown a helicopter knows that when picking the helicopter up into a hover left pedal is required to counteract the torque produced from the main rotor; however, as the pilot starts to takeoff, eventually the tail rotor is no longer operating in disturbed air. The left pedal that was needed during a hover is now excessive and the helicopter tends to yaw to the left as the tail rotor is operating in clean air.

What is felt during takeoff?

When a helicopter begins to takeoff, either from a hover or the ground you will feel a pitch up. This pitch up is a combination of dissymmetry of lift and transverse flow effect (blowback). The helicopter will also tend to roll to the right due to transverse flow effect and yaw to the left because of translational thrust. The pilot must continuously apply forward and left cyclic along with right pedal as the helicopter builds airspeed until established in forward flight.

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