User manual

DodoSim 206 FSX User Manual

Page 45

3.4.1.14 Rotor Droop on Torque Application

Increasing collective pitch creates greater drag on the rotor blades and the rotor RPM will drop if

engine power is not increased to compensate for the drag. The engine’s Fuel Control Unit (FCU)

includes a governor that attempts to automatically compensate for rotor RPM fluctuations by

increasing or decreasing fuel as required to produce greater or lesser power. However, sudden

increases in drag, (i.e. abrupt or large increases in collective pitch), can overwhelm the governor

and the rotor RPM can drop before the governor has sufficient time to compensate.

Very high torque loads may also overwhelm the tail rotor’s ability to compensate for the torque-

induced yaw produced by large torque demands and an uncommanded yaw to the right may

develop.

Pilot Information: If excessive or abrupt collective application begins to bog the rotor, the pilot

should reduce collective pitch immediately to aid recovery of rotor RPM. Failure to do so may

cause the rotor system to stall and the helicopter to descend rapidly and potentially irrecoverably.

3.4.1.15 Vortex Ring State

Vortex Ring State (VRS) is a condition where the helicopter’s main rotor system re-circulates the

air that is being displaced through it, rather than dragging fresh air through and so rotor thrust is

severely reduced. VRS can occur at low airspeeds during high rates of descent.

Entry into VRS is typically accompanied by a noisy buffeting emanating from the rotor system due

to the convoluted nature of the airflow through it, followed by an uncommanded descent that can

quickly escalate to a very high rate as lift is reduced. The pilot must avoid the temptation to try and

recover the situation by applying more power with the collective lever, as this only serves to

exacerbate the problem by increasing the size of the vortices being re-circulated.

As VRS becomes fully developed and all lift is lost, the noisy buffeting may reduce as the confused

airflow through the rotor changes reverses from downwards to upwards.

VRS can only be alleviated by reducing power, thus reducing the size of the re-circulated vortices,

followed by use of the cyclic to fly the helicopter out of its own down-wash. Once airspeeds in

excess of approximately 30 knots are achieved, power can be reapplied with the collective lever

and the descent can be fully arrested.

Pilot Information: To ensure VRS is not encountered, pilots should avoid situations where the

airspeed is less than 30 knots and the descent rate greater than 500 feet per minute. In actuality,

VRS may not develop until the airspeed is lower or the descent rate higher, as these values include

a healthy margin of safety.

3.4.1.16 Transverse Flow

As the helicopter’s airspeed increases when accelerating away from the hover, induced flow is

reduced on the front half of the rotor disc and increased on the rear half, effectively increasing the

angle of attack on the advancing blade. This causes the blade to flap up, but due to gyroscopic

precession the effect is felt at 90° out of phase, causing the rotor to pitch up and to the right.

Positive forward and left cyclic is required to counter this behaviour and should be anticipated when

accelerating from a hover into forward flight.

Hover-taxiing is normally performed below the speed, (16-24 knots), where transverse flow is

encountered in order to minimise pilot workload.

Pilot Information: Remember that a helicopter’s main rotor has no concept of “forward”.

Transverse flow will be encountered when the helicopter is flying in any direction, the rotor reaction

being up and to the right of the direction of travel and the pilot must react accordingly.