User manual

DodoSim 206 FSX User Manual

Page 66

insufficient to maintain height and the helicopter will begin to sink. Therefore, an increase in power

using the collective lever is required to compensate.

As the helicopter transitions into forward flight, between approximately 12 and 24 knots the rotor

encounters Transverse Flow (see section 3.4.1.16), and a distinct pitch up and roll to the right occurs.

Positive opposing cyclic input will be required to overcome this behaviour.

As airspeed increases, the rotor begins to develop Effective Translational Lift (ETL) as it begins to leave

its own vortices behind and move into undisturbed air. This increases the aerodynamic efficiency of the

rotor and more lift is generated for the given power setting. The helicopter will begin to climb if the

power is not reduced using the collective, (which may or may not be desired.)

As airspeed continues to increase and the main rotor vortices are left behind, the helicopter’s tail rotor

also gains efficiency from the clean air and the aerodynamic effects of the fuselage and vertical

stabiliser begin to take effect, reducing the need for anti-torque pedal application. Above approximately

40 knots, only small pedal inputs may be required in order to keep the “ball” centred and the helicopter

in trim.

To summarise, as the helicopter transitions from the hover into forward flight, the pilot must work the

cyclic to maintain pitch and roll attitude, reduce left pedal input to correct for the loss of torque-induced

yaw, and reduce collective pitch if a constant altitude is to be maintained. It will require a lot of practice

before a seamless transition can be achieved without the helicopter even momentarily deviating from

the intended heading or attitude.

4.1.4.2 Cruising

In forward flight, cyclic control is primarily used to control airspeed, and collective control is primarily

used to control altitude. To travel faster, greater forward cyclic deflection is applied and to climb,

collective pitch is increased to generate more thrust. However, the two are interrelated in that climbing

will occur if forward cyclic pressure is reduced as part of the forward thrust is deflected vertically.

To maintain forward flight, constant forward deflection of the cyclic is necessary to counter “flap-back”,

where the rotor tries to oppose the direction of travel. To increase the cruise speed, greater forward

cyclic is required.

The helicopter’s maximum achievable straight and level speed is not dictated by the engine

performance, but rather by the maximum cyclic input achievable to overcome flap-back. Eventually, the

cyclic can be pushed no farther forward and airspeed will not increase any further. Note that VNE

(Velocity to Never Exceed) is not necessarily indicative of an achievable speed in normal operation, but

a safety limit, where the helicopter may sustain damage or become uncontrollable above. A helicopter

may exceed VNE in a rapid descent, in which case there will be insufficient forward cyclic input to

overcome flap-back and the helicopter may pitch and roll out of control.

Pilot Information: VNE is not an absolute figure, but is dependent upon altitude and aircraft loaded

weight. I.e.:

For weight < 3000 lbs, VNE = 130 Knots – 4 Knots per 1000 feet

For weight > 3000 lbs, VNE = 122 Knots – 7 Knots per 1000 feet

As greater forward cyclic inputs are made to maintain higher forward speeds, thrust needs to be

increased using the collective lever to counter the deflection of thrust horizontally to maintain a constant

height. The pilot must be careful not to over-torque the transmission, otherwise damage may occur.

Pilot Information: Maximum continuous torque limit is 85%. This may be exceeded for short durations

and during take-off, i.e.:

Take-off torque limit = 100% for five minutes after take-off.

Transient torque limit (5 second max) = 110%