Ex. 5 Attitude and Power Changes

Flapback is an aerodynamic side effect which affects a rotor system when the strength or direction of the relative wind changes.
The relative wind change may come about as a result of a commanded speed or direction change controlled by the pilot, or as an uncommanded change due to say a gust of wind.
In simple terms the rotor disc will flap away from the relative wind as though the wind has 'got under' the disc, almost like a piece of cardboard in the wind.

Power is controlled by raising or lowering the lever. The exact method depends on the aircraft being flown. Turbine engines are usually governed and power is controlled solely by raising and lowering the lever. Piston engines however are not normally as sophisticated and power is controlled by raising and lowering the lever and manipulating the throttle at the same time. Raising the lever will require a throttle increase to maintain RRPM, and lowering the lever will require a throttle recduction to maintain RRPM. Most small piston helicopters are fitted with a 'correlator' which will assist with these changes, but it will not maintain ERPM at a fixed value. The exceptions to this rule are the Robinson range of helicopters which are fitted with a throttle governor, which is only allowed to be disengaged during emergency proedures training.

Any change of power will result in a change of applied torque. Unless corrected the torque change will result in a yaw, in a direction opposite to main rotor rotation for an increase in torque, and in the direction of main rotor rotation for a decrease in torque. To counter this whilst making a torque change we use the anti-torque pedals to maintain the heading of the aircraft during the change and check the slip ball is in its balanced position when we have finished making the change.

Engines necessarily have limits imposed upon them to ensure that they will reach their expected service life.
Any exceedance of the specified limits will not necessarily mean any damage is done, but should always be reported to the engineers.
The limits normally specified for an engine will specify maximum and minimum values for parameters such as

• Engine RPM
• Engine Oil Pressure
• Engine Oil Temperature
• Fuel Pressure

Helicopters are naturally unstable aircraft. Any disturbance felt by the aircraft will not subside of its own accord and will ultimately result increasingly large oscillations about the trimmed condition immediately before the disturbance. This is immediately evident in the workload experienced by the pilot compared to flying a fixed wing aeroplane. This results in the constant need to fly the helicopter which even when correctly trimmed will diverge from its present flight path without constant minor corrections from the pilot in a matter of seconds.

The diagram shown here is a breakdown of the power required across the airspeed range of a typical light helicopter.

The points to note are

• The Minimum Power Required Speed.
• The Front Side of the curve where power demand increase as airspeed increases.
• The Back Side of the curve where power demand increases as airspeed decreases.
• Rotor Profile Power does increase but only slight across the whole speed range.
• Induced Power reduces as speed is increased.
• Parasite Drag Increases as speed is increased.