User manual

DodoSim 206 FSX User Manual
Copyright© 2009 DodoSim. All Rights Reserved.
Page 42
drop and the Power Turbine (N2) and rotor RPM (NR) needles will stabilise as the governor regains
control.
Important note: The clutch connecting the Power turbine (N2) and Rotor shaft (NR) can be
thought of as a one-way bearing: i.e. The N2 can drive the NR up and the NR can drive the N2
down, but failures notwithstanding, the N2 cannot drive the NR down and the NR cannot drive the
N2 up. However: Friction within the clutch does enable one to drag slightly on the other.
3.4.1.8 Control Cross-Couplings
Helicopters generate large gyroscopic forces as their main and tail rotors circulate. Pilot inputs to
one control intended to produce a specific change in attitude often generate smaller secondary
attitude changes on different axes, termed “cross-coupling”. Helicopter manufacturers design out
cross-couplings as far as is possible, but some unwanted couplings in various regions of the flight
envelope will remain. The resultant effect is that pilots must often adjust one control to
compensate for intentional changes in another. (This is the “rubbing tummy whilst patting head”
analogy that is often used to describe helicopter piloting.) Extensive practice will be required
before the secondary responses become second nature and the helicopter appears to fly with little
effort on the part of the pilot.
3.4.1.9 Enhanced Low-Speed Behaviour
The DodoSim 206 FSX includes various enhancements to low speed behaviour, including hovering
instability and pendulum stability, whereby the helicopter will feel as if it is balancing on a ball of air
but can “fall off” if constant cyclic input is not applied to maintain it. By virtue of the fact that the
helicopter’s fuselage acts like a pendulum weight hanging below the rotor, the fuselage exhibits a
tendency to swing underneath the rotor if cyclic input is not maintained to prevent it. Whilst this
pendulum tendency can be utilised by the pilot to hold a steadier hover, failure to anticipate the
behaviour can lead to over-controlling and resultant pilot-induced-oscillations.
3.4.1.10 Low Pressure Fuel Starvation
Above 6,000 feet pressure altitude, fuel pressure may be insufficient to continue engine
combustion and a “flame-out” may occur unless the fuel boost pumps are utilised. In the event of
an electrical generator failure, the pilot must descend below 6,000’ as once the battery is depleted,
the pumps will fail and the engine will likely quit.
Fuel pressure may also be lost as the tank nears empty. If both pumps are in use, the usable fuel
limit is 5 Gallons. If only one is in use then the usable fuel limit is 10 Gallons.
Pilot Information: To avoid potential problems, the pilot should ensure that both fuel pumps are on
at all times during flight. In the event of a single pump failure, the pilot should descend below
6,000’ in case the remaining pump were to fail also.
3.4.1.11 Retreating Blade Stall
The helicopter’s blades rotate counter-clockwise when viewed from above. Consequently, as
described in the “Dissymmetry of Lift” section above, when the helicopter is moving forward, the
advancing blade on the right side, (starboard), of the helicopter is passing through the air faster
than the retreating blade on the port side. Retreating blade stall occurs when the difference
between the rearward speed of the inboard section of the retreating blade and relative speed of the
airflow it is travelling through diminishes below the blade’s stall speed.
The consequence of retreating blade stall is that lift is lost on the left side of the rotor disc and the
helicopter will roll to the left. Due to gyroscopic precession, this effect is felt at 90° out of phase
and so the helicopter will exhibit a tendency to pitch up as well as roll to the left. Retreating blade
stall is a factor that ultimately limits the helicopter’s maximum speed. (Refer to the Cruising sub-
section with the Standard Procedures section later in this manual for information on calculating