Arma 3: Advanced Helicopter Flight Model
This document relates to Helicopter Flight Model Config (XML)
Changes introduced by RTD libraries 4.1.1 in Arma 3 Helicopters DLC
1) <Fuselage CoPX= CoPY= CoPZ= changed to forceExertionPnt*=
2) <Engines> maxPower= changed to emergencyPowerHP=
<DriveTrains> torque maximumContinuous= and takeoff= have no use, were removed
3) <ControlSystem> <FCSComponent renamed to <CSComponent
Added new value rotorBrake
4) <Rotors> <ModelConfiguration updateSubstepCount="360" /> were replaced by <ModelConfiguration maxIntegrationStepSizeDeg="1.0" />
<InducedVelocity groundEffectFactor= > <InducedVelocityTable>
5) <GroundContacts> removed <GeneralConfiguration gearDragCoefficient= and gearReferenceArea="0"
Notes about VRS (Vortex Ring State):
When the vortex ring state is entered depends on vertical velocity, horizontal velocity and current thrust (roughly collective position). Generally, VRS occurs at low horizontal speeds and relatively large vertical speeds (comparable to induced velocity at the given thrust setting). Although there are different numbers mentioned in the Internet they are usually over-generalizations. VRS entrance conditions differ from helicopter to helicopter and it differs also depending on the current mass of the helicopter. A loaded helicopter requires more thrust, which results in larger induced velocity. In this configuration, blade tip vortices are blown quickly downwards and entrance to VRS is hence harder.
VRS starts with vibrations. As the vertical speed increases VRS condition deepens and pitch,yaw,roll and thrust oscillations can be observed. Another important thing to observe is sudden loss of thrust; climb rate will fall quickly in VRS however it will stabilize after some time.
Once the VRS is fully established the thrust will not be sufficient to come out. Actually, if the helicopter is relatively lightly loaded and has powerful engines, it might eventually brake the descend but usually this takes more time than simply increasing horizontal speed. Getting a little bit ETL will clear the rotor from vortex rings.
RTD Implementation: There are two aspects of VRS
- Average induced velocity increase
- Changes in distribution of the induced velocity on the blades
For the first one an empirical, table look-up based model is used. ( see <InducedVelocityTable> ). As is customariy in the literature, the axes and values in this table are non-dimensionalized by dividing to v_h = sqrt(Thrust/(2*airdensity*rotorArea))
More details about this simulation approach can be found in this paper 
The second one is partly responsible for the thrust loss (increase in average induced velocity also plays a major role) and also vibration and oscillations.
- Loss of thrust: Distribution of the induced velocity at the blade elements are automatically modified in VRS.
- Vibration: vibration is to be visually cued at the application level (e.g. vibrating console etc. ).
- Oscillations: RTD provides a configurable means for creating thrust and moment oscillations.
Configuration of the behavior in VRS is made in <VortexRingState> section of the XML. In <Perturbation> section, oscillations for thrust and moments can be configured with their periods and amplitudes.