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| L-1011 AC Pack |
Simulating the L-1011 Air Conditioning and Pressurization for the simulator will be a lot of fun. As you saw in part one of this posting, the basics of the pressurization system for the aircraft is realized in the Laminar Research x-plane simulator. However, the Air Conditioning system is not implemented at all. To make the simulation complete these systems need to be simulated in software as well
The image above shows a single AC PACK for the L-1011, there are 3 of them in the system. Each of them has a number of switch controllers. For simplicity of this blog posting I will only talk through one of the packs (pack 2) but the others work exactly the same way.
So we start from top to bottom:
1. Pack Flow Control Switch
Pack Flow control is a switch light that shows either a FLOW BAR or OVHT (Overheat). Here are the two elements on the actual panel and in the schematic:
The schematic shows a small [S] next to the valve. This indicates that this valve is spring loaded to shut. In other words, when pressure is coming from the bleed air system the valve will be pushed open by the pressure and the flow bar will illuminate. When the valve gets too hot (bleed air too hot) the OVHT warning illuminates. So why is it a switch if all it takes to illuminate the bar is pressure on the valve. When the switch is "latched" (in) the valve is in normal operation and does exactly what I just describe. When the switch is unlatched (out), however, the valve is permanently closed and will never open, even with bleed air pressure.
2. Hot Air Switch
The hot air switch allows for bleed air to be directly fed to the hot air planum. The valve is spring loaded [S] and opens automatically with pressure from the bleed air. When the valve is pushed open, the flow bar will illuminate on the switch. This is the normal operation when the switch is latched. When the switch is unlatched the valve will remain closed even with bleed air pressure. When the switch is unlatched the OFF legend illuminates.
3. Temperature Control
Bleed air from the pack flow control valve (1) is cooled through a primary and a secondary heat exchanger. The cooling provided by the heat exchangers is controlled by the position of the RAM AIR exhaust doors which determine the flow of ambient outside air across the heat exchangers. This is the primary way of cooling air. Additional cooling is provided as needed by the Air Cycle Machine (ACM). The ACM is a combination compressor and turbine powered by the bleed air flowing through the pack. When the ACM is working primary air from the primary heat exchanger passes through it and is heated by compression, cooled again by the secondary heat exchanger and the rapidly expanded by the turning for additional cooling.
The amount of cooling provided by the ACM is controlled by the Turbine Bypass Valve.
The system either operates in an automatic or manual mode as selected by the switch-light. Latched (in) the mode is automatic and ram air outlet and turbine bypass are controlled by the planum temperature demand. When unlatched (out) the system is in manual mode and the WARM and COOL momentary switches drive the position logic for TURB BYP and RAM AIR.
4. System Monitoring
There are a number of controls and gauges that allow the monitoring of temperatures in the system. All monitoring happens on the the ECS Monitoring panel:
There are a total of 5 temperature parameters that can be monitored for each AC Pack (controlled by the PACK SELECT switch on the ECS Monitor panel). They are:
- Pack Inlet Temperature taken right before the Pack Flow Control Valve
- Compressor Discharge Temperature measured before air from the ACM enters the secondary heat exchanger.
- Turbine Inlet Temperature measured after the secondary heat exchanger and prior to air entering the turbine
- Air Cycle Machine (ACM) Discharge Temperature is measured after the air leaves the pack and water separator and prior to it being mixed into the Cold Planum.
- Hot Manifold Temperature is taken prior to hot air entering the Hot Air Manifold.
Lastly, the air flow in Cubic Feet per Minute is displayed on the PACK FLOW meter also on the ECS Monitor Panel.
Software Considerations
All in all the system seem pretty complicated, however, from a software perspective it should not be too bad to model a somewhat lifelike system. Each PACK will be an object with attributes and parameters. The code will not attempt to compute actual air flow or heat exchange but will simply roughly approximate what the system would do under certain ambient air and airspeed conditions.
Ambient outside air at altitude and airspeed we can both get from x-plane as a dataref:
sim/weather/temperature_ambient_c
sim/cockpit2/gauges/indicators/airspeed_kts_pilot
With these two I should be able to develop an approximation of cooling efficiency of the air at altitude and simulate the automatic or manual operation of the ACM systems. As mentioned earlier, many of the spring loaded valves open automatically with bleed air pressure in the system. For bleed air there are also a number of datarefs that will come in handy:
sim/cockpit2/annunciators/bleed_air_off ... for each engine
sim/cockpit/warnings/annunciators/bleed_air_fail ... for each engine
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