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| Litton LTN-72RL CDU Development Setup with X-Plane |
Still working on the Instrument Data page of the Litton LTN-72RL CDU. The data page actually incorporates a great number of X-Plane data refs and I finally got them all to work. A few of them, like drift angle, turned out to be a bit of head-scratcher.
Here is the list of X-Plane data refs that feeds the CDU's Instrument Data page:
gTAS = XPLMFindDataRef("sim/flightmodel/position/true_airspeed");
gWindDir = XPLMFindDataRef("sim/weather/wind_direction_degt");
gWindSpeed = XPLMFindDataRef("sim/cockpit2/gauges/indicators/wind_speed_kts");
gAltitude = XPLMFindDataRef("sim/cockpit2/gauges/indicators/altitude_ft_pilot");
gAxialG = XPLMFindDataRef("sim/flightmodel/forces/g_axil");
gHeadingMag = XPLMFindDataRef("sim/cockpit2/gauges/indicators/heading_AHARS_deg_mag_pilot");
The first image shows the aircraft sitting on the runway end, ready for take-off. You see that the "Wind Data" line is blanked because we have not reached the minimum target speed of 100KT yet. The wind data is only really relevant in flight and is used on this page to compute the Drift Angle. The track heading that is dialed into the INS is a course from Seattle Tacoma direct to JFK (a bit unlikely ... but good enough for the test.
In the image above you see that on the HSI we are expecting the course pointer to point to 063º for our track and the the compass card to indicate a heading of 161º. Both are magnetic as indicated by the "M". You see that we have a ground speed of 0 but a True Air Speed indicated of 2KT ... that's our headwind component that points directly into the pitot :)
Once we are airborne the Instrument Data page can be used as a cross reference to the HSI, the Airspeed Indicator and the Altimeter. In the real aircraft installation, the INS would actually send all this data to the HSI if it is selected as the source for the HSI.
In the image above and below you see the direct correlation between the X-Plane simulated ADI and HIS as well as the output on the Litton LTN-72RL CDU. This is a few minutes into flight and we are climbing through an altitude of 5600ft on a heading of 034º magnetic towards our track of 064º magnetic. I tried to take the two images as close to each other as possible, but you see that the LTN-72RL indicates a ground speed of 244KT and X-Plane shows roughly the same (246KT).
The Instrument Data page of the LTN-72RL has more than 5 lines of data. To reach the additional data the display needs to be scrolled down with the down arrow. The additional lines give data for body acceleration along the track (G's) as well as Drift Angle and Altitude in Feet and Meters (this data is not corrected for pressure). The image below shows the X-Plane rendered altitude, data and the corresponding data output on the Litton LTN-72RL Instrument Data page.
On climb-out and as we are approaching our target altitude of FL200 (20,000ft) the simulated weather outside gets smoother and the LTN-72RL indicates a longitudinal axis acceleration of 0.095 Gs. You are probably familiar with this when you fly on a regular commercial jet, it's that gravity pull that makes you feel just a little heaver in your seat. However, 0.095Gs you would probably not feel and it would be quite comfortable. Given the wind at 18,400ft we have also computed a Drift Angle (DA) of 6.4º nose right ... as indicated by the "R" in the DA line.
Writing the integration for the LTN-72RL is really a lot of fun. Again, the device uses ARINC 575 as the data transfer from the computer simulated INS to the display. The INS simulation runs on the same computer as x-plan ... so all it really requires is my laptop and the ARINC 429/575 interface.
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| Litton LTN-72RL - On Ground |
In the image above you see that on the HSI we are expecting the course pointer to point to 063º for our track and the the compass card to indicate a heading of 161º. Both are magnetic as indicated by the "M". You see that we have a ground speed of 0 but a True Air Speed indicated of 2KT ... that's our headwind component that points directly into the pitot :)
Once we are airborne the Instrument Data page can be used as a cross reference to the HSI, the Airspeed Indicator and the Altimeter. In the real aircraft installation, the INS would actually send all this data to the HSI if it is selected as the source for the HSI.
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| X-Plane ADI and HSI showing the corresponding INS data. |
In the image above and below you see the direct correlation between the X-Plane simulated ADI and HIS as well as the output on the Litton LTN-72RL CDU. This is a few minutes into flight and we are climbing through an altitude of 5600ft on a heading of 034º magnetic towards our track of 064º magnetic. I tried to take the two images as close to each other as possible, but you see that the LTN-72RL indicates a ground speed of 244KT and X-Plane shows roughly the same (246KT).
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| Litton LTN-72RK showing heading and speed data. |
The Instrument Data page of the LTN-72RL has more than 5 lines of data. To reach the additional data the display needs to be scrolled down with the down arrow. The additional lines give data for body acceleration along the track (G's) as well as Drift Angle and Altitude in Feet and Meters (this data is not corrected for pressure). The image below shows the X-Plane rendered altitude, data and the corresponding data output on the Litton LTN-72RL Instrument Data page.
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| X-Plane ADI Altitude Band Output |
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| LTN-72RL Altitude Information on Instrument Data Page |
On climb-out and as we are approaching our target altitude of FL200 (20,000ft) the simulated weather outside gets smoother and the LTN-72RL indicates a longitudinal axis acceleration of 0.095 Gs. You are probably familiar with this when you fly on a regular commercial jet, it's that gravity pull that makes you feel just a little heaver in your seat. However, 0.095Gs you would probably not feel and it would be quite comfortable. Given the wind at 18,400ft we have also computed a Drift Angle (DA) of 6.4º nose right ... as indicated by the "R" in the DA line.
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| Litton LTN-72RL Track Acceleration and Drift Angle |
Writing the integration for the LTN-72RL is really a lot of fun. Again, the device uses ARINC 575 as the data transfer from the computer simulated INS to the display. The INS simulation runs on the same computer as x-plan ... so all it really requires is my laptop and the ARINC 429/575 interface.
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