Slow Data Read rate for pressure (0x28C8)

[AirPanther]

Good day,

I am able to successfully read most FSX offsets at low-mid millisecond rates (pitch, bank, altitude, heading, etc.). I'm using this data to feed real aircraft parts and the data has been working great. However, my altimeter and airspeed indicators use static pressure and ram air pressure not altitude and airspeed. Here are my issues:

Static Port Pressure Issue

I wasn't sure exactly which offset to use, so I'm using 0x28C8 (Ambient air pressure). This appears to produce the right value, but at a rate too slow to use. Even in FSInterrogate, when I poll this value, it only changes about every second or so, unlike altitude and other values, which are super smooth. I get the same behavior when I use offsets 0x0020 or 0x0B4C to read altitude instead of 0x0570.

Ram Air Pressure Question

Again, I wasn't entirely sure which offset to use, so I'm using 0x30D8 (Dynamic Pressure). The value seems to correlate with the airspeed, but I'm not sure if this is what I should be reading for ram air pressure. Thoughts are appreciated.

 

Thanks very much for any guidance,

Robert

P.S. - Pete thanks for the great product and for ALL your years supporting it.

[ckovoor]

Interesting!

But isn't Ram Air Pressure (or Total Pressure) = Static Pressure (or Ambient Pressure) + Dynamic Pressure  ?

So if you needed Ram Air Pressure to feed your hardware with, you would need to add the values as above.

Also wouldn't Static Pressure (or Ambient Pressure) be a value which doesn't change very rapidly with time ........ unless, of course, you are travelling very fast indeed (vertically through altitude layers or horizontally across isobars) within the atmosphere?

Just my two cents.

Chakko.

 

[AirPanther]

Awesome! I tried your suggestion for total air pressure and it appears that was the answer I needed, thank you very much! I didn't think about Bernoulli's Equation.

This solves my issue for Ram / Total Air Pressure!

Still trying to figure out the Static Pressure issue. I would expect it to change proportional to the altitude like in the real aircraft. It seems to be doing that, just with really low resolution. Unlike the dynamic pressure, which is updating in near real-time (low-mid millisecond rate), the static pressure still seems to be updating about every second or so.

Robert

 

[John Dowson]

Hi Robert,

As Chakko says, the ambient pressure doesn't change very quickly, which is why you are seeing slow updates. If you monitor offset 28C8 and go into slew mode and slew up very fast, I see the following logged:

  1115813 Monitor IPC:28C8 (FLT64) = 2071.1340
  1115813 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2071.13404498
  1115891 Monitor IPC:28C8 (FLT64) = 2070.8867
  1115891 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2070.8867444
  1115985 Monitor IPC:28C8 (FLT64) = 2070.6393
  1115985 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2070.63931635

  1116078 Monitor IPC:28C8 (FLT64) = 2070.3920
  1116078 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2070.39201578
  1116125 Monitor IPC:28C8 (FLT64) = 2070.1446
  1116125 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2070.14458772
  1116219 Monitor IPC:28C8 (FLT64) = 2069.8974
  1116219 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2069.89741462
  1116297 Monitor IPC:28C8 (FLT64) = 2069.6500
  1116297 SimRead: 28C8="AMBIENT PRESSURE" [also 28E8]
            FLT64: 2069.64998657
 

The interval here varies between 80-100ms.

Note that there is a delta value set on 28C8 (i.e. changes less than this are not reported) of 0.00009F, which should be fine.

With further tests, also monitoring/logging 3498 (ambient pressure in millibars) and the altitude (6020), it looks like the sim is only updating on millibar increments above 0.12-0.16 or so (or about a metre).

Interestingly, slewing down slowly (Z) seems to go at a rate of about a meter per second, logging the ambient pressure roughly once per second, while slewing up slowly (F3) seems to go at about 15m per second, logging the pressure around 12-15 times per second.

In summary, I think the rate is down to SimConnect rather than anything in FSUIPC5.

Cheers,

John

[AirPanther]

John,

Thank you very much for your response! What you are experiencing aligns with what I'm experiencing, and it makes sense that this wouldn't be a limitation caused by FSUIPC. So question is, is there another way (or offset) I should be reading to get the static pressure in real time?

Robert

[Pete Dowson]

6 hours ago, AirPanther said:

So question is, is there another way (or offset) I should be reading to get the static pressure in real time?

If you check through the SimConnect variables available to read, and do find something else other than the ones already supported in FSUIPC, hen by all means it can be added. But I'm afraid we are not hacking into the internal code of the Sim these days. 

The list of available SimConnect variables can be found here, under the heading "Simulation Variables" in the SimConnect API section.http://www.prepar3d.com/SDKv4/LearningCenter.php

Good luck. I think you'll find that FSUIPC already reads all those concerned with air pressure.  Both John and I are of the opinion that it isn't re-calculated "in real time" but only based on QNH or altitude changes of a metre or close.   Maybe your only recourse is to interpolate for lesser changes.

Pete

 

[AirPanther]

Pete,

As usual, I sincerely appreciate you taking the time to help me think through this. I will definitely give some thought to interpolation. Will also look at some of the altitude equations to see if I can calculate static pressure from altitude and temp.

Thanks again Pete/John for all your years of support!

Robert