Mach / IAS doesn't change with altitude

[BAW019]

Hi Peter,

I'm using the new (and frankly excellent) Ellie Systems EADI and we've noticed something a bit odd with the AP speed offsets. Not sure if it's something you can deal with or not but I thought it was probably worth asking, espcially since it made us realise that the behaviour also shows itself in the PFD of the default FSX aircraft.

As you know, Mach numbers have equivalent airpseeds that change with altitude. If I were to set my AP to maintain 250KIAS the plane would do exactly that, and as my altitude increased the airspeed would stay at 250KIAS while the ground speed slowly built up. On the speed strip my magenta speed bug would be stuck to 250 because I'm in SPD mode.

However if I select MACH and set 0.72 on the autopilot, the quivalent airspeed changes with altitude. Although the aircraft obediently maintains that MACH speed it's groundspeed and airspeed both change with altitude.

So when I'm using MACH to command my autopilot the magenta speed bug should slowly drift along the speed strip of the PFD, reflecting changes in the IAS at different altitudes but it doesn't - it stays put until you tweak the AP's mach knob. Then it jumps to the correct position on the strip.

There's obvously something inside FSX that can calculate airspeeds from mach speeds correctly - like I said it corrects itself if you move the MACH knob - but it doesn't do this constantly which means the PFD shows the wrong infomation most of the time.

I guess my question for you is this: is there any code you could (and would be willing to) add to FSUIPC to spot that the autopilot is in MACH mode and get it to automatically populate the AP IAS offset with the correct value for the altitude? (I posted a couple of equations over on the Ellie Systems forum that appear to do this if you're interested).

Quite understand if you can't or don't want to touch this but it would be a nice fix for a curious bug in FSX,

All the best

Ian

[Pete Dowson]

is there any code you could (and would be willing to) add to FSUIPC to spot that the autopilot is in MACH mode and get it to automatically populate the AP IAS offset with the correct value for the altitude? (I posted a couple of equations over on the Ellie Systems forum that appear to do this if you're interested).

Rather than use equations wouldn't simply making a quick small change to the Mach setting, and back, work, if you've found that twiddling the knob re-computes the IAS target? At least that way the result would be correct according to FSX's equations (and approximations) rather than someone else's?

I'll take a look.

If I add this as a "Miscellaneous" fix it would only apply to Registered users. Is that what you would expect? These days, with SimConnect gradually taking over most of the application interface duties, I have to add as many benefits as possible for Registered users.

Regards

Pete

[BAW019]

I'd be happy with that - I'm a paying customer :wink:

It's an interesting idea about the tweaking - I haven't tried doing so programmatically but I'll give it a bash after work tonight.

Cheers Peter,

Ian

[Pete Dowson]

It's an interesting idea about the tweaking - I haven't tried doing so programmatically but I'll give it a bash after work tonight.

Results of experiments here:

First off, the IAS/Mach C/O button in the FS gauges do NOT change anything detectable inside FS itself unless the A/T is active. It is merely a local display option. There's no way FSUIPC or anything can detect that you are displaying Mach rather than IAS, and I could not risk updating the IAS from the Mach without knowing you are displaying Mach in case it should be the other way around.

So, whatever fix is adopted will only work when a speed control mode is active. Is that what you wanted? I have no idea how the "Ellie" gauges you are using handle the display change or what it displays, but it seems to me that if this is to be completely fixed, the local display operation would need modifying too.

Second, I've found that all that is needed is for me to rewrite the Mach value as it is -- it doesn't need to be a different value. That makes it simpler to implement -- I don't have to run the risk of making two changes with the possibility of ruining a user change in between. There could still be a danger of a user mach change getting temporarily nullified by one of the automatic re-writes.

I'll experiment further and if okay send you a version to test.

Regards

Pete

[BAW019]

Hi Peter, from what you've said I think it makes very little sense to risk a stable version of FSUIPC. The maths behind deriving the correct IAS for a given MACH/Altitude is relatively quick and easy and I'd imagine it would be more prudent to change the rendering program (Ellie) than mess with the data by changing FSUIPC.

If you've other things to do (!) I wouldn't spend any time on it but I'd be happy to beta test anything you may decide to do!

Thanks for looking into it,

All the best

Ian

[Pete Dowson]

Hi Peter, from what you've said I think it makes very little sense to risk a stable version of FSUIPC.

No risk. It only does something when the option is enabled AND you are using the A/T in mach mode. And then all it does is occasionally (once every 15 frames or so) re-write the current AP Mach setting.

The maths behind deriving the correct IAS for a given MACH/Altitude is relatively quick and easy and I'd imagine it would be more prudent to change the rendering program (Ellie) than mess with the data by changing FSUIPC.

But that won't help non-Ellie users -- after all it affects the default FSX gauges too.

If you've other things to do (!) I wouldn't spend any time on it but I'd be happy to beta test anything you may decide to do!

It's done. It wasn't hard ;-). Try it:

http://fsuipc.simflight.com/beta/FSUIPC4263.zip

The option is on the Miscellaneous tab.

Regards

Pete

[BAW019]

Yep that's done the job. Thanks Peter, I'll tell the developers at Ellie you've sorted it,

Thanks again for looking into this!

All the best

Ian

[Pete Dowson]

Yep that's done the job.

Okay. I've released 4.263 in the FSX Announcement above, now.

Regards

Pete