AI Aircraft direction not matching ownship

[skelley]

Hello all,
I am working on a refueling scenario for the F-35 using a KC135RT for the tanker. For ease of setting up I have both planes at the same latitude with a small difference in longitude (about .5 nm) and both set with a starting direction in the FXML file of 90.0 degrees. Also all of the [pbhx]VelBodyAxis settings are and the yVelBodyAxis to set level flight on startup. When I start the scenario the F-35 is showing 98 degrees and the TSD compass says the F-35 is not pointing due east but the tanker is. As the planes fly they separate along x. The icon for the tanker on the TSD appears headed due east as expected but the ownship is not. Its as if the F-35 is slewing 8-9 degrees left and the flight path drifts slowly to the north. The plane's indicated direction has to be between 98 and 99 degrees to stay on the same latitude.

Prepar3D settings:
Realism settings are all 0.
Turbulence is off.
Weather is set to none.

Scenario variables:
Rudder is centered.
Yoke[XY] are centered.
ElevatorTrim = 53.91
Heading (for both planes) = 90.00
[pbhx]VelBodyAxis are 0
YVelBodyAxis = -32.435179080017562
ZVelBodyAxis 684.02608191093509

The xml file contains no commands for any of the control surfaces and the tanker is flying toward a waypoint with exactly the same latitude. I have attached a dropbox link to a screen capture of the TSD right after startup that shows the planes at different angles. Can anyone explain what is going on?
https://www.dropbox.com/s/26ibtgsizkjyxsk/TSD.png?dl=0

[skelley]

I'm adding information and thoughts to my original post.
The tanker described in the original post flies true east as expected based on the initial heading and the waypoint having the same latitude as the tanker and its latitude does not seem to change over time. If I let the scenario run for a few minutes and then save as a new scenario the tanker's latitude is the same in the original .fxml file and the new one. The F-35 however creeps north as though there is a cross wind coming from the south. The difference between the compass heading and the specified heading would also indicate a cross wind but the tanker does not behave as though there is a cross wind. Also, as indicated in the first post, the weather is set to 'none':
<Section Name="Weather">
<Property Name="WeatherType" Value="0" />
<Property Name="ThemeName" Value=" " />
<Property Name="ThemeTime" Value="0" />
</Section>

I am not a pilot and am not 100% sure what I should expect but plane behavior in this scenario is not what I would expect. If there is a cross wind I would expect both planes to be affected and the weather setting should be no wind anyway.

Any help in understanding what is going on would be greatly appreciated.

[Adam Breed]

Hi skelley,

Each aircraft has slightly different flight dynamics.

There could be some control noise or other forces causing the F-35 to slightly drift.

To help rule a few things out, could you swap the F-35 with another vehicle and see if the issue can be recreated?

Regards,
Adam

[skelley]

Thanks for the response. I suspect what I am seeing is correct, I just don't understand the 'why' of it. I have a call in to our SME (he may not get back to me since the contract is up) to ask him if what I am seeing is what I should expect.

[skelley]

Our SME says the symbol that represents the tanker on the TSD should indicate the tanker's magnetic heading, not the absolute heading, so the symbol is not being rendered correctly.