Position aircraft without reset

[weliwarmer]

Hi,

I'd like to slowly increase the altitude as if the aircraft is being lifted by a balloon. Is there a way to position the aircraft without resetting the flight?

If I use the Position Aircraft, it resets the simulation and disconnects my SITL script. I've been flying the aircraft up to the desired height which triggers the script to run but it's not really a valid test.

Any pointers in the right direction would be most welcome.

Version 2018.2.2 on Linux.

Thanks, Tim

[ludomotico]

Ctrl+U instantly increases the altitude in steps of 1000ft. If you need other steps, you can configure keyboard.xml globally, or only for a specific aircraft in the .xml describing the aircraft.

[Thorsten]

Do you want to simulate the slowly being lifted part? In this case, external forces in the FDM are the tool of choice (I've been using a rather sophisticated variant of that to drag the Shuttle on top of a 747 to glide tests).

Or do you just want to get higher by teleportation - in this case writing to /position/altitude-ft gets the job done rather efficiently.

[weliwarmer]

Thanks guys. I think the Ctrl+U will help me to test the script in the short term but I'd really like to model the lift by simulating a weather balloon attached to the aircraft. Could you point me to any documentation on where to start with external forces please? Ideally there would be two simulated craft, the UAV and the balloon.

I've started reading http://wiki.flightgear.org/JSBSim_Buoyant_forces

Maybe the development stages could be:
1. Ctrl+U
2. External buoyant force directly on the aircraft - linear 5m/s lift while under 100,000ft
3. Model a weather balloon with attachment to UAV. Balloon simulates a 7.5kg upward force and would burst at 120,000ft. The UAV triggers a separation from balloon at 100,000ft. This would allow testing of emergency scenarios such as early balloon burst.

Meanwhile I'll start with Ctrl+U

[Thorsten]

The JSBSim reference manual is your friend.

[wkitty42]

i was going to mention the UFO's HOME and END keys implementations which raise and lower the UFO... the longer you hold them down, the faster the rising or lowering... the UFO uses its own FDM, though, so it may not be helpful...

[wlbragg]

Between the Shuttle 747 ride (most likely to be closer to your use case) and what is in the cargo hauling of the AirCrane dev repository https://github.com/wlbragg/AirCrane, you probably have all the working examples you would need to do what you want to do. Being able to read it and immediately understand how it works might be another story.
Another example (jsbsim) is the mooring code in the latest c172p and j3cub. It is an example of forces at work between two objects that could be adapted for your use case.

[weliwarmer]

Great info, thanks again.

Need to find out how interaction between simulated aircraft works, e.g. the AirCrane, so I can get the Weather Balloon model to lift the UAV... https://sourceforge.net/p/jsbsim/code/c ... r-balloon/

Looks like I've got a lot of studying to do....

[Thorsten]

It's not terribly difficult, but you'd do well to work through the JSBSim manual.

[weliwarmer]

Knickers! Ctrl-U stops working when simulation is connected to my script (ardupilot SITL).

[Thorsten]

What is your script actually doing?

[weliwarmer]

I'm connecting to FlightGear on udp:5503 using the Ardupilot SITL (http://ardupilot.org/dev/docs/setting-u ... linux.html). There is also a python script running on a virtual companion computer which watches the altitude, ready to trigger UAV release from the balloon and start the descent flight (https://github.com/EqualExperts/eesa/bl ... mission.py)

When ardupilot SITL is connected to FlightGear, the CTRL+U does not work.

[Thorsten]

Well, the site you linked says

Developers can optionally install the FlightGear Flight Simulator and use it (in view-only mode) to display a 3D simulation of the vehicle and its surroundings. (emphasis by myself)

So if the actual simulation is done elsewhere and FG does just the visualization, naturally you can not influence the state of the simulated object FG-side - you have to prod your simulation software to alter altitude.

[weliwarmer]

Yeah, boo. I was hoping that there's a simple way to increase altitude. Even setting up a Generic Protocol UDP listener and injecting /position/altitude-ft does not work, I can see the altitude jump to the required value but then it bounces right back to the home altitude.

I don't (yet) understand how the SITL script runs as an external FDM enough to modify it. Time for more reading....

Thanks again for your help.

[Thorsten]

I'm not really sure on what you ultimately want to do - but as long as you use FG for visualization, the position data comes from eslewhere, so all you do inside FG is immediately overwritten when the next update from the external FDM arrives.

You also can't use JSBSim, buoyancy or external forces in that case - all of that has to be in your external FDM - FG just provides what you see at the position it is told from elswhere.