Simple contrail system for flightgear
Lauri Peltonen, A.K.A. Zan


This is a simple contrail system which tries to be somewhat physically accurate.
Though  because FlightGear's environment model is not so good in what comes to RH,
the contrails usually look just the same...

Only player's model can cause contrails I think, if someone knows if this is possible
to add to AI and MP models please give me a message :)

Anyways, here's the readme:

- Copy contrail.nas to aircraft's Nasal/ directory.
- Copy contrail.xml and puff.rgb to aircraft's Models/ directory.

- See attached examples from f18. You need to edit aircraft's -set.xml file to
  include the contrail.nas. Example is in f18-set.xml.

- You also need to place contrail launcher behind every engine of your aircraft 
  for best results (meaning 4 launcher for huge jumbojets etc, looks nicer that way :)
  See the example in f18.xml. You need to edit your aircraft's Models/<aircraft>.xml.
  Add multiple contrail models with differet offset to obtain best results.

- At the top of contrail.nas is some default values for modeling the contrail graphics. 
  Feel free to modify them to find options that suit your model best.



A note on the physics:

I used mostly sources like these: 
- http://itg1.meteor.wisc.edu/wxwise/AckermanKnox/chap15/contrail_applet.html 
- http://cimss.ssec.wisc.edu/wxwise/class/contrail.html

This modeling requires that temperature is below -43 degrees Celcius for any possibility 
for contrail to appear. I calculate "real" RH that is different from the usual RH in 
very cold temperatures. That RH is then used in combination with temperature to calculate 
the dissipation time and density (and other factors for nicer graphics) of the contrail.
All formulas are rough approximations, mostly calculated with curve fitting from values 
obtained from that contrail applet.

Because of the "broken" RH model of the flightgear, the dissipation time is always 
maximum, and density depends almost only on temperature -> altitude. Jump to 
10km (30 000ft) to surely see the contrails.




If any questions, better "cloud" model than the puff.rgb or some improvements to the nasal
or the particle launcher, please give me a message. Easiest way is through IRC, I'm Zan on 
#flightgear ang Â#fg_cantene.

Have fun.