How do you modify the f-15 air flow ramp

[AniSkywalker]

It lower during startup and sonic speed?

[Felix/FFDS]

If you are creating a new F-15 model, you would need to make the animated nozzle part with dual condition:

At startup to xxx rpm it lowers, and above yyy speed it lowers.  

or

#2are you trying to change an existing (compiled model) = which you can't.