Dynamic shine effect from FSX with Photoshop?

[Darth Mop 6]

Hi, well I'm not a newbie when it comes to repaints but I'm not a master either, so I was wondering how we can create a (static) shine on the aircraft like FSX does it dynamicly, check this shot for example (it's my own repaint):as you can see FSX creates a dynamic shine on the plane which causes the top to be brighter and the bottom to be darker, this effect is widely used on the POSKY Embraer series (official repaints)so how can we add this (i guess as a separate layer) with Photoshop?

[Fspaintshop 0]

Its done with the alpha layer or channel. Add the alpha then hit crtl L to adjust how much, black is shiny metal [chrome] less black less shine. Most paints you will go with a very light gray....