I wish i knew ho to do that 
Well, you're using Photoshop, that's a big plus already. Basically, what you do is you create a new layer that goes over your painting layer, whichever that is, and stick a simple gradient in there using the Gradient Tool, then selecting the preset white to black gradient. You make sure the white is completely see-through (opacity at 0%), then drag the tool top to bottom over your livery. Then just crop out the bits of the gradient you don't want to have in your livery, basically everything that's not the airplane, adjust the opacity of the layer until you think it works, and you have a very, very basic shading.