ruban: Thanks, I like the dual rotors too. :D Well, for the blades, I'll try, quite suck at drawing 45degree.
yrizoud: Noted. Will go try it :)
Looks a bit flat. You could use darker cast shadows, it would help 'outline' the helis. And the steps on the bottom and right side should get a bit of cast shadows, too.
For the middle heli I would suggest turning the blades 45 degrees, as they're blocking the heli as it is.
I'll agree with Luc on this, dual rotor is best.
Thanks Lucanius. Yeah, seems that I forgotten to turn off the white bg when saving.
O_O Whoa! I love the dual rotor one the most. Faved. 
You could probably make the white bg on the detail image transparent like you did the preview.
Thanks =)