This thing screams with a .25 FX, and vertical is absolutely amazing! I've got a .25 LA lying around, and I was actually thinking of swapping that in to save a little weight. I couldn't find a larger fuel tank that would fit, so I just have the stock tank, and I set my timer for 6min flights. I originally intended to build this as an electric, but then I had a .25 FX lying around, and decided to try that.

I would guess it would fly fine with a .15 LA. I think they recommended a Magnum .15 ball bearing engine, which puts out more power than the LA. Mine is very overpowered with the .25 FX, but then a .15 LA has only half the power.