As stated, When the engine range on a plane is stated, you want to at least the top engine stated, if not a little more depending on your flying style. Something like a cub is ok to stay with less power, but some planes just won't fly. I remember my first one stated 25 to 40. It was a long time ago when the engines don't have what they do today. I can't imagine a 25 flying it. The 40 was almost not enough. I think they are tested on cool days at sea level to get the engine sizes. Unfortunate and kind of miss-leading, but put the 15 in a drawer and save it for another plane. I have a couple of 15 size planes that are rockets. What a blast.