A slow fly prop is the more delicate of any other prop models, its light weight, usually has wider blades. A Sport prop would be the beefier prop, takes hits better but also weighs more. An electric only is somewhere in between. If you have a light wing loading slow flyer like a Nutball the slow fly props are great because you can put a bigger prop on a smaller motor, less rpm with wide lofty blades is more efficient, especially on lighter batteries like 2 or even 3s 1000mah