I have the ST 51 and love it. One pop with the starter and it's roaring. At times, I fear I'll shear the wings off. I'm flying it on a Wolff Pak Thrush with a 60" wing span. It's a small model, only about 5 lbs, but man that .51 pulls it with plenty of power to spare. I fly mine with an 11X6 APC prop.