All I know is I tried very hard and talked to a lot of respectable pilots in different flying categories (pattern, IMAC, and 3D) and found that it is very difficult to find a happy combination of 25% plane and gas engine. 26cc is not enough power (for aerobatics) and 40 cc is too much weight. What is the wing loading on the Yak with the 40cc? What happens when it dead sticks? I thought about getting something very similar, and these were the questions I was asked. And of course there are always exceptions, and you can get things to work that are not optimum.