To add to my confusion, I have just bought an OS 95v as well. So now I have almost all possible engine combinations to use on the Calypso. It is a good problem to have but it is confusing nonetheless. Hehehehe! But I am now leaning towards the OS 95v since it will be mounted inverted and can be vitually interchangable with the OS 61vf or Hanno Special if I decide to go with a 2 stroke. The OS 55ax, on the other hand, would have to be mounted at a 45 degree angle to allow proper pipe placement. This would make it more difficult to re-engine the plane if it is underpowered. Hahaha. I really don't know what I am going to use yet. All I know is that I will have to decide by the weekend.