GB is right I may be all wrong but lets see? Seems to me its a long ways from 64 to 71inchs especially a biplane. And I'am betting that long way is just about the differance between a ninety and a one twenty FS's.

If one makes the asumption that a 120FS is ideal for GP's airplane and I do based upon flying two of them with a 120FS and the other with a G23 that would seem to indicate the ideal engine size for the Stirling airplane would be a 90FS considering modern taste's and power to weight ratios. Another factor here while yes the GP airplane is light by modern standards but its a tank by RC standards of half a century ago. I don,t know your airplane but its quite probably a lighter built structure.

OK guys sorry for playing the devils advocate but hey whats the point of asking if everyone agrees, So yup I think a 120 is overkill. Gotta run now Incoming, I.am heading for cover

John