Sarpolus 90% Challenger engine

[Marty Hammersmith]

I’m plans building a 90% Sarpolus Challenger. The plans call for .35 engine. Would a modern ball bearing .28 be appropriate? I can do that or .25 ball bearing or I have an Evolution .36 already converted to CL but that seems likely overkill especially since this is 90%.

[BYU]

Try an OS 25 LA or FP or an Enya 30ss

[Phil Hawkins]

At 90% a .35 stunt engine would probably still be fine. Whatever you decide, try to leave room to change up or down if you want to change engines. I have a couple planes that the engine bay is a perfect for the original design, but too snug to upgrade without major surgery. Maybe make a test stand and run the engines you have and get an idea which one will give you the best propeller choices. Plus weight. Also build as light as you can get it. This might be a great platform to go electric!