What happens when an engine is under-propped?
What is the lowest pitched 12 in. prop I could run on a .70
Surpass?

Here's TH's specs:

Suggested Prop(s): Sport & Stunt models: 11x8,11x9,12x7,12x8,12.5x6
Scale models: 13.8x8,14x7,15x6,16x6
3-Blade: 11x7

Thanks Stick Jammer

The manual does recommend those props, but states it is a starting point. Slightly larger or smaller props can be used.

Guess I should have mentioned the engine is in a Big Stick .40.

I am wondering if the engine can be damaged, or if a plane just won't fly if it is under-propped?

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Yes, under propping can cause engine damage due to higher RPM's than the motor was designed for.

an underpropped engine will over rev and it will definetly shorten an engines life, particuarly 4 strokes. what engine?

Hey, formation flying!

.70 Surpass, but you knew that.

The avitar fooled me. Thought you were Stick Jammer.

4 strokes can be damaged if they over rev. You need to keep the rpm's in the suggested area for best performance. If you over rev you can float the valves and that can create a problem. It is kind of like driving your car way past red-line. Something is going to give.

Guess I will get a tach and a selection of APC's and do a little experimenting.

Thanks guys,
Steve