I have a 91 OS 2 stroke I just purchased and I have the identical one in a Carl Goldberg Super Chipmunk. There is a large selection of props listed starting from a 13/8 all the way up to a 17/6. I have been using a 13/8 with my chipmunk and plan to put the new engine on the Hobbico Sukhoi. I am really happy with the the power on my chipmunk, but it is a bit fast and could use a bit more vertical. The engine turns up about 11500 rpm. What are the pros and cons on diameter versis pitch? If I go to a wider prop, say a 14/8, the RPM's will drop I am sure but what kind of changes will I see with the way the plan flys? The manual recommends a 15/6 which will lug the engine down to the 9000 rpm range I think.

Best regards,
Randy

basically I think it boils down to higher pitch smaller diameter = more speed, more rpm. more diameter less pitch == better verticle but less top speed and lower rpm which also facilitates longevity in an engine!

I dont know much about the 91 , but 9,000 isnt to bad, .. try the 14 x 8 or perhaps even a 14 X 7 but from what you say about going a bit to fast and needing more verticle the 15 X 6 might be the way to go

Go to the Master Airscrew web page. There is quite a discussion over RPM, performance, etc from props. Basically, it is a way to arrive at the prop that will deliver the best performance. Turning fast does not in itself produce the best delivery of power. And if you want more vertical, you want more power.

Cheers,

Chip

I had a world models chipmunk with an os 91fx on it - it flew well with an apc 14x6

Hi!
I would try a 14x6 APC! Master Airscrew is no option as those are louder and doesn't provide the speed nor thrust needed.
Regards!
Jan K
Sweden

I would back off the prop load slightly and let the engine turn a little faster. 11,500 is a little slow for a two stroke.

Prop relative load
13-8 1061
13-7 929
14-6 923

Both 13-7 and 14-6 should turn around 13,300 to 13,500 and make considerably more power. My experience indicates that 13-7 will pull the up line longer and also fly faster.

Bill

Thank you all for your help... I will try a smaller higher pitch on both planes. I like the way chipmunk works now, but I always back way off on the throttle on the down leg in fear of ripping her apart, and since I built the chipmunk, I know it is better engineered then the hobbico sukhoi.

Best regards,
Randy