Hi all

i I have questions regarding propeller sizing. I have a 60 size piper cub , fitted with a OS 61 FX 2 stroke engine. I am running normal 15% nitro glow fuel.

When i purchased the plane , it had fitted , a 12x6 master airscrew propeller. When I flew the plane , it seamed to fly well , but however the " nice cruse speed" was around 1/4 throttle. I want this speed to be up about 1/2 to 3/4.....not so low down on the stick. I have made sure my carburettor barrel is proportional to stick position ie carb is at 1/2 when stick is at 1/2 and carb fully open when the stick is fully open.

So , in my wisdom I decided to change the size of the prop to 13x6. The plane still flys well , and the nice cruse speed is about 1/2 where I want it to be. The only problem I have with it now , is that is all I get. I get to about 1/2 throttle plane flys well , but I have no more power past this point. If I push the stick to full throttle the plane does NOT seam to go faster. I'd like to have a bit more go when I give it full power to get out of a sticky situation if I need too.


I have have started to look at propeller sizing on the net , what I should expect to see when changing props etc , but I am finding it very hard to understand. ESP when referring to the length of prop , I understand pitch kind of , but the relationship between the two I am having trouble with , and what I should be doing to get the desired results from my cub.

I I have asked a few people from the field , and the best answer I have been getting is just try and see. Well , there must be more to it than just trying the props to see what happens......

it it would appear that , according to OS manual , the 12x6 is a good propeller for this engine. However , like I mentioned above , the speed that seams to be a nice cruse speed was roughly 1/4 throttle. I want it to be up around 1/2 to 3/4 , so I guess a slower prop......so I have to give a little more throttle to be at the same speed I was with the 12x6?


sorry if this seams like I am reading too much into it , but I like to have a good firm understanding of how and why , not just " try another one" and see..........


hopeing someone here might be able to help!

thanks heaps Craig.