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.

When you say nice cruise you are basing this on what you see. Well that's ground speed. Airspeed we can't measure. Stall speed is based on airspeed. Just be careful you don't get too slow of an airspeed and stall.
The way you make it sound it sounds like your throttle isn't set up correctly. The engine should increase in speed the full range of throttle movement.
The 13x6 should be just fine but Master Airscrew have a lot of flex to them try and APC prop

You went the wrong direction with a 13-6 your 61FX will be far better off with an 11-7 and the level speeds will increase and the shorter prop will improve throttle response.

John

Thanks guys. I wasn't too sure. It seams funny how it doesn't seam to go much faster even tho I give it more throttle. I'm guessing here it is the same as a car going up a hill in a particular gear. If that gear is too high , ie 4th , even if you push the pedal further down , it doesn't go up faster........select a higher gear ie 2 or 3 , and your away. Would a similar thing be going on here with my prop? Maybe I will drop down again to the 12x6 or even an 11x7. Yes I am basing this on what I see. Up until this point I was flying a 40 size boomerang , fitted with a OS 55AX , with and 11x6 prop. I got used to that speed , and this plane seams to go slower........at full stick , for me it's like 1/2 the speed of my boomerang , and I worry about stall. At present the 13x6 is flat out at 1/2 power and anything from 1/2 up doesn't seem to have much effect at all......yet when I ground the aircraft and check the carb barrel , it seams to open and close as it should Ie at 1/2 power it opens 1/2 way and at full power it opens fully. However , when in the air 1/2 to full seams to do nothing , which makes me suspect I have done something wrong with prop selection. If not the prop , what else could cause this? Cheers

Hey it is a Piper Cub! They are many things but fast is not one of them.

Stick with the 12x 6

Thanks heaps guys. I really appreciate your comments. I love the cub! I may not be a speed machine but it's such a pleasure to watch and fly. I am seriously considering getting a giant scale one , maybe a decathlon too. Can't beat the old classics !

Craig.

You are welcome Craig. And yes Cubs are fun, everyone loves a Cub once in a while This is also a good illustration of the importance of prop selection and not being afraid to experiment a little. Every modeler/flyer needs a prop wall. Props are relatively cheap and every time I am in a shop or ordering I always get extra and select odd sizes as well.

John

Go back to the 12x6 and adjust the linkage to your carburetor. If you will point your servo arm nearly straight away from the carb, you will get much finer movement at the lower throttle settings. 50% throttle stick shouldn't give you 50% barrel movement if you want a linear response. About 1/3 throttle at 50% stick is closer to being right. So you want the throttle arm pointed almost directly away from the engine at idle, then just a bit past 90 degrees to the engine at full throttle for max movement. Making that mechanical change will reduce the sensitivity at low throttle settings and increase it at high throttle settings, giving you that midrange control you are looking for.