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Prop sizes for J-3 Cub

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Prop sizes for J-3 Cub

Old 01-17-2007, 02:33 AM
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Default Prop sizes for J-3 Cub

I have a J-3 Piper Cub .40 with an 82" wingspan and a Saito 72 4-stroke in the nose. What size prop should I go with? This may seem sad but I never understood what those "prop numbers" mean (ex: 10x6, 11x6, etc.). I like having a versatile prop so that I can fly scale and sport during the same flight. Let me know what you think! Thanks.
Old 01-17-2007, 05:41 AM
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Default RE: Prop sizes for J-3 Cub

Hi!
13x6 or 14x4...
The first number is the diameter in inches..the second number is pitch in inches (the distance the propeller will travel forward during one revolution).
Old 01-17-2007, 03:50 PM
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Default RE: Prop sizes for J-3 Cub

Your Cub is roughly 1/5 scale. the full scale used a 72 inch prop, so 14 inch is the closest diameter for 1/5 scale. I have the Hangar 9 Cub, has an 80 inch wing, and also have a 1/5 scale Sig cub with an 84.5 inch wing. The H-9 has a .72 Saito, and the Sig has a .65 Saito. I use a Master Airscrew 14-6 K series prop on both, and find it's the best all around size for speed and performance. Both engines are more than the planes need for basic flight, but allow you to fly at lower throttle and get that nice "putta-putta" sound. Both engines will easily turn a 14-6 prop. A 14-4 will work, too. The difference between the 4 pitch and 6 pitch is this:

At a given rpm, the 4 pitch will fly the plane at a slower airspeed than the 6 pitch, or

if you want to fly the plane at a faster speed without running the engine at a higher speed, then use the 6 pitch.

The 4 pitch will pull better, but not accelerate as fast. Think of them as gears in a transmission. The 4 pitch is like low gear, good to pull from a start, but only goes so fast when the engine is wound out. The 6 pitch is like 2nd gear, it will also start the plane off, but takes more power, but will also go faster. 6 is as high as you need to go on a Cub, it wasn't meant to haul a**, and any higher pitch is at the limit of the engine size.
Old 01-17-2007, 04:10 PM
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Default RE: Prop sizes for J-3 Cub

Thanks for your help khodges, that really clears things up....seeing as we both have the same engines in basically the same plane, I'm gonna try both 14x4 and 14x6 props in my Cub. One last thing though, I haven't broken in the Saito 72 yet....is there an ideal size to use during break-in?
Old 01-17-2007, 05:38 PM
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Default RE: Prop sizes for J-3 Cub

Either of the 14's will work fine. I ran about three tanks of fuel through mine, run it about half throttle most of the time, occasionally go to full for 5-10 seconds, then drop to 1/4 throttle, just mix it up and don't run full throttle all the time for the first few tanks. (Break it in like you were flying it on the ground). Many people recommend checking valve clearance after break-in, but I went two years on my .65 before checking clearances, and they were spot-on when I finally did check them.

Personally, I think you'll like the 14-6 better, I tried a -4 and it was okay, but the engine was running higher rpm all the time, and the Saito just sounds so good at mid-throttle, and the plane sounds more "scale". My .65 will turn a 14-6 at about 9600 rpm on 15% Cool Power, the .72 goes to about 9850. Not a lot of difference, it seems, but it makes a lot of difference flying. Both my planes are heavy for their size, at just over 11 pounds each (floats on the H-9, lots of mods and a few repairs on the Sig). I can cruise at a click or so less than 1/2 throttle on the Sig, and the H-9 will cruise at 1/4 throttle. The larger diameter prop has wider blades than even a 13, and they move a good bit of air. Even taking off on water only requires a touch over 1/2 throttle to get airborne in under a 100 feet (nice and scale).
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Old 01-19-2007, 05:12 PM
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Default RE: Prop sizes for J-3 Cub

khodges, I love that picture of your Military Cub. The Vice-President of our Club is giving me his old Cloth kit J-3 Cub. I think it's the Goldberg or Great Planes Version. I'm not sure at the moment, he had a 90 in it the last time I've seen it. Anyway, He was landing it one day 3 years ago, and a cross wind flew him in a wire mesh fence at slow speed. Not much damage, he did however did a sloppy patch job on it... he didn't build it for show, and he only flew it in his back yard.

When he gives it to me next Sunday, I will want to replace the Cloth. When he put the Stars on it... he put them on upside down.[sm=lol.gif] He's an old fellow... has big hands and building planes is never in his plans for being show pieces. But what he builds for what I've seen, they operate and fly very well.

I figured I'll Post the rebuild here at RCU this summer or fall. This will be my first in taking on such a feat.

Do you or any of you know where I could get Vinyl Decals for a 1/4 Scale and possible other scales?

One thing I hate is decals that have a shine to it. I want to do a great job... and make him proud that he gave it to me to begin with. He did have plans on burning it in his Burning Barrel... glad he changed his mind. I figure I'd practice with a few rebuilds... and then try and build a kit myself. In the meantime I'm sticking to my Hanger 9 Cub and Pulse.

My wife and I are talking about having me turn the Cub into a Captured Cub by the Luftwaffe. Piper had a contract with Finland just before the war. When Germany took over, they had a whole fleet of J-2's and 3's.

I think it would be a great conversation piece... and very historical. I'm just glad I was rated in a J-3 and have flown many hours in them in my youth. I have never met anyone who didn't enjoy flying the Cubs. Large or small they are a fantastic plane.

Peter Dowling aka Luftwaffe Oberst
AMA District II # 865404
Aero Modelers Club
Pulaski, NY
Old 01-19-2007, 08:55 PM
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Default RE: Prop sizes for J-3 Cub

Thanks, Peter. That picture was taken by our club prez at our Warbird fly-in last year. He's made some really good pictures, he is a partner in a group called "Cameras In Action" (CIA), they shoot for NASCAR, and some of his airplane pics have made it into RC Report.

There's several places in the rags that advertise vinyl graphics. Our club, and the builders in this area, are very fortunate to have a guy that does custom work for us at a pittance. He's retired and has a sign shop in his garage, and all we have to do is bring him a sketch of what we want and give him the dimensions. The nose art, stars and numbers on my L-4 were done by him, and there's hardly a warbird in the area that doesn't have something made by him on it.

Good luck on refurbishing the Cub.
Old 11-19-2010, 08:29 PM
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Default RE: Prop sizes for J-3 Cub

I have a Hanger 9 1/4 scale Piper Cub ARF. I was gluing the windshield on the other day and when I went to pulled the painter's tape away from the windshield and adjacent area of the fuselage, I pulled a clear film away from the black stripe on the side of the fuselage. Is this clear film protective covering for shipping or should it remain?

Thanks for any input.
Old 11-20-2010, 12:00 AM
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Default RE: Prop sizes for J-3 Cub

I had that cub several years ago with several different engines over its lifespan (It is currently sitting in Minnesota basement somewhere...) For that size engine the 13-6 is the best size for all round performance


Old 11-21-2010, 03:01 AM
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Default RE: Prop sizes for J-3 Cub

I like to run a 12 X 6 on my .70 FS engines. I play around allot on the ground with practicing one wheel T&G's and tail high taxi in the grass. I usually run larger tundra tires and with the shorter 12" prop, I get the ground clearance I need. Plus I get better throttle response with less turning mass.




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