Always wanted to ask this question.
In props up to 13" in dia 4" pitches are easy to find. However when you go to 16" and 17" dias lower pitches are rare.

Secondly, on the sim I was trying out a 160FX on the Reactor 50cc. I found that a 16x4 or 5 just wouldnt give sufficient thrust whereas a 16x8 did seem to do well.

Even in the manuals for larger engines, recommended props seem to have higher pitches. So my 55ax has recomended 12x8 where as the 160fx says 16x12 or 17x10.

What gives?

Ameyam

Large engines can't rev as high as the smaller engines can so to have a reasonable airspeed (which basically is what the pitch gives) they need a higher pitch. Because the engines are much larger they also have much more torque (although at lower revs) which is what allows them to turn those big props.

I like the pic, you need more eye shadow.

sticks

Increasing pitch can increase thrust slightly but not as much as diameter. Same goes for 4 strokes, they need much higher pitch props fly properly. For example my Magnum 52 FS like the MA 11X7.5 whereas my 2 stroke 53 like an 11X6.

Ok. Getting to the point, I intend to put it in a Funtana 125, talking regarding a 120AX here. I was going to order the prop from Tower but decided to postphone it till the time I actually order the 125 which will be some time April / May. I actually obtained the engine from my LHS, they had a 120AX ringed that wasnt selling because it didnt have a muffler.

What I wanted to know is, will a 17x4 be enough or should I get a higher pitch prop?. Note that the Funatana is 3D in intent and not designed to fly fast. Also, which propo to use to break-in?

Ameyam

Prop pitch isn't hard to understand, if you run a search there are bunches of threads about it.
The stock answer is: A longer prop with a smaller pitch will give you better thrust/torque and a smaller prop with a heavy pitch will give you more speed. When you look at 3-D pilots props your going to find some long props with very light pitch.
It's still a testing required thing to find the prop that will fly the plane the way the pilot likes, there is no magic number or one size fits all, it's up to the pilot to find the prop they like.
I like my planes to fly slower but have shorter take offs and better virticle thrust so I tend to run longer props with smaller pitch. This thinking works most the time but I have some plane engine combos that just fly better with a bigger pitch.
I think that in the tower catalog they have props listed for 3-D, sport and pattern for the APC props. If you look at that you should see the pitch is different for the same size prop.
You can't just buy one size and think it's the perfect prop for the engine plane combo you have.
Buy a few and test to see what you like. It turns into a long day but it can be fun and an eye opener.
Gene

Yes, I will get a couple. Atlease a 16x8 or 17x8 (will decide when I place the order) and a 17x4/5 if its available.

I have a related question. I have a 15x5 MAS on a OS FS91SII on a Phoenix Topstar (same as UCD 60 and Flip). I use the airplane only for 3D. Earlier I haed problems with a 75 AX on it so I switched to the 91 but have flown it only once since then. I found that the airplane flies very slowly. Should I change my prop and if so which one?

Ameyam

A 17 x 4W or 17 x 6 APC would be about as big as you want to go on a 1.20AX
They both should give about the same load on the engine...a 17 x 8 would be a bit too much.

Hard to say without seeing the plane fly, 3-D planes fly slow anyway. This is why we all say the prop selection or the magic prop is what YOU like. When you see people tossing out an answer stating one size for any engine as the best prop you know that's just silly. Everyone likes there planes to fly differently and a lot of that difference is controled by the prop size and pitch. It will change from brand to brand of the same size too. As long as the engine is kept within it's working RPM range your OK.
I can state an 11X6 APC is about perfect for a .46 engine but it's not really, it's just a starting point for testing, I may or may not end up with an 11X6 APC depending on how it flys the plane the engine is attached to. My .91 FS mounted on a 60 size Kaos ended up with a 16X4 or 16X6 APC prop. Most people will tell you that's not a very good combo but it flew the plane just the way {I} liked. We on the net can't tell you what is slow or fast, it may be just right for some of us and not for others.

On this site http://www.mvvs.nl in the "Utilities" section, is a prop power calculator that was developed by Pe Reivers. You can use this spreadsheet to estimate speed and thrust that a prop might produce at a certain rpm, and the horsepower required to turn that rpm.

While everything in the spreadsheet is good to know, and fun to play with on a rainy day, it may or may not mean much when it comes to flying a plane. Gray Beard runs a 16x6 on a 91 four stroke and it pulls his plane the way he likes. I mostly use a 14x6 on 91 four strokes.

It's written over and over. Prop selection is a very personal thing. With identical engines and airframes I might think a 12x4 is the best prop ever and you may think a 11x6 is perfect.

And what I use isn't etched in stone, I have broken a prop on that plane and not had another of the same size to replace it with in my box. I have used props like a 15X8, 15X4, 14X10. You get the idea. There are a number of prop sizes that can be used but I like a slower plane with a lot of pull. The Kaos air frame was designed to be a speedster back in the 70s running a .61 engine, I over powered it a little and if I run a prop that is smaller and has a higher pitch the plane tends to fly faster then I like.
The prop Carr is using is more in the norm for most people.
Just for grins I will tell you about my CG Extra. I run a YS 1.20 in this old heavy plane. I'm running a 15X8 APC prop. During testing I discovered for one reason or another the plane became unstable {very hard to describe} with other size props?? I tried a lot of different props but even just changing the pitch of the 15 inch prop changed how the plane flew. This is the only plane I have ever had that reacts this way to the prop. I have never over thought the why of it and don't plan to, the 15X8 is the prop the engine/plane likes. In this case it was more the plane then the pilots choice. I don't worry about it or think about it, I just make sure I have an extra 15X8 APC with me at all times.