I have an OS 120ax engine... I used a 16x4W prop with 20% nitro fuel content... I snapped the blade on second flight, both sides of the 2bladed prop gone... my question is when going up in nitro one needs a larger prop probably from what I have experienced... if I go down to 5% (still have extra 5L left) can I use then a smaller prop like a 15x6? thats what OS recommends(prop), but on tower they recommend again 16x10-12 or 15x10-12... but they don't mention with which fuel... (naturally by decreasing nitro content you get less engine power?)

I really don't think the nitro content will make that much of a difference.....I ran 15% when I flew glow in everything including 4 strokes.......When you snapped a prop I promise it wasn't from the nitro content of the fuel....probably from a defect in the prop.....I had a prop that failed after the first flight...cracked down the blade long wise.....
Run whatever prop you like as long as it is in the range recommended......
20% nitro will run a little hotter.....give you a better idle....I doubt you'd see the difference in top end

The 120 Manual recommends a 16x8-10 or 15x10-12.

Both the 16x4 and 15x6 will be too small.

thought I could get away with it... a well thanks guys... will get largest possible prop... the engine needs to be killed

Hi!
You don't choose prop according to what fuel you use..you choose prop according to what plane and engine you plan to use.
Soo ...What plane is it you intend to use the OS 1.20 AX in???

Funtana X100... the engine is way too strong... I can fly at 1/4 throttle... swapped it for my Raptor Titan, wouldn't have been my engine preference for this plane... but I have it now and want to fly it... think I will go with a 16x8...

I am flying a pattern type bird with a 120AX for power its swinging a 17X6 prop. Plenty of power unlimited vertical burning 10% nitro for fuel.

I suppose 18 will be too much? I can get a 18x4 at the hobbyshop...

Switch_639,

dont scare that engine with your prop selection.

No jsut kidding!. Like Minnflyer said the manual recommensd 16x8-10. By rule of thumb, you can get 16x8 or 17x7 or 18x6. You need to note that the larger engine rev slower so need more pitch in the prop. So, for example, if you want to 3d on a 55ax, then a 13x4 would be ideal. I found through experience that on a 75AX that would be 14x4 to 14x6 and on FS91SII that would be 14x6-8. On a 95ax that would be 15x6 and on a 120ax it would be 16x8 or 17x7 if there is such a prop. Of course, this will change with prop, airplane and altitude but just a guide.

A fellow filer at my field is flying a Phoenix Yak 60 with the 120AX and a 16x8 prop and I noticed he is not able to maintain elevation in hover- his airplane keeps rising inspite of throttle management. But then again thats a 60 size airplane with a 120ax!

16x4 is too small for the 120ax, if you are looking for a 3d prop, try a 18x4 though I dont think that would make much of a difference, it will give a very low cruising airspeed


Ameyam

I would go with the 18x4 then... the engine needs load on it to protect itself from overreving like it did with the 16x4 and a slow cruising airspeed is just what I am looking for... it was too fast with the 16x4, no wonder it snapped...