I am running a 13x8 with no heat problems and plenty of RPM. I have not tached it though. OS Manual calls for a 13x8-10. I just want a go fast hotrod.

Based on my tach readings on my Tower 75, my guess is that you get about 10000 rpm on that 13-8 if it is an APC, which is to low for the engine to produce its best hp. If you want to go fast you also need rpm`s and not just the biggest prop you can find. A 12-8 will be faster in your 50 size plane because the 12" diameter is plenty for such a small plane and it will turn alot more rpm`s with the same pitch (8) and your plane will go faster.
As a general rule OS ALWAYS recommends to large propellers for their engines, this is only because they want to keep noise down
Lug the engine down with a too large propeller and it will not produce its max horsepower and you will not be able to go as fast as you could have been.

You need to get yourself a tachometer to learn what prop sizes your engine will turn well

APC 13x8 gets 10,900 on my .75AX

Not bad, that is better than the Tower 75[X(] But what I wrote previously still applies, it will be faster with a 12-8, as long as it`s not a really draggy plane.
12-12 is just trying to kill the engine.

Sevans16. If you are this hungry for maximum speed you really should have bought the 91 FX instead. It will turn the same prop faster, or turn a bigger prop just as fast and it is basically almost the same engine when considering the weight and physical size.

OS .55AX spins a 12X7 no problem. I would think the .75 would handle the extra pitch to 12.

Some OS 1.20AX owners are running a 17X6 at 9600. That rivals a Saito 1.50!

One thing that I'm concerned about is OS lists the .75AX at 19.4oz. They list the 1.20AX at 21oz. These two engines can't possibly be within two ounces of one another.

well as far as the "designed to run bigger props" comment goes for the 55AX, Im running mine all stock, 15% omega fuel with a 9x9 APC at 15400 rpm and 9x8 APC around 16K rpm.. Very nice performance IMO without a tuned pipe or a tuned muffler..

Ive also got the 120AX, but im running it at 9100 rpm with a 17x6 JXF for 3D. Note that this engine is rated max HP @ 9000 rpm and max usable advertised as 9500.

Now the 55AX is advertised as 17k and 75AX also 16K. So I see no reason not to run smaller props on the 75AX such as 10x9 11x8 12x7 or 13x6 to gain RPM and total power output..

15x6 APC on a .75 2 stroke?? come on!..

I have an aero-works 540T, 60" wing span, rated for 61 two stroke engine. I put the 75 AX in with a 13 6 APC, run most of the time at half throttle. It tacs at approximately 11,800 with Powermaster 15%. At full throttle it is very fast and has great acceleration. Transition from idle is great, no hesitation, and very low extended idle. Love the engine and am puting one in the Skylark 70 next.

Just thought I would add my number to the mix.

Fuel homebrew: 5% nitro, 11% castor, 11% Coolpower blue, 73% methanol.
Altitude ~1700ft
plug OS#8

APC 11x9 ~12,150rpm

APC 12x9 (cut down to 11.5x9) ~11,000rpm

APC 12x8 (cut down to 11.5x8) ~11,550rpm

I did have some figures for an 11x10 APC but can't remember them off the top of my head. Got about 35 flights on it now (12oz tanks) and still running the same plug I ran it in on. I don't have any idle RPM figures but it's low enough for what I need.


I'm tempted to drop the oil back to 18% (9%castor, 9%coolpower) but I'm happy how it runs and it doesn't mess up my plane. Ran it in as per the manual on a test stand, full throttle alternating between rich and lean. The nip at TDC has dropped off a fair bit but I'm logging the flights (and rpm every now and then) and will see how it holds up over time. The first throttle up after taxiing to the strip is a bit gluggy, but in the air is spot on for how I fly (sport pattern).

60" Royal FW190 with empty weight 4.4kg (fuelled weight 4.75kg)
Running a Master Aircrew 13x8 3-Blade prop @9,600 RPM.
Model cruised at 60% throttle above calculated stall speed of 44km/hr

yes, too much,, for a 2 stroke you will need a 91 (even larger) for an apc 16X4W. as for a 4 stroke buy a Saito FA125

Jim

A 13 X6 APC prop flys my heavy bi wing Arrowmaster with extended upper wing for IMAA legal events. All flight characteristics are normal. It appears to fly a bit slowly, and has short vertical climbs, but the airplane is totally controllable. This plane would have a bit more of the Circus plane zip with the OS 75 AX, and experience tells me to go long and light on pitch. This Always makes OS happy. This characteristic emanates from the way engineers at OS spec out their engines. I would use a 14 X 6 prop and let the engine fully breath after take off as she unloads and RPM grow. This engine is stronger than you think and CAN handle 14 size props. We all know that looking for a vertical climb the higher pitch props lose RPM's too fast.and the short prop with low pitch will take away the speed element.
Enjoy the OS 75AX, I'm planning to own mine soon.
samiamaustin

As has been noted by Great Planes/Hobbico Tech in the past; the AX engines do better when propped for a ground peak around 12k. The lighter the prop load, the better they unload in the air.

And…..
I just so happen to have one available

Lol.
hpw about a nice 82 ?

Do you need one?


My favorite FA-82 is the FA-72

Quote:

Do you need one?

no. but I thought I don't have one, and if the price is right, you know?