I have a OS 46 FX on a WM P-51. The baffle has been removed from the muffler. I use OS-8 glow plugs. I am currently using Wildcat with 10% Nitro and 18% synthetic oil.

Which prop should I use if I want to go fast?

I've tried a APC 12x6 (too slow), APC 11x7 (acceptable), APC 10x7 (a little faster). I am currently using a APC 10x8 which gets me good speed at 11,500 RPM. I have a APC 10x9 waiting to be tried.

If my overall goal is speed, is there anything wrong with going with a small diameter to get more RPM and high pitch to get more speed?

I know I can get the RPM higher than 12,000 RPM if I lean it out, but I don't know what limit I should set for max RPM. The guys at my field use the OS 46 FX on various planes and normally use props with larger diameter and limit their WOT RPM to 11,000 RPM. Although I want speed, I don't want my OS 46 FX to live a long life.

What is the practical maximum RPM for this engine?

Thanks,
Juice

Juice,
I'm using a MAS 11x5 on my 46FX and it's a good match and I'm still running rich for break in. I also had good luck with an APC 10x8.Pete

9 x 6, ar 9 x 7 APC

I would go for an 11 x 6. This is the best compromise for the OS 46 FX for resonable RPM and reduced engine wear...

Also consider fitting a Master Airscrew as it is lighter than the APC.

Finally, as a general rule , if you want a torquy plane you have to choose big disc small pitch, and for speed you should prefer small disc big pitch....

Good luck

Juice,

You are doing it right.
To get more speed, you must get more RPM and use a higher pitch prop.

But drag is also an issue. The P-51 is one of the fastest planes of WWII, but you WM still makes some drag, especially because despite its scale structure, it does not have a laminar flow wing, like the real thing.

To get more speed out of your model, you can reduce the drag by flush fitting gear doors and smoothening the control surface hinging surfaces.

But you need a lot more power. Your OS.46FX is not the power paragon you apparently think it is.

You should get a Jett .50, or another very strong engine.
Yes it could cost you. It will get your P-51 to high speed, due to high RPM on a 10x6.

If you go for a Jett .60, or a YS .61 two-stroke, you can get the same high RPM on a 10x8, or a 10x9, which can bring you to a 30% higher speed.

It doesn't matter how much nitro you use, a common .60 engine, will not bring you to the RPM you need, on a usable prop, for the speed you want. Neither will your .46FX.

You have been going in the right direction and train of thought! The 10x7/10x8 is probably the best prop for the OS on a muffler. Use an APC or MA Scimitar series prop. Stay away from the standard black MA or even the TopFlite props. Experiment with some 15% fuel. The speed freaks usually go with a smaller diameter prop. Throw on a 9x10 APC. If you really want some more umph then get a pre-tuned pipe setup from MACS. This will give you at least another K RPM.

You have to try to get the engine to run in the 14000 - 15000 rpm range. This is where it develops its optimum power output.

The 10x9 and 10x8 props pose too much load on the engine to achieve this.

First, pipe the engine (although it will ruin the looks of your Stang). A standard muffler will hold you back.

Try 9x8 and 9x7 props.

The others are right though, that .46 FX will never make your Mustang go ballistic.

If you want real speed, try one of the common speed designs: Magnum, Dust, Whip, ...

All good advise posted here. We use to race these planes with that very engine. After trying every prop under the sun, we had best results with either an APC 10x6 or APC 10x7. Anything bigger, did not go any faster, and the smaller 9" props just didn't have enough thrust.

If you haven't removed your muffler Baffle, you can do that for a small boost. A cheap way of increasing speed would be to purchase a Tower Muffler for your engine. This will increase your RPM. The Ultrathrust muffler would work even better.

The only problems with these mufflers, is that the installation will be tough unless you run the engine upright, or inverted. A standard side mount won't let the muffler clear the bottom of the fuze.

Thanks for all your responses... they are all good.

I don't want to spend more money on this plane and engine. I know the 46 FX is no powerhouse. I just wanted to find the max speed for this combo.

I know I'm running the engine a little rich still because I've been too chicken to go over the 11K RPM limit I've imposed on myself. The muffler baffle has already been taken out. I guess the next thing to do is to try to find the Max RPM - 300 RPM for each of my 10" props. Then I'll try it all again with higher Nitro content, after I finish my current gallon of 10% Nitro.

daven... It's good to hear some first hand experience with this combo. What fuel did you use and what RPM did you achieve? Did you have any problems cooling the engine? Mine is inverted and I cut out 2.8125 sq-in from the bottom of the cowl behind the head of the engine.

Thanks,
Juice

Juice,

The 10x8 size will give you as much load as an 11x6. If you use the Master Airscrew prop, it will not be as heavy as an APC and your engine will be turning approximately 11.8K.

If you change to the Hobbico .46 muffler (I am told it is the exact same as the Tower muffler, at a little less cost.), you will be getting 12.3K.

A gain of 500 RPM is 13% in horsepower output, which is a difference you can really feel.

A 10x8 at 12.3K will give you more calculated speed than a 10x6 at 14.5K.

At $13, it is much less expensive than any "real" pipe.

If you want speed all you have to do is look at what the pylon boys are running. A .46 on 15% PowerMaster, a #8 plug, and a 9.5 X 7 APC prop seems to be about optumum. Usually the ground RPM is set around 14.6k figuring it will gain more when it "unloads" in the air. In the WM T-34 which is raced a lot that equates to around 92 MPH on a straight run using radar to measure. Funny thing is engines seem to do as well or better with the baffel in. Oh well, check with your local pylon racers. They know how to go fast and turn left.

Roger