I am flying a World Models P-51 Mustang, with an OS 46FX. Using an APC 10-6 prop, I can turn 13,500 RPM, on the ground. Changing to a 10-7 APC, I can turn 12,500 on the ground. I cannot see any great difference between them, but I suspect that the more horsepower (and speed) will be in the upper RPMs, although I really have no idea what the power curve looks like. What direction should I take with props in order to achieve higer speed. Please offer other comments as well if it helps to achieve greater speed. Also, the canopy is not tightly sealed onto the fuse. Will the gaps cause any problems in high speed flight?
thanks for your insight and assistance.

The higher pitch prop would be better, depending on how draggy the plane is. does it have retracts? that wouldhelp too. if it's a clean enough plane,9-8 prop might work also. just watch RPM. A lot of it is trial and error to find the right prop.

Yes it has retracts, and as Mustangs go, it is pretty slick. It is not a thick airfoil design. I will try to run up a 9-8 and see what RPMs it turns.

I ran a WM P-51 with a 46FX like yours, fun plane, weak landing gear.

Reading your RPM's it sounds like you still might have the baffle in the muffler. I would recommend the 9.5x 7.0 APC prop. This series of props will do better on the top end. It should get the motor to spin in the 15,000 range with out the baffle installed. A 7 pitch would still allow good pulling power to get it off the ground and give good top end for what I would assume as a 5 or 6 pound model. You may have to contact APC directly if your LHS can't help.

I also had one of those tower mufflers which helps a good deal. But you need a spacer between the muffler and the motor to help clear the cowl. Tape up that canopy and go fly. Good luck

I had fuel delivery problems and had to plug one of the two exhaust exit tubes on this Pitts muffler. They are quite large in diameter and just allowed no muffler pressure at all. Plugging one of the tubes gave me the fuel pressure I needed...no more flame outs when going to low throttle..I didn't notice any decrease in rpm, either.
It's a fun plane...gets lots of lookers at the field, but it's not the rock steady flyer that my pattern ship is. To hold this plane in knife edge is a real wrestling match. It'll never be my favorite, but it'll be a regular flyer. I will seal that canopy, and I'll make a trip to the LHS for a bag of props. Thanks for the tips.

My Best advice is to use a tacho and test different props on the ground.
See how big you can go in diameter and pitch before you start to lose rpm. This should give you a good idea as to what will work well in the air.
Obviously High rpm doesn't always relate to high speed. without a tacho to measure rpm you'll be flying blind as it were.
For instance I have A Great planes P51 mustang with a Saito 65...... a 13 x6 inch prop gives considerably more rpm than the 13 x 8 which I fly it on. But the larger pitch pulls the aircraft around far better. The interesting thing is to the human ear you cant tell the difference in rpm between the to props...Good luck

Capt. Jim,

RPM on the ground, at zero airspeed, only tells a small part of the story. In fact, it tells a misleading story. We often hear people talk about props for thrust vs. props for speed. This is not really a correct view. The tradeoff is between thrust at low ( or zero ) speed and thrust at high speed. Generally, lower pitched props give higher thrust at low airspeed, and higher pitched props give higher thrust at higher airspeed. Because the engine only has so much power, you can't increase pitch without reducing diameter. Unfortunately, lower diameter props are less efficient, so there is another tradeoff. Ideally, what you want is a prop with enough pitch to give you high speed, large enough diameter to be fairly efficient, and which your engine can spin at close to its peak power. This is complicated, which is why it is a trial and error process. I would suggest trying something a bit higher pitch, with smaller diameter to let it turn faster. A tachometer reading on the ground can give you some idea whether the diameter is small enough to let your high-pitch prop spin fast, but it is misleading, because a high-pitch prop will 'unload' at high airspeed more than an low-pitch prop will.

Good luck,

banktoturn

Capt. Jim,

Hey, I know this will sound a little crazy, but put on a APC 9x6 and give that a try. Your RPM's should go to the 15-16000 range. With your muffler tube blocked so you can maintain low RPM pressure, you'll be seriously restricting the high end. Best Regards, jeff