I have the evo trainer engine on my sig LT40. When testing it the other day, it seems to really scream WOT. I had changed the prop from the 3-blade trainer prop to a 10x7. I also had taken the weight off the flywheel. Would a 11x6 slow the engine down a few rpms?? I took the limiters off the high speed needle and there is oil and smoke coming out the exhaust. It also wants to scream louder doing the pinch test so I feel its tuned right. Just nervous with this high RPM.

a 10x7 is a bit small for that engine, try an 11x6 like you think, I use an 11x6 on my p-51 pts with really good results, when I want to do high speed passes I go to a 10x9 apc with no issues, I tried an 11x7 and it worked ok but the vertical suffered.

To answer your question exactly the 11x6 will slow it, please be sure you are not to lean, I like the engine to run for a couple of seconds when the line is pinched not just rev then die.

I also have the pts mustang ( mk 1 ) so the change in engine sounds confused me. Thanks for the info and I'll swing by the hobby store on the way home.

My most favorite qoute is "don't confuse noise with power". Get the prop that works, and keeps your RPM within the operating range of that engine. And, don't confuse the power ratings by the manufacturer with real, working RPM ranges. Keep it below 12,000 RPM and get a prop that works for you. That may mean getting three or four, but you will always use the ones you didn't use this time on something else.

Remember... the prop sizes, such as 11-7 denote this: the 11 is the width. the 7 is the pitch. The pitch is how much the prop would move through the air in normal operation, or under ideal conditions.. in this case, 7 inches per revolution. At 12,000 RPM, this prop will move 84,000 inches per minute, or 1400 inches per second, or 116.6 feet per second, or 79.5 mph. And, that is under ideal conditions, which is never attainable.

The RPM range I suggested is probably normal for most of our sport engines. Now there are some that will operate at twice that RPM range, but they ususally don't last very long. Our engines, most sport engines, are just fine within the normal power range. For the engine you are talking about, that is about 12,000 RPM.

For your information, larger engines, like my favorite OS 1.20 AX, operate at about 9,000 or so RPM but can turn a larger prop with a greater pitch. So, they can really move the plane through the air quite smartly.

I hope this helps.

CGr.

CG says it way better then I could, use the prop that makes the plane fly the way YOU think it should.I found the 11x6 works for what I want to do, you may not like it.

Yes, an 11x6 or 11x7 is good for that engine.

One more little ditty that may also help you out.

The pitch, the second number in the prop size, is what will set the speed of the aircraft, as well as how well it "pulls".

Think of the pitch as the gears in your car. If you use a lower gear, for instance, your car will accelerate faster, but it will not have a very high top speed, yet the engine is turning very fast. As you change gears, shift from first to second, for instance, you car will pick up speed, but do it at a slower rate and the engine will be at a lower RPM for the same speed as it was in first gear.

So, if you decrease the pitch, you gain power but go slower, but accelerate faster. As my above post pointed out, with a given pitch, and a given RPM, you have the potential of reaching a certain speed. Increase the pitch, you will have a higher speed but less "power" to do it. (your engine has the same power, but it is working harder to turn the prop with a higher pitch than it was with the lower pitch, so it gets to max speed quicker).

CGr.

Thanks CGR. I'll take both props with me to the field and have my instructor check them out. I'm still learning.