I have an OS 40 LA motor on my Sig Kadet LT-40. I have been using an 11x5 prop. When I was at the flying field last week, someone gave me a 12 1/4 x 3 3/4 prop. It seemed to give the motor more power. Is it OK to use this prop on this engine? It does not show up in the recommended list for the motor. Will using it cause an damage to the motor?

Both of those props seem to me to be a bit much for the engine in question. Will they hurt it? No. It will require a higher idle speed due to the extra load on the engine as opposed to using a 10x6 or so. At about $3-$4 a prop for Master Airscrew get a few different sizes and experiment with them untill you come up with a combination YOU like and that may just be that 12.25x3.75 you have. As far as I know APC is the only producer of that size prop and it was desinged for the .46 Fx/Ax and similarilly powered engines (TT .46, etc.) for 40 size 3D planes. Thus it costs more than the afor mentioned MAS props.

Put a APC 10-6 on it, if you think that 12.25-3.75 perked it up. It will extra perky.

Yep, use a 10X6. That 12.25X3.75 is for 3D and hovering. Don't think you need that on a Kadet.

I think the Black MAS 10-6 works better on the 40 LA

I would use a 10*5 10*6 or 10*7 a 12 inch prop is a bit much for a .40, and the OS LA's are actually i little weaker than most .40's

Austin

APC makes a 10.5 x 4.5 that should be good for that engine. As said, get several props and find the best for that engine and airplane. Don't throw the rest away as they will be the best for another engine/airplane.

I use a 10x6 and I love it. I don’t like my engine thought

A 9X7, 10X6, 11X4 or 11X5 would work well with the LA. The 12" prop you have won't cause damage to the engine if the plane stays on the ground. If you put a 12" prop on that .40 in a .40 size plane, you might have enough power to take off, but turns are going to require a lot of altitude. The LA series are concidered generic type engines. They won't have enough power to take the plane through a turn without losing altitude or stalling. The prop you have is better suited for a high end .46, a .55 or .61 engine. On your LT-40 / .40LA combination I would recommend a 10X6 or 11X4.

Hi!
If you fly at sea level a 11x5 or 11x6 APC is what works best in a high winged trainer.
10x6 is not a good prop size if used on the OS LA. 40 or .46 if you have a trainer in mind.The reason for this is that the LA engines doesn't like to rew that much...but they are good stump pullers and can handle rather large diameter props!

I agree. I had excellent results using APC 11x5 props on the older .40 FP, an older engine that's similar to the .40 LA. This results in excellent climb and reasonable top speed for high wing trainer style aircraft. These engines are like four-strokes and make their best power at lower rpm. Putting a tiny prop on these engines because you think that it is written in stone that an engine has to spin 14,000 to 16,000 rpm just because it's a two stroke leads to the dog reputation of these engines.

You said it seems to give the engine more power, which means it suits the engine/airframe combination. That says it all. If the engine were overheating you would notice it. You might try an 11x4 or the suggested 10.5x4.5 and see how it does. Props are a pretty cheap part of the hobby and it is worth experimenting, as you are finding out.

I'm not surprised your prop worked well, because the LT-40 is a big draggy air frame, so high pitch props are apt to be wasted on that plane.

Also, I agree with others who point out that this engine does fine on largish props. I had an OS 40 FP that seems to do best on an 11x5. But that was a small plane.

Jim

Don't confuse noise with power. If it revs higher RPM, this may not be indicative of power increases, and may, in fact, hurt the engine. Use the recommended prop for that engine. If the plane came with a particular size prop, stick with it unless you really know what you are doing or if someone with a lot of experience helps you out. Don't try to make this plane do something it was not designed to do by trying to over or under prop'ing it.

CGr