I guess many people know that im a newbie in the flying hobby... I have a Superstar MK II it came with an OS .40LA in the manual it is recomended using an 11X7 prop for running in, then says after a good run in period you can use a 10X6, 10.5X6, or 11X7. I was just wondering... What exactly do these numbers mean, I am pretty sure it has to do with the length and pitch of the prop,

If my assumption on lenght and pitch are incorrect what are the numbers, and how exactly is flying affected by these numbers?

you are right about the numbers being the first one number length and the second number pitch

On a trainer with a 40LA engine you might try the 10x6 or 11x5 props. The 11x7 is a bit big for the 40LA. First number is diameter in inches and the second is the pitch - one revolution of the prop would move the plane forward 6 inches with a 10x6 prop.

The length of the prop is equivalent to the amount of 'traction' that it has, i.e. the more surface area it has to affect airflow. The pitch is the forward movement ( in inches ) of the prop per revolution. So a 7 pitch prop will pull the plane 7" per revolution. Of course that is a simplified explanation but its a close approximation and a comparative measurement.

For a given engine like your .40LA, a range of props is given that depend on how you want to fly and the type of aircraft flown. For instance a trainer might best work with a prop that has a lot of "traction" (length) but not so much speed (pitch). Rpm (rev. per minute) is also affected inversely by prop length and pitch. A pylon racer would prefer a greater pitch while spinning smaller length prop to gain some rpm.

If the manual is recommending a larger length and greater pitch to break in the engine, it is probably to keep the rpms low until the engine is broken in.

For that engine I would use a Master Airscrew 10-5 or 10-6 prop. [8D]

The first number is the length of the prop, tip to tip. The second number is the pitch. This number is the number of inches the prop will THEORETICALLY move through a SOLID material in one revolution. The capitalized words are important. Because air is compressible, and not solid, there's a lot of slippage so the REAL amount of movement is never the real pitch number.

Now, what does this second number really mean?

GIVEN THE SAME RPM:
A large diameter/low number pitch prop will give more pull and less top speed.
A small diameter/high number pitch prop will give more speed and less pull.

It's like low gear/high gear in a car.

Since you want a trainer to not be too fast, try a 11 x 4 on that LA.

Dr.1

What manual suggested the 11x7 for break in? I use the OS 46AX which is more powerful than the LA 40 and the recommended breakin prop is 10X6. My regular flights after that have been made with the 11x6. I have a buddy who uses an 11x7 on his identical plane. It seems to fly ok but my LHS thinks that prop is a bit much for the motor...however, the manual does include this size as a recommended one.

I would go with the 10X6 for your trainer. The 6" pitch is sort of a "happy medium" you get enough speed, and you still get good pull. I used a 10X6 on my 40 LA and I really loved it.

LT-40

According to the owner's manual (page 13) for the .40 LA/.46 LA/.65 LA, the recommended prop for the .40 LA is 11x5. This happens to be a good choice for .40-sized trainer aircraft:

http://manuals.hobbico.com/osm/40-46-65la-manual.pdf

for a 40 LA,, use a 10x6 master airscrew glass filled prop.. i've found it to be best for basic flying.

for a 46 FX, you have more choices based on what kind of flying you want to do
use an 11x5 master airscrew for most flying.. a 12x4 can be used for hovering..
i use a APC 12.25x3.75 on an OS 46 FX, and with a mouse can pipe, it hovers very well

Hi!
Sorry! But a master Airscrew prop is never good! You should use a APC 11x5 or 11x6 for best flight performance with your OS LA .40, at least if you fly at sea level.

I now realize that i used the wrong numbers in the post... Ive been using an 11X5 for breaking in the engine... I repeat an 11X5 not the larger size previously listed in the post.