I have a Hobbico Trainer 40 with an OS .46AX engine. How should I determine the prop size to use. 11x6 or 11x7? Or is another size more appropriate. The trainer manual says to use the motors recommendations of which are several sizes to choose from.

I have flown before. I am using this model to get back into it after 10 years off.

If you are just starting out use an APC 11x5. It will give a bit slower speed but lot's of pull to get you out of those moments when you need it.

If you have some experience use the APC 11X6 and with good expereince even the 11X7. The 11X7 will make it a speedster but it will be very hard to slow the plane down.

When you are first learning, it is best to choose a prop that has a higher diameter and lower pitch. This will give you better takeoff performance, and also allow the plane to slow down more for landing. One thing that could limit the prop diameter is the length of the landing gear. Too long a prop and it will hit the ground while you are taking off and landing.

If you've got the ground clearance, you could even try a 12x4 on the 46 AX. With a trainer, you'll get lots of thrust to get off the runway quickly. Also, think about using an APC prop. I maidened a Tower Trainer with an OS LA 46 Monday for another club member. He was using the provided Nylon 11X5 prop, and takeoffs were long and iffy off our grass field. We swapped it for an APC 11X5, which apparently has a different air foil, and performance went up markedly. It turned about 300 rpm faster, but more importantly, it was up off the grass in about half the distance.

Brad

G'day Mate, An 11x6APC a 12x4APC if you have enough ground clearance, this prop will give you about 13,000RPM, with an AX or on my significant others plane using an OS 46AX, we use a 12x6APC, & an idle bar plug to help keep it running smoothly for take off, all we had to do was put 3" wheels on it, for ground clearance.
Good Flyin Mate.
Allan.