I bought a GP .40 without a prop. What prop do I want for this? Thanks; Don

Depends on the plane. Have a look at the manual and pick from the manufacturer's recommendations. For a draggy plane or for a plane you want a lot of vertical performance from, you'll want to stick with the big diameter/low pitch options. For speed, go the opposite route.

There is a prop chart sticky right here on beginners at the page top.

11 x 4 with a trainer or slower flying model.

10 x 6 for something sportier.

APC props are among the more efficient but are a little more brittle than say the grey Graupners.

The owner's manual for the GP 42 recommends a 10x6.

I bet the owners manual did not mention anything about aircraft type. For a trainer with a good idle and throttling use the 11x4 or 11x5 and only use the 10x6 for what most would characterize as a hot rod airplane.

John

Pretty good advise from John, prop size has been discussed here over the years with the rule of thumb posted so many times it could fill a page all it's own. The owners Manual shows props in the working range of the engine then it's up to the pilot to test props until they discover the prop make and size they like the best. I have my new little Up-Roar I scratch built with the FOX .40 and started out with a 10X6 master airscrew. I am looking for a slow flight with as much thrust as I can get. At the moment the plane is a bit faster then I was looking for and I haven't started prop testing yet. I'm betting an 11X4 or 6 will be what I end up with. I just couldn't locate enough props in the sizes I was looking for to start testing. The MAS 10 inch props with a 6 pitch was were the only props I could find at my LHS. I found a bunch of APC props in the sizes I was looking for but they were all reverse thrust?? The LHS must have gotten some kind of deal on them??