You most likely have the .45 NT engine on your Alpha. I've found the 11 X 5 that Hobbico sells for the NextStar is a great prop for trainers and should work just fine. It'll give a bit better climb and just a tad less speed than an 11 X 6 but that is fine for the Alpha - but its not like your after willing a Q-500 pylon race with it. You will find the overall performance will increase with the change to a 2-blade prop.

Hogflyer