I do not know the wt or wingspan of the trainer but I would try the 11x6 you will get plenty of thrust at a lower rpm you do not need
13K on a trainer I have a couple of VMAR apache lls that are HEAVY and the irvine 40 diesels in them turn a max of 10000 and
they do fine they are swinginq 12x6 props the ST40 is a fine long lasting engine a great value, think the tower price $50 and ringed
martin