ORIGINAL: dieseldan
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
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The one way to seriously limit a two-stroke's useful lifetime is run it excessively hot. The way to do that is to load it down with too much prop. Super Tigres were designed to rev up to "normal" rpm. This normal rpm is higher than what many of today's younger 3D flyers are used to, but they are valid just the same. The problem is that the ignition timing is partially controlled by the high speed needle. If set rich enough, all is well. If not, excess heat is generated. Heat reduces the engine's useful lifespan. One way to reduce heat is to reduce the load placed upon the engine. Finding that perfect compromise between lower temperatures and what is needed to fly the model properly/satisfactorily is the goal.
I'm not saying that you can't run an 11x6 on a .40, but the operator will have to be careful that they do not get off too lean. The same is true with all props, but a 10x6 or 11x5 is less prone to build up as much heat.
Ed Cregger