I am at 4000ft And I have found that a 9x4 has worked the best. For what I want slow and low. The 10 just never found the power band. I think If I was lower the 10x4 ma would be the best. Now I am wishing I would have put a 46 on it. I have craseh it 5 times in 2 days. No damage. I have never even come close to crashing my topcap. THese things are a gas. Thanks again. Dan