Brian, I'm going to have to go back and do some work to answer your question. I printed out the data for 5% and 15% static margin for the Axiome, and it would suggest I could go further back yet with my cg. (This is with std. stab efficiency). The problem is I'm at 33% of MAC now and I'm not between 5 and 15 on the static margin. Doesn't sound right to me but maybe that's what happens with these swept wings. Anyone know where CPLR's Axiome cg is/was?
Jim O
I ran the calculator with different static margins until the cg matched my actual cg and it came out about 17%. This is with the stab efficiency set to low.
I believe the cg calculator looks good for finding the MAC and the AC but is suspect when it comes to the neutral point. It can calculate the tail volume coefficient with the data supplied, but needs to make a bunch of assumptions regarding lift curve slopes and downwash angles. I ran through the calculation making my best guess assumptions including a stab efficiency of .5, and came up with a neutral point an inch forward of what the tool calculated. It is still way aft of the AC suggesting I still have a lot of static margin. The tail volume coefficient kind of dominates the results and maybe we shouldn't be surprised as we do have long tail moments and now relatively small wings on our models.
I'd have to make a major change in the battery mount to move the cg back any further and I don't want to add any more tail weight so this may be the end of the discussion. Then again I'd sure like to see how it would fly with a lower static margin like you'd expect and want on an aerobatic airplane.
Jim O
Edit: I just noticed that my scratch calculation of the NP is nearly right on the money if I compare it to the tool's calculation with low stab efficiency. In other words the difference between using standard and low changed the neutral point by one inch which would change the static margin by 7.5%. This kind of makes me believe it works.