"B A R R Y" wrote in message
...
I think the aft CG will result in a higher cruise speed because the arm
between the center of lift and CG will be reduced (assuming CL is
behind the CG) and the tail will have to do less work.

I would think the CG that requires the least amount of trim would be the
most efficient. If that is in fact true, the most efficient CG would vary
for different aircraft.

I agree that there's a secondary effect with respect to control surface drag
related to the trim position. However, the primary issues are a) downforce
balancing the CG (presumably forward of the center of lift), which has to be
compensated for in the form of more lift (so more induced drag) and b) the
induced drag from the horizontal stabilizer/elevator itself (adding to the
total induced drag directly). Both of these directly correlate to the CG
position.

The parasitic drag due to trim position is only likely to be significant
when the CG is already relatively far aft and the increase in induced drag
isn't large. So yes, the *exact* place where the least total drag occurs
may not be precisely at the rear-most CG position available, but I suspect
in most cases it is (hopefully airplane designers aren't normally creating
trim systems that require a non-neutral trim position for a neutral elevator
force), and when it's not, it's not going to be far from there.

Pete