On 2014-08-24 10:55:45 +0000, plantain said:

So, my understanding is that as a general rule in flying, the further
aft the centre of gravity, the less drag.

Well, no. Once the CofG is far enough back that the elevator has to
produce lift then you are inneficient again, because the elevator has a
worse aspect ratio than the wing.

The safe rearward CofG limit is well beyond the most efficient rearward CofG.


Q1) I understand that negative flaps reduce the amount of torque
generated by the difference between centre of lift and centre of
gravity, but I'm not quite sure how - I assume the centre of lift has
to move forwards because I don't see how the centre of gravity would
change. Surprisingly there seems to be very little information about
this on the internet.

If the flaps aren't producing lift then something else further forward
has to instead.


Q2) My next puzzle comes from flying my unflapped glider aft, usually
90% aft. This means that when I am at cruising speed, I need
substantial forward stick/elevator deflection to maintain that speed.

This doesn't feel efficient - so my question is, is it more or less
efficient than if I had the centre of gravity forward such that neutral
elevator produced my intended cruise speed?

That definitely sounds inefficiently far rearwards.