What aircraft design elements contribute the most to sustained flight? If I wanted to design an aircraft for autonomous operation and maximum endurance, what design philosophies provide the greatest benefit?  I'm imagining a powered glider.  Does not have to be highly maneuverable.

I'm interested in relative proportions, wingspan, length, center of gravity, center of pressure, wing placement, dihedral/anhedral, engine placement, fuel type, etc.  


Is there a "sweet spot" for center of gravity relative to center of lift so as to maximize stability?  
Since it will be mostly autonomous (computer-controlled), can I design for less inherent stability?
What engines/fuel types are the most efficient?
Natural stability would be highest on the list for me. Look at free flying model airplanes for inspiration. They have their own characteristics:


Very low wing loading for minimum sink speed
High natural stability, and consequently a very narrow speed range
High aspect ratio wing, thin airfoil with high camber
Extremely short fuselage ahead of the wing (no payload to carry)
Extreme fuselage length aft of the wing which provides ample aerodynamic damping
Strong dihedral of the outer wing.


This in combination means that your control software can be very simple. Relaxed stability will require many sensors and built-in redundancy, and the more parts you need to add, the more possibilities of failures you will end up with. In contrast to that, the laws of physics never fail, use them to your advantage. See below for an example which I copied from the Wikipedia page on free flight models:



For airfols, c.g. location and all other details I recommend to look at several free plans until you see a common pattern.

Regarding the type of fuel: Depends how noisy and messy you tolerate the engine to be; some people will only fly electric while others are happy with two-stroke engines.