# 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?
• What kind of an aircraft are you thinking of? In what ways might it be similar to and different from a Reaper, say? Feb 10 '15 at 5:36
• the reaper is a good starting point, at least in terms of using it as an example for why the design looks like it does. i was thinking something more in the tactical realm, perhaps no bigger than 15-20' wingspan, or something hobbyists could put together on the weekend. Feb 10 '15 at 5:56
• A balloon :-). ${}$ Feb 10 '15 at 6:20
• What exactly is a powered glider? Do you mean a motor glider that has an engine for take-off only? Feb 10 '15 at 11:02
• @RedGrittyBrick there are a number of different kinds of motor gliders: self-launching (instead of a tow or winch), sustaining (in case you fail to find lift), and touring (can operate like an airplane).
– rbp
Feb 10 '15 at 12:59

Natural stability would be highest on the list for me. Look at free flying model airplanes for inspiration. They have their own characteristics: