How Can We Design a Solar Drone with Long Duration Flight and High Payload?

[yuiop]

Recently, Facebook suggested solar powered drones could be used to provide internet coverage in remote areas and they would basically act as low altitude 'geostationary' satellites. The current duration record for a solar drone is 2 weeks using energy stored in batteries during the day, to keep the aircraft airborne overnight. http://www.barnardmicrosystems.com/UAV/milestones/solar_powered.html

What is the best way forward to design a drone that can stay up for months or even years? For example another way to store energy during the day is to gain altitude and glide overnight, eliminating the requirement for heavy and expensive batteries. I think the key parameter here is the sink rate rather than the glide ratio as the glider would have to keep itself in the air for roughly 12 hours each night without losing too much altitude.

Is wind a problem at the altitudes considered (around 50,000 ft) requiring higher wing loading and glide speed to penetrate into the winds and hold station? What other considerations are there for a long duration solar aircraft with a significant payload and requiring power to transmit internet data? Are flight durations of years feasible?

 

[CWatters]

Several years ago (>15) I attended the Militky Cup event in Switzerland. One event was an informal competition for solar powered model planes of around 3m span. They were typically very marginal on performance and occasionally had difficultly maintaining height if the sun went in. No doubt things have improved since then but I bet it's still a challenge to build a small plane that can fly on the low power available.

I recall the best planes were flying wings covered in cells. They had fancy electronics including variable pitch props so that they could adjust the load presented to the cells and keep them operating close to their sweet spot. If you used your hand to cast a shadow on the cells you could see the prop pitch change!

There might be some vids on youtube but there were a lot of other competition classes as well that weren't solar powered.

 

[CWatters]

Photo from 1997...

 

[yuiop]

Photo from 1997...

Are pusher props pretty standard design for solar powered models? If so, what is the thinking behind that?

 

[AlephZero]

Are pusher props pretty standard design for solar powered models? If so, what is the thinking behind that?

It probably helps keep the center of mass in the right place, if there is no fuselage and tail.

 

[yuiop]

So altitude and overnight gliding or batteries for extreme duration solar drones with a realistic payload? (Or hybrid?)