As far as I am aware, there have not been any model aircraft that have successfully circumnavigated the Earth without stopping. Few trans-Atlantic attempts have succeeded (see: The Spirit of Butts' Farm). My question is: what makes long-range model aviation so difficult? Is the small size of model aircraft disadvantageous when traveling long distances? Or is success just a question of improving the materials and propulsion, and flying during good weather?
So many different issues, I study a lot of the FAA regulations closely but have contemplated this same question a few times.
- The power and weight of gear is heavy, so to make it all the way around is not going to happen without a lot of hacking. (solar proof of concept or similar notwithstanding)
- Efficiency of solar (power and weight) is a problem. The square feet of solar needed is quite large and disproportionate to how we conceptualize "RC planes". These vehicles are really flying solar arrays.
- Data, control, tracking, and safety assurance is much harder half way around the globe, even with satellite assistance
- Satellite data and connectivity hardware is still big leaving this type of task to the larger military units.
From a regulatory perspective
- FAA would rather cut their hand off than issue BVLOS waiver
- Patchwork problems internationally exist not only for aviation, but also frequency use . EU radio freqs are very different. When traveling to fly internationally it takes a while to make sure I am legal or have legal gear.
- Clearing airspace for these attempts is a serious undertaking of budget and politics.
Weather is a huge concern for small craft. Variations in the strata of the atmosphere are no joke. Be it thin air at altitude, winds, cold/hot temperature, all of this is hard for a small craft to manage.
"At heights of 7 to 16 kilometres, the air in jet streams moves at constant speeds of over 25 metres per second or 90 kilometres an hour." source
The US Military is one of a few orgs with advanced programs for this type of use. The U.S. military UAS groups has Tier 2+ with a '3,000-nautical-mile (6,000 km) radius'. These devices range from sub-55 pound to 2+ ton vehicles.
For what it is worth, I know some people with ties to a successful cross Atlantic RC plane flight. The stories they tell underscore how hard this task is to pull off.
In the end our number one obstacle for extended UAV flight of this type (and in general) is the same problem we have for phones and laptops. Our current battery tech is 20-30 years behind the technology it powers. The physics community engaged with this part of the equation frequently try to move the ball forward but the person to make a lighter and better battery will win the internet. Other fuel types are out there too, for example Hydrogen where the MFD3000 by Bobby Watts is available with a hydrogen fuel cell providing some interesting performance and represents good progress.
I think anyone with some expertise in mechanical, electrical, aviation, fuel and power train, etc could stop by and add a few more reasons why this is generally out of reach right now.