Would a piston or turbine engine be best suited for a small UAV flying 8000miles?
Not to burst your bubble, but doing this &quot;cheap&quot; isn't possible.
First look at range. Light aircraft that have a 2-ton+ payload have limited range, look at the upcoming Cessna 408, it can carry 3 tons but only has a 1,000 mile ferry range. In order to increase the range, you need to carry more fuel, reducing your load. So lets say you dedicated 1 ton to a fuel tank (neglecting the weight of the tank/piping), that gives you an extra ~300 gallons, maybe an extra 500 miles (I don't have info on fuel burn rates at this time, just a generous guess).
Now, you want 8,000 miles, so you need something that can carry a lot of fuel. Now you're in the 767-ish sized territory, so payload won't be a problem, nor will range (when you add , but it will be expensive. A new 767 cost around \$100 million, or you can lease it for between \$600,000 and \$650,000 per month.
If you need an aircraft with air-drop capability, now you need to find something ex-military. A C-130 won't do (range is only about 2000 miles), even the gigantic C-5 Galaxy only has a 4,800 mile range. There just doesn't exist an aircraft to do this.
Add on top of that the cost of developing an autonomous flying system with take-off and landing capability, you are well into the $200 million range at this point.
So now what? You've got a 767 with large payload capability (but no air-drop) that can fly somewhere and back. Time to fly right? Nope! You now (and have been for some time) need to deal with regulatory requirements for multiple jurisdictions since you are crossing a lot of unrelated airspaces in your flight. You need to coordinate demonstration flights and paperwork, along with all the compliance needed for a &quot;one-of-a-kind&quot; aircraft. Many won't want you flying over populated areas, or may deny you completely. Might have to do some lobbying ($$$).
Ok... got it, regulations approved, flight approved, aircraft bought, cargo bought, time to fly? Almost, try getting insurance. It will be expensive, probably hundreds of thousands per month.
A flight crew is the least expensive thing here
If you are serious about this, save yourself \$100m to \$150m and hire a qualified crew. You don't have to go through the development of this automated system, the regulatory approvals, the test flights, the insurance costs, the reluctance to have a 90+ ton aircraft flying without any operator intervention at 500 miles per hour and just hire somebody to fly it.
Pilots aren't the most well-paid, but lets say you have 2 crews making \$100,000/year each ($400,000/year total) with benefits. That's \$33,333/month, for the cost of development (lets say \$100m) of the automation system, you can employ these people for 250 years.