How to maximize flying distance
 

I will build a surveillance UAV, and I want to maximize the flying distance so it can cover as many kms as possible on one full tank. I'm looking at the "TAM-5/The Spirit of Butts' Farm" for inspiration. Size and performance looks very much like what I need. However that was a one-off event, maybe hard to replicate. But on the other hand, it had a max weight limit of 5 kg that I can ignore.

How would you go about building such a plane? Hopefully mostly off the shelf parts.

(I tried to link to Wikipedia, but it was not allowed, but you can Google "The Spirit of Butts' Farm" to read more.)

Engine: I assume 4-stroke or is there a more fuel efficient type? Optimum fuel mix? Fuel price not an issue. Reliability is an issue (24+ hrs in the air), so what brand and model would you prefer? I'm looking at the 11cc Saito FG-11 as a starting point.

Body: 150-200 cm wingspan seems about right. Acrobatics and looks unimportant. Powered glider or traditional? Any kit suggestions?

Any suggestions, or links to similar discussions/projects?

Thanks,
Joe

Don't know about the 24 hours, but a diesel is the most efficient. A 2 stroke would be ok. They take a while to warm up and can't be overcompressed, a bit of an art to it. 4 strokes are ok too, but a bit heavy. A smaller than stock carb in any case will help mileage.

Most countries have pretty strict regulations on the flying of such aircraft nowadays for obvious reasons. If you let us know what country you intend to fly this surveillance UAV in , we can give you some guidance as to the hoops you need to jump through in advance.

An off the shelf 4 stroke petrol engine possibly with a smaller carb will give you long flight times.

A Senior Telemaster at 2.4 m span is a very stable off the shelf solution for an airframe.

If you are in the US, you'll need to check out the UAV laws as they govern non-line of sight flight. There is education and licensing involved. Don't give all of us RC hobbyists a bad name by trying this without it.
24 hours in the air isn't going to happen with anything off the shelf. To carry camera gear that would be meaningful for a vehicle like you are talking about, you'll need a much bigger plane. I'm guessing people don't do surveillance with a GoPro, right?