Why would you choose to use forward props vs rear props vs jets on a small aircraft? What are their respective benefits?
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You can have either two types of props: pull props or push props. Pull props go in front, and “pull” the craft forward, push props go in the rear and “push” the craft forward.
Push props are good because they don’t get in the way of cameras or missiles. But ALSO, they decrease drag on the craft and increase efficiency. They do this by “eating” some of the turbulent air after it passes over the craft.
Since UAVs often spend a lot of time in the air “loitering”, this lets them stay up even longer! Drones like the predator are cheap and expendable, so we value efficiency over raw performance (dodging anti air missiles or fighters) so they get the push props.
Future UAVs that are more like fighters will not use props at all.
When talking about airplane engines the big questions are thrust vs weight and efficiency bands. Propeller engines have a relatively low thrust to weight ratio. This means for the weight of the engine it doesn’t produce much thrust. That being said they produce more thrust at lower engine speeds than a jet would. So a propeller engine consumes less fuel at low speeds than a jet would.
Side note: there is no real advantage to a rear facing propeller. Most airplanes that do that are gimmicks or proof of concepts for engineers. The only thing it allows is the plane can be shorter in length because you can fuse the tail section with the engine housing. But that doesn’t really do much for the function of the plane in a practical sense.
Now jets on the other hand have a high thrust to weight ratio. This means that they can create a lot of force compared to the weight they add to the aircraft. Jets are also more efficient at high engine speeds that propeller engine. This means that jets can make small things go really fast or lift heavier things than a propeller engine could, but they consume a lot of fuel to do it. Less than a propeller engine would for an equal amount of work, but a lot of fuel nonetheless.
Edit: I’ve seen a couple mentions of drones. Drones have rear push propellers for space saving and forward visibility. It doesn’t add any efficiency. While you or I may be able to see through the rapidly spinning propeller, a camera would likely catch snapshots or even be completely blocked by the rolling shutter effect. But that’s a different problem altogether. All you need to know is there is no efficiency increases, it’s merely a consequence of engineering around a plane whose main job is gathering visual intelligence.
You also have turboprops which use a jet engine to turn a propeller. And most jet engines are turbofans, where 80-90% of the thrust is produced by the big fan in the front just pulling air through without burning it. Kinda like a prop with a ring around it.
Generally speaking:
propellers have better efficiency at lower speeds, but limit your top speed, because you get issues with the blade tips reaching the speed of sound otherwise. This would be really loud, and have bad efficiency.
Modern jet engines are Turbo Fans, which are basically a duct around the propeller. They slow the air down before it enters the large Fan (we call the propeller Fan now) (the diameter at the intake is smaller than the Fan diameter. This Expansion slows the air down), which allows the to encounter this speed of sound issue later. (the tips of the blades are however still supersonnic, but because they are in the duct, this isn’t that loud, and produces fewer efficiency issues). Jets can thus fly at higher speeds.
Both engine types are however powerd by the same thing (in larger aircraft, with let’s say more than 600 hp per engine), a turbine. On small aircraft, normal piston engines are used.
Right now, several companies are trying to develop a mixture between a propeller and fan (again), the propfan. This might be used on the future version/replacement of the 737 and a320.
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