So propeller engines are more efficient than jet engines? Is this true regardless of speed, plane size? I always thought jet engines were revolutionary, were they really just quieter?
Jet engines were revolutionary, but not because they are quieter. Pure jet engines are actually incredibly loud. The only way the noise was reduced in modern airline engines was the advent of high bypass engines where most of the air going through the engine is only slightly accelerated.
However, for modern airliners, fuel consumption is important. So a second class of jet engines was used that send a small portion of the air through the "core" of the engine, where it is burned and power is extracted, and the majority of the air is accelerated through the fan without being combusted. This is much more efficient (and much quieter, since most of the noise you hear is from the interaction of the high speed air coming out of the engine with the surroundings). The open-rotor concept described in this article is just a natural extension of the high-bypass engine. You get a much more efficient engine when you can increase the ratio between the amount of air that you accelerate through the fan compared to how much you send through the core and burn (this is called the bypass ratio). Since the weight and drag penalty increase exponentially as you increase the size of your engine, removing the duct around the fan becomes necessary once you get to very large bypass ratios.