Wings are great for crusing over long distances and carrying heavy loads, but they aren’t that great if your aircraft needs vertical agility. Rotors, on the other hand, are great for vertical agility, but they aren’t that great for long distances and heavy loads. Any aircraft that wants to fly efficiently can be designed for cruising or hovering, but not both.
Lots and lots of people have tried to figure out a way of making some sort of compromise work. Mostly, this involves stapling as many vertical rotors as you have a budget for to a fixed-wing aircraft and just calling it a day: When you want to go up or down, you use the vertical rotors, and the rest of the time, you use whatever other rotors you can afford to have mounted horizontally. If you’re very clever, maybe you come up with a design that uses one set of rotors for both vertical and horizontal flight, either with some kind of rotating wing or with a vehicle that can pitch over in flight; but the fact remains that your design is wasteful—either you have useless rotors when flying horizontally, or useless wings when flying vertically.
At ICRA this year, researchers from the Singapore University of Technology & Design introduced a new kind of flying robot called THOR: Transformable HOvering Rotorcraft. THOR manages to achieve very high structural efficiency by using all of its aerodynamic surfaces in both vertical and horizontal flight modes, transforming from a flying wing into a sort of whole-body spinning bicopter thing that you really need to see to believe.