The UAV's airframe refers to the overall design and configuration of a UAV. This is the first decision to be made when buying or designing a UAV, and has a large impact in the UAV's capabilities. The most popular UAV airframe types can be organized into three main categories: airplanes, multicopters and helicopters. Each category has its own advantages and disadvantages that must be considered when choosing a UAV.
Airplanes, also known as fixed-wing aircraft, are the more traditional form of UAV. They are composed of one or more wings that generate lift and stabilize the aircraft as it moves forward, and a propulsion unit to propel the aircraft forward. Control surfaces on the wings provide control during flight. This configuration is the most aerodynamically efficient of the three and provides the longest flight time, speed and range. Also it is mechanically and electronically simple, and thus cheaper and easier to build and fix than the other options. The main disadvantage of this configuration is its inability to hover or take off vertically. The airplane needs to be moving through the air at all times, otherwise the wings cannot generate lift. Not being able to hover means the airplane cannot stay in one point in the sky in order to observe a point in the ground, and it also means it needs a runway to be able to take off and land. These limitations can be overcome to some degree by using catapults to launch the aircraft and nets to recover it, and circling over a point in the ground.
Helicopters overcome the main disadvantage of airplanes, as they are able to hover and take off vertically from small areas. They do this by using a large rotor, which is basically a set of rotating wings. The wings can change pitch using the mechanisms known as collective and cyclic pitch control, in order to control the orientation of the aircraft. Since rotating the main rotor creates a torque on the vehicle, a small tail rotor is required to cancel this torque. The tail rotor can vary its thrust, either by varying its speed or blade pitch, to control where the helicopter points to. A mechanical system on the main rotor called the flybar is required to stabilize the aircraft by mechanical means. Recently some helicopter designs known as flybarless helicopters are removing this system and substituting it with an electronic stabilization system. This configuration is less efficient than an aircraft and generally provides lower range, flight time and top speed. The main disadvantage of this configuration is its mechanical complexity. This means that the helicopter is more expensive, harder to set up and maintain, and much harder and costly to fix after a crash.
The multicopter or multirotor is one of the most popular types of UAV airframe configurations. This configuration uses multiple propellers to provide lift force, instead of using a single lifting rotor like a helicopter. The multicopter is able to control its orientation by adjusting the speed of each of the propellers independently, causing differential thrusts and torques. Multiple types of multicopter exist, and are classified according to the number of propellers they use and how they are arranged. Other variations of the multicopter concept exist, such as those using variable-pitch propellers or ducted fan propulsion units.
The main advantage of the multicopter configuration is that it maintains the vertical takeoff and hover capability of a helicopter, but with the mechanical simplicity of an airplane. The use of multiple propellers eliminates the need of a complex rotor mechanism found in a helicopter. This mechanical simplicity comes at the cost of electronic complexity and cost, as the multicopter requires a flight controller to stabilize the aircraft and calculate appropriate motor speeds according to control inputs. Also, the multicopter requires multiple motors, speed controllers, and a power distribution board. However, recent advances in electronics have made these components smaller and cheaper, and have made the multicopter a viable alternative for general UAV use. This UAV configuration is also less aerodynamically efficient than a helicopter, and generally provides lower flight times and payload capabilities.
The advantages and disadvantages of each airframe type must be weighed in order to select the best UAV type for a given task. The table below compares the benefits of each airframe category.